Hydrocarbons in Contractional Belts (Geological Society Special Publication 348)

Hydrocarbons in Contractional Belts

The Geological Society of London Books Editorial Committee Chief Editor

Bob Pankhurst (UK) Society Books Editors

John Gregory (UK) Jim Griffiths (UK) John Howe (UK) Rick Law (USA) Phil Leat (UK) Nick Robins (UK) Randell Stephenson (UK) Society Books Advisors

Mike Brown (USA) Eric Buffetaut (France) Jonathan Craig (Italy) Reto Giere´ (Germany) Tom McCann (Germany) Doug Stead (Canada) Maarten de Wit (South Africa)

Geological Society books refereeing procedures The Society makes every effort to ensure that the scientific and production quality of its books matches that of its journals. Since 1997, all book proposals have been refereed by specialist reviewers as well as by the Society’s Books Editorial Committee. If the referees identify weaknesses in the proposal, these must be addressed before the proposal is accepted. Once the book is accepted, the Society Book Editors ensure that the volume editors follow strict guidelines on refereeing and quality control. We insist that individual papers can only be accepted after satisfactory review by two independent referees. The questions on the review forms are similar to those for Journal of the Geological Society. The referees’ forms and comments must be available to the Society’s Book Editors on request. Although many of the books result from meetings, the editors are expected to commission papers that were not presented at the meeting to ensure that the book provides a balanced coverage of the subject. Being accepted for presentation at the meeting does not guarantee inclusion in the book. More information about submitting a proposal and producing a book for the Society can be found on its web site: www.geolsoc.org.uk.

It is recommended that reference to all or part of this book should be made in one of the following ways: Goffey, G. P., Craig, J., Needham, T. & Scott, R. (eds) 2010. Hydrocarbons in Contractional Belts. Geological Society, London, Special Publications, 348. Cook, B. S. & Thomas, W. A. 2010. Ductile duplexes as potential natural gas plays: an example from the Appalachian thrust belt in Georgia, USA. In: Goffey, G. P., Craig, J., Needham, T. & Scott, R. (eds) 2010. Hydrocarbons in Contractional Belts. Geological Society, London, Special Publications, 348, 57–70.

GEOLOGICAL SOCIETY SPECIAL PUBLICATION NO. 348

Hydrocarbons in Contractional Belts

EDITED BY

G. P. GOFFEY PA Resources UK Ltd, UK

J. CRAIG Eni Exploration and Production Division, Italy

T. NEEDHAM Needham Geoscience Ltd, UK

and R. SCOTT CASP, University of Cambridge, UK

2010 Published by The Geological Society London

THE GEOLOGICAL SOCIETY The Geological Society of London (GSL) was founded in 1807. It is the oldest national geological society in the world and the largest in Europe. It was incorporated under Royal Charter in 1825 and is Registered Charity 210161. The Society is the UK national learned and professional society for geology with a worldwide Fellowship (FGS) of over 9000. The Society has the power to confer Chartered status on suitably qualified Fellows, and about 2000 of the Fellowship carry the title (CGeol). Chartered Geologists may also obtain the equivalent European title, European Geologist (EurGeol). One fifth of the Society’s fellowship resides outside the UK. To find out more about the Society, log on to www.geolsoc.org.uk. The Geological Society Publishing House (Bath, UK) produces the Society’s international journals and books, and acts as European distributor for selected publications of the American Association of Petroleum Geologists (AAPG), the Indonesian Petroleum Association (IPA), the Geological Society of America (GSA), the Society for Sedimentary Geology (SEPM) and the Geologists’ Association (GA). Joint marketing agreements ensure that GSL Fellows may purchase these societies’ publications at a discount. The Society’s online bookshop (accessible from www.geolsoc. org.uk) offers secure book purchasing with your credit or debit card. To find out about joining the Society and benefiting from substantial discounts on publications of GSL and other societies worldwide, consult www.geolsoc.org.uk, or contact the Fellowship Department at: The Geological Society, Burlington House, Piccadilly, London W1J 0BG: Tel. þ44 (0)20 7434 9944; Fax þ44 (0)20 7439 8975; E-mail: [email protected]. For information about the Society’s meetings, consult Events on www.geolsoc.org.uk. To find out more about the Society’s Corporate Affiliates Scheme, write to [email protected]. Published by The Geological Society from: The Geological Society Publishing House, Unit 7, Brassmill Enterprise Centre, Brassmill Lane, Bath BA1 3JN, UK (Orders: Tel. þ44 (0)1225 445046, Fax þ44 (0)1225 442836) Online bookshop: www.geolsoc.org.uk/bookshop The publishers make no representation, express or implied, with regard to the accuracy of the information contained in this book and cannot accept any legal responsibility for any errors or omissions that may be made. # The Geological Society of London 2010. All rights reserved. No reproduction, copy or transmission of this publication may be made without written permission. No paragraph of this publication may be reproduced, copied or transmitted save with the provisions of The Copyright Licensing Agency Ltd, Saffron House, 6 –10 Kirby Street, London EC1N 8TS, UK. Users registered with the Copyright Clearance Center, 222 Rosewood Drive, Danvers, MA 01923, USA: the item-fee code for this publication is 0305-8719/10/$15.00. British Library Cataloguing in Publication Data A catalogue record for this book is available from the British Library. ISBN 978-1-86239-317-2 Typeset by Techset Composition Ltd, Salisbury, UK Printed by MPG Books Ltd, Bodmin, UK Distributors North America For trade and institutional orders: The Geological Society, c/o AIDC, 82 Winter Sport Lane, Williston, VT 05495, USA Orders: Tel. þ1 800-972-9892 Fax þ1 802-864-7626 E-mail: [email protected] For individual and corporate orders: AAPG Bookstore, PO Box 979, Tulsa, OK 74101-0979, USA Orders: Tel. þ1 918-584-2555 Fax þ1 918-560-2652 E-mail: [email protected] Website: http://bookstore.aapg.org India Affiliated East-West Press Private Ltd, Marketing Division, G-1/16 Ansari Road, Darya Ganj, New Delhi 110 002, India Orders: Tel. þ91 11 2327-9113/2326-4180 Fax þ 91 11 2326-0538 E-mail: [email protected]

Contents GOFFEY, G. P., CRAIG, J., NEEDHAM, T. & SCOTT, R. Fold –thrust belts: overlooked provinces or justifiably avoided?

1

ROEDER, D. Fold –thrust belts at Peak Oil

7

HILL, K. C., LUCAS, K. & BRADEY, K. Structural styles in the Papuan Fold Belt, Papua New Guinea: constraints from analogue modelling

33

COOK, B. S. & THOMAS, W. A. Ductile duplexes as potential natural gas plays: an example from the Appalachian thrust belt in Georgia, USA

57

TURNER, S. A., COSGROVE, J. W. & LIU, J. G. Controls on lateral structural variability along the Keping Shan Thrust Belt, SW Tien Shan Foreland, China

71

ROURE, F., ANDRIESSEN, P., CALLOT, J. P., FAURE, J. L., FERKET, H., GONZALES, E., GUILHAUMOU, N., LACOMBE, O., MALANDAIN, J., SASSI, W., SCHNEIDER, F., SWENNEN, R. & VILASI, N. The use of palaeo-thermo-barometers and coupled thermal, fluid flow and pore-fluid pressure modelling for hydrocarbon and reservoir prediction in fold and thrust belts

87

CAPOZZI, R. & PICOTTI, V. Spontaneous fluid emissions in the Northern Apennines: geochemistry, structures and implications for the petroleum system

115

RODRIGUEZ-ROA, F. A. & WILTSCHKO, D. V. Thrust belt architecture of the central and southern Western Foothills of Taiwan

137

HESSE, S., BACK, S. & FRANKE, D. Deepwater folding and thrusting offshore NW Borneo, SE Asia

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Index

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Fold –thrust belts: overlooked provinces or justifiably avoided? GRAHAM P. GOFFEY1*, JONATHAN CRAIG2, TIM NEEDHAM3 & ROBERT SCOTT4 1

PA Resources UK Limited, Waterfront, Winslow Road, Hammersmith, London W6 9SF, UK

2

Eni Exploration & Production Division, via Emilia 1, 20079 San Donato Milanese, Milan, Italy 3

Needham Geoscience Limited, Lynnthorpe, 4 Easby Drive, Ilkley LS29 9BE, UK 4

CASP, West Building, 181A Huntingdon Road, Cambridge, CB3 0DH, UK *Corresponding author (e-mail: [email protected])

Abstract: This volume results from a conference intended to assess the exploration and exploitation primarily of onshore fold–thrust belts. These are commonly perceived as ‘difficult’ places to explore and therefore are often avoided by companies. However, fold– thrust belts host large oil and gas fields and barriers to effective exploration mean that substantial resources may remain. This volume shows how evaluation techniques have developed over time. It is possible in certain circumstances to achieve good 3D seismic data. Structural restoration techniques have moved into the 3D domain and simple thermal constraints can be enhanced by using more sophisticated palaeothermal indicators to more accurately model burial and uplift evolution of source and reservoirs. Awareness of the influence of pre-thrust structure and stratigraphy and of hybrid thick and thinskinned deformation styles is supplementing the simplistic thin-skinned fault-bend and fault propagation models employed in earlier exploration. The ‘learning curve’ in fold –thrust belt exploration has not been steep and further improvement seems likely to be a slow, expensive and iterative process with information from outcrop, well penetration and slowly improving seismic data. Industry and academia need together to develop and continually improve the necessary understanding of subsurface geometries, reservoir and charge evolution and timing.

This Geological Society Special Publication contains a selection of the papers presented at a conference on ‘Fold–Thrust Belt Exploration’, held in London from 14 to 16 May 2008. The conference was conceived by Enzo Zappaterra and organized by the Petroleum Group of the Geological Society of London, in collaboration with the Geological Society’s Tectonic Studies Group. The conference was intended to assess the current state of the scientific evaluation, exploration and exploitation of onshore contractional ‘fold –thrust’ belts in both academia and industry. A primary objective of the conference was to consider whether fold–thrust belts could still host substantial undiscovered hydrocarbon resources which might be located through commercial exploration efforts. Put another way, given what we know, are fold– thrust belt provinces overlooked or justifiably avoided? Onshore fold–thrust belts are typically perceived as ‘difficult’ places to explore for several reasons (which have been easy for exploration management to perceive as prohibitive). Typically, rugged terrain makes access difficult and substantially increases the cost of exploration. Seismic data quality is often insufficient to allow unambiguous interpretation or to differentiate between alternative models. Reservoir development and

charge history are frequently difficult to unravel because of uncertainty regarding the burial history, while the tectonic regime tends to lead to a perceived high risk of trap breach. On the other hand, some fold– thrust belts do host very large oil and gas fields. Do the technical and logistical barriers to effective exploration mean that there are substantial hydrocarbon resources still to be found in the world’s fold–thrust belts? The Fold –Thrust Belt Exploration conference was intended to address this question by drawing on experience from both industry and academia. Exploration and exploitation techniques used to evaluate the hydrocarbon potential of fold–thrust belts have in many respects evolved substantially over the last few decades. Seismic imaging and interpretation tools have improved significantly, section restoration and balancing software has evolved into the 3D domain, and computer modelling of burial history and fluid flow in the context of tectonic loading/unloading and shortening has been developed. Simultaneously, the oil and gas industry has moved into deepwater provinces where contractional tectonic styles are common and where high quality 3D seismic imaging has allowed richly detailed analysis of structural development. Have these improvements produced sufficient new

From: Goffey, G. P., Craig, J., Needham, T. & Scott, R. (eds) Hydrocarbons in Contractional Belts. Geological Society, London, Special Publications, 348, 1–6. DOI: 10.1144/SP348.1 0305-8719/10/$15.00 # The Geological Society of London 2010.

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insights to reinvigorate exploration under higher oil prices or are the negative perceptions of the challenges of onshore exploration in contractional regimes still justified?

Do fold – thrust belts host substantial undiscovered hydrocarbon resources? Figure 1 shows the worldwide distribution of foreland fold–thrust belts. Roeder (2010) notes that fold–thrust belts contain more than 700 billion barrels of oil equivalent (BOE) of known hydrocarbon reserves. The Zagros fold– thrust belt contains the vast majority of these reserves (c. 517 billion BOE), with the remaining 183 billion BOE distributed across some 30 other fold–thrust belts. Even with the Zagros fold–thrust belt excluded, much of the reserve is concentrated in just a few provinces and the reserves distribution has a long ‘tail’, with some 23 fold– thrust provinces each having established reserves of between 2 and 5 billion BOE. In this context Roeder asserts that much of the yet-to-find reserves in the world’s fold–thrust belts will inevitably be in relatively modest sized fields of, perhaps, less than 100 million BOE, in fold– thrust belts which host a few billion barrels of oil equivalent each. The Zagros fold– thrust belt is clearly a key area for hydrocarbon exploration in fold–thrust belts. In an unpublished presentation at the conference, Sepehr et al. and colleagues from the National Iranian Oil Company showed profiles from 3D seismic surveys over several fields. These illustrate a level of structural complexity not portrayed in the limited published literature, including relatively recent discoveries in sub-thrust traps. A careful study of structural timing in the Zagros belt in southeastern Lurestan, Iran, by Blanc of StatoilHydro and co-workers from National Iranian Oil Company and CSIC Barcelona, indicates that the initial growth of many of the large folds pre-dates deposition of Eocene – Early Miocene strata. A previously unrecognized phase of Early Palaeogene compression is interpreted to account for up to half of the measured shortening. Early structural growth has also been demonstrated in the Dezful Embayment and foreland basin area of the Zagros (Abdollahie Fard et al. 2006) as well as the Lurestan. This can be dated as latest Cretaceous and results from the interaction of basement structures on ‘Arabian’ and ‘Zagros’ trends. The structures extend into the NW Persian Gulf (Soleimany & Saˆbat 2010). The major implication of this work is that some areas of the Zagros previously considered unprospective, because structural development was thought to post-date hydrocarbon charge, may in fact be locally prospective. In

contrast to Roeder’s conclusions of only modest yet-to-find resources in the globally prevalent lower ranked fold–thrust belts, these interpretations hint that a substantial yet-to-find resource may still exist in the world’s most richly endowed fold– thrust belt, the Zagros. In another unpublished conference presentation, Cooper observed that the six largest fold–thrust belts in the world have different structural characteristics, implying that deformation style is not a critical factor in the resource density of these belts. The volume of hydrocarbon resources in any given fold–thrust belt seems to be more closely linked to the presence of an effective source rock. Fold – thrust belts tend to have a high density of structural traps and this was seen by Cooper as the differentiating factor from other structural provinces. Roseway, in another unpublished conference paper, presented a comparative study of 33 foreland basins and fold– thrust belts. This showed that the overwhelming majority of source rocks in these systems are Cretaceous –Tertiary oil-prone marine shales. The dominant top seal is mudstone, some 40% of reservoirs are shelf carbonates and 40% are marine shelf and continental sandstones. Some 60% of fold– thrust belt reservoirs examined in this study are fractured, with unfractured carbonate reservoirs being uncommon.

What techniques are being employed in fold – thrust belt exploration and development and how are these developing? Section modelling and restoration of fold–thrust belts have moved emphatically into the 3D domain. At the conference, Gibbs and co-workers from Midland Valley Exploration, discussed how traditional section restoration and balancing algorithms using flexural slip, simple shear, fault-parallel flow and mixed-mode combinations introduce bias into kinematic prediction and interpretation style, and that such algorithms are often selected on the basis of limited observations. Dominantly planestrain assumptions allow these approaches to be applied in 3D restoration but further limit their utility. In the view of Gibbs, 3D approaches are still powerful, notwithstanding these limitations, and techniques are evolving towards kinematic predictions that are not limited by plane-strain assumptions or by user choice of tectonic transport direction. Analogue modelling is also providing valuable insights into the evolution of fold–thrust belts. At the conference, Sassi and co-workers from the Institut Franc¸ais du Pe´trole (IFP) presented the results of numerical digitization of a sandbox experiment that modelled the evolution of force folding within a

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Conventional Oceanic Plate Boundaries & Motions: Mid Ocean Ridge & Transform Faults

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Oceanic Subduction Plate Velocities: a. Diverging b. Converging

ICE

a

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Oceanic Crust (undifferentiated) Oceanic Plates

Brooks Range Parry Islands Timan-Pechora

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Mostly Oceanic Plates Associated with Backarcs

PA - Pacific Plate NZ - Nazca Plate CO - Cocos Plate JF - Juan De Fuca Plate CAL - Caroline Plate

PH - Philippine Plate CA - Carribbean Plate SC - Scotia Plate

Continental Boundaries & Structures:

Baikal-Predpatom

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associated with breakup of Gondwana parts of Pangea SA - South America

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associated with breakup of Gondwana parts of Pangea

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*Including Ill-defined sub-plates within diffuse plate boundaries as follows: O - Okhotsk T - Tarim NC - North China

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Folded (Orogenic) Belts

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Submarine Regions where non-closure of plate circuits indicates measurable deformation; deformation mostly inferred from seismicity.

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Fig. 1. Index map showing orogenic belts worldwide, age of continental crust and distribution of foreland fold– thrust belts. Open red boxes denote areas covered by specific papers in this volume. Map courtesy of Albert Bally and C & C Reservoirs.

FOLD– THRUST BELTS

transpressive regime. The experiment was used to create an evolving 3D model of interfaces and faults and to describe the 3D geometric evolution over time. Hill et al. (2010) used centrifuge analogue modelling to examine structural styles in the Papua New Guinea Fold Belt, by varying the mechanical stratigraphy until the structural styles generated match those interpreted in the subsurface. Analogue models with pre-cut faults, albeit of appreciably lower angle than would be expected for inherited extensional faults, produced early inversion structures similar to that observed at the Kutubu Oilfield, suggesting that pre-thrust weaknesses may play a significant role in the development of the Papua New Guinea Fold Belt. Such techniques provide geometric and kinematic insights which can assist in the interpretation of seismic data. As always, information from outcrop studies is extremely important, especially where integrated with other data. Wilson and colleagues from Oil Search Limited presented a paper on the Moran Field in Papua New Guinea at the conference. Geological complexity here is the result of shortening being influenced by pre-existing structures and partial detachment, within Early Cretaceous mudstones, between the oil-bearing Upper Jurassic reservoirs and the Miocene limestone carapace at surface. Seismic data quality is poor over the field due to a combination of steep and variable dips and the presence of a rugged surface topography of karstified limestone. Careful integration of multiple datasets and techniques, including 87Sr/86Sr isotopic dating of the limestones and the use of produced oil volume mass balance techniques, has helped define cross-cutting faults at reservoir level which play a significant role in oil entrapment and production. It was conspicuous in the conference that it is still relatively rare for subsurface models and geometries to be adequately constrained by seismic data. In a paper on the Kutubu Oilfield in Papua New Guinea, Bradey and colleagues from Oil Search Limited showed that five 2D seismic lines, of variable quality, had been acquired across the Kutubu field, some years after the commencement of production, at a cost of US$ 80 000/km. Seismic acquisition in Papua New Guinea is notoriously expensive, even by fold–thrust belt standards, due to the rugged karstified terrain, dense rain forest and absence of access routes. Kutubu exemplifies the perceptions of the utility of seismic data in these terrains: expensive, difficult and generally of poor quality. However, in an unpublished paper, Duque and colleagues from BP gave some reason for optimism that high quality seismic imaging can eventually be achieved in fold–thrust belts. Based on BP’s

3

experience in the Llanos foothills of Colombia, the authors showed that there have been improvements in acquisition and processing techniques over time. On the acquisition side, improvements in data quality have resulted from increased reliability of the recording equipment and the ability to handle substantially more channels. On the processing side, the key issues are statics, velocities, noise attenuation, migration and imaging. Specific techniques which have been helpful in improving seismic data quality include refraction tomography to address statics issues and pre-stack imaging in 3D datasets. Derivation of the appropriate velocity field requires a good understanding of the geology and is an iterative process that evolves as well data are acquired and seismic data quality progressively improves. Duque et al. showed that an iterative approach to processing, using several different processing contractors working in parallel on the same dataset, enabled BP to achieve outstanding success in imaging to depth in the Cusiana– Cupiagua area of the Llanos foothills. Comparison of the latest seismic dataset with earlier processed versions of the same dataset in this area show that it is possible to acquire high quality seismic data in onshore fold–thrust belts, but it takes significant time and very substantial effort. Several speakers at the conference and authors of papers in this volume presented case studies in which the understanding of structural geometries evolved with both increasing data and the use of techniques that allowed model development to move beyond well-established fault-bend and fault propagation fold geometries. For example, Newson demonstrated the value of continued re-evaluation of existing models in the Moose Mountain Field in the Canadian fold– thrust belt. The Moose Mountain Field had previously been interpreted using a faultbend fold model, but re-evaluation using balanced sections, dip-domain analysis and down-plunge projection indicated a detachment fold origin with several thrust sheets forming an antiformal stack. The re-interpretation resulted in new pool gas discoveries that have doubled the original field size. Similarly, in a paper on the implications of a recently drilled deep geothermal well in the Variscan Aachen fold and thrust belt in the Rhenish Massif in Germany, Becker from RWTH Aachen University, showed how integration of core and wireline log data allowed the revision of interpreted structural geometry at the front of the thrust belt. The implications of this re-interpretation include the recognition that the displacement of individual thrust sheets is probably much less than previously estimated. The evolution in structural understanding of fold– thrust belt fields with expanding datasets appears to be common. Hill et al. (2010) reviewed

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how the structural model of the Kutubu field in Papua New Guinea has evolved from a simple thinskinned fault-bend fold, through an overturned fault propagation fold, with break-thrust and small-scale duplexes at reservoir and seal level respectively, to a hybrid model of apparently inversion-related basement uplift overlain by thin-skinned folds. This evolution reflects the progressive incorporation of data from 50þ wells and eight seismic lines acquired over the field during its development. Cook & Thomas (2010), in the context of an interpreted ductile duplex developed within a recess in the Appalachian thrust belt, draw attention to the apparent role of pre-thrust extensional structures in the thickening of basal weak shales within a deep and subsequently inverted thick-skinned graben. This extensional mechanism is considered the most likely mechanism for the observed substantial thickening of ductile shales below the main roof thrust and its recognition has provided a possible solution for volume balance problems encountered in the palinspastic restoration of sections across the recess. The role of pre-existing extensional structures was also assessed by Turner et al. (2010) in the Keping Shan thrust belt of NW China. Here preexisting extensional structures give rise to partitioning of the belt and provide marked stratigraphic breaks which strongly influence the structural architecture of the later thrust belt. Bump et al. from BP in another unpublished conference paper drew attention to the variation in structural complexity in the Llanos foothills of Colombia, depending on whether or not the interpretation is constrained by well data. Sections drawn without well control tend to show simple structures with long, continuous horses, whereas those drawn with well control tend to show tighter folds, more faulting and ramps and flats within horses. A significant limitation of 2D seismic data in this area is the tendency for apparent frontal structures to be either seismic artefacts or to be too shallow to form traps at reservoir level. Roeder (2010) draws attention to examples in the East Venezuelan fold belt, where the interpretation of structural geometries has been compromised by unrecognized limitations in the application of fault-bend fold models. It appears there has been a historical reliance on thin-skinned geometric models in the interpretation of fold–thrust belts. These models were originally developed in predominantly thin-skinned belts such as the Rocky Mountains, where the pre-deformation stratigraphy was treated as having been deposited in a broadly consistent layer cake on a relatively undeformed ramp margin. The Rocky Mountains are now sufficiently well described to show that the effects of lateral facies changes can in fact be incorporated into sections. For example, dominated by folds to the north and with ‘classic’ large thrust

sheets of carbonate ‘beams’ in the south (McMechan 1985; Spratt et al. 2004). Basement control is also now recognized here so that there is a move away from a pure thin-skinned approach; this is particularly the case for structures lying at a high angle to the thrust belt (Fermor 1999). Thin-skinned models have, however, historically been applied widely to fold–thrust belts even though thin-skinned shortening may not be the primary shortening mechanism. In areas where pre-existing extensional and early low strain inversion structures provide strong control on structural development, thin-skinned geometric models may not be applicable or only of value with considerable adaptation. Recognition of the limitations of applying a narrow suite of models and awareness that structural geometries are often strongly influenced by pre-existing weaknesses and variations in the mechanical stratigraphy and reflect thick-skinned inversion, or hybrid thick-skinned inversion with thin-skinned deformation in the carapace, may only come after significant, and perhaps unsuccessful, exploration efforts. The implications in terms of magnitude of shortening and the nature, timing and location of potential traps are profound, yet are often overlooked. Would the progressive improvement in subsurface understanding in Papua New Guinea described by Hill et al. (2010) have taken place if early drilling had not found several giant fields and so given a commercial imperative to develop a thorough understanding of the subsurface and to the drilling of more exploration, appraisal and development wells that helped constrain the structural models? Roeder (2010) highlights the existence of several fold belts where exploration has been based on very limited seismic datasets and with limited geometric models. He suggests that these areas may offer opportunities to reassess the remaining prospectivity. It is not difficult to envisage that some prospective fold–thrust belts may remain underexplored because early drilling was unsuccessful, understanding was limited by sparse data and, perhaps, by the use of inappropriate models, so that there has been little commercial imperative to develop the level of understanding achieved elsewhere. The timing of hydrocarbon charge and the palaeo-burial and palaeo-thermal history of source and reservoir rocks tends to be subject to wide uncertainty. Historically, analysis has been dependent on measurements of the maturity of organic matter, such as Ro and Tmax. Roure et al. (2010) observe that apatite fission track data and analyses of hydrocarbon-bearing fluid inclusions can reduce the range of uncertainty and allow better prediction of the timing of hydrocarbon generation and the pressure/temperature conditions in the reservoir during cementation and hydrocarbon trapping. Capozzi & Picotti (2010) show how the analysis

FOLD– THRUST BELTS

of brines and hydrocarbons leaking at the surface can further constrain the maturation –migration using examples from the Northern Apennines in Italy. Wiltschko & Roa (2010), with reference to the southern Taiwan orogen, show how a multimodal approach to constraining cross sections using thermal maturity and thermochronological data in addition to GPS velocities allow the influence of large pre-existing normal faults to be determined and cross sections to be better constrained. While papers and posters on deepwater fold– thrust belts were presented at the conference, the focus of the conference was on onshore fold– thrust belts and this focus is reflected in the content of this volume. However, as a reminder that there is scope to transfer understanding between onshore and deepwater contractional belts, Back et al. (2010) present an analysis of the relative roles of crustal shortening versus gravity-driven shortening in the NW Borneo deepwater fold–thrust belt. Cross sections based on 2D seismic data are used to show that gravity-driven shortening decreases and basement-driven compression increases from south to north along the fold–thrust belt. Several contributions to the conference touched on the wider context and significance of fold– thrust belts for adjacent basins. Scott’s presentation on Novaya Zemlya was specifically on this topic. Although the Novaya Zemlya fold–thrust belt itself is not prospective, studies of the fold–thrust belt are valuable because it developed after subsidence had begun in important hydrocarbon basins on both the foreland and hinterland side. Fold– thrust belt geometry provides valuable information about the location and timing of this subsidence. With the development of a fold–thrust belt having the potential to affect sediment dispersal patterns, reservoir geometry and lithology, hydrodynamics and charging in adjacent, more-prospective basins, this aspect should not be overlooked.

Are fold – thrust belts set to re-emerge as a major new exploration target? In an unpublished conference paper, Graham presented a pessimistic view, observing that fold– thrust belts elevate rocks that were previously deeply buried and that this has a detrimental effect on key elements of their petroleum systems. Notwithstanding the potential for large traps and often limited prior exploration to condemn prospectivity, fold–thrust belts tend to exhibit complex internal geometries that are often inadequately imaged on poor quality seismic data. In essence, paraphrasing Graham, hydrocarbons trapped in fold–thrust belts are ‘Goldilocks’ accumulations, requiring special circumstances to allow structures to develop in time to

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receive hydrocarbon charge and then to retain trap integrity as the fold–thrust belt continues to evolve. Roeder (2010), revisits several failed exploration ventures in fold–thrust belts to analyse whether they share common characteristics. In an idiosyncratic view, Roeder argues that many fold–thrust belts could be re-evaluated and exploration re-commenced, having been under-explored in the past with low drilling and seismic density and with wells that were often sited on the basis of geological models rather than observed data. However, based on a review of the distribution of reserves in fold–thrust belts worldwide, and notwithstanding the scope to re-explore, Roeder concludes that only the fold–thrust belts in the Middle East offer a globally significant target for finding new hydrocarbon resources.

Conclusions The technologies, techniques and models available to the exploration and production industry today are a significant improvement on those of the past and offer the possibility of more effective exploration in fold–thrust belts in the future. However, the improvement has been incremental, not a stepchange, and the establishment of even a basic, reliable understanding of regional and trap-scale structural geometries remains highly problematic in many fold–thrust belts. Limited or unreliable understanding of large-scale geometries will undermine the use of some of the available techniques. Undoubtedly, fold–thrust belts are still immensely challenging places in which to explore costeffectively. It seems likely that in the near to medium term, any significant success in fold– thrust belt exploration will be in areas where politics, not geology or technology, has been the barrier to effective exploration. This is well illustrated by recent successes in the Zagros belt in the Kurdistan region of northern Iraq. This area has been barely touched by industry for several decades and major undrilled traps are comparatively easy to locate. Elsewhere, despite all the progress in exploration and modelling techniques, and notwithstanding the undoubtedly high density of remaining traps, it seems that progress will continue to be slow. If hydrocarbon entrapment relies on a ‘not too hot, not too cold’ set of ‘Goldilocks’ circumstances, and while effective exploration remains an imprecise iteration of remote-sensed and field observations, loosely constrained by a few widely spaced well penetrations and expensive seismic data of limited quality, it will continue to be difficult to mitigate risk and reduce uncertainty in the exploration of fold–thrust belts. As exploration has moved away from areas with simple and predictable mechanical stratigraphy where the widely used

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geometric models were first developed, to fold– thrust belts with more complex mechanical stratigraphy, pre-contractional features and where thick and thin-skinned structures have developed together, the understanding of the subsurface structure will continue to be very uncertain. The identification of the traps most likely to be hydrocarbon charged will, inevitably, remain a major challenge. Opportunities do exist in many fold–thrust belts to re-evaluate their hydrocarbon potential and to re-commence exploration from a hopefully more enlightened perspective, being fully aware of the possible failings of previous exploration. It would appear that, outside the Middle East, the potential yet-to-find resource in fold–thrust belts may be mostly in fields of tens to low hundreds of millions BOE size in provinces which currently host less than 5 billion BOE each. These will offer attractive possibilities for some companies but, given the cost and exploration risk, these areas will be avoided by many others as long as there are less challenging opportunities elsewhere. The learning curve in global fold–thrust belt exploration is not steep and as a result it seems likely that fold–thrust belts will still be capable of providing new discoveries long after the world’s shelf and deepwater basins are thoroughly explored. Progress, however, is likely to be slow and will involve an expensive iterative process of maximizing the lessons learned from all well penetrations, slow improvement in seismic data quality and industry working closely with academia to develop and continually improve the understanding of subsurface geometries, reservoir and charge evolution. Fold–thrust belt exploration is clearly not for the ‘faint-hearted’ and is unlikely to reward individuals and companies that are either unwilling or unable to devote the necessary time, skills and expenditure. Even with an appropriately resourced effort, the entrapment and preservation of hydrocarbons in fold–thrust belts seems to depend on ‘Goldilocks-like’ circumstances which will continue to challenge the combined predictive abilities of both industry and academia. It is, perhaps, still too early to judge whether fold–thrust belts are overlooked provinces or justifiably avoided, but barring the development of some unforeseen new enabling technology, a major shift in the focus of the oil and gas industry towards renewed exploration of the world’s fold–thrust belts does not seem likely.

References Abdollahie Fard, I., Braathen, A., Mokhtari, M. & Alavi, S. A. 2006. Interaction of the Zagros Fold– Thrust Belt and the Arabian-type, deep-seated folds in the Abadan Plain and the Dezful Embayment, SW Iran. Petroleum Geoscience, 12, 347–362.

Capozzi, R. & Picotti, V. 2010. Spontaneous fluid emissions in the Northern Apennines: geochemistry, structures and implications for the petroleum system. In: Goffey, G. P., Craig, J., Needham, T. & Scott, R. 2010. Hydrocarbons in Contractional Belts. Geological Society, London, Special Publications, 348, 115–135. Cook, B. S. & Thomas, W. A. 2010. Ductile duplexes as potential natural gas plays: an example from the Appalachian thrust belt in Georgia, USA. In: Goffey, G. P., Craig, J., Needham, T. & Scott, R. 2010. Hydrocarbons in Contractional Belts. Geological Society, London, Special Publications, 348, 57–70. Fermor, P. 1999. Aspects of the three-dimensional structure of the Alberta Foothills and Front Ranges. Geological Society of America Bulletin, 111, 317– 346. Hesse, S., Back, S. & Franke, D. 2010. Deepwater folding and thrusting offshore NW Borneo, SE Asia. In: Goffey, G. P., Craig, J., Needham, T. & Scott, R. 2010. Hydrocarbons in Contractional Belts. Geological Society, London, Special Publications, 348, 169–185. Hill, K. C., Lucas, K. & Bradey, K. 2010. Structural styles in the Papua New Guinea Fold Belt; constraints from analogue modelling. In: Goffey, G. P., Craig, J., Needham, T. & Scott, R. 2010. Hydrocarbons in Contractional Belts. Geological Society, London, Special Publications, 348, 33–56. McMechan, M. E. 1985. Low-taper triangle-zone geometry: an interpretation for the Rocky Mountains foothills, Pine Pass-Peace river area, British Columbia. Bulletin of Canadian Petroleum Geology, 33, 31– 38. Roeder, D. 2010. Fold–thrust belts at peak oil. In: Goffey, G. P., Craig, J., Needham, T. & Scott, R. 2010. Hydrocarbons in Contractional Belts. Geological Society, London, Special Publications, 348, 7– 31. Roure, F., Andriessen, P. et al. 2010. The use of palaeo-thermo-barometers and coupled thermal, fluid flow and pore-fluid pressure modelling for hydrocarbon and reservoir prediction in fold and thrust belts. In: Goffey, G. P., Craig, J., Needham, T. & Scott, R. 2010. Hydrocarbons in Contractional Belts. Geological Society, London, Special Publications, 348, 87–114. Soleimany, B. & Sa`bat, F. 2010. Style and age of deformation in the NW Persian Gulf. Petroleum Geoscience, 16, 31– 39. Spratt, D. A., Dixon, J. M. & Beattie, E. T. 2004. Changes in structural style controlled by lithofacies contrast across transverse carbonate bank margins— Canadian Rocky Mountains and scaled physical models. In: McClay, K. R. (ed.) Thrust Tectonics and Hydrocarbon Systems. AAPG Memoir, 82, 259–275. Turner, S., Cosgrove, J. W. & Liu, J. G. 2010. Controls on lateral structural variability in along the Keping Shan Thrust Belt, SW Tien Shan Foreland, China. In: Goffey, G. P., Craig, J., Needham, T. & Scott, R. 2010. Hydrocarbons in Contractional Belts. Geological Society, London, Special Publications, 348, 71–85. Wiltschko, D. V. & Rodriquez-Roa, F. A. 2010. Thrust belt architecture of the central and southern western foothills of Taiwan. In: Goffey, G. P., Craig, J., Needham, T. & Scott, R. 2010. Hydrocarbons in Contractional Belts. Geological Society, London, Special Publications, 348, 137– 168.

Fold –thrust belts at Peak Oil DIETRICH ROEDER Murnau Geodynamics, 9225 West Jewell Place No. 107, Lakewood, CO 80227, USA (e-mail: [email protected]) Abstract: Outside the Middle East, onshore fold–thrust belts (FTB) of Tertiary to Recent age contain a significant part of the globally developed petroleum, but far less of the oil and gas remaining undiscovered. Depending on high quality data and on deep drilling, renewed exploration of former failures is commercially attractive, and it will help in exploring the deepwater belts of compression. In FTB with a defined petroleum system, an under-explored trend may be the informally named ‘deep-updip’ or DUD trend. Shale-gas-prone formations in FTB require new exploration strategies, but in the public domain, this type of prospect has not yet been discussed. FTB discoveries require geological insight, persistence and exponentially rising investment. The paper includes examples from the Northern Alps, from the Llanos foothills of Colombia, from Eastern Venezuela and from the Po Valley basin of Italy.

Fold –thrust belts (FTB) are the shallow and partly petroliferous fringes of the global orogenic belts. Many FTB, their location, geographical extent and key elements of their structural style, as well as their stratigraphy and petrology, can be studied in mountainous exposures. However, their tectonic and economic understanding requires subsurface data. Beginning in earnest about 50 years ago, reflection seismography and deep drilling have been, and still are, steering the global exploration of the geologically youngest FTB. Significant academic merits notwithstanding, the present and future understanding of FTB depends on funding and other incentives offered by the petroleum industry. The geological advantage of petroleum systems in Tertiary and younger FTB settings (Marc Cooper, pers. comm. to DHR) over objectives in shallower basin parts is found in their more complete stratigraphic record, in a basin setting combining source and reservoir, and in the favourable thermal configuration of stacked thrust sheets. Peak Oil is a concept of global economy. It defines an economic state during which the demand for petroleum exceeds the technically possible rate of supply from the Earth’s developed petroleum reservoirs. The demand for petroleum increases through growth of economies and populations. Limits to rates of supply are not set by the amount of remaining reserves. Rather, they are set by the naturally declining rate of production in all oilfields, by faltering rates of new discoveries, and by the geographic and political inequality between oil-producing nations and nations with growth of demand. For each developed field, producers maintain a Maximum Efficiency Rate (MER). MER is a declining and constantly monitored composite of reservoir physics, wellhead crude price, and cost

of investment, operation and product transport. Peak Oil is generally perceived as a state of instability of conventional energy supply.

Impact of future FTB discoveries At a seemingly critical point in time, the question arises: Can we restore the stability of conventional energy supply by developing more and cheaper oil and gas in the planet’s onshore fold–thrust belts? In the present paper, the short answer is ‘No’ for all onshore FTB outside the Middle East. This ‘No’ does not condemn FTB exploration, but it responds to its limited global impact. It is based on published reviews of global petroleum reserves, of typical productive fold–thrust belts and of their undiscovered potential. For the poorly known and understood deepwater belts, this paper’s short answer is ‘Perhaps yes’. Finally, the shale-gas potential of FTB remains as an unknown economic complexity added to the extant FTB complexity. Its part of the short answer would be ‘Not now’. Even if FTB provinces cannot delay the problems of Peak Oil, their continued exploration is vital to humankind. The present contribution of FTB geology is academic, and it will be needed in exploring the deepwater belts. However, reexploring productive FTB preserves and uses sunk costs, live expertise and extant facilities (brains, seismic data and pipelines). Re-exploring FTB makes business sense. Therefore, it is an acceptable international and corporate task.

Reserve estimates Today, the standard reference for global data on petroleum reserves (Klett et al. 1997) must take

From: Goffey, G. P., Craig, J., Needham, T. & Scott, R. (eds) Hydrocarbons in Contractional Belts. Geological Society, London, Special Publications, 348, 7–31. DOI: 10.1144/SP348.2 0305-8719/10/$15.00 # The Geological Society of London 2010.

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account of 2800 billion barrels oil equivalent (BOE) plus new discoveries and minus about 12 years of production increasing by an annual 4%. This reference has grouped the global oil-prone area into 406 provinces, has ranked them by their known reserves, and displays the sizes of reserves of entire provinces and their log-normal distribution. This reference is ageing but still valid. In Figure 1, the Klett– Ahlbrandt data are displayed in a stack of three Cartesian histograms of globally ranked petroleum provinces. Using these log-normally distributed data and applying subjective estimates, we assume that undiscovered FTB production will comprise about 1% of the global reserves. In the top histogram, the global reserves in all geological settings appear dominated by two super-giant provinces, the Zagros –Mesopotamide foothills (533 billion BOE) and West Siberia (356 billion BOE), closely accompanied by other giant Middle East sites. In the second histogram we exclude these giants, and we count only listed and named basins clearly associated with FTB tectonics. In the third histogram, the Zagros – Mesopotamide FTB system is excluded, and the known FTB reserves still amount to a staggering 177 billion BOE, while at present, the global

consumption is about 12 billion BOE per year and rising (Edwards 2001). The third histogram is repeated at two scales in Figure 2. The top version is detailed and shows that among the non-Middle East FTB provinces, the seven richest provinces contain about 60% of the reserves. Each of the lesser 23 FTB provinces (outlined in white) contains reserves of between 5 and 2 billion BOE. It is within this field of known reserves that most likely any future FTB discoveries will be made. Their field sizes will depend on an undefined proportionality, say perhaps of 1 in 10, between known and undiscovered reserves. The proportionality depends on the present state of exploration. For conducting renewed exploration in any FTB basin ranked in the white-outlined field, industry can hope for field sizes of between 10 and 100 million BOE. However, field sizes of less than 10 million BOE are up to 23 times more likely than sizes of 100 million BOE. This hopeful guess is based on sparse geological evidence, as well as on the assumed but poorly understood proportionality. Exploration failures have not yet been treated probabilistically, but certainly they will improve the odds for success in discovering new small fields.

Fig. 1. Cascade of three log-normal histograms ranking the world’s known petroleum provinces or basins by their quantities of known and producible hydrocarbons, designed after Klett et al. (1997). Vertical scales are linear in billions of barrels oil equivalent. Horizontal scales are ranking numbers of named and measured petroliferous basins. Discussion in text.

FOLD–THRUST BELTS AT PEAK OIL

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Fig. 2. Cascade of two scaled versions of third (bottom) histogram shown in Figure 1, after data by Klett et al. (1997). Vertical scales are linear in billions of barrels oil equivalent. Horizontal scales are ranking numbers of named and measured petroliferous basins. Discussion in text.

Cost of studying fold – thrust belt geology Beyond surface mapping and applying popular model concepts of geological interpretation, FTB exploration advances only through the use of seismic surveys and deep wells. These and other items must be financed from returns on investment. Also, FTB exploration must compete with easier and less challenging opportunities. Usually, exploration expenditures rise exponentially with increasing detail and better defined objectives. During a South Alpine venture and between 1982 and 1985 (Anschutz and partners, see Roeder & Bello 2003), seismic-controlled surface mapping and a modern thrust interpretation became available for a famously low US$ 0.87 per acre (US$ 2.15 per hectare). This work sufficed to define the potential, to be granted concession areas, and to attract investing partners. However, to define trends, leads and drillable prospects, the consortium needed an improved seismic survey at a cost of US$ 200 per acre (US$ 500 per hectare). Today, this survey is still incomplete. The deep wells needed to explore FTB leads within both Alpine flanks must reach or exceed 6000 m. The recorded expenditures for circumAlpine wells have a log-normal distribution and have cost an average of US$ 2000 per drilled metre between 0 and 5000 m, and of US$ 4000

per metre in wells between 5000 and 8000 m deep (Spoerker in Brix & Schultz 1993). Everywhere, more surface geology, the use of all available seismic data, new seismic acquisition and high wellhead prices are required for resuming failed or simply abandoned FTB plays.

Failed ventures, Cordilleran origin of FTB understanding Well-prepared FTB ventures have failed, and will fail, before their target has been reached, for a variety of common errors, or for technical shortcomings, or for non-technical business decisions. Remembering and re-enacting these ventures during an era of better technology and higher petroleum prices may globally evolve into a trade of secondary or tertiary exploration, here loosely defined as exploration resumed with revised concepts and increased expenditure. Modern understanding of FTB geology evolved in the foothills of the Canadian Rocky Mountains between 1950 and 1970, when the international petroleum industry applied single-fold reflectionseismic profiling and focused on deep drilling for thrust-faulted leading-edge traps. Discoveries and failures occurred simultaneously, and the competing community of explorationists and mappers learned

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jointly (Bally et al. 1966; Roeder 1967; Dahlstrom 1969, 1970 and many others). The Canadian vintage of FTB experience has been exported into every FTB province on Earth and into all modern textbooks of structural geology. Figure 3 combines partial segments of two seriated structure cross sections of the Canadian Rockies in Southern Alberta, showing named and unnamed exploration wells, producing gas fields and undrilled ventures of renewed exploration. A: Imperial Scalp Creek was drilled in 1956. The well encountered the roof thrust of a duplex structure, but the sub-thrust objectives were beyond the capacity of the rig on location. Drilling to sub-thrust objectives would require an improved access road. B: Shell Limestone Mountain did not explore a sub-thrust prospect that had not been visible in the seismic data of 1955 vintage. C: Wells were located on a shallow and un-prospective structure or missed the leading edge of a potential reservoir. D: Chevron Fording Mountain did penetrate the sub-thrust zone of a classical duplex structure, but it encountered unexpected structural complications. Re-exploring this structure for its Mississippian reservoir and early Mesozoic source rock appears promising. E and F: Savanna Creek has been producing wet gas from Mississippian dolomites, and it may have missed a Devonian play nearby. G: the triangle zone of the Southern Canadian FTB may be a not fully explored trend of gas production from Mississippian dolomite.

Deep-Updip Lead (DUD) Figure 4 is a structure cross section explaining conceptually and hypothetically a top-rated FTB trend or lead just updip of the thick-skinned front of any FTB. This lead may be informally named ‘deep-updip’ lead or DUD lead. In many structural configurations of the thick-skinned front, the DUD lead clearly is absent, but it is productive in Shell’s gas fields of southernmost Peru (Roeder & Chamberlain 1995), in the Tesoro block of Southern Bolivia (Dunn et al. 1995), in the Tecate and El Furrial fields of Eastern Venezuela (Hung 2005), and in the Cupiagua and NW trend fields of eastern Colombia (Martinez 2006). The DUD setting has sometimes been missed by the edge of regional seismic coverage, such as on the Alto Beni block of Bolivia (Roeder & Chamberlain 1995) and in the Folded Molasse ventures of Germany and Switzerland, see Figures 5, 6 and 7. The Macal venture of Eastern Venezuela failed because of a misinterpretation of the DUD setting. The Covenant field of Utah is perhaps located in the DUD setting (Sprinkel & Chidsey 2008). In the Canadian and Alaskan parts of the Cordillera,

the DUD setting awaits scrutiny and exploration. The Doonerak window of the Romanzoff Mountains (Mull et al. 1987) condemns the DUD setting for the east part of the Alaskan Brooks Range. Other FTB settings notwithstanding, the search for the DUD lead would be useful in any scrutiny of the remaining FTB potential.

North Alpine ventures (Figs 6 – 9) Since about 1950, oil and gas exploration of the Alpine north front evolved, succeeded, failed and resumed as competitive or national or consortial efforts. In the present paper, this complex history is sampled by the review of four events. An early and largely non-geological effort in Germany failed but was repeated and improved twice. An international consortial effort drilled two rank wildcats of geological significance, but did not achieve commercial production. A special part of a national venture discovered significant FTB-based oil and gas by close cooperation between one drilling engineer, two structural geologists and a funding and encouraging management. Together, these ventures demonstrate the need for reliable and adaptable models, for old and modern models and modern data, for the continued ability to read and use obsolete data, and for enough encouragement and funding. Collectively, the five decades of North Alpine exploration produced much published Earth science, notably Mueller 1978; Bachmann et al. 1982; Lemcke 1988; Wessely 1993; Schwerd et al. 1995 and many others. However, some fundamental aspects of North Alpine petroleum trapping are still conjectural. Preussag’s North Alpine venture on German and Austrian territory (1950–1975) was based on surface geology and on the new electronically processed analogue reflection-seismic data. It also was based on learning from early FTB discoveries in Austria and Southern Alberta (Canada). Preussag’s early explorationists did not clearly define their leads (Figs 6 & 7). The available seismograph technology was marginal at best. There was no tectonic model to put order into the abundant and wellmapped surface data. New fixistic views (Richter & Schoenenberg 1955; Kockel 1956; Jacobshagen 1975) were proposed to replace mobilistic views (Ampferer 1906; Ganss & Schmidt-Thome 1955; Tollmann 1976). Later, the state-supported and consortial Vorderriss well (see Fig. 7; Bachmann & Mueller 1981 and Bachmann et al. 1982) did build a pre-seismic and clearly mobilistic FTB model for Alpine exploration. The well was located on the flank of a tight surface anticline, but its seismic support is plainly

FOLD–THRUST BELTS AT PEAK OIL Fig. 3. Two structure cross sections selected from an inventory of sections from the Southern Canadian Rockies redrawn from D. H. Roeder (2000). The sections display several producing and failed (dry) exploration wells. The letters identify the wells. West is to the left. Bar scales indicate horizontal and vertical scales (VE ¼ 1). Black surfaces: grouped Mississippian and Devonian carbonates. Shaded surfaces: Cambrian sediments. White surfaces: Mesozoic to Palaeogene sediments. Discussion in text.

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Fig. 4. Structure cross section of an imaginary FTB illustrating the concept of the DUD trend. Designed after data for an academic test in structural geology at Colorado School of Mines. Black: potential reservoir horizon. Light shading: passive margin sediments. Dark shading: basement. Bar scale indicates horizontal and vertical scale (VE ¼ 1).

illegible (see Dohr 1981). In 1977– 1978, Preussag drilled the well to 6468 m at an unknown cost. The well defined the Austro-Alpine allochthon, its sole, and details of its internal polyphase compressional structure. However, its implied frontal prospect of the DUD type remains unconfirmed and untested, even with far better CDP-type seismic data and a second consortial well (Schwerd et al. 1995). The Austro-Alpine allochthon, a mapped tectonic unit, has High Alpine topography and therefore is an unlikely prospect for sealed hydrocarbon reservoirs. In Austria NE of Vienna and the River Danube, the Austro-Alpine is productive and is covered by the Vienna Basin, a Neogene combined foredeep and extensional successor basin (Fig. 8). Uebertief, a uniquely successful FTB venture by the Austrian state corporation OMV, discovered

commercial hydrocarbons in buried Austro-Alpine rocks. Until 1992, Uebertief had generated a cumulative production of 70 million barrels (MMB) of Alpine light oil and 995 billion cubic feet (BCF) of Alpine methane. Hydrocarbons (Fig. 9) are trapped in tightly folded Triassic carbonates. They are sealed unconformably by an inter-tectonic shale series (Gosau) and by the shaly and extensional Vienna Basin fill. The largely vertical reservoir beds are not visible on conventional seismic data. Exploration focused on drilling, logging, biostratigraphy and structural geology. As part of an overall programme of Austrian national resource development and with persistent and effective funding, the Uebertief venture combined the teamwork of at least three leading people, Hermann Spoerker, drilling engineer, and Godfrid Wessely and Wolfgang Zimmer, geologists. The

Fig. 5. Stratigraphic colour legend and brief survey of the stratigraphies for coloured versions of structure cross sections in the Northern Alps and in Eastern Venezuela, composed for use with Figures 6– 9 and 16.

FOLD–THRUST BELTS AT PEAK OIL Fig. 6. Structure cross section No. MW-2-SB-17 of the Alpine north front in Bavaria (Germany) showing three identified pre-seismic foothills wells that have missed the deep belt of potentially prospective imbricates. There is minor seismic control (not shown). The frontal and triangle part of the foothills or Folded Molasse belt is based on seismic data (Lohr 1969). North is to the left. Scale bar indicates horizontal and vertical scale (VE ¼ 1). More discussion in text.

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14 D. ROEDER Fig. 7. Montage or composite of two seriated structure cross sections of the Northern Alps in Bavaria, redrawn from D. H. Roeder (2001). Well names are 1 Eberfing, 2, 4, 5, Murnau, 3 Staffelsee and 6 Vorderriss. The more westerly and more northerly segment (drawn after Mueller 1978) shows the structural setting of the old Murnau wells. The montage does not equalize the flexural curvature of the basement top taken from Sommaruga (1997). Scale bar indicates horizontal and vertical scale (VE ¼ 1). More discussion in text.

FOLD–THRUST BELTS AT PEAK OIL

Fig. 8. Regional structure cross section through the middle part of the Vienna Basin, strongly simplified after Zimmer & Wessely (1996). Of the numerous wildcat wells and producers, only the well symbols are shown. Inset square shows location of more detailed Figure 9. Bar scale indicates horizontal and vertical scale (VE ¼ 1). A: The dotted line is the Sommaruga North Alpine standard flexural basement top located radially 29 km above the seismic-controlled Moho. B: Implied but not documented external massif required to move the North Alpine thrust wedge during the Neogene. C: Rheno-Danubian Flysch and suture-associated late Cretaceous sediments. D and E: Basement parts (‘Altkristallin’) of the allochthonous imbricate complex, outcropping in Leitha hills, redrawn after Zimmer & Wessely (1996).

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16 D. ROEDER Fig. 9. Structure cross section of the frontal Austro-Alpine allochthon with two stacked and up-righted thrust sheets unconformably covered by inter-tectonic upper Cretaceous clastics and by post-compressional Vienna Basin clastics. Wells of the Uebertief venture encountered and missed commercial oil and gas accumulations. At this detail and limited control, the standard tectonic models lose their applicability, not their validity. Redrawn after Zimmer & Wessely (1996). Red ¼ gaseous petroleum. Green ¼ oil.

FOLD–THRUST BELTS AT PEAK OIL

venture includes the hitherto deepest European well for hydrocarbon exploration, Zistersdorf UT-2A, with total depth (TD) at 8553 m (1992). The well data developed during the Uebertief venture contain numerous geological details of locally vital importance, but of unlikely regional or textbook significance. Nevertheless, OMV’s Alpine exploration is a global milestone in applying and modifying the FTB style as recognized in the Canadian Rockies. The Gosau seal in the Uebertief area may or may not encourage Austro-Alpine exploration outside the Vienna Basin. Even during the years of academic campaigning for an Alpine fixistic reinterpretation, the OMV team maintained an inventory of seriated structure cross sections, in part with seismic control and built with modern structural geology. The inventory was used to plan and drill a major series of sub-thrust wildcat wells with a mixture of production and dry tests (Zimmer & Wessely 1996). Between 1994 and 2002, two American independents, Anschutz and Forest Oil, resumed exploration in the old Preussag concession with a new inventory of field-checked seriated cross sections. The attraction of risk-sharing partners failed because of German legal restrictions to the re-use of Preussag’s state-supported consortial and early Common-Depth Point (CDP) seismic data. The most recent and unlikely last North Alpine venture, again by the Austrian corporation of OMV and in the German part of the Molasse basin, is

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continuing the work resumed by Anschutz and Forest. OMV’s exploration is focusing on deep thrust imbrications near the south edge of the Molasse basin, by using 3D seismic surveys and extant surface geology. Results have not yet reached the public domain. However, one older well, Au, near the Austrian– Swiss border, may have reached a shallow analogue of the DUD trend but was dry and abandoned (Wessely 1993).

More Cordilleran ventures An FTB setting is evident in three of the four giant Cordilleran petroleum provinces of South America, and two of them are reviewed here: the Oriente foothills of Colombia and the Serrania foothills of Venezuela (Fig. 10). Both clearly show that it is the abundance and maturity of source rock that makes a petroleum province. They also show that it takes the development drilling after a commercial discovery to reveal and confirm the structural style. At least one of them shows significant limits to the guiding or misguiding concepts of structural styles. The South American Cordillera is an assembly of thick-skinned and locally pluton-invaded cratonic basement blocks. They are separated by crustal strike-slip faults that strain-partition (Fitch 1972) the Peru –Chile and Caribbean subduction (Figs 10 & 14). The north part of the Cordillera affects an Atlantic– Tethyan passive margin series

Fig. 10. Sketch map of northwestern South America after GSA Tectonic Map of South America and Bellizzia & Dengo (1990) showing in three shades of grey from west to east, the Andean tectonic belts, high blocks with outcropping basement, many strike-slip faults, and the Subandean foredeep east of the Cordilleran frontal fold belt. The map also shows two rectangular areas of ventures and structural inventories reviewed in the present paper.

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of post-Jurassic age with unevenly distributed petroleum source rock. A Cordilleran foredeep series of Tertiary to Recent age covers the passive margin series and migrates diachronously northward and eastward. The thick-skinned Cordilleran east front is accompanied by a discontinuous or vaguely coherent FTB. In Colombia, the FTB style is more clearly imbricate. In East Venezuela, the frontal FTB is a belt of detached and internally coherent folds dissected by several thrust faults. Cordilleran petroleum is sourced in Cretaceous to Palaeogene shales. It is trapped in intercalated sandstones within compressional intra-Cordilleran belts, such as the Middle Magdalena valley, in extensional intra-Cordilleran belts (Maracaibo), and in foreland FTB (Oriente, Serrania, Trinidad). The Orinoco tar belt suggests long-range updip migration of petroleum. There are no public-domain data on the geographic distribution of the associated and free gases, of their origin, and of their geochemistry.

Oriente of Colombia Klett et al. (1997) rank this petroleum province as No. 53, with 5.4 billion barrels (BB) of liquid, 10.3 TCF of gas, and 7.4 billion barrels oil equivalent (BOE). Geologically, the Oriente is well described (Colletta et al. 1990; Cooper et al. 1995; Martinez 2006; unpublished talk by R. Graham from Fold-and-Thrust belt conference). It is still under active exploration after world-class discoveries at Cusiana and Cupiagua. The Oriente belt could serve as a global type locality for interfering thrust progradation and triangle back-thrusting, as well as for a successful history of inter-related commercial and public FTB ventures. Figures 11– 13 are cross sections sampling the geology of the Oriente FTB. Its significance started with the Farmout by Triton company (see Triton Colombia, Inc. 1982) showing an abundance of source rock, a somewhat discontinuous layer of fluviatile sands, and classical seismic-documented foothills architecture. Detrimental to industry was its remote location relative to tidewater, and the absence of any local infrastructure. Today as well as initially, security problems and environmental problems are costly and common in the Llanos province. Ecopetrol, the Colombian state petroleum company, persisted in its regional seismic exploration, but had only limited economic success in the Medina segment of the Oriente FTB.

East Venezuela basin (EVB) Three points distinguish this world-class FTB. First, its production rate and its potential are blessed by an abundance of mature source rock. Secondly,

fatal errors in tectonic analysis were caused by unrecognized or acknowledged but ignored limits to the Bally–Dahlstrom –Suppe model of faultbend folding. The errors suggest that this enormously successful concept of structural geology needs restraint and control by data. Thirdly, East Venezuela exploration shows that even with a nearperfect digital system of data retrieval, key data can get lost among hundreds of carefully logged wells, thousands of miles of high quality seismic data, and dozens of well-designed seriated cross sections. Along its cratonic south flank, the thick-skinned and strike-slip-faulted Cordillera of Eastern Venezuela (Fig. 14) is lined by a thin-skinned and petroliferous FTB. Its details are described by varieties of the Bally–Dahlstrom –Suppe structural style (Roure et al. 1994; Passalacqua et al. 1995; Audemard & Serrano 2001; Hung 2005). Its petroleum reserves, as an East Venezuela Province, are ranked (Klett et al. 1997) as No. 13, with 30.2 BB of liquid to solid petroleum, 129.7 TCF of biogenic and thermal gas and 52.6 billion BOE. East Venezuela contains dozens of producing or shut-in anticlinal and thrust-faulted oil and gas fields, hundreds of new field wildcats, and thousands of kilometres of CDP 2D seismic data, all managed and funded by PDVSA, the Venezuelan state petroleum corporation. Geological maps play a key role in defining the northern limits of the productive basin. Also, geological maps are key data for a valid tectonic model. Surface-controlled photogeological maps of the Cordillera and its foothills date back to PDVSA’s precursors such as Esso-Creole Petroleum, renamed Lagoven. Between about 1950 and 1965, and using the same personnel, Geophoto of Denver and Calgary together with Esso– Imperial– Exxon, simultaneously mapped East Venezuela and the productive foothills of Southern Alberta (Canada). In contrast to the setting and style of Alberta, the East Venezuela maps show no thrust faults but internally coherent open and steep-flanked folds. Given the common history of these maps, this contrast of style probably is well observed. There is no record of Exxon –Lagoven’s original structure cross sections. Between 1990 and 2000, exploration by a new generation of geologists and their modern CDP seismic data applied the key elements of the modern Suppean FTB style (Suppe 1983). New structure cross sections (Figs 15–17) show problems with the updated style and their solutions. Two versions of a structure cross section appear in Figure 15. Originally (top image), it proposed a shallow and large main thrust covering a wide field of footwall imbrications (Roeder 2001). A revision to comply with well data soon became needed.

FOLD–THRUST BELTS AT PEAK OIL Fig. 11. Two cross sections of productive foothills structures. West is to the left. Two identical scale bars, both implying equal horizontal and vertical scales (VE ¼ 1). Top: line tracing of one of the pre-discovery 2D seismic lines across the Cusiana field, with shading applied to the Palaeogene stratigraphic interval containing the petroleum system. Groups of letters identify details of the local stratigraphy, after Valderrama (1982), not explained in the present paper. Redrawn after Roeder & Chamberlain 1995. Bottom: Foothills of the Southern Canadian Rocky Mountains in Southern Alberta. Black: Mississippian and Devonian passive margin carbonates. The Mississippian is involved in two productive thrust sheets, Turner Valley and Highwood–Quirk Creek. Redrawn after Bally et al. (1966). 19

20 D. ROEDER Fig. 12. Two parallel and partly seismic-controlled structure cross sections of the Llanos foothills, east front of the Eastern Cordillera, Colombia. West is to the left. Scale bars show equal horizontal and vertical scales (VE ¼ 1). Black: Palaeogene interval of the passive margin or rift sequence, containing the petroleum system. Dark grey: Palaeozoic and older rocks forming the basement of the Eastern Cordillera. Lighter grey, Jurassic and Cretaceous sediments. Prepared during Ecopetrol’s bid round study for JNOC in 1998.

FOLD–THRUST BELTS AT PEAK OIL 21

Fig. 13. Composite structure cross section of the productive Llanos foothills (top) and its partial SNIP restoration (bottom). West is to the left. Bar scale is valid for horizontal and vertical scales (VE ¼ 1). Black: Palaeogene interval of passive margin or rift sequence containing the petroleum system. Grey: unidentified older and younger intervals illustrating the structural setting. This section and its restoration display a gradual or step-wise dissection of a triangle zone by late and out-of-sequence thrusts. Simplified and redrawn from material by Martinez (2006). More discussion in text.

22 D. ROEDER Fig. 14. Crustal structure cross section of Cordillera and Caribe Borderland of Eastern Venezuela, sketched after published data, unpublished geological maps courtesy PDVSA, and the position of the subducting Caribbean slab (Van der Hilst & Mann 1994). North is to the left. Bar scale is valid for horizontal and vertical scales (VE ¼ 1). Black: Cretaceous and Palaeogene sediments. Light grey: continental crust. Dark grey: oceanic crust and mobilized continental crust.

FOLD–THRUST BELTS AT PEAK OIL 23

Fig. 15. Structure cross section of the Serrania del Interior in Anzoategui (East Venezuela), two dated versions. Top: A pre-Macal section (Roeder 2001) assuming a style of thin-skinned thrust stacking at and below the major and shallow Urica–Pirital thrust or Cordilleran main thrust. Bottom: A revised version after the Macal evidence of moderate and thick-skinned displacement within a field of internally coherent detached folds. In both versions, the Urica–Pirital thrust, lettered X, cuts up-section and blind-soles beneath the Neogene of the East Venezuela basin. The bar scale is valid for horizontal and vertical scales (VE ¼ 1). The colouring does not conform to the colour legend of Figure 5. Grey shading: Barranquin and older Cretaceous sediments. Thick black line: El Cantil Cretaceous limestone, a seismic marker. Colours are as labelled in Figure 5. Well symbols: Extant well and proposed location of New Field Wildcat Capiricual (2001). The bottom of the section is the gravimetrically assumed and very uncertain basement top.

24 D. ROEDER Fig. 16. Manual line tracing of seismic data with colours showing a stratigraphic interpretation. Seismic data are two combined and overlapping versions of reprocessed seismic line ET88-17A, courtesy PDVSA, near the south front of the Serrania del Interior and near the state line of Anzoategui and Monagas (East Venezuela). The patchwork shows two wells and two geological interpretations. North is to the left. Colours are as labelled in Figure 5. A and D: Lower Cretaceous El Cantil limestone, a seismic marker. B: Shallow high-strain Cordilleran main thrust assumed to define duplex target for MAC-1X. C: interpreted trace of Cordilleran main thrust or Pirital or Prepirital thrust. E: Assumed allochthonous duplex or horse unit disproved by MAC-1X. This combination of an older seismic line with a newer reprocessed patch illustrates a failed attempt to reinterpret a thick-skinned FTB front in terms of a high-strain version of the Bally– Dahlstrom–Suppe style.

FOLD–THRUST BELTS AT PEAK OIL 25

Fig. 17. Two versions of a regional structure cross section illustrating the impact of the Macal failure. Top: pre-Macal version composed of the work by Saul Osuna and Leroy Hernandez. This version shows a large and shallow Urica–Pirital thrust. It detaches the Cordilleran field of Biotian folds and covers a field of prospective Jurassic to Neogene imbricates. Bottom: As suggested by two Macal wells and by the seismic line ET88-17A, the Urica– Pirital thrust, moderately dipping, is the sole of the thick-skinned main part of the Serrania del Interior. There is major established production only south of this thrust. Light grey: pre-Cretaceous sediments and basement, undivided. Black: Lower Cretaceous El Cantil limestone, a seismic marker. Dark grey: Palaeogene foredeep fill containing the petroleum system. The bar scales are valid for horizontal and vertical distances (VE ¼ 1). Colours are as labelled in Figure 5.

26 D. ROEDER Fig. 18. Top: Structure section across the Pirital front and the foothills field of El Furrial. Bottom: Line tracing of the seismic support for this cross section. Production of this field comes from several sand levels within the (light-brown) interval of Palaeogene foredeep fill above the (dark brown) Vidono source rock (Hung 2005). The Pirital thrust is strongly discordant to its hanging wall and less discordant relative to its footwall. Its displacement is 25–30 km. The Pirital hanging wall shows strong mountainous topographic relief infilled by the Morichito-Las Piedras successor basin, essentially the Orinoco delta. To the right (south) of El Furrial, foredeep fill may be present below the Morichito–Las Piedras unit. Colours are as labelled in Figure 5.

FOLD–THRUST BELTS AT PEAK OIL

The large thrust fault had to blind-sole southward, and the poorly controlled north-dipping basement top presented space problems. Surface data do not support a frontal thrust, even though it has been inferred and mapped tentatively as the Urica fault (Ostos 1990), a local branch of the more regional Pirital thrust. South-dipping panels of foredeep fill suggest a triangle zone and the blind south front of the assumed main thrust. The Macal project (2001–2002) involved two deep wells and was targeted for Duplex-type thrust imbrications (Dahlstrom 1970; Suppe 1983) or horses enclosed between the shallower Pirital thrust and the deeper Prepirital thrust. This interpretation assumed a Pirital thrust overlap of 40– 50 km. In a graphically combined form, Figure 16 illustrates the Macal pre-drilling and post-mortem play concepts. The well MAC-1X bottomed in Cretaceous or in older tight and sterile sediments, well below the assumed Pirital or Quiriquire thrust. It never encountered the anticipated Tertiary sourcereservoir system. Later, a paper copy of dip line ET88-17, hand-interpreted by Dr Peter Varrell

27

(about 1992) was rediscovered. This line shows the Pirital fault as a much deeper thick-skinned thrust fault with a setting of constant cut-off angle, with a ramp-on-ramp geometry, and with an estimated throw of 30 km. This setting is drastically different from the anticipated setting of a shallow Pirital –Prepirital system. The Macal failure appears in Figure 17 with two versions of a regional structure cross section. Two wells (only one of them shown) and a forgotten seismic line have strategically limited the Cordilleran mountain front area prone to contain unexplored leads. They also have limited the preseismic assumptions to a moderate-strain version of the Bally–Dahlstrom –Suppe style of FTB tectonics. On the positive side, they have opened the field for exploring detached Biotian folds. A new and critically revised systematic account of East Venezuela leads and prospects is needed to support PDVSA’s petroleum strategy. The El Furrial field (Fig. 18) appears as a duplex in sub-thrust position beneath the Pirital thrust. This thrust fault shows about 25 km of separation, and it discordantly dissects folds of the detached Serrania

Fig. 19. Three stratigraphic well profiles or formation-top logs of two wells in Lombardia of Northern Italy. The actual Cascina Riviero well (formerly Zandobbio) contrasts strongly with its prognosis and with the incompletely drilled Franciacorta well (labelled F-Corta) 5 km to the SE. Vertical scale in metres. Black: Upper Triassic Dolomia Principale, an important reservoir in the Po Valley basin. Dark grey: shaly and in part organic-rich Rhetic or uppermost Triassic. Light grey: Other Mesozoic and Tertiary formations. Black lines in Franciacorta log: High-energy seismic events.

28 D. ROEDER Fig. 20. Two seriated and parallel structure cross sections, Alpine south front in Lombardia, Italy, showing productive or shut-in fields of Malossa and Adda and dry consortial well Cascina Riviero, formerly Zandobbio. Sections are based on surface geology, on partial seismic control and on an assumed Bally– Dahlstrom–Suppe model of thinskinned FTB tectonics just updip of the thick-skinned (Orobic) front. Black: Lower Cretaceous Majolica and upper Triassic Dolomia Principale. Light grey with parallel lines of equal depth below basement top: Pre-Triassic crystalline rocks or basement. Dark grey: Upper Triassic and Jurassic passive margin sediment, also (in foreland part of bottom section) Gonfolite Flysch or Alpine –Apennine foredeep fill of Neogene age, Shallow black unit above Malossa field: Gasiferous Messinian boulder beds. North is to the east, bar scales are valid for horizontal and vertical distances. Both sections display a significant and not entirely controlled difference in depth to foreland basement. The deepest slice in the Alpine frontal stack of thrust sheets is visible on some but not all seismic lines. If present as designed in these sections, the Main Dolomite reservoir of the productive Foothills unit could be reached in wells 7.6 km and 6.8 km deep.

FOLD–THRUST BELTS AT PEAK OIL

type. A second, more internal footwall imbrication may or may not form a drillable prospect, shown in the present interpretation.

South Alpine venture This failed and unfinished venture could be re-opened given recent attempts to lessen Europe’s dependence on West Siberian gas. In Northern Italy, Anschutz and consorts (1982–1998) tried to discover wet gas on acreage down-dip from producing fields and outside of the exclusive ENI Reserved Area. The global rank of the Po Valley basin (Klett et al. 1997) is 84, with 0.4 BB of liquid, 18.9 TCF of gas, and 3.2 billion BOE. Geologically, the venture aimed at a DUD trend, but it failed with the first well Agip–Chevron Cascina Riviero-1 dry and abandoned. The well neither confirmed nor condemned the objective, that is, a deep imbricate directly down-dip from extant production. A seismic image of the objective appears visible, deep and tectonically covered by units of the thick-skinned Alpine south front. Success in this play would require not only the definition and drilling of the trapping tectonic unit but also firm knowledge of the burial history at the Alpine front, and from the start, a full consortial control over all relevant data both inside and outside the ENI Reserved Area. Feasible locations for the first well were extremely difficult to obtain. The Alpine foothills terrain is composed of densely populated valleys and steeply sloped and forested ridges. At the site finally chosen, the well did reach the tight and dry top of the reservoir in a flank position and not at the crest. The short reservoir interval opened was dry. Finally, the well failed conceptually (Fig. 19) because of a major error in extrapolating the northward thickening of the passive margin wedge containing both the source rock and the reservoir. This error dates back to inaccurate consortial cross sections designed by Dietrich Roeder (see Roeder 1992). Two structure cross sections of the South Alpine front have been in part revised but not re-balanced (2008, Fig. 20) to account for the Cascina Riviero well. In both sections, the potential reservoir horizons, productive in the Po Valley basin, include the (lower Cretaceous) Majolica limestone and the (upper Triassic) Dolomia Principale. Of several known source rocks, the (upper Triassic) Riva di Solto is the most likely local candidate.

Revisiting the South Alpine target Encouraged by the data available from Cascina Riviero, by the political need to replace West

29

Siberian gas, and by the economic need to replace European atomic energy, a second search for a productive DUD trend at the Alpine south front would be based on a restudy of all relevant 2D seismic data. This search would focus on defining a deep slice conterminous with the buried and productive foothills belt. Based on the cross sections shown in Figure 20, the undrilled but seismically visible deep slice perhaps involves the upper Triassic reservoir at an estimated depth of 7.6 km near Cascina, and 6.8 km near the Adda River. The thermal state of a deep foothills slice depends on the kinematic sequence and on the tectonic ages. In the common progressive sequence, the deep slice forms by piggybacking the thick-skinned front, and therefore it is over-mature. In an anomalous setting generated by the Messinian sea-level lowstand (see Roeder 2004), emplacement of the thick-skinned Alpine front may be late and analogous to the Friuli seismicity (Slejko et al. 1987). In this case, the present blind front may have overrun an extant and still cold foothills imbricate.

Conclusions Globally, there are few onshore FTB areas not controlled by state petroleum interest. However, the challenges for states and private industry are the same. Few FTB areas are still in the frontier state, and renewed exploration will be required in most future FTB ventures. Modern exploration is vastly superior to the technical state that was valid during the era of FTB failures. Modern visualization easily replaces the old hand-designed structure cross sections, but it depends on the availability of 3D seismic data. The typical FTB failures occurred in domains of low-fold and obsolete 2D data not feasible for visualization. Investment in modern 3D data must be based on a technical estimate of potential return. Therefore, a typical restudy of FTB failures, preparatory to any decision on investment, requires an intelligent use of obsolete data. Discovery will be possible if the old data are organized and reinterpreted, if high quality seismic data of any vintage can be obtained, if complex new dynamic models are used, and if the new exploration can be integrated with earlier attempts. Local and global shortages of marketable natural gas may or may not help to maintain the high gas prices that are needed to account for the high risk of very deep drilling. Enzo Zappaterra invited the author to attend the Geological Society of London conference on petroleum exploration LGS conference on petroleum exploration of fold –thrust belts and to write the present paper. Enzo

30

D. ROEDER

had been in charge of the Zandobbio-Cascina Riviero well and its geological preparation based on incomplete and uncertain data (1991– 1996). After the LGS conference (2008) he freely shared his revised and better understanding of the drilled prospect. The present paper profited from suggestions by Marc Cooper, Rodney Graham, Rob Butler, Ken McClay and Francois Roure. Finally, reviews by Graham Goffey and an anonymous referee substantially improved this paper.

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Edwards, J. D. 2001. Twenty-First Century Energy: Decline of fossil fuel, Increase of renewable nonpolluting energy sources. In: Downey, M. W., Three, I. C. & Morgan, W. A. (eds) Petroleum Provinces of the Twenty-First Century. AAPG Memoir, 74, 21–34. Fitch, T. J. 1972. Plate convergence, transcurrent faults, and internal deformation adjacent to SE Asia and the Western Pacific. Journal of Geophysical Research, 77, 4432–4460. Ganss, O. & Schmidt-Thome, P. 1955. Die gefaltete Molasse am Alpenrand zwischen Bodensee und Salzach. Zeitschrift der Deutschen Geologischen Gesellschaft, 105, 402–495, Hannover. Hung, E. J. 2005. Thrust belt interpretation of the Serrania del Interior and Maturin subbasin, eastern Venezuela. In: Venezuela, H. G., AveLallemant, V. B. & Sisson, B. (eds) Caribbean– South American Plate Interactions, Venezuela. Geological Society of America, Special Paper, 394, 1– 346. Jacobshagen, V. 1975. Zur Struktur der su¨dlichen Allga¨uer Alpen. Gebundene Tektonik oder Deckenbau? Neues Jahrbuch fur Geologie and Paleontologie Abhandlungen, 148, 185– 214, Stuttgart. Klett, T. R., Ahlbrandt, T. S., Schmoker, J. W. & Dolton, G. L. 1997. Ranking of the World’s Oil and Gas Provinces by Known Petroleum Volumes. US Geological Survey Open File Report, 97–463. Kockel, C. W. 1956. Der Umbau der no¨rdlichen Kalkalpen und seine Schwierigkeiten. Verhandlungen der Geologischen Bundesanstalt, 205–214, Vienna. Lemcke, K. 1988. Geologie von Bayern 1: Das bayerische Alpenvorland vor der Eiszeit. Erdgeschichte, Bau, Bodenscha¨tze, E. Schweizerbart’sche Verlagsbuchhandlung Stuttgart, 175p. Lohr, J. 1969. Die seismischen Geschwindigkeiten der ju¨ngeren Molasse im ostschweizerischen und deutschen Alpenvorland. Geophysical Prospecting, 17, 111– 125, Den Haag. Martinez, J. A. 2006. Structural evolution of the Llanos foothills, Eastern Cordillera, Colombia. Journal of South American Earth Sciences, 21, 510–520. Mueller, M. 1978. Miesbach 1 und Staffelsee-1, two basement tests below the Folded Molasse. In: Closs, H., Roeder, D. & Schmidt, K. (eds) Alps Apennines Hellenides. Inter-Union Commission on Geodynamics, Science. Report 38, 64–68. Mull, C. G., Roeder, D. H., Tailleur, I. L., Pessel, G. H., Grantz, A. & May, S. D. 1987. Geologic sections and maps across the Brooks Range and Arctic Slope to Beaufort Sea, Alaska. Geological Society of America Maps and Charts Series, MC-28S, 1:500 000. Ostos, Marino 1990. Evolucion tectonica del margen Sur-Central del Caribe basado en datos geoquimicos, GEOS No. 30, 1– 294. Passalacqua, H., Fernandez, F., Gou, Y. & Roure, F. 1995. Crustal architecture and strain partitioning in the eastern Venezuelan ranges. In: Tankard, A., Suarez Soruco, R. & Welsink, H. (eds) Petroleum Basins of South America. AAPG Memoir, 62, 667–679. ¨ ber der Bau der Richter, M. & Scho¨nenberg, R. 1955. U Lechtaler Alpen. Zeitschrift der Deutschen Geologischen Gesellschaft, 105, 57–79, Hannover.

FOLD–THRUST BELTS AT PEAK OIL Roeder, D. 1967. Rocky Mountains – Der Geologische Aufbau des Kanadischen Felsengebirges. Beitr. Regionale Geologie der Erde, 5, Borntraeger, 318p. Roeder, D. 1992. Thrusting and Wedge growth, Southern Alps of Lombardia (Italy): final reports. European Geotraverse (EGT). Tectonophysics, 207, 199–243. Roeder, D. 2000. Canadian Rocky Mountains – Inventory of Cross Sections. BERTFEST, Albert W. Bally Spring 2000 Symposium & Fest, Rice University, Houston, TX. Roeder, D. 2001. Eastern Venezuela and Cutofito Fold– Thrust Belts: Constructing a Series of Structure Cross Sections: A report of a regional structural review for 2001: PDVSA internal report. Roeder, D. 2004. Messinian Salinity crisis and SouthAlpine tectonics. Part 1 – thin thrust wedges: – NAFTA, Year 55, No. 3, (March), Zagreb, R. Croatia, 107–116. Roeder, D. & Bello, H. 2003. Global Inventory of Fold– Thrust Belts: Petroleum Geology, Geodynamics (2 volumes). A short course for AAPG International Conference, Barcelona, Espana. Roeder, D. & Chamberlain, R. L. 1995. Eastern Cordillera of Colombia: Jurassic– Neogene Crustal Evolution. In: Tankard, A., Suarez Soruco, R. & Welsink, H. (eds) Petroleum Basins of South America, AAPG Memoir 62, 633 –646. Roure, F., Carnevali, J., Gou, Y. & Subieta, T. 1994. Geometry and kinematics of the North Monagas thrust belt (Venezuela). Marine and Petroleum Geology, 11, 347– 362. Schwerd, K., Huber, K. & Mueller, M. 1995. Tektonik und regionale Geologie der Gesteine der Tiefbohrung Hindelang 1 (Allga¨uer Alpen). Geologica Bavarica, 100, 75–115, Mu¨nchen. Slejko, D., Carulli, G.-B. et al. 1987. Modello Sismotettonico dell’Italia Nord-Orientale: Consiglio Nazionale delle Ricerche. Grupo Nazionale per la difesa dai terremoti, Rendiconto No. 1, 1 –82.

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Sommaruga, A. 1997. Geology of the Central Jura and the Molasse Basin: New Insight into an EvaporiteBased Foreland Fold and Thrust Belt. Memoire de la Societe Neuchateloise des Sciences Naturelles, Tome XII, 176p. Spo¨rker, H. 1993. Technik des Bohrens. In: Brix, F. & ¨ sterreich. Schultz, O. (eds) Erdo¨l und Erdgas in O Verlag Naturhistorisches Museum, Wien, 100– 114. Sprinkel, D. A. & Chidsey, Th. C. 2008. A Review of Petroleum Exploration in the Central Utah Thrust Belt with an Emphasis on the Failures and How they guide Current Research. In: Emme, J. & Carr, M. (eds) Rocky Mountain “Dusters” Lessons Learned and Opportunities Created. RMAG Seminar, Denver September 22, 2008. Suppe, J. 1983. Geometry and kinematics of fault-bend folding. American Journal of Science, 283, 648– 721. Tollmann, A. 1976. Der Bau der No¨rdlichen Kalkalpen. Franz Deuticke, Wien, 449p. TRITON COLOMBIA, INC. 1982. Farming Possibilities in Colombia. Unpublished public-domain brochure. Valderrama, R. 1982. Desarollo de facies en la cuenca de los Llanos Orientales Colombianos. In: Roberto, L. (ed.) Exploracion Petrolera de las Cuencas Subandinas. Volume 3, Bogota´ 1982. Van Der Hilst, R. D. & Mann, P. 1994. Tectonic implication of tomographic images of subducted lithosphere beneath northwestern South America. Geology, 22, 451– 454. Wessely, G. 1993. Beilage 9, Geologischer Tiefbau Flysch-Kalkalpenzone. In: Brix, F. & Schultz, O. ¨ sterreich. Verlag Naturhis(eds) Erdo¨l und Erdgas in O torisches Museum, Wien. Zimmer, W. & Wessely, G. 1996. Exploration results in thrust and subthrust complexes in the Alps and below the Vienna Basin in Austria. In: Wessely, G. & Liebl, W. (eds) Oil and Gas in Alpidic Thrustbelts and Basins of Central and Eastern Europe. EAGE Special Publication No. 5, The Geological Society, London, 81–108.

Structural styles in the Papuan Fold Belt, Papua New Guinea: constraints from analogue modelling KEVIN C. HILL1*, KATIE LUCAS2,3 & KEITH BRADEY1 1

Oil Search Limited, Angel Place, 123 Pitt Street, Sydney, NSW Australia 2000

2

Geological Sciences and Geological Engineering, Miller Hall, Queen’s University, Kingston, Ontario, K7L 3N6, Canada 3

Present address: Premier Gold Mines, 401-1113 Jade Court, Thunder Bay, Ontario, P7B 6M7, Canada *Corresponding author (e-mail: [email protected])

Abstract: Cross sections, seismic data and centrifuge analogue modelling reveal the structural styles in the oil-producing areas of the Papuan Fold Belt. They include inverted basement faults, detachment faults in the Jurassic section 1 –2 km beneath the Neocomian Toro Sandstone reservoir, and tight, overturned folds in the reservoir sequence with stretched and boudinaged forelimbs, cut by break-thrusts. Additional features include highly variable thicknesses in the Cretaceous Ieru Formation, the regional seal sequence, including through-going detachments that isolate the overlying thick Miocene Darai Limestone. Centrifuge analogue modelling of intact, plane-layered strata determined that the mechanical stratigraphy and the thickness of weak beds above the lower de´collement horizon exert the greatest control on the structural style. Large-offset thrust faults were only produced in models with pre-cut faults, generating early inversion and then large ramp anticlines, similar to those in the Kutubu Oilfield, which has reserves of .350 million barrels. It is suggested that the Kutubu Oilfield trend was underlain by a large normal fault and that, by analogy with the Vulcan Sub-basin, oil-rich source rocks may be confined to the hanging wall or north side of this fault. Oil would have been generated and expelled during thin-skinned deformation.

The aim of this paper is to describe the structural style of hydrocarbon traps in the Papuan Fold Belt (Fig. 1), particularly the thin-skinned structures, and to relate the structures to mechanical stratigraphy through physical, centrifuge analogue modelling. Structural interpretation has incorporated data from the many wells that have been drilled, from surface mapping and from 2D seismic and other geophysical surveys. However, the jungle-covered mountains in Papua New Guinea (PNG) severely hamper geological mapping and result in poor quality seismic data, so there remains much ambiguity in the structural interpretations. The ambiguity is greatest in the deeper, undrilled, parts of the structure and in the sheared forelimbs, both areas that are being investigated for additional hydrocarbon potential. In order to constrain the structural interpretation and hence assess this potential, physical scaled-modelling was carried out in a centrifuge using horizontal layers of plasticine and silicone putty to reflect the mechanical stratigraphy of the main beds. The models are generic rather than designed to replicate specific structures, but by varying the thickness and competence of the individual beds it was possible to obtain a very good fit to the known structures.

Oil and gas exploration commenced in PNG in the late 1920s, drilling shallow wells on seeps near major rivers on the Fly Platform. Exploration in the fold belt commenced in the 1950s on the accessible mountains, resulting in significant gas discoveries such as Barikewa in 1958 and Juha in 1983. Commercial oil was discovered in the Iagifu – Hedinia (Kutubu) anticlines in 1985 (Bradey et al. 2008) followed by nearby discoveries at Agogo, SE Mananda, Moran and Gobe, which have collectively been on production since 1992. Initial recoverable reserves in the known fields were well over 500 million barrels of low viscosity oil. The giant Hides gas field (Johnstone & Emmett 2000), with over 5 trillion cubic feet (TCF) reserves, was discovered in 1986 and will be the core of the gas development project planned in the near future. This project, combined with new technologies, has opened the door to a renewed phase of exploration for deeper, more cryptic oil and gas plays (Hill et al. 2008). Currently, over 250 wells and sidetracks have been drilled in the Papuan Fold Belt, and more than 3000 km of 2D seismic data have been acquired, generally of poor to moderate quality. There has been considerable surface geological

From: Goffey, G. P., Craig, J., Needham, T. & Scott, R. (eds) Hydrocarbons in Contractional Belts. Geological Society, London, Special Publications, 348, 33– 56. DOI: 10.1144/SP348.3 0305-8719/10/$15.00 # The Geological Society of London 2010.

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Fig. 1. Sun-shaded, digital elevation model showing the main features of the Papuan Fold Belt. The main structural belts are labelled, after APC (1961). The Darai Plateau is a very large asymmetric anticline overlying an inverted extensional fault that was active from Triassic to Miocene times. The structure is offset by a tear fault, the Bosavi Lineament, to underlie the mountain front in the NW part of the fold belt. All the producing oil and gas fields lie within the Strongly Folded Belt. Key wells are labelled and oil field and gas field outlines are shown in green and red, respectively. Wells within the fields are shown by white dots but, for clarity, are not labelled.

mapping, aided by 87Sr/86Sr isotope dating of the Miocene surface limestones (Hornafius & Denison 1993) and by analyses of synthetic aperture radar images. Structural interpretation has also been improved by the acquisition of regional and high-resolution aeromagnetic surveys, gravity surveys, and of earthquake seismic data. In this paper, a regional cross section is presented and two oil-producing structures and one breached structure are discussed, each of which has been drilled by numerous wells with dipmeter data, has been covered by widely spaced 2D seismic data and has good surface outcrop data. The structures are the Moran, Agogo and Paua anticlines (Fig. 1).

Tectonics, stratigraphy and structure of the Papuan Fold Belt Tectonically, the island of New Guinea comprises the northern margin of the Australian continent

that has undergone Miocene to Pliocene oblique convergence with the Pacific Plate resulting in collision with intervening microplates. The reader is referred to Hill & Hall (2003) for a recent discussion of PNG tectonics. The Papuan Fold Belt straddles the middle of the island and comprises precipitous mountains of heavily karstified Miocene limestone covered with dense equatorial jungle. The Fold Belt is made up of folded and thrust Mesozoic clastic rocks and Tertiary limestones and is bound to the south by the Fly Platform containing similar rocks, but undeformed. Compression in the fold belt occurred mainly in the Late Miocene and Pliocene (Hill & Raza 1999) directed roughly from NE to SW until the Middle Pliocene. Crowhurst et al. (1997) proposed that there was then a change to east –west compression in PNG, continuing to the present and resulting in increased strike-slip deformation. The stratigraphy of the Papuan Fold Belt is summarized in Figure 2a and the simplified stratigraphy

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Fig. 2. Stratigraphy across the Papuan Fold Belt. (a) Simplified lithostratigraphic section (after Hill et al. 2000) flattened on the top Miocene. The Mesozoic section is dominantly mudstone, but contains the Upper Jurassic to Neocomian Iagifu, Hedinia, Digimu and Toro sandstone reservoirs. These are collectively modelled as Toro Sandstone. The Cretaceous Ieru Mudstone is the regional seal and is unconformably overlain by the thick Miocene Darai Limestone and Orubadi Marls. The Upper Triassic and Lower Jurassic syn-rift sequence is schematic on this section. (b) Stratigraphic column used for mechanical modelling in a centrifuge, showing the mechanical stratigraphy and real versus model thicknesses. P ¼ plasticine, SP ¼ silicone putty. 1 mm in the model is equivalent to 1 km in the prototype (see Table 1). In the later models, a thicker analogue Imburu sequence and pre-cut fault were used as shown.

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used in centrifuge analogue modelling (discussed later) is shown in Figure 2b. Beneath the Fly Platform and the fold belt, ‘basement’ comprises Upper Palaeozoic rocks, mainly Permian, that were deformed in the Early Triassic New England Orogeny and intruded by Middle Triassic granites (Van Wyck & Williams 2002; Crowhurst et al. 2004). The sequence was deeply eroded and the granites were exposed at that time. In the Late Triassic and Early to Middle Jurassic, the area was subject to extension and rifting (Home et al. 1990), depositing the syn-rift Kana Volcanics, Magobu Coal Measures and Barikewa Mudstone, the latter two being probable source rocks (Fig. 2a). Regional Late Jurassic subsidence flooded the margin allowing deposition of the Imburu Formation, Toro, Digimu, Iagifu and Hedinia sandstone reservoirs and the Cretaceous Ieru Formation seal. In distal facies of the northeastern Fold Belt, both the Imburu and Ieru mudstones are hydrocarbon source rocks. During the latest Cretaceous to Paleocene, southern PNG was uplifted, probably associated with northern Tasman and Coral Sea rifting. Subsequent erosion stripped some Upper Cretaceous sediments in the fold belt and Fly Platform area and deposition did not resume until Late Oligocene flooding allowed widespread deposition of Miocene shallow marine carbonates, the Darai Limestone (Fig. 2a). Carbonate deposition was halted by the Late Miocene onset of compressional deformation, which was also responsible for generation and migration of most hydrocarbons.

Structural models of the Fold Belt APC (1961) divided the Papuan Fold Belt into three NW–SE-trending belts, illustrated in the regional section shown in Figure 3 (discussed below). In the SW was the ‘Gently Folded Belt’, including the giant but low relief Darai anticline, 40 km wide and 100 km long. Structures in this belt are generally considered to be inverted basement structures (e.g. Hobson 1986; Hill 1991). The second belt, c. 30 km wide, commenced NE of the Darai anticline and comprised the ‘Strongly Folded Belt’. All the hydrocarbons to date have been found in this belt and its structural style is addressed in this paper. The area further to the NE was classified as the ‘Imbricate Zone’ and consists of thrust repeats

of Miocene limestone and uppermost Cretaceous shales, which are recorded in outcrop. This area is not considered here, but was analysed by Hill et al. (2000) and is currently being explored by oil companies. As little was known about the Papuan Fold Belt, early structural models followed those from betterstudied fold belts in North America and Europe, for instance fault-propagation folds (Smith 1965), imbricate thrusts (Findlay 1974; Jenkins 1974), duplexes (Hobson 1986) and fault-bend folds (Hill 1991). It was only with the detailed analysis of individual anticlines following the drilling of several wells that it was found that the structures comprised detached folds with overturned and/or thrusted forelimbs (Lamerson 1990; Eisenberg 1993; Franklin & Livingston 1996). Regional sections continued to show underlying basement thrusts or basement inversion structures (Buchanan & Warburton 1996; Thornton et al. 1996; Cole et al. 2000). Drilling and seismic acquisition over the Moran and Paua anticlines indicated breakthrust structures with strongly sheared forelimbs (Davis et al. 2000; Lingrey 2000). Recently, Hill et al. (2008) and Bradey et al. (2008) used seismic and potential field data to confirm along-strike partition of the fold belt into zones with differing basement involvement. To interpret and model the structure of the Papuan Fold Belt, it is important to know or infer the pre-compression configuration of the margin. For instance, was it a relatively undeformed ramp as beneath parts of the Canadian Rocky Mountains (e.g. Bally et al. 1966) or a highly faulted margin with abrupt thickness changes as recorded in parts of nearby Indonesia (e.g. Chambers et al. 2004)? Cooper et al. (1996) interpreted two regional seismic sections across the Timor Sea, a margin along strike to PNG with a similar Mesozoic history. There they found that the major basinbounding fault abuts the stable Londonderry High, and that two large faults bound the Jurassic Swan Graben, but otherwise the Mesozoic section gradually thickens seaward over a distance of 180 km (Fig. 4). They also noted a significant offset of structures across the Paqualin transfer zone (e.g. Woods 1992). Cooper et al. (1996) proposed that this area is a good structural and stratigraphic analogue for the Papuan Fold Belt prior to thrusting.

Fig. 3. (Continued) Regional cross section over the Papuan Fold Belt based on projected well data, surface geology and, in part, on poor to fair quality seismic data. The seismic data were most useful in defining regional dip and elevation of basement and occasionally the dip and shape of the base of the Darai Limestone. The bulk of the section results from structural interpretation of well and surface data. The lowermost section, at half-scale, shows a restoration of the Gently Folded and Strongly Folded Belts such that the shortening has not yet propagated over the hypothetical graben containing source rocks. See Figure 1 for location and text for detailed discussion.

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Fig. 3.

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Fig. 4. (a) Location map of regional seismic lines across the Vulcan Sub-Basin in the Timor Sea, along strike to PNG. Cooper et al. (1996) inferred that this area was a good analogue to the pre-deformation stratigraphy and structure of the Papuan Fold Belt. Jabiru, Challis and Skua are oilfields, each with recoverable reserves of 50– 150 million barrels. (b) Block diagram of the top Callovian in the Vulcan Sub-Basin showing the Upper Jurassic rift geometry, after Cooper et al. (1996). Note the large growth fault adjacent to the Londonderry High similar to the Darai Fault in PNG (Fig. 3). Oil source rock is largely restricted to the syn-rift fill in the Swan and Paqualin grabens (Kennard et al. 1999) and may be similarly restricted in PNG.

Regional structure of the Papuan Fold Belt A regional cross section was constructed in 2DMove from the foreland across the Darai Plateau, the Strongly Folded Belt and the Imbricate Belt

(Fig. 3). The section was drawn to honour all stratigraphy and dips from outcrop and ten boreholes in addition to synthetic aperture radar images and limited potential field data. It was constructed along or close to seismic lines, particularly the semiregional line PN05-404 (shown in Hill et al. 2008)

STRUCTURAL STYLES IN THE PAPUAN FOLD BELT

across the Fold Belt. In general seismic data quality was moderate, ranging from poor to occasionally good. Migrated seismic lines were used for all interpretations, although regularly checked against those with stack and wave-equation processing. The interpreted horizons were imported into GXII and vertically depth stretched using laterally varying interval velocities for each of the main stratigraphic units. The resulting depth horizons were then imported into 2DMove and were used to guide the form of structures. Due to the moderate quality of the seismic data, they were not relied upon in detail to construct the cross sections. The top and base of the Darai Limestone were usually reasonably imaged so the underlying structure was interpreted by projecting down using known stratigraphic thicknesses. The section was incrementally restored using 2DMove to help validate the interpretation and show the likely structural evolution. Due to the complexity of the structures, several methods were used in restoration. Late-stage basement inversion structures were restored using a tri-shear algorithm that accommodated local changes in thickness of the overlying sediments. The tri-shear algorithm was a good approximation to the deformation for the Mesozoic section, but not for the competent Darai Limestone, that required separate, fault-parallel flow restoration. Relatively simple fault-bend fold structures were restored using fault-parallel flow algorithms. More complex structures, such as the overturned SE Hedinia and Moran anticlines, had late break-thrusts restored by fault-parallel flow, were partially unfolded using a flexural slip unfolding algorithm, and then again restored using fault-parallel flow. Usually a small degree of area balancing was required for the core of the structures. The regional section was restored incrementally in six stages, but only one is shown in Figure 3, illustrating restoration of the Gently Folded and Strongly Folded Belts.

Fly Platform and Gently Folded Belt The thickness of sediments above basement within the Darai Plateau is more than double that of the adjacent Fly Platform (4800 m v. 2200 m) indicating inversion of a previously extensional growth fault (Fig. 3). The extensional fault was probably a major basin-bounding fault, as the sedimentary thickness to the north remains at 4–6 km. The growth appears to be continuous through geological time, suggesting a relatively stable platform to the south, similar to the Londonderry High of Cooper et al. (1996; Fig. 4). Offset of the Darai Limestone and continuing earthquake activity suggest that Darai Plateau inversion was Pleistocene to Recent. However, a component of Late Miocene inversion cannot be ruled out.

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There is clearly a significant unconformity between the Cenomanian upper Ieru Formation and the Late Oligocene basal Darai Limestone. Apatite fission track analyses (e.g. Hill & Gleadow 1989, 1990) combined with vitrinite reflectance profiles in the Kanau-1 well indicate .2 km erosion of the uppermost Ieru Formation beneath the Darai Plateau, but ,1 km erosion beneath the foreland, prior to Darai Limestone deposition. This suggests that in the Early Tertiary the old normal fault was inverted and that the hanging wall was eroded prior to regional Oligo-Miocene subsidence. Although they appear to have structural closure, neither the Kanau-1 nor the Bosavi-1 wells drilled on the Darai Plateau recovered hydrocarbons. This is thought to be due to lack of charge. It is notable on the cross section that the Toro Sandstone reservoir in both wells is near sea-level. To the SW, the Toro Sandstone almost abuts the basal Darai Limestone across the Darai Fault. It is considered likely that along strike the Toro connects to the basal Darai Limestone and hence is in pressure communication with the foreland, consistent with the low pressures recorded in the Kanau-1 well.

Strongly Folded Belt Where the gently NE-dipping limb of the giant Darai Plateau meets the frontal fold belt structures, such as Zongwe (Figs 1 & 3), strong linear reflectors were observed on seismic at c. 3 s (see Fig. 9 in Hill et al. 2008). These are interpreted to be Magobu Coal Measures overlying basement. Using the depth conversion methods outlined above, these reflectors record a consistent dip of c. 68 for over 12 km to the NE beneath the Strongly Folded Belt suggesting a planar, relatively undeformed Jurassic sequence above basement such that the overlying Zongwe, Ai-io and SE Hedinia structures are detached within the sedimentary section (Fig. 3). The Zongwe anticline is interpreted to be the leading edge of the thin-skinned thrusting, in that there is a Darai Limestone repeat at surface along a fault detached within the Ieru Formation. This is thought to be underlain by a reactivated basement thrust creating a large gentle fold in the Toro reservoir. The evidence for the basement thrust from seismic data is equivocal as the data quality in that area is poor. However, on the synthetic aperture radar image (Fig. 1), the Zongwe and Ai-io anticlines together can be interpreted as a 3–5 km wide asymmetric structure that resembles a mini Darai Plateau. The SE Hedinia anticline has been drilled by three wells and the detachment is inferred to be near the top of the Koi-Iange section. The SE Hedinia wells show that the anticline is tight, probably with an overturned forelimb, or with a forelimb sheared out by thrust faulting. Both are typical

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structural styles along strike. Due to the steep dips, the core of the structure is not effectively imaged on seismic data. Between SE Hedinia and Kutubu East (Figs 1 & 3), there is a consistent SWdipping panel from basement through to the basal Darai Limestone indicating uplift of basement to the NE on a significant basement fault. Based on seismic interpretation and section balancing, the basement faulting is interpreted to deform the top Koi-Iange Formation detachment so that at least some of the basement faulting occurred after the overlying thin-skinned deformation. The Kutubu East anticline was drilled by two wells, which encountered minor gas and very high pressures, as opposed to normal to low pressures in the Strongly Folded Belt to the SW. The conundrum of having a breached structure that preserves very high pressures is currently being investigated, but it clearly shows a sealing fault underlying the Kutubu East anticline. Regionally, this fault separates the high pressure belt to the NE from the adjacent SE Hedinia Gasfield and Kutubu Oilfield to the SW.

Imbricate Belt Over a distance of 10 km to the NE of the Kutubu East anticline, interpretation of the seismic data (Hill et al. 2008) indicates a planar, gently NE-dipping panel of strata from basement to top Darai Limestone. Geological maps show that this panel is overlain by thrust repeats of Darai Limestone and thin upper Ieru Formation, the start of a major Darai duplex that crops out over a band that is 24 km wide from Lake Kutubu to the Wage anticline. Beneath the mapped Mubi anticline the step-up in basement, inferred from seismic data, appears to fold the overlying Darai Limestone duplex, so the basement thrusting occurred after the thin-skinned deformation. The Darai Limestone duplex exposed at surface represents considerable shortening in the Darai Limestone and upper Ieru Formation. Below, or to the NE, this must be balanced by equivalent shortening in the lower Ieru Formation to Koi-Iange Formation. On the cross section, this has been represented as a simple duplex forming the Mount Castle and Wage anticlines. However, numerous other interpretations are possible, including basement involvement. A recently acquired regional seismic line across these structures may resolve the subsurface geometry.

Centrifuge analogue modelling Background In order to create a more accurate representation of the subsurface structure of the Papuan Fold

Belt, and to improve exploration success, scaled physical analogue models were employed. These centrifuge models simulate mechanical stratigraphy and can help determine which factors control the deformation and thus predict the structural style, the deformation sequence and the geometry of hydrocarbon traps. Dixon (1996) showed that when a pre-existing dip-slip fault was inserted into the model to simulate an old basement or extensional fault, it was reactivated early in the deformational history, prior to thin-skinned deformation. Recent centrifuge modelling of facies changes and reefs within fold and thrust belts, such as the Canadian Rocky Mountains, clearly demonstrated that mechanical stratigraphy not only affected the structural style, but also the sequence of structural deformation (Dixon 2004). This modelling also demonstrated how the strength of the basal de´collement surface influences the style of deformation. A de´collement of moderate strength produces forelandverging folds in weak, basinal facies and thrusts in competent platform facies. A weaker basal detachment promotes upright folding in the basin facies and forethrusts and backthrusts associated with upright buckling on the platform (Dixon 2004). Dixon (1996) applied generic analogue models to the Papuan Fold Belt, and showed detached or loosely linked structures in competent beds, depending upon the relative strength of the intervening weak layer. He illustrated the potential for complete detachment between the reservoir and near-surface structures. He also showed that pre-existing faults in a ‘basement’ analogue are reactivated early in the deformation and remain as important features. However, the amount of reactivation varied inversely with the dip of the pre-existing fault such that reactivation was barely noticeable if the initial preexisting fault was steeper than about 458. Dixon’s (1996) pre-existing faults extended through the analogue Mesozoic section, with growth in the competent reservoir analogue, but not in the underlying weak Jurassic syn-rift section. Dixon (1996) also discussed the limitations of the modelling. He pointed out that it cannot simulate variables such as geothermal gradient, pore-fluid pressure and syntectonic erosion. Furthermore, the modelled stratigraphic sequence rests on a rigid baseplate that is not involved in the deformation.

PNG model parameters The centrifuge experiments were performed in the Experimental Tectonics Laboratory at Queen’s University, Canada. The centrifuge modelling technique used is discussed in detail by Dixon & Summers (1985), Dixon & Tirrul (1991), Liu & Dixon (1991) and Dixon & Liu (1992). The centrifuge employed in these experiments (Fig. 5) can

STRUCTURAL STYLES IN THE PAPUAN FOLD BELT

Fig. 5. Centrifuge modelling. (a) An oblique view of the centrifuge used for analogue modelling, showing the location of the chamber within the centrifuge rotor. (b) Close-up of the centrifuge rotor, indicating how the model fits into the chamber. (c) Schematic diagram of the initial model configuration. The rigid base plate represents the basement of the model, and consists of aluminium plates. Shortening of the foreland stratigraphic succession is caused by the gravitational collapse of the hinterland wedge.

subject the model to a centripetal acceleration up to 20 000 g, such that it simulates the Earth’s gravity. All experiments were subjected to an acceleration of 4000 g (where 1 g ¼ 9.8 m/s2, normal Earth

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gravity). Each model underwent two to four deformation stages, each stage lasting for five minutes at the maximum acceleration of 4000 g, with an additional seven minutes for the acceleration and deceleration of the centrifuge. The models were photographed in plan view and cross section after each stage. Table 1 outlines the model scaling ratios used, after Liu & Dixon (1991). The models were constructed of plasticine modelling clay and silicone putty, with internally layered units of differing mechanical strengths. These materials exhibit a contrast in their competencies and are suitable analogue materials for the different rock types in the Papuan Fold Belt (Dixon & Summers 1985). In the models, the plasticine represents competent units such as limestone and sandstone, and a combination of silicone putty and plasticine represents incompetent units such as shale. The strength of a mechanical unit can be altered by changing the ratio of plasticine to silicone putty, with an increase in plasticine corresponding to an increase in strength. Building on Dixon’s (1996) work, the models presented here were designed to specifically simulate the known stratigraphy (Fig. 2). From a mechanical point of view, this comprises the strong Miocene Darai Limestone; the weak Cretaceous Ieru Formation, intermediate strength Upper Jurassic Iagifu, Hedinia, Digimu and lowermost Cretaceous Toro sands (here collectively termed Toro) and the weak Jurassic clastic sequence, mainly Imburu Formation (Fig. 2b). The thicknesses and competencies of the layers were varied in each model to demonstrate how different mechanical stratigraphies, and different combinations of preexisting faults, control the overall structural style of the fold belt (Table 2). Importantly, the thickness and strength of the Jurassic syn-rift section was varied to see if it resulted in different structural styles. If a characteristic style could be attributed to a specific syn-rift thickness, it may be possible to predict the location of old normal faults in the Papuan Fold Belt by analysing structural style. The initial Papuan Fold Belt modelling focused on changes within the overall mechanical stratigraphy, the strength and thickness of the lower detachment horizon and the competency of the Ieru Formation. Further centrifuge experiments incorporated previous modelling work by Dixon (1995) and Dixon et al. (1996), that show how primary extensional faults can disrupt the typical deformation sequence of fold and thrust belts. The models created for these experiments contained a pre-cut fault in the lower unit. The fault was cut at an angle of 188 dipping to the hinterland. This very low angle was used as steeper dipping faults were found to exhibit less reactivation and then lock up (Dixon 1995, 1996; Dixon et al. 1996).

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Table 1. Model scaling ratios used for this study, after Liu & Dixon (1991) Ratio (model: prototype)

Equivalence (model ¼ prototype)

lr ¼ 1.0  1026 rr ¼ 0.6 tr ¼ 1.0  10210 ar ¼ 4.0  103 sr ¼ rr lr ar ¼ 2.4  1023

1 mm ¼ 1 km 1.60 ¼ 2.67 (bulk value of stratigraphic column) 1023 s21 ¼ 10213 s21 (for example) 4000 g ¼ 1 g Calculated from other ratios

Quantity Length Specific Gravity (mass) Time (strain rate) Acceleration Stress

Table 2. Construction details for plane-layered models KL10, 12, 17 and 19 and for pre-cut fault models 20 and 22 Model

Prototype unit

Construction materials

Number of internal laminae

Total unit thickness (mm)

P:SP thickness ratio

Total model thickness

KL10

Darai Ieru Toro Imburu Darai Ieru Toro Imburu Darai Ieru Toro Imburu Darai Ieru Toro Imburu Darai Ieru Toro Imburu

P P/SP P P/SP P P/SP P P/SP P P/SP P P/SP P P/SP P P/SP P P/SP P P/SP

8 16 4 16 8 16 4 32 8 16 4 32 8 16 4 32 8 16 4 32

1.5 1.0 0.5 1.0 1.5 1.0 0.5 2.0 1.5 1.0 0.5 2.0 1.5 1.0 0.5 2.0 1.5 1.0 0.5 2.0

1:0 1:1 1:0 1:0 1:0 1:1 1:0 1:0 1:0 2:1 1:0 1:0 1:0 2:1 1:0 2:1 1:0 2:1 1:0 1:0

4 mm

KL12

KL17

KL19

KL20 KL22

5 mm

5 mm

5 mm

5 mm

Abbreviations: P, plasticine, SP, silicone putty.

Vaseline petroleum jelly was applied to the fault surface to prevent the hanging wall and footwall from re-adhering after being cut. The initial model configurations for these experiments are shown in Figure 2 and Table 2. Regrettably, the seismic data in PNG were not of adequate quality in the basement, typically at 3 –6 s two-way time (twt), to determine the dip of faults to compare with the model (see discussion).

Plane layer, mechanical stratigraphy modelling The initial model had a relatively thin basal unit, simulating c. 1 km of Jurassic section as observed beneath parts of the Fly Platform (Fig. 3). Forelandverging imbricate duplex structures developed as thrusting initiated early during deformation and

only low-amplitude folds were able to form prior to the development of break-thrusts (Fig. 6a, KL10). Two de´collement surfaces exist within the Imburu and Ieru analogues, creating differential shortening and thrust spacing between the competent units. The second model shows an increase in the thickness of the basal unit from 1.0 mm to 2.0 mm, simulating c. 2 km of Jurassic section above rigid basement, as observed beneath the Darai Plateau (Fig. 3). This modification decreased the amount of foreland vergence and created more upright structures (Fig. 6b, KL12). There was an increase in folding versus thrusting, with increased fold amplitude in both the Toro and the Darai analogue units. The de´collement horizon within the Ieru analogue was not active consistently throughout deformation, allowing regions of both harmonic and disharmonic deformation to develop.

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Fig. 6. Centrifuge analogue modelling of intact, plane-layered strata with initial conditions as shown in Figures 2 and 5 and Table 2. Note the variation in structural style as the thickness and relative mechanical stratigraphy are varied. Observed faults have been highlighted with thin black lines. Model (a) has a 1-mm thick Imburu analogue as opposed to 2 mm for models (b) to (e). In model (c) the mechanical strength of the Ieru is increased and in model (d) the mechanical strength of the Imburu is increased. Model (e) has a pre-cut fault dipping at 188 from the base Imburu to the base Darai analogue. See text for discussion.

The third model incorporates an increase in the mechanical strength of the Ieru analogue, creating a mechanical linkage between the overlying Darai and underlying Toro competent units (Fig. 6c, KL17). By forcing the linkage of the competent

units, the upper three units act as a single beam deforming above a weak, relatively thick, de´collement horizon. The upright, fold-dominated structures are harmonic between the competent units, with surface structures situated directly above the

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structures at depth. However, in detail it can be seen that the Toro analogue is dominated by tight folding, the Ieru analogue manifests substantial changes in thickness and the Darai analogue records both open folding and thrust faulting. The Ieru analogue acts as a local or diffuse detachment zone connecting the folds and minor faults in the Toro analogue to the faults in the Darai analogue. The structural style is very similar to that recorded in the Moran and Usano structures (Franklin & Livingston 1996; Davis et al. 2000; Lingrey 2000) and is discussed further below. The fourth model includes an increase in the mechanical strength of the Jurassic Imburu analogue (Fig. 6d, KL19). This strong lower de´collement horizon produced a thrust-dominated structural style, containing duplexes with foreland vergence. Upright structures did not develop due to the enhanced foreland vergence. Early developed folds had very low amplitude, and thrusts developed early in the deformation. A significant second de´collement surface developed within the Ieru analogue, allowing the structures between the Darai and Toro analogues to become laterally displaced from each other. Varying the mechanical stratigraphy used in the fold and thrust modelling of plane-layered strata resulted in four different structural styles

(Fig. 6a–d). The structural style that most resembles that recorded in the Papuan Fold Belt is shown in Figure 6c, perhaps with elements from Figure 6d. Thus the modelling suggests the presence of a relatively thick and weak Jurassic section and a slightly more competent Ieru section. The models are discussed further below when compared to cross sections of the Papuan Fold Belt.

Modelling of pre-existing faults Recently acquired and/or reprocessed seismic data across the Kutubu Oilfield suggest a low-angle thrust fault with 4–8 km of displacement in that area of the Papuan Fold Belt (Bradey et al. 2008), as illustrated in Figure 7. The Kutubu seismic line (Fig. 7) also shows an unusual ‘double-hump’ structure in the hanging wall with the Toro Sandstone and Darai Limestone both gently folded, except in the forelimb where overturned Darai was encountered at the base of one well. The structure has been confirmed by 43 wells drilled over the field (Bradey et al. 2008). The double-hump structure in part results from thrust displacement over a series of ramps and flats, but it requires a relatively large thrust displacement. No such large-offset thrust or double-humped anticline was recorded in the plane-layered models presented above. It has

Fig. 7. Seismic line PN88-IAG-1 across the Iagifu and Hedinia anticlines that comprise the Kutubu Oilfield. See Figure 1 for location. Note the inferred relatively large offset of the Toro and two-humped nature of the Toro anticline, similar to that in Figure 6e. Note also the required internal deformation within the Ieru in the anticline cores, as shown by tighter folds in the Darai than in the Toro.

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long been suspected that Jurassic extensional faults were a controlling factor in the structural evolution in the Papuan Fold Belt (e.g. Buchanan & Warburton 1996) and that the Kutubu Oilfield is underlain by a Jurassic normal fault that may have been reactivated. These concepts were tested by models that included one or two faults cut into the pre-deformation model, each of which was lubricated by Vaseline petroleum jelly smeared along the fault surface. In order to obtain the large thrust offset, the faults were pre-cut at 188 to bedding and lubricated as steeper pre-cut faults tend to lock up during deformation (Dixon 1996). A model was run with a single pre-cut fault from the base of the Imburu analogue to the base of the Darai analogue (Fig. 6e, KL20). The stratigraphy used was the same as that in Figure 6c (Table 2). The pre-cut fault was reactivated early in the deformation and accommodates much of the shortening. The dip of the pre-cut fault evolved to become steep as it passed through the break in the Toro analogue, shallow as it continued through the incompetent Ieru analogue, and steeper again where it propagated through the Darai analogue to the surface of the model. Much of the shortening was channelled to the incompetent Ieru unit, creating laterally displaced structures between the Darai and Toro analogues. A second large fault with a shallower dip developed in the Darai analogue on the foreland side of the reactivated fault. The slip on the pre-cut fault at Toro level is roughly equal to the sum of the slip on these two Darai faults indicating that the pre-cut fault splays upwards, linking to both faults. Furthermore, some of the displacement was channelled towards the foreland along the Ieru de´collement, creating greater shortening and a different structural style within the Darai analogue than in the Toro analogue. The differential shortening laterally displaced the Darai analogue further towards the foreland with respect to the structures that developed in the Toro analogue. This model bears a strong resemblance to the structure of parts of the Kutubu Oilfield (Fig. 7) and to that shown in the SE Hedinia area on the regional cross section (Fig. 3). In particular, the model shows a two-humped Toro anticline in the hanging wall of the pre-cut fault, as seen in the Kutubu anticline and large displacement along the fault. Further, the model shows a detachment in the Ieru, with splays cutting through the Darai towards the foreland of the Toro structure, as recorded in the Zongwe and Ai-io structures on the regional section (Fig. 3). To test the possibility of multiple normal faults beneath the Papuan Fold Belt, two faults were pre-cut in the model, each penetrating from the base Imburu to half way through the Ieru

45

analogue unit. Displacement along the pre-cut faults was simultaneous within the resolution of the experiment, each with equivalent shortening that was channelled to both competent units (Fig. 8). The region between the two pre-cut faults acquired the least amount of deformation, creating a large zone of uplift where the panel between the faults was transported up the frontal fault towards the foreland. Initial thrusting along the pre-cut fault can be seen after only 5% total shortening (Fig. 8a). At 16% total shortening, the faults remain as the dominant feature of the model (Fig. 8b), but as deformation progresses to 44% shortening, younger fold– thrust structures develop independent of the reactivated faults (Fig. 8c). Importantly in these models, the pre-cut faults were reactivated first creating anticlines akin to inversion structures and the fold and thrust structures formed subsequently. If true in the Papuan Fold Belt, it would have important implications for migration and charge of structures (see discussion). Comparing the model shown in Figure 8c with the regional cross section (Fig. 3) it is apparent that there are some broad similarities. Perhaps the structure above the leading pre-cut fault is equivalent to the broad SE Hedinia –Kutubu East culmination, in other words the Strongly Folded Belt. The long hinterland-dipping limb above the pre-cut fault in the model is very similar to that interpreted on seismic data (Hill et al. 2008) used to construct the regional cross section. Similarly, the structure above the more hinterland pre-cut fault in the model could equate to the broad culmination defined by the Mount Castle and Wage structures on the regional section. If this comparison is valid, then it suggests that the two culminations may be associated with pre-existing faults, perhaps Jurassic extensional faults.

Structural style from well and seismic data The Moran anticline The Moran anticline was first drilled in September 1996 and was found to contain an 800 m oil column in Digimu and Toro sandstone reservoirs. Davis et al. (2000) stated that ‘the Moran structure is a narrow, elongate SW-vergent fault-bounded anticline with a moderate to steeply dipping (308 –508) backlimb and an overturned near vertical forelimb’. Davis et al. (2000) and Lingrey (2000) presented cross sections through the Moran 1X, 2X, 1XST and 2XST wells, two of which drilled through the hanging wall across the thrust and tagged the footwall. The sections indicated that the structure was folded first then a break-thrust broke

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Fig. 8. Centrifuge analogue modelling of plane-layered strata with two faults pre-cut from the base Imburu to middle Ieru as shown in Fig. 2b. Initial conditions are recorded in Table 2 and Fig. 2. Note that the pre-cut faults are reactivated first making large anticlines (a and b) and the thin-skinned deformation encroaches at higher degrees of shortening (c). See text for discussion.

through the stretched, faulted and boudinaged forelimb. The structure is strongly compartmentalised (Hill et al. 2008), with a fault across the centre separating an 800 m oil column to the west from an equivalent higher-pressure water column at the

same elevation to the east (shown by the abrupt eastern edge of the field boundary on Fig. 1). Here, we present two seismic lines across the Moran anticline and adjacent structures and a cross section based on well data, surface dips and

STRUCTURAL STYLES IN THE PAPUAN FOLD BELT

seismic data and utilizing the centrifuge analogue modelling presented above. Figure 9 shows uninterpreted and interpreted versions of recently reprocessed, migrated seismic line PN96–302 across the Moran anticline through the Moran 1X, 2X, 1XST and 2XST wells. The c. 5 km seismic line illustrates the difficulty in acquiring good quality data across this steep, mountainous, jungle-covered terrain with karstified limestone at surface. Data acquisition is exacerbated by crooked line paths, air-filled caverns, deep fissures, an irregular low velocity weathered zone and a velocity inversion from the Darai Limestone to the underlying Ieru Shale (Lingrey 2000). Seismic acquisition is also limited by the current cost of US$ 100 000/km, in part due to the necessity of helicopter-supported operations. Although the seismic data quality is only poor to moderate, it is still useful in helping to interpret the structure when used in conjunction with surface geology. Indeed, now that the obvious surface anticlines in the Papuan Fold Belt have been drilled, such seismic data are vital in future exploration. On the uninterpreted section of Figure 9, the broad form of the strongly reflecting Darai Limestone can be seen, revealing the position of the Moran Thrust. The inclination of the backlimb can also be determined, both at surface and at depth, away from the core of the structure. It is also possible to infer a potential sub-thrust structure at Toro level, as shown, although this is not proven and relies in part on structural and modelling analogues. Unfortunately, below the Darai Limestone in the core of the structure, almost all of the reflectors are spurious, as shown from the interpretation post-drilling. The steep dips and probable small-scale internal faulting of the beds make imaging of the core of the structure almost impossible. In areas without well control, interpretation of the core relies on structural and analogue models. Figure 10a shows a structural interpretation of the Moran and Paua anticlines as well as the NE limb of the large Mananda anticline that is imaged on the seismic line (Fig. 9). The Moran section is similar to those presented by Lingrey (2000) and Davis et al. (2000), except that the borehole dips have all been reprocessed, there are additional surface dips and the seismic data have been reprocessed. The Mananda part of the section is constrained by additional seismic lines and eight wells on the Mananda anticline along strike and the Paua structure is constrained by two wells projected onto the section. It should be noted that the number of dips shown is only a small sample of those used to construct the section and that the Mesozoic stratigraphic zonation is very fine, with many more horizons correlated than can be

47

shown. Thus the hanging wall anticlines are well constrained. The Middle Jurassic to basement portion of the section is largely unconstrained and schematic. A key part of the interpretation is the ‘regional’ level of the relatively undeformed Middle and Lower Jurassic section. Projecting those beds downdip from the foreland suggests that the top of the Middle Jurassic should be at c. 5 km subsea, consistent with the regional section (Fig. 3). However, interpretation of the seismic data suggests that the top of the Middle Jurassic is at 3–4 km subsea beneath Moran and modelling of earthquake seismic data in the area (Hill et al. 2008) indicates high velocity basement is at 6 km subsea. Therefore, reverse faults in basement have been inferred on the interpretation, as shown. These may be reactivated extensional faults, but this is unproven. As can be seen from comparison of Figure 10a, b, there are strong similarities between the section interpretation and the centrifuge model shown in Figure 6c. Both sections show open folds in the Darai but tight to overturned folds in the Toro– Iagifu (reservoir) section, with large changes in thickness of the intervening Ieru Formation. Both show brittle faulting in the Darai, but more folding in the reservoir section. Furthermore, a detachment within the Ieru is manifested in both sections, connecting the faulting in the Darai above Moran to the thrust fault underlying the Paua anticline. Such structures may well have formed out-of-sequence as the Moran anticline developed in front of and below the Paua anticline. A significant difference between the interpreted and analogue model sections (Fig. 10a, b) is the level of detachment beneath the reservoir. The Moran wells and a few other wells drilled to the fault in the Papuan Fold Belt, combined with the seismic data, strongly indicate a detachment in the Imburu Formation just above the Koi-Iange Formation, c. 800 m below the top of the Toro Sandstone. In contrast, in the analogue model the fundamental detachment is scaled to be 2.5 km below the top of the Toro. However, it should be noted that the level of detachment varies in the Papuan Fold Belt and that there is good evidence from seismic data (better quality than in Fig. 9) that the detachment is deeper, c. 1.5–2.0 km below the top Toro, in the core of the Mananda anticline, the core of the Kutubu Oilfield (Fig. 7) and the northeastern part of the regional section (Fig. 3). Another difference is the degree of thickness changes in the Ieru, which is considerably greater in the analogue model. It may be that the interpreted cross section underestimates Ieru thickness changes in the synclines; for instance the Darai keel between the Moran and Mananda

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Fig. 9. Seismic line PN96-303 through the Moran-1 and Moran-2 wells, blank and interpreted. The well ticks are horizon tops, not dips. The data illustrate the limitations of seismic acquisition and interpretation in PNG. However, it is possible to infer the shape of the Darai Limestone, the location of the Moran Thrust and potential sub-thrust structures. See text for discussion.

STRUCTURAL STYLES IN THE PAPUAN FOLD BELT

Fig. 10. (a) Structural cross section of the Moran and Paua anticlines based on detailed well dips and stratigraphy, surface mapping and seismic data. The well ticks are representative dips. This section has not been balanced. (b) Expanded view of the middle of Figure 6c showing a strong similarity between the interpreted section and the analogue model.

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anticlines could be shallower, as it is only constrained by poor quality seismic data. Detailed analysis of the Moran cross section (Fig. 10a) reveals several features that suggest a structural evolution (discussed later). In the core of the structure a low-angle, folded thrust fault was encountered at three locations in the boreholes, with c. 50 –100 m offset. The wells that drilled the forelimbs of both the Moran and Paua structures found a relatively complete stratigraphic sequence, but 30 –50% of the normal thickness and cut by faults. The Moran Fault was interpreted to comprise a zone of faults through these stretched beds and is itself offset by a backthrust encountered in three wells. Within the Ieru Formation, the Bawia Mudstone is a weak and mobile bed in which the thickness is highly variable, probably due to tectonism. This horizon is interpreted to be the main detachment horizon within the Ieru Formation as dips are highly variable above it with common thrust splays but dips below are relatively consistent. One further feature is the interpretation of ‘out-of-the-syncline’ thrust faults (Dahlstrom 1970), for which the main evidence is offsets and increased thickness of the Darai Limestone. Combining those features suggests the following structural evolution: (1) Minor low-angle thrust faults developed through parts of the structure. (2) A fault-propagation fold developed with a stretched overturned forelimb, including folding of some of the existing low-angle thrusts.

(3)

With continued shortening, the fold evolved into a break-thrust. (4) The next structure towards the foreland, the Mananda anticline, was thrusted and folded, jacking up the existing structures and generating out-of-the-syncline thrusts and backthrusts due to the resultant space problems. In terms of hydrocarbons, it seems likely that the 800 m oil column is preserved, in part, due to tight folding of the reservoir so that it is encased in mudstone of the Ieru Formation, the regional seal. The forethrusts and backthrusts have effectively made the crest of the anticline a pop-up structure that may have helped to isolate and preserve the hydrocarbon column. It is notable that the adjacent Paua structure encountered a residual oil column, minor gas and high pressures and that the same was true for the eastern half of the Moran anticline across an important tear fault (Hill et al. 2008). The Mananda structure to the southeastern preserved a small oil column at low pressure in an isolated crest at its SE end, the SE Mananda field (Fig. 1).

Moran – Agogo structure Figure 11 shows c. 9 km of an interpreted version of a recently reprocessed, migrated seismic line PN07–505 across the Agogo Oilfield and the southeastern part of the Moran anticline. This SE part of the Moran structure tested water at high pressure instead of oil. Although imaging of the core of the Moran structure is still poor, probably due to

Fig. 11. Structural interpretation of seismic line PN07-505 across the Agogo oilfield and SE Moran anticline. See Figure 1 for location. The Agogo anticline, the adjacent syncline and even the underlying basement are relatively well imaged.

STRUCTURAL STYLES IN THE PAPUAN FOLD BELT

steep dips and faulting, the seismic and nearby Moran-3X well data (Fig. 1) suggest that the structure is a thrust ramp, with a less well developed forelimb than the oil bearing part of the structure shown on Figure 10. The SE Moran structure is perhaps more like the thrust Toro structure near the centre of Figure 6e than the folds with stretched forelimbs near the centre of Figure 6c. The Agogo anticline is along strike, but slightly en echelon to the SE Mananda anticline, hence is further away from the Moran Thrust. Thus the syncline between the Moran and Agogo structures is preserved rather than faulted out as it was on Figures 9 and 10. In consequence, the Darai Limestone to Jurassic section is much better imaged, particularly within the Agogo structure. It is also possible to infer some underlying basement structure as shown on the section. It is notable that the structural style on this seismic line is not exactly like any of the centrifuge analogue models presented here. In particular, the consistent large thrust offset of Toro Sandstone and Darai Limestone differs from the models, but resembles the Toro offset near the centre of Figure 6e and towards the right hand side of Figure 8c.

Discussion Centrifuge analogue modelling The influence of mechanical stratigraphy upon structural style was demonstrated by the experiments presented here, as shown by the different structural styles in Figure 6. The thickness of the competent beds remained the same in these experiments, so the variations in structural style were due to subtle changes in the relative strength and thickness of the weaker layers. Thin alternating strong–weak –strong– weak layers with large competence contrasts resulted in imbricate thrusts in the strong layers with thrust spacing proportional to the bed thickness (Fig. 6a). The intervening weak layers acted as ductile detachments that accommodated the variable strain, so that thrusting in the strong layers was disharmonic, as previously recorded by Dixon (1996). Doubling the thickness of the lower weak layer above ‘basement’ profoundly changed the structural style with more buckle folds developing that were variably harmonic and disharmonic in the overlying competent units (Fig. 6b). The basal weak layer above basement in PNG is the syn-rift sequence, so modelling thickness changes as in Figure 6a, b suggests how structural styles might change across a syn-rift growth fault. Unfortunately it was not possible to run a model with a step in basement. However, Dixon (2004) modelled lateral changes in mechanical stratigraphy

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to represent reef versus off-reef facies, showing abrupt changes in structural style at the boundary. The modelling presented here indicates that in PNG and elsewhere, the structural style would similarly change dramatically across an old extensional fault with thick, weak syn-rift strata on one side (Fig. 6b) and thin equivalent strata on the adjacent high (Fig. 6a). The large folds preferentially formed over the syn-rift strata (Fig. 6b) would be more prospective as hydrocarbon traps. Slightly strengthening the upper weak layer in the models produced dominantly buckle folds in the middle competent layer, the Toro reservoir analogue, and thrusting in the upper, thick competent layer, the Darai analogue (Fig. 6c). These models were most like the known fold belt structures, such as in the Moran anticline. Significantly, the models were able to produce tight, overturned folds in the Toro reservoir section with break-thrusts through the attenuated forelimb as recorded in the Moran, Paua and SE Hedinia anticlines (this paper) and the Usano and Hedinia anticlines (e.g. Lamerson 1990; Franklin & Livingston 1996). Furthermore, the models recorded open folds in the more competent Darai and common faults cutting the Darai, many of which were splays from a Ieru detachment that rooted back to the previous Toro anticline, towards the interior of the fold belt. The models also recorded dramatic thickness changes within the Ieru as proven in many wells. The importance of the models is not in the area of known structures, but as an aid to interpretation in areas under exploration with little subsurface data and poor to moderate quality seismic data. When a lubricated pre-cut fault dipping at 188 through the Mesozoic analogue was introduced to the model (Figs 6e & 8) it was reactivated early in the deformation and recorded a large displacement. In the hanging wall an overturned fold was generated at the leading edge (Fig. 8). This large offset, particularly in the centre of Figure 8c, is similar to the structure interpreted within the Kutubu Oilfield, the largest oilfield in PNG (Fig. 7). The modelling did not otherwise produce faults with large offset. A comparison of Figure 6c and e shows the impact of the pre-cut faults in sections that otherwise had identical stratigraphy. With the exception of the pre-cut fault the structural style is the same. This suggests that a pre-existing fault or weakness was necessary to produce the Iagifu –Hedinia anticlines that contain the Kutubu Oilfield (Fig. 7). It is debatable whether or not a pre-cut fault dipping at 188 can represent the effect of an old extensional fault, which would be expected to have a dip of 45 –608. Dixon (1996) modelled the reactivation of pre-cut faults, but found that with steeper pre-cut faults the amount of reactivation

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decreased, such that there was minimal reactivation of faults dipping steeper than c. 458. An important consideration is that Dixon’s pre-cut faults did not have thicker, weak (syn-rift) beds in the hanging wall so did not simulate a Jurassic growth fault offsetting basement. However, Dixon’s later experiments with an abrupt facies change from weak to strong beds across a vertical contact (Dixon 2004) did generate a fault with large offset at the contact, albeit at relatively high levels of shortening. Importantly these structures had an overturned fold in the hanging wall, as recorded by drilling at the leading edge of the Hedinia anticline in the Kutubu Oilfeld (Bradey et al. 2008). Combining the results from the models shown in Figure 6a, b with the pre-cut fault models and Dixon’s (2004) facies change models, it seems very likely that an old extensional fault with a thick, weak syn-rift section would be reactivated early in the deformation as a thrust fault. Furthermore, this fault would propagate to have a large displacement compared to other faults in the area. The Hedinia Thrust beneath the Kutubu and SE Hedinia oil and gas fields (Figs 3 & 7) is probably such a fault.

PNG structural style and hydrocarbon prospectivity In the introduction, the Timor Sea area was proposed as an analogue for the pre-deformation architecture of the Papuan Fold Belt (Fig. 4; Cooper et al. 1996). In order to assess hydrocarbon prospectivity, this concept is reviewed further here, incorporating the results from the structural sections and centrifuge modelling experiments. The Timor Sea lies between the Bonaparte Basin to the NE with gas reserves of 28 TCF and the Browse basin to the SW with gas reserves of 30 TCF (Australian Government 2007). Within the Timor Sea, the Vulcan Sub-basin (Fig. 4) contains a handful of medium-sized oilfields with total reserves of 357 million barrels (Longley et al. 2002), similar in size to those of the Papuan Fold Belt. Rifting to form the Vulcan Sub-basin occurred in Late Jurassic to Early Cretaceous times (Pattillo & Nicholls 1990) following Late Triassic to Mid Jurassic rifting in PNG (Home et al. 1990), as part of the same break-up of the north Australian margin (Veevers 2000). Kennard et al. (1999) concluded that oil generation and expulsion were restricted to Oxfordian– Kimmeridgian syn-rift source rocks principally within the Swan and Paqualin grabens and the deepest (SW) portion of the Cartier Trough (Fig. 4). Thus the oil source rock was focussed in local deep graben within an otherwise gas-prone area.

The regional cross section presented here (Fig. 3) illustrated a number of different structural styles across the Papuan Fold Belt, which can be related to features within the Vulcan Sub-basin. The section showed that the Darai Fault (Fig. 3) is an important basin-bounding fault across which there is significant growth in several stratigraphic units demonstrating long-lived extensional activity. This is akin to the major basin-bounding fault that abuts the Londonderry High such that the sediments beneath the Darai Plateau may be comparable to those in the Skua syncline (Fig. 4b). In PNG, the basin-bounding extensional fault is interpreted from seismic data to continue along much of the front of the Papuan Fold Belt, although occasionally offset across tear faults such as the Bosavi Lineament (Fig. 1). The Paqualin Transfer across the Vulcan Sub-basin (Fig. 4a; Woods 1992) is considered to be a basement-controlled feature that probably resembled the Bosavi Lineament prior to compressional deformation. The Darai Fault was probably inverted in Early Tertiary times, was definitely inverted in Pliocene times and remains active, as indicated by compressional earthquakes. Importantly, the current basement inversion occurred prior to any thin-skinned deformation as the nearest thin-skinned structure is 40 km to the north (Fig. 3). This is consistent with the results of the centrifuge analogue modelling with a pre-cut fault (Fig. 8 and Dixon, 1996) in which the existing faults were reactivated in compression at low amounts of shortening prior to the onset of thin-skinned deformation. If inverted early in the deformation sequence, it would be reasonable to expect structures such as the Darai Plateau to trap any subsequent hydrocarbon charge, yet all wells in that area are dry with few oil shows. As the structures to the north are charged, there must be a barrier to migration between the Strongly Folded and Gently Folded Belts (Figs 1 & 3). The Strongly Folded Belt contains all of the commercial oil reserves found in PNG, with large gas discoveries to the NW at Hides and to the SE in the Gulf of Papua. The belt is underlain by a large-offset thrust fault and has accompanying large hanging wall anticlines (Figs 3, 7 & 11) that appear to be less common elsewhere in the fold belt. The centrifuge analogue modelling suggests that a pre-existing weakness is required to generate such large-offset faults. It seems likely that, prior to compressional deformation, the Kutubu, Agogo and SE Hedinia oil and gas fields (Figs 1, 3, 7 & 11) were underlain by a significant normal fault, perhaps the southern bounding fault of a deep graben similar to the Swan Graben on Figure 4 (see hypothetical graben on Fig. 3). The graben could have been the source kitchen for all the oil and been responsible for the development of the

STRUCTURAL STYLES IN THE PAPUAN FOLD BELT

large-offset fault that created the Strongly Folded Belt, hence supplying both trap and charge. The significant normal fault may be the barrier to migration into the Gently Folded Belt structures to the SW.

Proposed structural evolution Combining the structural observations from the cross sections presented and the analogue modelling the following structural evolution is suggested. † The pre-compression architecture of the margin consisted of a stable platform in the south bounded by large Late Triassic to Mid Jurassic extensional faults with thick syn-rift sediments beneath the future Darai Plateau. Further north, beneath the future Strongly Folded Belt, a second major normal fault is inferred, bounding the area to the north that contained the main oil source rocks. This may have been a deep graben (Fig. 3). † During early compressional deformation, in the Late Miocene, there was probably minor reactivation of basement faults as shown on Figure 8a, including the fault beneath the Strongly Folded Belt. † Thin-skinned thrusting occurred along an Imburu detachment that was in some areas between the Iagifu and Koi-Iange sandstones (Fig. 10a) and in other areas below the Koi-Iange (Figs 3, 7 & 6c). The transition from one detachment level to another probably occurs across transfer zones that may be old basement faults similar to the Paqualin Transfer on Figure 4. † In the Strongly Folded Belt, the inverted normal fault was reactivated as the Hedinia Thrust and accommodated 4– 8 km of shortening, building the Iagifu and Hedinia anticlines that comprise the Kutubu Oilfield (Figs 1 & 7). † As the thin-skinned structures propagated over the source kitchen beneath the Strongly Folded Belt, oil was generated and expelled. † In areas such as Moran, a fault-propagation fold developed with a stretched overturned forelimb, including folding of some of the existing thin-skinned thrusts. This evolved into a break-thrust. † Thrusting and folding of the next structure towards the foreland occurred, jacking up the existing structures and generating out-of-thesyncline thrusts and backthrusts due to the resultant space problems. † As orogenesis propagated further towards the SW, inversion occurred creating the Darai Plateau and was probably accompanied by renewed basement reverse faulting beneath the fold belt, perhaps reactivating old extensional faults.

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Future work To determine the hydrocarbon prospectivity of the Papuan Fold Belt, understanding the structural style is vital and this paper attempts to address that issue. However, equally as important is understanding which faults seal and which faults leak and/or breach the structures. Furthermore, the timing of fault sealing with respect to hydrocarbon charge is important as fault seal parameters will change with the changing stress regime (e.g. Castillo et al. 2000). Crowhurst et al. (1997) argued that the compression direction was NE –SW in the Late Miocene, but changed to more E-W in the mid Pliocene. Such a stress rotation would have a significant effect on the sealing capabilities of faults, particularly cross-cutting faults. The importance of sealing or breaching faults is demonstrated by the Moran and Paua structures (Figs 1 & 10). The Moran oilfield resides in the NW half of the anticline where there is an 800 m oil column, separated by a sealing cross fault from the SE portion of the structure that tested water at elevated pressures. To the NE the Paua anticline encountered very high pressures, a residual oil column and minor gas and is thought to have been breached and then resealed. Clearly the dip-slip faults between Moran and Paua must seal, yet may previously have been open to allow hydrocarbon charge. Resolution of this issue is beyond the scope of this paper, but is the aim of ongoing studies.

Conclusions (1)

(2)

(3)

(4)

(5)

Centrifuge analogue modelling of intact, plane-layered strata determined that the mechanical stratigraphy and the thickness of weak strata above the lower de´collement horizon exert the greatest control on the changing structural styles of the Papuan Fold Belt. The models most like known structures had a thick, incompetent Jurassic shale sequence, a competent Toro reservoir sequence, an intermediate competence Ieru sequence and a competent Darai sequence. These models produced tight overturned folds in the Toro, thickness changes and detachments in the Ieru and open folds and thrust faults in the Darai, all recorded in the Moran anticline. Centrifuge analogue modelling with pre-cut faults in the Mesozoic section produced early inversion anticlines as recorded in the Papuan Fold Belt by inversion of the Darai Fault. With continued shortening the pre-cut faults accommodated much of the shortening as large-offset thrust faults that did not otherwise occur in the models.

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(6)

(7)

(8)

(9)

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The large-offset faults produced a ‘doublehump’ hanging wall similar to that in the Kutubu Oilfield, PNG’s biggest oilfield with .350 million barrels original oil in place. The undeformed PNG margin probably comprised a small number of large basin-bounding or graben-bounding faults across which there was substantial growth in the stratigraphic section. Away from those faults the Mesozoic section probably thickened gradually towards the NE. The seismic data, structural sections and analogue modelling defined the important elements of the PNG structural style. These are: (a) Inverted basement faults. (b) Thin-skinned faults detached in the Imburu at various depths of 1–2 km below the top Toro. (c) Tight and overturned anticlines in the Toro with long, continuous, steep backlimbs and stretched and boudinaged forelimbs cut through by a zone of break-thrusts. (d) Highly variable thickness within the Ieru, with the major detachment within the incompetent Bawia Member. This detachment is often reactivated out-ofsequence as the next structure towards the foreland forms. (e) Open folds in the competent Darai Limestone, cut by numerous thrust faults linking back into the Ieru detachments. (f) Cross-cutting or tear faults in places linked to old basement faults. These faults often seal and compartmentalise the reservoirs. The transition from one detachment level to another probably occurs across these faults. Understanding the structural style is only the first step in determining the hydrocarbon prospectivity. The next, more difficult, step is to determine the hydrocarbon charge, fluid pressures and sealing and/or breaching nature of faults.

Centrifuge analogue modelling was made possible by the support and guidance of Dr John M. Dixon and the research facilities at Queen’s University. The modelling research was supported by the Ontario Graduate Scholarship programme, the Natural Sciences and Engineering Research Council of Canada (NSERC) and the industry sponsors of the Fold– Fault Research Project (FRP). Peter Hamilton and Simon Skirrow of Oil Search Ltd are thanked for excellent drafting. Oil Search Ltd and Joint Venture partners gave permission for publication of this paper and use of any proprietary seismic data, sections and images within it. This study builds on the previous work of many geologists and geophysicists, particularly at

Chevron, BP, Esso, Mobil and the PNG Geological Survey. The paper greatly benefited from the helpful comments of Chris Elder and an anonymous reviewer. The interpretations presented here are strictly those of the authors and do not necessarily reflect the views of any of the companies involved.

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Liu, S. & Dixon, J. M. 1991. Centrifuge modelling of thrust faulting: structural variation along strike in fold–thrust belts. Tectonophysics, 188, 39– 62. Longley, I. M., Buessenschuett, C. et al. 2002. The North West Shelf of Australia – a Woodside perspective. In: Keep, M. & Moss, S. J. (eds) The Sedimentary Basins of Western Australia 3. Proceedings of the Petroleum Exploration Society of Australia Symposium, Perth, WA, 27–88. Pattillo, J. & Nicholls, P. J. 1990. A tectonostratigraphic framework for the Vulcan Graben, Timor Sea region. Australian Petroleum Exploration Association Journal, 30, 27– 51. Smith, J. G. 1965. Orogenesis in western Papua and New Guinea. Tectonophysics, 2, 1 –27. Thornton, R. C. N., Emmett, J. K., Lyslo, J. A. & Gottschalk, R. R. 1996. Integrated structural and stratigraphic analysis in PPL 175, Papuan Fold Belt, Papua New Guinea. In: Buchanan, P. G. (ed.) Petroleum Exploration, Development and Production in Papua New Guinea. Proceedings of the Third PNG Petroleum Convention, Port Moresby September 1996, 195–216. Van Wyck, N. & Williams, I. S. 2002. The age and provenance of basement metasediments from the Kubor and Bena Bena blocks, central Highlands, Papua New Guinea – constraints on the tectonic evolution of the northern Australian cratonic margin. Australian Journal of Earth Sciences, 49, 565–577. Veevers, J. J. 2000. Billion-year Earth History of Australia and Neighbours in Gondwanaland. GEMOC Press, Sydney. Woods, E. P. 1992. Vulcan Sub-basin fault styles – implications for hydrocarbon migration and entrapment. Australian Petroleum Exploration Association Journal, 32, 138–158.

Ductile duplexes as potential natural gas plays: an example from the Appalachian thrust belt in Georgia, USA BRIAN S. COOK1,2* & WILLIAM A. THOMAS1 1

University of Kentucky, Department of Earth and Environmental Sciences, 101 Slone Building, Lexington, Kentucky 40506, USA 2

Southwestern Energy Company, 2350 N. Sam Houston Pkwy E. Ste. 125, Houston, Texas 77032, USA *Corresponding author (e-mail: [email protected]) Abstract: In a well-defined small-scale recess in the Appalachian thrust belt in northwestern Georgia (USA), two distinct regional strike directions intersect at c. 508. Fault intersections and interference folds enable tracing of both structural strikes. Around the recess, tectonically thickened weak stratigraphic layers – shales of the Cambrian Conasauga Formation – accommodated ductile deformation associated with the folding and faulting of the overlying Cambrian– Ordovician regional competent layer. The structures in the competent layer are analogous to those over ductile duplexes documented along strike to the SW in Alabama, where gas production has been established from the deformed shale. The analogy with structures in Alabama suggests a ductile duplex and natural gas potential within the recess in Georgia. The tectonic thickening of the weak-layer shales is evident in palinspastically restored cross sections, which demonstrate a nearly 100% increase in volume over the restored state cross sections. The dominant cause of the surplus shale volume is likely pre-thrusting deposition of thick shale in a basement graben that was later inverted. The volume balance of the ductile duplex is critical for palinspastic reconstruction of the recess, and for the kinematic history and mechanics of the ductile duplex.

In the southern part of the Appalachian thrust belt in eastern North America, recently established gas production from ductile duplexes in Cambrian shales in the state of Alabama has generated interest in further exploration. In the southern Appalachians, the regional de´collement is in Cambrian shales near the base of a Cambrian –Pennsylvanian stratigraphic succession above Precambrian crystalline basement rocks. In Alabama, the depositional thickness of the Cambrian shale was controlled by basement grabens, the boundary faults of which later localized frontal thrust ramps (Thomas 2001). Beneath some frontal thrust ramps in Alabama, the Cambrian shale is tectonically thickened in ductile duplexes (mushwads), above which the overlying regional competent layer (Cambrian –Ordovician massive carbonate rocks) is non-systematically deformed. A ductile duplex is formed by a thick, weak stratigraphic layer that is deformed in incompetent horses between floor and roof thrusts (Thomas 2001). Exploration for gas in the Gadsden mushwad in Alabama (Fig. 1) began in 1985, and production was established in 2005. Fourteen wells have been drilled into the Cambrian shales (Conasauga Formation); initial production test rates ranged from 26 to 233 thousand cubic feet per day (Mcfd) (Williams 2007). In August 2007, eight wells produced a total of 6.8 million

cubic feet (MMcf), and the most productive of these wells produced 2.684 MMcf (Alabama State Oil & Gas Board 2007). The purpose of this paper is to consider possibly analogous structures along strike to the NE in the state of Georgia as potential targets for natural gas exploration.

Regional setting of southern Appalachian structures in Georgia Bends in the gross-scale structural trend of the Appalachian thrust belt have been recognized for well over 100 years (Willis 1893). Regionally, the Appalachian thrust belt includes the gradually curved Tennessee salient, convex toward the craton in the direction of thrust translation, and the more angular bend of the Alabama recess, concave toward the craton (Thomas 1977). At a small-scale recess in northwestern Georgia, north-northeastward-striking thrust faults and related folds in the southern arm of the Tennessee salient intersect east-northeastwardstriking thrust faults and related folds that diverge from the predominant strike of the eastern arm of the Alabama recess (Fig. 1). The Sequatchie anticline (Fig. 1), along the northwestern structural front of the southern Appalachian thrust belt (Thomas & Bayona 2005), has

From: Goffey, G. P., Craig, J., Needham, T. & Scott, R. (eds) Hydrocarbons in Contractional Belts. Geological Society, London, Special Publications, 348, 57– 70. DOI: 10.1144/SP348.4 0305-8719/10/$15.00 # The Geological Society of London 2010.

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Fig. 1. Structural outline map of the Appalachian thrust belt in Alabama and Georgia, modified from Thomas (2007). The black rectangle shows the location of the more detailed map in Figure 2. Names of faults are in all capital letters. The Floyd synclinorium is labelled as Fs, Gadsden mushwad as Gm. The label ‘Birmingham’ shows the location of both the surface thin-skinned Birmingham anticlinorium and the subsurface Birmingham basement graben.

a remarkably straight axial trace trending c. 040 and extending from the front of the Alabama recess on the SW to a tangent near the apex of Tennessee salient on the NE. The straight trace crosses the foreland with no deflection in strike at the smallscale recess in Georgia. Parallel to and SE of the Sequatchie anticline, the frontal Appalachian structures are characterized by narrow anticlines and broad flat-bottomed synclines, the southeasternmost of which is the Lookout Mountain syncline (Fig. 1). In contrast, along the trailing edge of the Appalachian sedimentary thrust belt, the Cartersville and Great Smoky faults mark the leading edges of metamorphic thrust sheets and intersect at c. 708 (Fig. 1). In Alabama, the Cartersville and related Talladega faults generally parallel the regional 040 trend of the thrust belt; but in Georgia, the Cartersville fault bends to 070 and intersects the Great Smoky fault, which trends c. 000 (Fig. 1). The intersection of the Cartersville and Great Smoky faults is the most pronounced surface expression of the two regional structural trends in the small-scale recess in Georgia (Thomas & Bayona 2005).

In the trailing part of the Appalachian sedimentary thrust belt (in the immediate footwall of the bend in the Cartersville/Great Smoky fault system), the trend of the Eastern Coosa fault bends abruptly from 020 on the north to 070 on the SW, framing the small-scale recess (Fig. 1). Where the fault bends abruptly in strike, several trailing splays extend southward, continuing along the direction of strike of the north-northeast-striking leading fault (Fig. 2). The intersection between the Eastern Coosa fault and the trailing splays in the hanging wall defines a clear interference pattern between the two dominant strike directions of the leading fault. Further southwestward in Alabama, the 070-striking segment of the Coosa fault merges into the predominant 040-trending Appalachian structures (Fig. 1). In intermediate structures between the sharply bent Eastern Coosa fault and the nearly straight frontal structures (e.g. Lookout Mountain syncline), the bend in strike is absorbed by various intersecting and interfering folds and thrust faults in the Kingston– Chattooga composite thrust sheet (Figs 1 & 2). The distinct structural intersection in

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Fig. 2. Geological map of the small-scale recess in Georgia, compiled from field data of authors, as well as Butts & Gildersleeve (1948), Cressler (1963, 1964a, b, 1970, 1974), Georgia Geological Survey (1976), Thomas & Cramer (1979), Coleman (1988), Osborne et al. (1988) and Thomas & Bayona (2005). Plunge directions of fold hinges are denoted by closed arrows. The Kingston– Chattooga anticlinorium is the structurally high outcrop area dominantly of Units 1 and 2 between Lookout Mountain syncline and Taylor Ridge monocline. The Floyd synclinorium (including Little Sand Mountain, Rock Mountain and Judy Mountain synclines, as well as other unnamed folds) encompasses the entire outcrop area of Unit 4 SE of the Kingston and Chattooga faults.

these intermediate structures, at an angle of c. 508 between two distinct elements of regional strike, characterizes the small-scale recess in Georgia. Structures striking 020 in the southern arm of the Tennessee salient and striking 070 in the eastern arm of the Alabama recess plunge from opposite directions into the depression of the Floyd synclinorium in the trailing part of the Kingston – Chattooga composite thrust sheet (Figs 1–3). The Rome thrust sheet, consisting of Cambrian shale-dominated facies of the Conasauga Formation, bounds the southern and eastern sides of the small-scale recess (Figs 1 & 2). Trailing the eastern side of the small-scale recess, the Rome fault has a highly sinuous trace, indicating a folded, subhorizontal fault surface (Fig. 2). Along the southern side, the Rome fault trace has an average trend of c. 090 but is highly sinuous in detail (Cressler

1970), indicating a subhorizontal envelope of folds of the fault surface that cuts obliquely across several thrust ramps and folds in the footwall. In addition to the irregular map trace, the shallow dip of the Rome thrust sheet is evident from the lack of seismic imaging of the near-surface fault (Thomas & Bayona 2005). The Rome fault truncates footwall folds that are coaxial with the folds of the fault surface; however, the fault-truncated footwall beds are folded more tightly than is the fault surface. The map relationships show that older footwall folds were truncated by an out-of-sequence Rome fault, and that the footwall folds were subsequently tightened, folding the Rome thrust sheet along with the footwall beds (Thomas & Bayona 2005). Further to the west in Alabama, the trace of the Rome fault curves to parallel the large-scale Appalachian structures (Fig. 1).

60 B. S. COOK & W. A. THOMAS Fig. 3. Geological cross sections illustrating the structural geometry of the small-scale recess in Georgia. The locations of the cross sections are shown on the inset map. Blue lines above present topographic surface show inferred pre-erosion extent of contacts between lithotectonic units.

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Fig. 3. (Continued).

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Structure and stratigraphy of the smallscale recess in northwestern Georgia An interference pattern in the structural intersection in the small-scale recess in Georgia, between east-northeastward and north-northeastward-striking folds and faults enables the tracing of both strike directions through parts of the intersection. Fold trains of both structural trends plunge into the depression of the Floyd synclinorium in the trailing part of the Kingston –Chattooga composite thrust sheet (Figs 1– 3). Unique structural expressions distinguish three structural domains: (1) the Kingston– Chattooga anticlinorium, which includes the frontal structures of the thrust sheet; (2) the Little Sand Mountain– Horn Mountain fold train, which trends c. 020 in the northern part of the Floyd synclinorium; and (3) the Simms Mountain –Horseleg Mountain fold train, which trends c. 070 in the southern part of the Floyd synclinorium (Figs 2 & 3). The Palaeozoic strata are divided into four lithotectonic units (Thomas 2001, 2007; Thomas & Bayona 2005) on the basis of general stratigraphic characteristics and mechanical behaviour during deformation (Fig. 2): Unit 1, the regional dominant weak layer, containing the regional de´collement, encompasses Lower to lower Upper Cambrian finegrained clastic rocks and minor thin-bedded limestones (Rome and Conasauga Formations); Unit 2, the regional dominant competent layer, which controls ramp geometry, is an Upper Cambrian – Lower Ordovician massive carbonate unit (Knox Group); Unit 3, a relatively thin, laterally variable, heterogeneous Middle Ordovician to Lower Mississippian succession of limestone, shale, sandstone, and chert; and Unit 4, an Upper Mississippian– Pennsylvanian synorogenic clastic wedge dominated by shale in the lower part and generally coarsening upward into sandstone and shale. The detachment of the Kingston–Chattooga composite thrust sheet is persistently in shale-dominated facies of the Middle to lower Upper Cambrian Conasauga Formation (Unit 1). In northwestern Georgia, Units 3 and 4 primarily are deformed passively over the underlying regional competent layer (Unit 2). Topography in northwestern Georgia is largely controlled by stratigraphy: most ridges are on Unit 3, and topographic flats are predominantly on shale-dominated strata in Units 1 and 4. Interestingly, the idea of dividing the regional stratigraphy into layers on the basis of relative rigidity and the inferences of how they affect structures in the southern Appalachians were first discussed by Hayes in 1891.

the Lookout Mountain syncline (Figs 2 & 3). Only a very gentle concave-cratonward curvature of the Kingston fault corresponds roughly to the more angular recess between the fold trains within the Floyd synclinorium further to the SE. The Chattooga fault and a leading imbricate parallel the trailing limb of the Kingston thrust sheet and end northeastward along strike, indicating that the Chattooga fault is a splay in a composite thrust sheet from the detachment of the Kingston fault (Fig. 2). The leading part of the Kingston– Chattooga composite thrust sheet forms the structurally high Kingston –Chattooga anticlinorium exposed in Units 1–3 (Figs 2 & 3). The anticlinorium is deformed by internal folds and faults. The trailing limb of the anticlinorium (the Taylor Ridge monocline) dips southeastward beneath the relatively deep Floyd synclinorium, which plunges into a regional depression within the recess between the oppositely plunging fold trains (Figs 2 & 3). The Taylor Ridge monocline is expressed at the surface primarily in Unit 3, striking c. 025.

Little Sand Mountain – Horn Mountain fold train The flat-bottomed Little Sand Mountain syncline, which is expressed at the surface in a sandstone in Unit 4, parallels the southeastern (downdip) side of the Taylor Ridge monocline, trending c. 020 (Fig. 2). The northwestward-verging Clinchport fault ramps through the trailing (SE) limb of the Little Sand Mountain syncline, and northeastward along strike, obliquely truncates the 000-trending Dick Ridge anticline (Fig. 2). Johns Mountain anticline is a cylindrical ramp anticline in the hanging wall of the Clinchport fault exposed in Units 2 and 3, trending c. 020. The Johns Mountain anticline ends in a southwestward-plunging, apparently conical fold associated with the southwestern end of the Clinchport fault (Fig. 2). Northeastward along strike, the Johns Mountain anticline merges with the up-plunge part of the 000-trending Horn Mountain anticline, which splits southward into a pair of southward-plunging anticlines (Fig. 2). Turkey Mountain anticline in the hinterland of the southwestern end of Johns Mountain anticline is a doubly plunging anticline exposed in Unit 3, trending 015. All of the anticlines rise steeply above the flat-bottomed synclines and have amplitudes of c. 650–1500 m. Spacing between the anticlines is c. 4– 7 km.

Kingston – Chattooga anticlinorium

Simms Mountain – Horseleg Mountain fold train

The northwestward-verging Kingston fault and a leading imbricate bound the southeastern limb of

On the southern side of the small-scale recess, eastnortheastward-plunging flat-bottomed synclines and

DUCTILE DUPLEXES AS POTENTIAL NATURAL GAS PLAYS

narrow, steep-sided anticlines diverge from the north-northeastward-striking Chattooga fault and Taylor Ridge monocline, and plunge northeastward into the depression of the Floyd synclinorium (Figs 2 & 3). Simms Mountain anticline plunges 075 into the deepest part of the Floyd synclinorium and, southwestward up-plunge, shows distinct fold interference with the Taylor Ridge monocline (Fig. 2). The flat-bottomed Rock Mountain syncline is expressed at the surface in sandstones of Unit 4 and trends 067. Lavender Mountain anticline is a cylindrical fold, forming a ridge of Unit 3, trending 064, and ending in a northeastward-plunging conical fold (Fig. 2). The southwestern up-plunge end of the Lavender Mountain anticline shows fold interference with the Turnip Mountain anticline, which is a ramp anticline exposed in Units 2 and 3, trending c. 020, roughly parallel with the Taylor Ridge monocline in the footwall (Fig. 2). Judy Mountain syncline, which is expressed in a sandstone within Unit 4, trends 067. Horseleg Mountain anticline is exposed in Unit 3 and trends 059 (Fig. 2). The anticlines rise steeply above the flat-bottomed synclines and have amplitudes of c. 650 –1000 m; spacing between the anticlines is c. 4 –7 km. Strawberry Mountain anticline, which is c. 17 km NW of Simms Mountain anticline and on the opposite end of the Little Sand Mountain syncline, trends c. 059, parallel with other anticlines in the Simms Mountain –Horseleg Mountain fold train. Strawberry Mountain anticline ends in both directions along strike by interference with the Taylor Ridge monocline to the SW and with Dick Ridge anticline and the Clinchport fault (part of the Simms Mountain –Horseleg Mountain fold train) to the NE (Fig. 2). Although the Strawberry Mountain anticline has the orientation of the Simms Mountain– Horseleg Mountain fold train, it is isolated within the Little Sand Mountain –Horn Mountain fold train, clearly showing interference between the two fold sets.

Cross sections and subsurface structure For this study, field measurements of structural orientation data and stratigraphic thickness have been obtained and compiled with structural data from other studies in the region (Butts & Gildersleeve 1948; Cressler 1963, 1964a, b, 1970, 1974; Georgia Geological Survey 1976; Thomas & Cramer 1979; Coleman 1988; Osborne et al. 1988; Thomas & Bayona 2005) to constrain construction of palinspastically restorable cross sections (Fig. 3). The subsurface geology is interpreted from seismic reflection profiles and projection of surface data. The depths to basement and thickness of a basal weak layer are measured from seismic reflection

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profiles, and structures of the overlying units are constructed by extending surface measurements (i.e. stratigraphic thickness and strike/dip, etc.) into the subsurface. The seismic profiles show two distinct packages of clear layered reflectors in most places. The lower package of layered reflectors corresponds to Unit 1, and the base of the package is near the base of the sedimentary cover above Precambrian crystalline basement (Fig. 4). The top of the lower package of layered reflectors marks the top of Unit 1. The upper package of layered reflectors evidently corresponds to Unit 3, and also defines the top of Unit 2. The elevation of Unit 2 (regional competent layer) illustrates structural highs and lows across the Kingston –Chattooga composite thrust sheet. On the NW, the Kingston–Chattooga anticlinorium is a broad structural high bounded on the SE by the Taylor Ridge monocline, which dips into the deeper Floyd synclinorium. Unit 2 is structurally lower within the Floyd synclinorium, which is partitioned on the SW by east-northeastward-plunging anticlines of the Simms Mountain–Horseleg Mountain fold train and on the NE by south-southwestwardplunging anticlines of the Little Sand Mountain– Horn Mountain fold train, including the Clinchport thrust ramp (Johns Mountain anticline). Seismic reflection profiles show that the top of Precambrian crystalline basement dips very gently southeastward and is broken by small steep normal faults (e.g. Thomas & Bayona 2005). In cross section, the difference in elevation between the base of Unit 2 in the Kingston –Chattooga thrust sheet and the top of basement constitutes a large area to be filled (Figs 3 & 4), requiring an interpretation of subsurface structure. Previous interpretations have consistently included imbricate thrust sheets of Units 1 and 2 in the core of the Kingston–Chattooga anticlinorium, as well as blind thrusts in the cores of the anticlines of the Little Sand Mountain–Horn Mountain and Simms Mountain–Horseleg Mountain fold trains (e.g. Thomas & Bayona 2005). The dual fault traces of the Kingston fault and leading imbricate and the Chattooga fault and leading imbricate have been interpreted to be the surface expression of long imbricate thrust sheets in the core of the anticlinorium. Although this structural configuration satisfies the geometric form of the structures, other observations suggest that this interpretation may not be appropriate. The Chattooga fault and leading imbricate both end along strike, suggesting a relatively small magnitude of displacement. Furthermore, the Kingston fault and leading imbricate apparently terminate southwestward along strike and extend into an unfaulted detachment anticline (Thomas & Bayona 2005). Seismic reflection profiles clearly image the southeastern limb of the

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Fig. 4. Seismic reflection profile of the ductile duplex, interpretation shown in lower panel. Location of the profile is near the northwestern end of cross section A –A0 in Figure 3, and the profile is oriented approximately parallel to the section.

Lookout Mountain syncline in the footwall of the Kingston fault; however, the profiles are ambiguous SE of the Kingston fault, where no coherent reflectors are shown above the basal package of layered reflectors above the basement (Fig. 4). The seismic reflection profiles lack resolution of any possible imbricate thrust sheets of Unit 2 beneath the surface-exposed thrust sheet (Fig. 4). Information applicable to the resolution of structural style in Georgia may be obtained by analogy from structures along strike to the SW in the Appalachian thrust belt in Alabama. In Alabama, deep drilling in the Gadsden mushwad (Fig. 1) has documented a minimum thickness of 2835 m of intensely deformed and tectonically thickened dark-coloured shale and thin-bedded limestone of the Middle to lower Upper Cambrian Conasauga Formation (Unit 1) (Thomas 2001). Seismic reflection profiles image dipping reflectors of the regional competent layer (Unit 2) both NW and SE of the Gadsden mushwad; however, the profiles show a distinct lack of coherent reflectors within the mushwad (Thomas 2001, fig. 7). The internal structure of the mushwad is inferred to include thrust faults that partition the ductilely deformed mass into internally deformed horses. Observations of outcrops and shallow core holes document disharmonic, tight, smallscale folds (amplitudes and wavelengths on the scale of a few metres) broken by faults of uncertain displacements. The mushwad structure is interpreted to be a ductile duplex beneath a roof thrust sheet of the regional competent layer (Unit 2) and a structurally attached uppermost part of Unit 1. The roof of the Gadsden mushwad has been eroded leaving the core of the duplex exposed; however, the structure of the roof can be inferred from bounding structures across strike (Thomas 2001). Further to the SW in Alabama, the crest of the Birmingham anticlinorium (Fig. 1) includes multiple thrust faults and folds, as well as backthrusts, exposed in

Unit 2 (Thomas 2001; Thomas & Bayona 2005). These structures form the roof of a separate subsurface mushwad, which is also shown in seismic profiles as a zone lacking coherent reflectors. Interestingly, prior to drilling of the first well into the Gadsden mushwad in 1985, the common interpretation was that the structurally high rocks at the top of the exposed Unit 1 reflect a subsurface stack of imbricate thrust sheets of Unit 2 and younger rocks (Thomas 1985, 2001, fig. 9). By analogy with ductile duplexes that have been documented along strike in the Appalachians in Alabama (Thomas 2001), the subsurface structure beneath the Kingston– Chattooga anticlinorium, as well as beneath the trailing part of the composite thrust sheet, is interpreted here as a ductile duplex. In this new interpretation, the mapped Kingston fault and leading imbricate, as well as the Chattooga fault and leading imbricate, are interpreted to be relatively low-magnitude thrust faults limited to the roof of the ductile duplex (Fig. 3). An interval of layered reflectors beneath Unit 2 shows that some strata in the uppermost part of Unit 1 are attached to the competent layer in the thrust sheet, and that the detachment of the Kingston –Chattooga composite thrust sheet is within Unit 1. The roof thrust of the ductile duplex places Unit 1 strata in the Kingston– Chattooga composite thrust sheet over ductilely deformed, tectonically thickened Unit 1 in the ductile duplex. The ductile duplex fills the space between the base of the thrust sheet and the top of the autochthonous lower part of Unit 1 overlying Precambrian basement beneath the de´collement (Figs 3 & 4). Seismic reflection profiles show that the leading edge of the ductile duplex forms a tectonic wedge under Unit 2 in the NW-dipping limb of the Kingston –Chattooga anticlinorium (common limb with the Lookout Mountain syncline) (Figs 3 & 4); a similar wedge is documented for the leading edge

DUCTILE DUPLEXES AS POTENTIAL NATURAL GAS PLAYS

of the Gadsden mushwad in Alabama (Thomas 2001, figs 5 & 7). The folds of the Little Sand Mountain –Horn Mountain and Simms Mountain– Horseleg Mountain fold trains are interpreted to be exaggerated detachment folds in the roof of the duplex, with the exception of the Clinchport faultrelated fold (Johns Mountain anticline) and Horseleg Mountain anticline, which are interpreted to be translated detachment folds in the roof of the duplex (Fig. 3).

Volume balance in the ductile duplex In the cross sections (Fig. 3), a large volume of ductilely deformed Unit 1 (Cambrian Conasauga shale) is shown to fill the space beneath the roof thrust at the base of the Kingston–Chattooga thrust sheet. A simplistic iteration of a palinspastically restored cross section, which employs only a linelength balancing of the competent layer (Unit 2), outlines the implications for an area-balanced reconstruction of the weak layer (Unit 1) (Panel 1 of Fig. 5). Such a reconstruction is applicable to the evolution of a detachment fold, in which the regional weak layer is tectonically thickened to fill the cores of detachment anticlines as the overlying competent layer is translated. In this palinspastic reconstruction (Panel 1 of Fig. 5), however, the restored area of Unit 1 is only about 50% of the deformed area of Unit 1 in the ductile duplex (Panel 2 of Fig. 5), clearly requiring a different explanation for the large excess in the area of Unit 1. Two end-member solutions may be suggested for the excess volume of Unit 1 in the deformedstate cross sections. First, deformation/flow of the weak-layer shales from out of the cross section planes could supply local excess volume. Secondly, a complex history of basement-fault movement may have resulted in the sedimentary accumulation of locally thick weak-layer rocks as a source for a ductile duplex. Tectonic thickening of Unit 1 as a result of outof-plane flow requires convergence of material into the tectonically thickened ductile duplex. The intersection of the two structural trends (defined by the Little Sand Mountain –Horn Mountain and Simms Mountain –Horseleg Mountain fold trains) suggests possible convergence from the depression of the Floyd synclinorium into the Kingston–Chattooga anticlinorium. The tectonic thickening of c. 100% in the ductile duplex would require withdrawal of ductile rocks from an area as much as twice the size of the mapped area of the ductile duplex. Such a withdrawal would likely generate structural depressions (e.g. structures in the competent layer plunging away from the centre of the recess), which are not recognized in the present outcrop

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geology. The documented plunge into the depression of the Floyd synclinorium, in contrast, suggests divergent (rather than convergent) flow. Given these observations, out-of-plane flow does not seem likely to account for more than a small fraction of the excess volume of Unit 1 in the ductile duplex.

Analogy with structures in Alabama A complex history of basement-fault movement has been demonstrated to be integral to the formation of the ductile duplexes in Alabama, where the boundary faults of the Birmingham basement graben are clearly imaged in seismic reflection profiles (Thomas 2007). Large-scale frontal ramps rise northwestward over down-to-southeast basement faults, and thick disharmonic ductile duplexes (mushwads) underlie anticlinoria in which the competent-layer roof rocks are non-systematically faulted (Thomas 2001). Palinspastic restorations of thrust belt structures provide a framework to interpret stratigraphic variations in the context of episodic reactivation and inversion of the basement faults. In palinspastic location, the Middle to lower Upper Cambrian Conasauga Formation includes a shale-dominated facies greater than 2000 m thick in the basement graben, and a much thinner carbonate facies that is less than 800 m thick outside the graben (Thomas 2007). The differences in facies and thickness indicate synsedimentary fault movement, and the sedimentary variations document the time and magnitude of fault movement. Upper Cambrian massive carbonate deposits (Unit 2) overstep the graben boundary faults, indicating cessation of fault movement during deposition of Unit 2 carbonate rocks (Thomas 2007). The upper part of the Cambrian–Ordovician Knox Group (Unit 2), however, is unconformably absent over the palinspastically restored Birmingham graben. The unconformity is marked by a karstic palaeotopography with tens of metres relief, as well as sporadically distributed chert-clast conglomerate at the base of the Middle Ordovician cover stratigraphy (Thomas 2007). Middle Ordovician limestone units onlap the erosionally truncated Unit 2 and thin over the graben. These relationships indicate tectonic inversion of the Birmingham graben in the Middle Ordovician during Taconic tectonic loading (Bayona & Thomas 2003; Thomas & Bayona 2005). The amount of truncation of upper Unit 2 strata, palaeotopography and thinning by onlap combine to indicate as much as 700 m of reverse slip on the basement faults during inversion of the graben (Thomas 2007). Stratigraphic and sedimentological data indicate some minor episodic movement of the Birmingham graben faults during Silurian– Mississippian time, followed by .900 m of normal slip during

66 B. S. COOK & W. A. THOMAS Fig. 5. Simplified palinspastic restoration (Panel 1) based on line-length balance of the competent layer (Unit 2). Note that restored area of Unit 1 in the ductile duplex is c. 50% of the area of the ductile duplex in the deformed-state cross section (Panel 2), showing that this interpretation of palinspastic restoration cannot be area balanced.

DUCTILE DUPLEXES AS POTENTIAL NATURAL GAS PLAYS 67

Fig. 6. Sequential cross sections illustrating a basement graben that is interpreted to be the source of the surplus volume of Unit 1 shales in the small-scale recess in Georgia. Panel 1 illustrates the thick Unit 1 succession in a synsedimentary graben, overlain by a uniform thickness of Unit 2 shallow-marine carbonates. Panel 2 illustrates graben inversion, leading to elevation of thickened Unit 1 and erosional truncation of the top of Unit 2 over the former graben. Panel 3 illustrates the deformed-state cross section (cross section A– A0 from Fig. 3) in which the thickened Unit 1 is at the centre of a ductile duplex. The dashed red lines in panels 1 and 2 represent future trajectories of thrust faults, including the floor and roof thrusts bounding the ductile duplex. The area of the ductile duplex is equal in all three diagrams.

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deposition of Upper Mississippian– Lower Pennsylvanian clastic strata. The ultimate composite vertical separation on the basement fault is c. 2255 m (Thomas 2007). Propagation of Palaeozoic, thin-skinned Appalachian thrust faults at a regional de´collement in Unit 1 encountered the thick, mud-dominated facies (Conasauga shale) in the basement graben, as well as a basement-fault buttress at the northwestern boundary of the graben. Ductile deformation generated thick mushwads beneath large-scale frontal thrust ramps of the regional competent layer (Unit 2) (Thomas 2001). The maximum structural relief on the roof of the mushwads is as much as 4500 m, indicating c. 3:1 tectonic thickening of the depositionally thickened Conasauga Formation in the mushwad (Thomas 2007).

Interpretation for structures in Georgia No large-magnitude basement faults are seismically imaged in the region of the Kingston –Chattooga composite thrust sheet in Georgia; however, minor disruptions in the basal reflector package show the locations of faults that presently have small displacement of the top of the basement. By analogy with the history of mushwads (ductile duplexes) in Alabama, the present fault offset of the top of basement may reflect a composite of successive displacements, some of which are inverted. Assuming a history similar to that of the Alabama mushwads, an area balance of the ductile duplex beneath the Kingston –Chattooga composite thrust sheet (Kingston–Chattooga anticlinorium and fold trains) requires an original depositional thickness of the Conasauga Formation c. 500 m greater than that in the foreland to the NW (Fig. 6). Accommodation of the greater thickness indicates a basement graben c. 500 m deeper than present basement elevation. Later inversion of the graben would have reversed part of the original slip. By analogy with stratigraphy in Alabama, inversion during Taconic (Middle Ordovician) loading may be recorded in erosion of the upper part of the Knox Group, Unit 2 (Fig. 6) (Bayona & Thomas 2003). Previously unexplained observations in Georgia include a local lack of the upper components that regionally comprise the Knox Group; specifically the Lower Ordovician Chepultepec Dolomite is unconformably absent in northwestern Georgia (Coleman 1988). The same stratigraphic unit (Chepultepec Dolomite) is unconformably absent in the area of the Gadsden mushwad and along the Birmingham anticlinorium in Alabama, where the top of the Knox Group is marked by chert-clast conglomerates. Similar chert conglomerates are found sporadically at the top of Unit 2 in northwestern Georgia. These observations suggest that inversion occurred along basement

faults in Georgia. Although the Birmingham graben shows subsequent reactivation in Alabama during late Palaeozoic (Mississippian–Pennsylvanian) thrusting and tectonic loading (Thomas 2007), this later episode of basement-fault reactivation is not documented by stratigraphy in Georgia. The maximum structural relief on the roof of the mushwad in Georgia is c. 2500 m, indicating c. 2:1 tectonic thickening of the depositionally thickened Conasauga Formation. Sequential diagrams (Fig. 6) illustrate the interpreted origins of stratigraphic variations necessary to area balance the ductile duplex beneath the Kingston– Chattooga thrust sheet. The deformed-state cross section (Panel 3 of Fig. 6) shows the present location and geometry of the interpreted ductile duplex. The cross section in Panel 1 of Figure 6 illustrates the depositional framework of a thick Unit 1 succession in a synsedimentary graben; after the end of fault movement, Unit 2 was deposited across the graben with uniform thickness. The top of Unit 2 is drawn nearly horizontal to reflect the interpreted shallow-marine shelf deposition of the carbonate rocks. Area-balance restoration of the deformed state of the ductile duplex requires Unit 1 to be c. 1700 m thick in the graben, in contrast to a regional average of 1200 m. The depositional thickening requires c. 500 m of vertical separation along the normal fault boundary of the graben (Panel 1 of Fig. 6). Panel 2 of Figure 6 shows inversion of the graben to elevate the thick graben fill (Unit 1) and cover (Unit 2), leading to erosion of the upper part of Unit 2. In this interpretation, the thickness of Unit 2 in the deformed-state cross section (c. 600 m) constrains the thickness of the eroded upper part of Unit 2, which is of the order of 300 m. The amount of truncation, plus palaeotopography and onlap, indicate c. 500 m of reverse slip during inversion, and that magnitude of inversion places the top of basement at the present structural level (Fig. 6). As a final note, the volume balance of the ductile duplex is critical for palinspastic reconstruction of the recess, and the understanding of the kinematic and mechanical history of the local structures. The intersection and fold interference exemplify a long-standing problem in volume balancing of palinspastic reconstructions of sinuous thrust belts. Cross sections generally are constructed perpendicular to structural strike, parallel to the assumed slip direction. An array of cross sections around a structural bend may be restored and balanced individually; however, restorations perpendicular to strike across intersecting thrust faults yield an imbalance in the along-strike lengths of frontal ramps. Similarly, the restoration leads to an imbalance in the surface area of a stratigraphic horizon. The inverted basement

DUCTILE DUPLEXES AS POTENTIAL NATURAL GAS PLAYS

graben provides a solution to the volume balance problems encountered in palinspastic restoration of the cross sections around the small-scale recess in Georgia.

Conclusions Around the small-scale recess in northwestern Georgia, tectonically thickened weak stratigraphic layers of the Cambrian Conasauga Formation accommodated ductile deformation associated with the folding and brittle faulting of the overlying Cambrian –Ordovician regional competent layer. Ductile deformation of the underlying structurally thickened weak layer allows the shales to fill the cores of anticlines in the competent layer. The ductile duplex in the core of the Kingston – Chattooga anticlinorium represents an excess volume of Unit 1 shales that elevates the structural level of the Kingston–Chattooga composite thrust sheet. The trailing limb of the anticlinorium is marked by the Taylor Ridge monocline, which dips into the structurally lower Floyd synclinorium. In the Floyd synclinorium, two fold trains of broad synclines and narrow anticlines plunge into the depression of the synclinorium with two distinct structural trends. Low-amplitude folds, which are the plunging ends of the fold trains, characterize the centre of the abrupt bend in Appalachian structural trends in the recess in Georgia. The area of the mushwad in deformed-state cross sections is approximately twice the area of the corresponding Unit 1 in the restored cross sections, and cannot be explained solely by tectonic thickening parallel to the direction of apparent shortening of a conventional palinspastically restored cross section. This imbalance may result from some combination of two mechanisms: transport of Unit 1 shales into the plane of cross section, and activation/inversion of a basement graben. The out-of-plane transport of material implies an as yet unrecognized deficit in Unit 1 thickness elsewhere in the thrust belt to balance the surplus in the ductile duplex. A new interpretation proposes that a basement graben accommodated deposition of a locally thicker Unit 1 succession (c. 1700 m, in contrast to c. 1200 m to the NW of the graben) prior to thrust deformation, analogous to the Birmingham graben along strike to the SW in the Appalachian thrust belt in Alabama. Subsequent Middle Ordovician reactivation/inversion of the graben, related to Taconic loading, resulted in uplift and the erosion of the upper part of the overlying Unit 2 (Bayona & Thomas 2003). Finally, thrusting and accretion of the weak layer into the ductile duplex occurred during tectonic shortening in late Palaeozoic

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times. The exposed competent-layer structures in Georgia are analogous to those over shaledominated ductile duplexes (mushwads; Thomas 2001), which are being developed for natural gas in the Appalachian thrust belt in Alabama; however, the total thickness of the Unit 1 shale-dominated ductile duplex in Georgia is somewhat less than in those in Alabama. Finally, the interpretation of a basement graben yields a solution to volume balance encountered during palinspastic restoration of the array of cross sections around the small-scale recess in Georgia. Field mapping by Cook (2007– 2009) was funded by support from the EDMAP component of the National Cooperative Geologic Mapping Program of the US Geological Survey, the American Association of Petroleum Geologists, the Geological Society of America and the Ferm Fund from the University of Kentucky Department of Earth and Environmental Sciences. Acknowledgement is made to the donors of the Petroleum Research Fund, administered by the American Chemical Society, for partial support of this research. The authors thank Tim Needham, Terry Engelder, and special editor Graham Goffey for their constructive reviews of the manuscript.

References ALABAMA STATE OIL AND GAS BOARD. 2007. An overview of the Conasauga Formation shale gas play in Alabama, November 2007. Bayona, G. & Thomas, W. A. 2003. Distinguishing fault reactivation from flexural deformation in the distal stratigraphy of the Peripheral Blountian Foreland Basin, southern Appalachians, USA. Basin Research, 15, 503 –526. Butts, C. & Gildersleeve, B. 1948. Geology and mineral resources of the Paleozoic area of NW Georgia. Georgia Geological Survey Bulletin, 54. Coleman, J. L. Jr 1988. Geology of the rising Fawn CSD. Alabama Geological Society Guidebook, 25th Annual Field Trip, 12– 40. Cressler, C. W. 1963. Geology and ground-water resources of Catoosa County, Georgia. Georgia Geological Survey Information Circular, 28. Cressler, C. W. 1964a. Geology and groundwater resources of the Paleozoic rock area, Chattooga County, Georgia. Georgia Geological Survey Information Circular, 27. Cressler, C. W. 1964b. Geology and ground-water resources of Walker County, Georgia. Georgia Geological Survey Information Circular, 29. Cressler, C. W. 1970. Geology and groundwater resources of Floyd and Polk Counties, Georgia. Georgia Geological Survey Information Circular, 47. Cressler, C. W. 1974. Geology and groundwater resources of Gordon, Whitfield, and Murray Counties, Georgia. Georgia Geological Survey Information Circular, 39. GEORGIA GEOLOGICAL SURVEY. 1976. Geologic map of Georgia. Georgia Geological Survey. Scale 1:500 000.

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Hayes, C. W. 1891. The overthrust faults of the southern Appalachians. Geological Society of America Bulletin, 2, 141– 154. Osborne, W. E., Szabo, M. W., Neathery, T. L. & Copeland, C. W. Jr (compilers) 1988. Geologic map of Alabama, NE sheet. Geological Survey of Alabama Special Map 220, scale 1:250 000. Thomas, W. A. 1977. Evolution of Appalachian – Ouachita salients and recesses from reentrants and promontories in the continental margin. American Journal of Science, 277, 1233– 1278. Thomas, W. A. 1985. Northern Alabama sections. In: Woodward, N. B. (ed.) Valley and Ridge Thrust Belt: Balanced Structural Sections, Pennsylvania to Alabama (Appalachian Basin Industrial Associates). University of Tennessee Department of Geological Sciences Studies in Geology, 12, 54–61. Thomas, W. A. 2001. Mushwad: Ductile duplex in the Appalachian thrust belt in Alabama. American

Association of Petroleum Geologists Bulletin, 85, 1847– 1869. Thomas, W. A. 2007. Role of the Birmingham basement fault in thin-skinned thrusting of the Birmingham anticlinorium, Appalachian thrust belt in Alabama. American Journal of Science, 307, 46–62. Thomas, W. A. & Bayona, G. 2005. The Appalachian thrust belt in Alabama and Georgia: Thrust-belt structure, basement structure, and palinspastic reconstruction. Geological Survey of Alabama Monograph 16. Thomas, W. A. & Cramer, H. R. 1979. The Mississippian and Pennsylvanian (Carboniferous) Systems in the United States – Georgia. United States Geological Survey Professional Paper 1110-H, H1– H37. Williams, P. 2007. Conasauga saga. Oil and Gas Investor. September 2007, 77–80. Willis, B. 1893. The mechanics of Appalachian structure. United States Geological Survey Annual Report, 13, Part 2, 211 –281.

Controls on lateral structural variability along the Keping Shan Thrust Belt, SW Tien Shan Foreland, China SEBASTIAN A. TURNER1,2*, JOHN W. COSGROVE1 & JIAN G. LIU1 1

Department of Earth Science & Engineering, Imperial College London, South Kensington Campus, London SW7 2AZ, UK 2

Present address: BP Exploration & Production, Chertsey Road, Sunbury-on-Thames, Middlesex TW16 7BP, UK *Corresponding author (e-mail: [email protected]) Abstract: Lateral structural variability and partitioning of fold –thrust belts often reflects lateral variations in the stratigraphy of the deforming foreland and interaction with inherited structures. The Keping Shan Thrust Belt, NW China, was initiated during the late Cenozoic and is a spectacular example of contractional deformation in a foreland setting. The belt is characterized by a series of imbricate thrusts which form a broadly arcuate salient and deform the thick (3– 6 km) Phanerozoic sedimentary succession of the NW Tarim Basin (SW Tien Shan foreland). Abrupt lateral changes in the thickness of the sedimentary succession are associated with a series of major preexisting basement faults which cross-cut the belt and which were probably initiated during early Permian times. These lateral variations in the basin template have impacted strongly on the structural architecture of the superimposed thrust belt. Variations in the thickness of the sediment pile affect the spatial distribution of thrusts, which increase in abundance where the sediment is thinnest. The inherited cross-cutting basement faults and the associated abrupt changes in sediment thickness combine to generate partitioning of the thrust belt.

Lateral structural variability and partitioning within foreland fold–thrust belts is commonly associated with lateral variations in the deforming basin. An increasing number of studies into the evolution of foreland fold–thrust belts have examined the interplay between pre-existing structures, variations in the thickness and rheology of the sediment pile, and lateral differences in the type and thickness of the detachment horizon (e.g. Liu et al. 1992; Marshak et al. 1992; Lawton et al. 1994; Macedo & Marshak 1999; Sepehr & Cosgrove 2004, 2007; Butler et al. 2006). Such variations have important consequences on the structural architecture of the fold–thrust belt, causing lateral variations in horizontal shortening, deformation style and the spatial organization of structures. Furthermore, such lateral variations are often accommodated by the formation of lateral ramps and strike-slip (transfer) faults which are oblique or perpendicular to the general structural trend of the fold–thrust belt. The interplay between the pre-existing basin template and a later fold–thrust belt therefore has important implications for hydrocarbon exploration in compressional belts, causing lateral compartmentalization of reservoirs and structural complexity. The Keping Shan Thrust Belt is one of several fold–thrust belt salients which have evolved in the foreland of the SW Tien Shan, NW China, during

late Cenozoic times. The Keping Shan is characterized by a spectacular series of imbricate thrusts, which deform a predominantly Palaeozoic sedimentary pile (Fig. 1). The thrusts have trends varying from east –west to NE– SW across the belt and broadly form an arcuate salient (Figs 1 & 2). The internal structural architecture of the Keping Shan is complex. A series of major strike-slip faults, which are oblique or perpendicular to the general trend of the thrusts, partition the belt into a series of structural domains, characterized by lateral variations in horizontal shortening and the spatial organization of structures. The aim of this paper is to examine the structural architecture of the Keping Shan Thrust Belt and to identify the underlying causes of lateral (alongstrike) structural variability. By examining the preexisting structure and stratigraphy within the NW Tarim Basin, relationships between the architecture of the late Cenozoic thrust belt and the inherited basin template can be examined and assessed in order to provide a new model for the long-term structural evolution of the region. The data used in this study were obtained through satellite image interpretation and field-based mapping. A regionalscale geological interpretation map (Fig. 1) was developed using Landsat ETM and high-resolution (5  5 m) SPOT satellite images. These maps were enhanced with structural measurements and

From: Goffey, G. P., Craig, J., Needham, T. & Scott, R. (eds) Hydrocarbons in Contractional Belts. Geological Society, London, Special Publications, 348, 71– 85. DOI: 10.1144/SP348.5 0305-8719/10/$15.00 # The Geological Society of London 2010.

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observations made at outcrop scale during two field seasons to the Keping Shan. These methods combined provide the basis for analysing the large and small-scale structure and investigating the stratigraphic framework of the region. The latter yields information concerning inherited structures and lateral variations in the rheology and thickness of the sediment pile, which could prove crucial in unravelling the causes of structural variability in the Keping Shan.

Geological setting The Keping Shan Thrust Belt actively deforms the 3–6 km thick Neoproterozoic –Recent sedimentary succession of the NW Tarim Basin, an area that equates to the foreland of the SW Tien Shan (Fig. 1). The sedimentary succession records the complex and protracted history of the Tarim Basin, which began in the Neoproterozoic. At this time, the underlying Tarim Craton rifted from Australia during a widespread rifting event (Li & Powell 2001; Chen et al. 2004). Subsequently, the Tarim Craton accumulated a thick passive margin succession prior to collision with the developing Eurasian margin during Late Devonian to early Carboniferous times (Carroll et al. 1995, 2001). The collision resulted in the formation of the Tien Shan orogenic belt (Burrett 1974; Burtman 1975; Coleman 1989; Jun et al. 1998) and sedimentation within the NW Tarim Basin occurred in a foreland setting (Carroll et al. 1995). A short but important phase of extension occurred in the early Permian (c. 275 Ma, Zhang et al. 2008), which was associated with magmatic activity and the generation of major normal faults, which impacted on the thickness of the sedimentary succession across the region. As a result, the NW Tarim Basin remained an intrabasinal high throughout the Mesozoic and did not receive any sediment during this period (Li et al. 1996). Thick Mesozoic successions are recorded in two isolated depocentres in the western and eastern Tarim Basin, recording a series of smaller collisions at the southern margins of the growing Eurasian continent during the formation of the Tibetan collage (Watson et al. 1987; Hendrix et al. 1992; Sobel 1999). The collision of India and Eurasia in the early Cenozoic marked the onset of renewed contraction across Central Asia. Ancestral mountain belts including the Tien Shan were reactivated and rejuvenated, shedding large quantities of sediment into the Tarim Basin. Flexure at the margins of the basin resulted in accumulations of Palaeogene– Neogene sediments which locally exceed 10 000 m (Bally et al. 1986; Yang & Liu 2002). Reactivation of the Tien Shan did not begin until c. 20 Ma, and has accelerated since c. 10 Ma (Abdrakhmatov et al. 1996; Sun et al. 2004). During this time the

belt has accommodated around c. 200 (+ 50) km of crustal shortening (Avouac et al. 1993; Abdrakhmatov et al. 1996). The initiation of folding and thrusting within the SW Tien Shan foreland (NW Tarim Basin) probably began during or shortly after this time (?10– 5 Ma). Seismicity along the boundary zone between the Tarim Basin and the Tien Shan indicates that folding and thrusting is presently active (USGS 2009), while geodetic (GPS) measurements suggest shortening rates of 8 (+3) mm a21 across the Keping Shan and adjacent Kashgar Fold Belt. This corresponds to c. 40% of the total shortening rate across the whole Tien Shan belt (Reigber et al. 2001) and serves to demonstrate the importance of foreland fold–thrust belts in accommodating crustal shortening across orogenic belts.

Structure of the Keping Shan Thrust Belt The morphology of the Keping Shan is characterized by major fault zones which were generated (or reactivated) during contraction in the late Cenozoic (Yin et al. 1998; Allen et al. 1999). Following the format of Sepehr & Cosgrove (2007), these fault zones are categorized for the purpose of this study according to structural trends. Major fault zones within the Keping Shan can be broadly divided into two categories: (1) belt-parallel fault zones; (2) belt-oblique fault zones (Fig. 2).

Belt-parallel fault zones Belt-parallel fault zones comprise NE –SW to east – west trending faults which are parallel to the trend of the Tien Shan orogenic belt (Figs 1 –3). Without exception, all the fault zones in this category are thrusts, which predominantly verge to the south towards the interior of the Tarim Basin. Allen et al. (1999) proposed that the thrusts detach onto a thin upper Cambrian salt horizon. Palaeozoic and Cenozoic strata of the basin are exhumed in the hanging walls of the thrusts, forming topographically prominent ridges, which rise up to 1200 m high relative to the piggyback basins that have developed between thrusts. These basins are narrow (6–15 km in width), internally draining, and filled with Quaternary sediments. The general strike of the thrusts varies across the Keping Shan, giving the belt an overall arcuate salient geometry (Figs 1 & 2). Thrust trends vary from NE–SW in the east to east –west in the west. The eastern end of the Keping Shan tapers into a 50 km-wide recess which we have termed the Aksu Re-entrant. To the west, there is a gradual transition from the Keping Shan Thrust Belt into the Kashgar Fold Belt (Fig. 1). In contrast to the Keping Shan, the Kashgar Fold Belt is dominated

Fig. 1. Geological map of the Keping Shan Thrust Belt showing the master structural elements described in this paper. Belt-parallel faults are characterized by east–west to NE–SW trending thrust faults that define a broad, arcuate thrust belt. The belt is partitioned by a series of belt-oblique (strike-slip and oblique-slip) faults that predominantly trend NW–SE. The northern margin of the Keping Shan Thrust Belt is defined by the South Tien Shan Fault, which separates the metamorphic rocks of the Tien Shan from the sedimentary rocks of the Tarim Basin.

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Fig. 2. 3D perspective view of the Keping Shan Thrust Belt, generated by draping a Landsat ETMþ (bands 321, 30 m resolution) satellite image over a digital elevation model. Major thrusts are identified by the surface expression of their hanging walls, which form long, arcuate ridges that exhume a predominantly Palaeozoic stratigraphic succession. Laterally, thrusts interact with major belt-oblique fault zones (marked) that partition the thrust belt into a series of structural domains.

by simple detachment folds which only affect Cenozoic strata and there is little surface expression of thrusts (Scharer et al. 2004). South Tien Shan Fault. This fault acts as the major bounding fault which separates metamorphic rocks of the Tien Shan from the sedimentary cover succession of the Tarim Basin to the south (Fig. 1). Cenozoic activity on the fault and related exhumation of the Tien Shan mountains began around 24 Ma, at the Oligocene –Miocene boundary (Sobel & Dumitru 1997). The fault is a steep (40– 508) north-dipping thrust that trends ENE –WSW. Within the metamorphic mica-schists immediately north of the fault, there is an abundance of shear structures and minor folds, which have east –west trending fold axes and axial planes that dip 50– 608 to the north (Fig. 4a). These features are attributed to intense ductile deformation associated with the fault zone. The fault juxtaposes the metamorphic rocks against the Phanerozoic sedimentary cover succession of the Tarim Basin. It is therefore assumed that the South Tien Shan fault zone continues to substantial depth (c. 20– 30 km), acting to allow the Tarim Block to be underthrust beneath the Tien Shan.

Keping (Frontal) Fault. This fault separates the present alluvial Tarim Basin from the Keping Shan. It is the most southerly of the belt-parallel thrusts and has the most topographically prominent hanging wall (Figs 1 & 2). In addition, it is the most seismically active of all the belt-parallel faults, with focal mechanism solutions indicating relatively pure thrust displacement (USGS 2009). These seismogenic and geomorphic attributes suggest that the Keping Fault is the youngest beltparallel fault and that each respective thrust to the north is progressively older, and that the thrust belt has largely evolved as a simple forelanddirected (piggyback) series (Dahlstrom 1970; Butler 1982). Along much of its length, cliff-forming Cambrian– Ordovician limestones which dip gently to the north define the base of the thrust hanging wall. Only in a few localities are the remnants of fault-related folding preserved, but it is postulated that folds of similar form were once continuous along the mountain front (Fig. 3). The southern limbs of these folds are steeply dipping and often overturned, while the northern limbs dip between 20 and 408. Where present, the cores of fault-related folds are characterized by internal deformation, and minor thrusts and folds are abundant (Fig. 4c, e).

74 S. TURNER ET AL. Fig. 3. Balanced cross section across the Keping Shan (A0 –A00 , see Figure 1 for line of section). Major thrusts detach onto a middle Cambrian evaporite surface (sensu Allen et al. 1999), predominantly dipping to the north and verging to the south. Once balanced, the horizontal shortening across the section is 33%. This section assumes that deformation is completely thin-skinned but in the absence of subsurface data we cannot determine whether basement faults are involved.

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Fig. 4. Field photographs illustrating aspects of structural deformation in the Keping Shan: (a) Minor folding of interbedded quartzites and schists within the shear zone of the South Tien Shan Fault; (b) South-verging thrust duplex in Palaeogene conglomerate immediately south of the South Tien Shan Fault; (c, d) Deformation within Ordovician limestones in the Mystery Canyon, in the hanging wall of the Keping Fault, showing a south-verging thrust and intense deformation above a minor thrust respectively; (e) North-verging backthrust in the southern limb of a thrust-related anticline above the Keping Fault; (f) Middle Devonian red sandstones juxtaposed against middle Ordovician limestones along the northern segment of the Piqiang Fault, near Piqiang.

Minor folds have axes which are subparallel to the local trend of the Keping Fault.

Belt-oblique (cross) fault zones Major fault zones which are oblique (by .458) or perpendicular to the general trend of the Keping Shan are termed ‘belt-oblique’ and comprise

oblique-slip and strike-slip (transfer) faults. Several of these faults have a prominent surface expression, while others are more subtle. In either case, belt-oblique faults have an important role in partitioning the thrust belt into a series of structural domains which are characterized by variations in the spatial distribution of belt-parallel thrusts. The initial formation of these faults pre-dates the late

76 S. TURNER ET AL. Fig. 5. Stratigraphic correlation panel across a series of stratigraphic sections taken from the hanging wall of the Keping Fault and restored to the base Cenozoic unconformity. The correlation panel was constructed by measuring the thickness of the sediment pile at nine sections along the hanging wall of the Keping Fault. Major faults across which the sediment thickness changes abruptly were interpreted from the surface expression of structural lineaments (cf. Fig. 1). There is a dramatic thinning of the Palaeozoic sediment pile across the central part of the Keping Shan, an area referred to as the Bachu Uplift. The depth to the middle Cambrian detachment layer varies from more than 6 km to just over 2 km between the Piqiang and Sanchakou Faults. Much of the thickness change occurs in the lower Permian succession, but further work is required to determine whether this is a syntectonic feature. An additional major fault is proposed for the west, between the Sulphur Canyon and Yingan sections, where sediment thickness increases substantially despite a lack of belt-oblique structures with surface expression in this area.

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Cenozoic evolution of the Keping Shan Thrust Belt. A 280 km stratigraphic correlation panel was constructed by measuring the thickness of the sedimentary pile at nine sections along the hanging wall of the Keping Fault (Fig. 5). When hung from the base Cenozoic unconformity, the correlation panel demonstrates the impact of the inherited faults on the thickness of the sediment pile during an earlier phase of tectonism in the Tarim Basin. Although the event responsible for the inherited faults remains the subject of debate, it is likely that they formed during a brief phase of extension that affected the NW Tarim Basin during the early Permian. This period was characterized by substantial basaltic magmatism, resulting in the emplacement of NW– SE trending dykes and extrusive basalt flows within the lower Permian stratigraphic succession. The basalts yield ages of 274 + 2 Ma (Zhang et al. 2008). Analysis of borehole data from the interior of the Tarim Basin has revealed that the basalts cover a total area of around 250 000 km2 and are thought to have been caused by a short-lived mantle plume in the early Permian (Jia et al. 2004; Jiang et al. 2004; Zhang et al. 2008). Incidentally, the area in which the sediment pile is thinnest in the Keping Shan (Fig. 5), across a structure known as the Bachu Uplift, correlates to an area proposed to represent the source region of the early Permian basalts which flooded much of the Tarim Basin (Zhang et al. 2008). Furthermore, Chen et al. (2006) identify this region as the central part of a much larger area affected by substantial crustal doming from the late Cisuralian to the Guadalupian (c. 270 –260 Ma). An alternative possibility is that the inherited faults relate to a later transtensional event which affected the NW Tarim Basin during the Jurassic (Sobel 1999; pers. comm., J. Suppe). Tectonism during this period was characterized by NW–SE trending strike-slip faults which formed deep and narrow transtensional basins in the western Tarim Basin (Sobel 1999). A complete absence of Mesozoic sediments within the Keping Shan means that it remains speculative as to which event was responsible for the formation of the inherited faults, although we maintain that early Permian extension was the most likely cause. In either case, given that the Keping Shan remained an intrabasinal high throughout the Mesozoic (Li et al. 1996) it is likely that a substantial amount of erosion occurred across individual fault blocks and the Bachu Uplift prior to the early Cenozoic, accounting for the substantially reduced thickness of Palaeozoic strata over the central parts of the Keping Shan. Piqiang Fault. This fault is expressed for more than 70 km as a prominent structural lineament that has a dramatic effect on the structural architecture of the Keping Shan Thrust Belt (Figs 1, 2 & 6). The

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fault trends approximately NNW –SSE (340 –3508) and is defined by a series of segments which either offset or completely decouple the east –west trending belt-parallel thrusts to either side, thereby acting to partition the thrust belt. The trend of the fault is subparallel to the SSE-oriented thrust transport direction. Examination of the individual segments of the Piqiang Fault reveals that the faulting mechanism changes along strike at c. 20 –25 km intervals, locally acting as either a strike-slip fault or as a lateral ramp. In plan view, the Piqiang Fault has an impact on the spatial organization of the belt-parallel thrusts to either side of it, expressed as a change in the number and spacing of thrusts from west to east (Fig. 6). To the west, there are three major thrusts (with surface expression), one of which terminates against the Piqiang Fault as a lateral ramp. To the east, there are five major thrusts which are more closely spaced, creating narrower piggyback basins. The greater abundance of thrusts to the east implies that the total horizontal shortening to the east of the Piqiang Fault is marginally greater than to the west. Examination of the stratigraphy across the fault zone indicates that there is a net loss from west to east (Fig. 7). To both the east and west of the fault, the Palaeozoic megasequences are separated from the Cenozoic megasequence by a major unconformity which spans the Mesozoic. The anomaly lies in the age of the youngest Palaeozoic strata on the eastern and western sides of the Piqiang Fault. To the east, the unconformity separates the Middle Devonian from the Palaeogene, while to the west, it separates the lower Permian from the Palaeogene (Fig. 7). The upper Carboniferous and lower Permian sediments that are absent from the eastern side of the fault are shallow-marine to fluvial carbonates and sandstones which were deposited in the late Carboniferous foreland basin that developed adjacent to the ancestral Tien Shan (Carroll et al. 1995). On both sides of the fault, Palaeogene sediments are interbedded fluvial sandstones and mudstones which maintain the same thickness across the fault, suggesting they were deposited onto a flat, peneplain surface in the early Cenozoic. In total, we estimate that c. 800 m of sediment is absent from the eastern side of the Piqiang Fault. Saergan Fault. Similar to the Piqiang Fault, the Saergan Fault is a prominent structure which crosscuts belt-parallel thrusts (Figs 1 & 2). It can be traced for c. 40 km across the belt, and acts as a major right-lateral strike-slip fault. Unlike the other faults described in this section, however, there appears to be little or no stratigraphic discontinuity across it. In addition, the orientation of the fault is notably different from other belt-oblique fault zones, which generally follow trends of NW–SE to north –south. This suggests that unlike many other

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Fig. 6. Structure of the Piqiang Fault: (a) Geological map derived from the interpretation of (b), Landsat ETMþ (bands 321, 30 m resolution) satellite image. Along strike, the faulting mechanism appears to change, acting either as a strike-slip fault (southern and northern segments) or as a lateral ramp (central segment).

belt-oblique faults, the Saergan Fault formed during the late Cenozoic evolution of the Keping Shan and was not part of the early Permian fault population. The fault may act to accommodate lateral changes

in horizontal shortening which are not suitably attained through other reactivated belt-oblique structures, but this will require a further, detailed study of the structure.

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Fig. 7. Stratigraphic correlation across the Piqiang Fault, showing the net loss of nearly 800 m of Middle Devonian, upper Carboniferous and lower Permian strata from west to east, and the unaffected Palaeogene –Neogene strata above the base Cenozoic unconformity.

Sanchakou Fault. This fault zone has little surface expression but correlates to an important change in sediment thickness and causes localized disruption to the Keping Fault. The surface expression of the fault crops out in the village of Sanchakou, to the south of the Keping Fault. Tracing the fault to the north, the fault interacts with the Keping Fault and causes it to branch into two faults to the west (Fig. 8). Based on the stratigraphic correlation across the fault (Fig. 5), the net loss of stratigraphy is 500 –550 m from east to west. Most of the stratigraphic loss occurs within the Middle Devonian, and only a few kilometres to the west a progressive thickening of upper Carboniferous strata is observed in what would have been the original downthrown block. The interaction of the Sanchakou and Keping Faults is characterized by a zone of structural complexity in which upper Neoproterozoic to lower Cambrian sediments are exposed (Fig. 8). These are the oldest sediments within the central part of the Keping Shan, and they have been exhumed from beneath the middle Cambrian detachment layer.

Yijianfang Fault. The surface expression of the Yijianfang Fault is prominent within the Tarim Basin to the south of the Keping Shan, representing one of the few structural features that penetrate the present-day basin surface. The interaction with the Keping Shan is less obvious, but tracing the fault zone to the north it is apparent that it interacts with the Keping Fault and causes a kink in the NE–SW trend of it (Fig. 1). In addition, the structural dip of beds within the hanging walls of belt-parallel thrusts are substantially reduced for several kilometres eastward, from the average 358 dip values recorded across much of the Keping Shan to c. 258. Across the Yijianfang Fault, the change in stratigraphic thickness is c. 500 m and is most prominently demonstrated by the substantial increase in the thickness of lower Permian sediments to the east (Fig. 5).

Discussion Lateral variations in the structural architecture and partitioning of the Keping Shan Thrust Belt

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Fig. 8. Structure of the Sanchakou Fault: (a) SPOT-5 (bands 431, 5 m resolution) false colour satellite image; (b) Geological map based on the interpretation of (a). The Middle Devonian thickens abruptly across the Sanchakou Fault. On interacting with the Sanchakou Fault, the Keping Fault branches across a zone of structural complexity in which late Neoproterozoic sediments are exhumed from beneath the middle Cambrian detachment layer.

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correlates with major lineaments and fault zones which are oblique or perpendicular to the general structural trend of the thrust belt. These fault zones pre-date the late Cenozoic thrusting and previously acted as basin-bounding faults which controlled the thickness of the sediment pile. Partitioning of the belt correlates directly with changes in the total thickness of the sediment pile above the middle Cambrian basalt detachment surface. These changes occur across major beltoblique faults, which, as noted above, suggests they were active prior to thrusting. Plausibly, these faults were generated during an extensional phase associated with a mantle plume in the early Permian (Zhang et al. 2008) or during a transtensional phase in the Jurassic (Sobel 1999). Studies from other fold–thrust belts and analogue experiments have demonstrated the impact of sediment thickness on the structural architecture of superimposed compressional structures. Liu et al. (1992) show through analogue sandbox experiments that thicker sediment piles produce thrust systems

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in which major thrusts are widely spaced, creating wider piggyback basins. Conversely, thinner sediment piles deform as a series of more closely spaced thrusts separated by narrower piggyback basins (Fig. 9a). This arises because when a sediment pile is thick, fewer thrusts are required to attain the topography that satisfies the critical angle within the deforming wedge, than when the sediment pile is thin. When applied to the Keping Shan, this theory certainly seems to be applicable. The impact is best explored across the Piqiang Fault, the most prominent belt-oblique structure which causes substantial lateral discontinuity between the thrusts to either side and acts to partition two parts of the thrust belt. The sediment pile to the west of the Piqiang Fault is c. 4 km thick, while to the east it is c. 2 km thick. This is expressed in the deforming sediment pile as fewer thrusts to the west, and a greater number to the east (Fig. 9a). Furthermore, given that the Piqiang Fault was a pre-existing structure over which this stratigraphic discontinuity occurs, it provides an ideal plane of weakness that

Fig. 9. Impact of sediment thickness on thrusting: (a) Abrupt and substantial lateral change across a major pre-existing fault zone, which results in the reactivation of the fault and lateral partitioning of the thrust belt; (b) Abrupt but small change across a pre-existing fault zone, causing a kink or branching of the superimposed thrusts but without the need to reactivate the fault. These models are based on the direct observations from the Keping Shan and supported by theoretical models proposed by Liu et al. (1992).

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Fig. 10. Schematic model of the Keping Shan illustrating the impact of lateral variations in the thickness of the sediment pile, major pre-existing fault zones, and the structural architecture of the superimposed (late Cenozoic) thrust belt.

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can be reactivated as a strike-slip (transfer) fault which effectively decouples the thrust belt to either side of it, accommodating the abrupt lateral change in horizontal shortening and spatial organization of structures. Where the change in sediment thickness appears to be less substantial, it follows that the lateral variation in the thrust belt will also be less substantial. Changes in sediment thickness in the order of several hundred metres, such as that observed across the Sanchakou Fault, still cause disruption to the belt-parallel thrusts. However, rather than creating major strike-slip fault systems that completely partition the thrust belt, the belt-parallel structures accommodate these changes by branching and splitting into two thrusts (Fig. 9b). Plausibly, this pattern continues to apply where the net loss of sediment thickness is smaller or the change is less abrupt, such as across the Yijianfang Fault. Such belt-oblique structures have only a minor impact on the superimposed thrust system, such as a minor kink in the trace of belt-parallel thrusts. There is no requirement to reactivate the fault to accommodate this lateral change and were it not for the surface expression of the structure to the south (Fig. 1), within the Tarim Basin interior, such structures would probably go unnoticed within the thrust belt. Taking these observations into account, and applying the theoretical models (Fig. 9), we present a schematic model for the whole of the Keping Shan (Fig. 10). The model shows that lateral partitioning and structural variability is strongly affected by lateral variations in sediment thickness above the detachment horizon, which in turn are associated with the presence of major pre-existing structures that were potentially most active during the early Permian. Comparison with other studies demonstrates that the primary causes of lateral structural variability in the Keping Shan are well documented in other settings. Within the Cordilleran Fold– thrust Belt, western USA, Lawton et al. (1994) show that abrupt discontinuities that form between segments (compartments) of the thrust belt arise because of rapid lateral changes in the thickness of the sedimentary section. Where the change in sedimentary thickness is less substantial, diffuse transition zones form without the requirement to generate major strike-slip fault zones. A similar case from the Apennines, Italy, is presented by Butler et al. (2006), where major belt-oblique lineaments which coincide with substantial changes in the thickness of Mesozoic to lower Cenozoic strata have impacted on the plan-view architecture of the more recently imposed fold– thrust belt. In the Zagros Fold– thrust Belt, Iran, major pre-existing structures such as the Kazerun Fault have long-lived histories

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and acted to control not just the sedimentary thickness but also the sedimentary facies. Early activity during the Cambrian controlled the distribution of the Hormuz salt, which has created a major lateral change in the detachment within the belt. Subsequent activity of these earlier structures during the Cretaceous impacted substantially on the thickness of the sediment pile, which has further enhanced lateral variability in the Cenozoic fold– thrust belt (Sepehr & Cosgrove 2004, 2007; Sepehr et al. 2006). These studies in other fold– thrust belts compare closely to the observations and interpretations we have drawn from the Keping Shan.

Implications for hydrocarbon exploration Lateral structural variability within foreland fold– thrust belts has important implications for hydrocarbon exploration in similar settings, which, as discussed throughout this volume, are likely to become of increasing importance in the future. To date, there have been no discoveries of hydrocarbons in the Keping Shan, which reflects the lack of a suitable source rock within the stratigraphic succession. However, the Keping Shan provides an ideal analogue for similar settings which may be rich in hydrocarbons. Abrupt discontinuities within fold–thrust belts, such as those described in this paper and in analogous settings such as the Cordilleran fold– thrust belt, the Apennines and the Zagros (Butler et al. 2006; Lawton et al. 1994; Sepehr et al. 2006), act to partition the fold–thrust belt into a series of structural domains and may thereby compartmentalize structural reservoirs of hydrocarbons. In addition, lateral variations in the spatial organization of structures between different compartments can enhance the reservoir potential of certain parts of the fold–thrust belt over others, by enhancing the concentration of structural traps. An important consideration, and one which has formed the basis of much of this paper, is the preexisting basin template and the presence of major lateral variations in the pre-existing stratigraphic framework. This may have additional implications for hydrocarbon distribution, locally removing important reservoir or seal formations from certain parts of the fold–thrust belt. Thicker areas of sediment may also be subjected to higher temperatures at depth, thus increasing the thermal maturity of source rocks and enhancing hydrocarbon generation in certain parts of the belt relative to others. In a fold-thrust belt such as the Keping Shan, where sediment thickness varies on the order of several kilometres along strike, the presence and state of the key play elements (source, maturation, reservoir, seal and trap) could vary significantly and abruptly across belt-oblique fault zones.

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Conclusions Partitioning of belt-parallel thrusts within the east – west trending Keping Shan Thrust Belt occurs across major north– south to NW –SE belt-oblique fault zones and lineaments. Although these fault zones have developed coeval to thrusting during late Cenozoic contraction, there is strong evidence of earlier activity linked to either a period of extension during the early Permian or regional transtension in the Jurassic. This extension generated a series of major north –south to NW– SE trending faults across which there are substantial variations in the thickness of the sedimentary succession. late Cenozoic contractional deformation, associated with foreland deformation adjacent to the Tien Shan orogenic belt, generated the Keping Shan Thrust Belt. The lateral structural variability and partitioning of the belt is strongly controlled by the pre-existing faults and associated variations in sedimentary thickness. In several examples, the inherited structures have been reactivated during thrusting either as oblique-slip or strike-slip (transfer) faults which cross the thrust belt and help to accommodate abrupt lateral changes in the structure. Understanding how inherited structures and their impact on the sediment pile combine to influence the partitioning of the fold– thrust belt has important implications for hydrocarbon exploration in foreland settings. The Keping Shan serves not only to provide an insight into earlier phases of tectonism within the hydrocarbon-rich Tarim Basin, but can be applied as an analogue to fold–thrust belts worldwide. This study was funded by NERC and undertaken as part of the PhD research by S. Turner. SPOT datasets were provided by CNES through the OASIS programme. We are especially grateful to Nick Brook, Gareth Morgan and the Xinjiang Seismology Bureau for logistical and scientific support in the field during the summers of 2007 and 2008. We also thank Richard Phillips and an anonymous reviewer for constructive and helpful reviews.

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KEPING SHAN THRUST BELT, NW CHINA Lawton, T. F., Boyer, S. E. & Schmitt, J. G. 1994. Influence of inherited taper on structural variability and conglomerate distribution, Cordilleran fold and thrust belt, western United States. Geology, 22, 339–342. Li, D., Liang, D., Jia, C., Wang, G., Wu, Q. & He, D. 1996. Hydrocarbon accumulations in the Tarim basin, China. American Association of Petroleum Geologists Bulletin, 80, 1587– 1603. Li, Z. X. & Powell, C. McA. 2001. An outline of the paleogeographic evolution of the Australasian region since the beginning of the Neoproterozoic. EarthScience Reviews, 53, 237– 277. Liu, H., McClay, K. R. & Powell, D. 1992. Physical models of thrust wedges. In: McClay, K. R. (ed.) Thrust Tectonics. Chapman & Hall, London, 71–81. Macedo, J. & Marshak, S. 1999. Controls on the geometry of fold –thrust belt salients. Geological Society of America Bulletin, 111, 1808–1822. Marshak, S., Wilkerson, M. S. & Hsui, A. T. 1992. Generation of curved fold– thrust belts: Insight from simple physical and analytical models. In: McClay, K. R. (ed.) Thrust Tectonics. Chapman & Hall, London, 83– 92. Reigber, Ch., Michel, G. W. et al. 2001. New space geodetic constraints on the distribution of deformation in Central Asia. Earth and Planetary Science Letters, 191, 157– 165. Scharer, K. M., Burbank, D. W., Chen, J., Weldon, R. J., Rubin, C., Zhao, R. & Shen, J. 2004. Detachment folding in the Southwestern Tian Shan– Tarim foreland, China: shortening estimates and rates. Journal of Structural Geology, 26, 2119–2137. Sepehr, M. & Cosgrove, J. W. 2004. Structural framework of the Zagros Fold– thrust Belt, Iran. Marine and Petroleum Geology, 21, 829–843. Sepehr, M. & Cosgrove, J. W. 2007. The role of major fault zones in controlling the geometry and spatial organization of structures in the Zagros Fold– thrust Belt. In: Ries, A. C., Butler, R. W. H. & Graham,

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R. H. (eds) Deformation of the Continental Crust: The Legacy of Mike Coward. Geological Society, London, Special Publications, 272, 419– 436. Sepehr, M., Cosgrove, J. W. & Moieni, M. 2006. Role of the Kazerun Fault Zone in the formation and deformation of the Zagros Fold– thrust Belt, Iran. Tectonics, 24, TC5005. Sobel, E. R. 1999. Basin analysis of the Jurassic– Lower Cretaceous southwest Tarim basin, northwest China. Geological Society of America Bulletin, 111, 709–724. Sobel, E. R. & Dumitru, T. A. 1997. Thrusting and exhumation around the margins of the western Tarim basin during the India–Asia collision. Journal of Geophysical Research, 102, 5043–5063. Sun, J., Zhu, R. & Bowler, J. 2004. Timing of the Tianshan Mountains uplift constrained by magnetostratigraphic analysis of molasse deposits. Earth and Planetary Science Letters, 219, 239–253. USGS (UNITED STATES GEOLOGICAL SURVEY) EARTHQUAKE HAZARDS PROGRAM 2009. Available online, http://earthquake.usgs.gov. Watson, M. P., Hayward, A. B., Parkinson, D. N. & Zhang, Z. M. 1987. Plate tectonic history, basin development, and petroleum source rock deposition, onshore China. Marine and Petroleum Geology, 4, 205– 225. Yang, Y. & Liu, M. 2002. Cenozoic deformation of the Tarim plate and the implications for mountain building in the Tibetan plateau and the Tian Shan. Tectonics, 21, 1059. Yin, A., Nie, S., Craig, P., Harrison, T. M., Ryerson, F. J., Xianglin, Q. & Geng, Y. 1998. Late Cenozoic tectonic evolution of the southern Chinese Tian Shan. Tectonics, 17, 1– 27. Zhang, C. L., Li, X. H., Li, Z. X., Ye, H. M. & Li, C. N. 2008. A Permian layered intrusive complex in the western Tarim block, northwestern China: Product of a ca. 275-Ma mantle plume? The Journal of Geology, 116, 269–287.

The use of palaeo-thermo-barometers and coupled thermal, fluid flow and pore-fluid pressure modelling for hydrocarbon and reservoir prediction in fold and thrust belts F. ROURE1,2*, P. ANDRIESSEN2, J. P. CALLOT1, J. L. FAURE1, H. FERKET1,3,4,5, E. GONZALES1,6, N. GUILHAUMOU7, O. LACOMBE8, J. MALANDAIN1,8, W. SASSI1, F. SCHNEIDER1,9, R. SWENNEN4 & N. VILASI1,4 1

IFP Energies Nouvelles, 1 – 4 Ave. de Bois-Pre´au, 92852 Rueil-Malmaison, Cedex, France 2

VU-Amsterdam, de Boelelaan 1085, 1081 HV Amsterdam, the Netherlands 3

IMP, Apartado Postal 14-805, 07730 Mexico DF, Mexico

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KU-Leuven, Celestijnenlaan 2000, B-300 Leuven, Belgium 5

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VITO, Boeretang 200, 2400 Mol, Belgium

Pemex, Av. Urano 420, Col. Ylang Ylang, Boca del Rio, CP 94298 Veracruz, Mexico 7

Museum nat. Histoire Naturelle, 61 rue Buffon, F-75005, Paris, France

8

University Pierre & Marie Curie, Paris VI, Laboratoire de Tectonique, 4 Place Jussieu, F-75252, Paris, France 9

Beicip-Franlab, 232 Ave. Napole´on Bonaparte, PO Box 213, 92502 Rueil-Malmaison, Cedex, France *Corresponding author (e-mail: [email protected]) Abstract: Basin modelling tools are now more efficient to reconstruct palinspastic structural cross sections and compute the history of temperature, pore-fluid pressure and fluid flow circulations in complex structural settings. In many cases and especially in areas where limited erosion occurred, the use of well logs, bottom hole temperatures (BHT) and palaeo-thermometers such as vitrinite reflectance (Ro) and Rock-Eval (Tmax) data is usually sufficient to calibrate the heat flow and geothermal gradients across a section. However, in the foothills domains erosion is a dominant process, challenging the reconstruction of reservoir rocks palaeo-burial and the corresponding calibration of their past thermal evolution. Often it is not possible to derive a single solution for palaeoburial and palaeo-thermal gradient estimates in the foothills, if based solely on maturity ranks of the organic matter. Alternative methods are then required to narrow down the error bars in palaeo-burial estimates, and to secure more realistic predictions of hydrocarbon generation. Apatite fission tracks (AFT) can provide access to time– temperature paths and absolute ages for the crossing of the 120 8C isotherm and timing of the unroofing. Hydrocarbon-bearing fluid inclusions, when developing contemporaneously with aqueous inclusions, can provide a direct access to the pore-fluid temperature and pressure of cemented fractures or reservoir at the time of cementation and hydrocarbon trapping, on line with the tectonic evolution. Further attempts are also currently made to use calcite twins for constraining reservoir burial and palaeo-stress conditions during the main deformational episodes. Ultimately, the use of magnetic properties and petrographical measurements can also document the impact of tectonic stresses during the evolution of the layer parallel shortening (LPS). The methodology integrating these complementary constraints will be illustrated using reference case studies from Albania, sub-Andean basins in Colombia and Venezuela, segments of the North American Cordillera in Mexico and in the Canadian Rockies, as well as from the Middle East.

Present geothermal gradients can usually be derived from BHT (bottom hole temperature) measurements. Seemingly, the overall distribution of conductivities in the overburden can be reasonably described by applying standard values for each

dominant lithology, provided the latter can be properly documented by means of well logs and extrapolated laterally by the use of seismic sequences and attributes. In nearshore segments of passive margins and in foreland basins, where lithosphere

From: Goffey, G. P., Craig, J., Needham, T. & Scott, R. (eds) Hydrocarbons in Contractional Belts. Geological Society, London, Special Publications, 348, 87– 114. DOI: 10.1144/SP348.6 0305-8719/10/$15.00 # The Geological Society of London 2010.

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and crustal thickness remained relatively constant and only limited erosion occurred, vitrinite reflectance (Ro) and Rock-Eval (Tmax) values measured along vertical profiles (i.e. geochemical logs in wells) are usually sufficient, when combined with 1D well modelling (burial v. time curves), to derive realistic values for the palaeo-thermicity of a given area. In contrast, calibration of petroleum modelling becomes more complex in areas where both crustal and lithosphere thickness have been strongly modified since the deposition of the source rock, either in the distal portion of continental margins near the continent–ocean transition or in the inner parts of the orogens, where slab detachment or asthenospheric rise could result in drastic changes in the heat flow. Large uncertainties are also recorded when addressing petroleum modelling in foothills domains, basically because of the lack of controls on the palaeo-burial estimates in areas which have been strongly affected by erosion, and where it becomes challenging to solve for each time interval and for each cell of the model two unknown parameters (i.e. both temperature and burial). This paper will first briefly describe the current state of the art and integrated workflow developed recently for addressing basin modelling in fold and thrust belts (FTB). It will then document, based on representative case studies, the use of various palaeo-thermo-barometers for reducing the error bars in petroleum modelling in such tectonically complex areas as FTB, where major erosional events prevent any direct access to the palaeo-burial.

Integrated workflow developed for hydrocarbon and pore-fluid pressure modelling in FTB Dewatering processes and coeval overpressures build up have been widely documented in modern accretionary wedges by means of seismic attributes and deep ODP–IODP (International Oceanic Drilling Program) wells. For instance, the increasing load of synflexural sediments deposited in foredeep basins results in a vertical escape of formation water and a progressive mechanical compaction of the sedimentary pile where pore-fluid pressures remain dominantly hydrostatic. However, this process ultimately induces a velocity increase of seismic waves from the surface down to a depth where the vertical permeability reaches a minimum, precluding any further escape of underlying fluids toward the seafloor. Undercompacted sediments occur beneath this compaction-induced regional seal, being characterized by slower seismic velocities and overpressures. Worth mentioning, this occurrence of an overpressured horizon in the foreland strongly

decreases the mechanical coupling and friction between deeper and shallower horizons, thus helping the localizing and propagating forelandward of the deformation front. Although FTB share many similarities with offshore accretionary wedges in terms of the modes of thrust emplacement and overall structural style, boundary conditions of these two geodynamic systems are rather different in terms of porosity/ permeability distributions and fluid flow regimes. This is due to (1) the age of the accreted series (usually restricted to the relatively young synflexural sequences in accretionary wedges, against dominantly pre-orogenic passive margin sequences in FTB), and (2) the origin of the fluids (mixing of sedimentary fluids with meteoric water in FTB, against entirely marine or basinal fluids in offshore accretionary wedges). Unlike in modern accretionary wedges, overpressures can usually not be detected by anomalies in the seismic attributes in FTB, making integrated basin modelling techniques an indispensable tool, as documented below, to predict the distribution of pore-fluid pressures and hydrocarbon (HC) potential before drilling.

Coupled kinematic, thermal and fluid flow modelling in the frontal part of Eastern Venezuelan FTB The El Furrial and Pirital thrusts developing at the front of the Eastern Venezuelan thrust belt have been the focus of a pilot modelling approach coupling various 1D (Genex) and 2D (Thrustpack and Ceres: Sassi and Rudkiewicz 2000; Schneider et al. 2002; Schneider 2003; Deville & Sassi 2006) basin modelling tools. A structural section was first compiled from the interpretation of seismic profiles, and integration of wells and outcrop data. This section was then balanced and restored to its pre-orogenic configuration, providing an accurate control on the initial spacing of future thrusts. Incremental 2D forward kinematic modelling coupling erosion/sedimentation and flexure was subsequently performed with Thrustpack by means of a trial and error process, until the result section of the model was consistent with (1) the present architecture of the El Furrial and Pirital thrusts, (2) the pattern of erosional surfaces and unconformities currently observed in the Morichito piggyback basin and adjacent Pirital High, as well as (3) the measured temperature proxies (Ro from wells and outcrops). Despite strong erosion on top of the Pirital allochthon, where late Miocene and Pliocene series of the Morichito thrust-top basin rest locally directly on top of Cretaceous series (Fig. 1), this thrust unit

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Fig. 1. 2D forward kinematic and petroleum modelling along a regional transect in Eastern Venezuela. (a) Location map; (b) Structural section across the El Furrial and Pirital structures, outlining the BHT and maturity indicators (Tmax and Ro) used for calibration; (c) Left column: Main stages of the kinematic modelling. Blue: Jurassic ¼ synrift; green ¼ Lower Cretaceous (Barranquin Formation); deep green: Querecual source rock; orange: Naricual Ologocene reservoir; brown, grey and yellow: Miocene Carapita Formation, La Pica and Morichito Pliocene formations. (d) Right column: Transformation rates of the main potential source rocks for the same time intervals.

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is adjacent to the El Furrial trend where a continuous Palaeogene and Neogene sequence is preserved. Because the Pirital allochthon is also close enough to the Maturin Basin, where the entire passive margin and flexural sequences remained undeformed, basal heat flow was assumed to always have been constant along the entire transect; that is, heat flow values derived from 1D Genex modelling in wells from the foreland and El Furrial trends could be extrapolated laterally along the entire profile for the 2D Thrustpack and Ceres modellings. This assumption results in a good agreement between observed and computed values of Tmax and Ro, the Upper Cretaceous Querecual source rocks being indeed overmature in the Serran˜a (allochthon), but still in the oil window in the footwall of the Pirital thrust, whereas the Miocene Carapita shales are still immature within the El Furrial anticline (Fig. 1). The intermediate geometries of the Thrustpack structural model were ultimately used as input data for the 2D Ceres pore-fluid pressure and fluid flow modelling. Pore-fluid pressures in the wells document a currently overpressured regime in the Oligocene Naricual sandstone reservoirs of the El Furrial trend, which are indicated by a white circle in Figure 2. These pressure values, amounting up to 80 MPa at depths between 4 and 5 km in the Oligocene reservoirs (Fig. 2c), were used for quality control of the modelling. Apart from confirming the HC potential of this part of Eastern Venezuela, this pilot modelling sequence provided useful information on the timing and boundary conditions for pore-fluid pressure building along the transect, as well as on the history of fluid flow velocity within the Oligocene reservoir of the El Furrial trend: (1) According to the model, overpressures in El Furrial reservoirs did not start until the structural closure was effective on both sides of the anticline, all compaction water previously escaping laterally toward the south, updip of the foreland flexure. However, this result is probably biased by the fact the litho-stratigraphic description of the geological section was oversimplified. In particular, neither lateral sedimentological nor porosity/permeability variations were taken into account here when describing the regional internal architecture of the Oligocene and Neogene series. Most likely, the integration of an additional step in the modelling, involving the forward simulation of the sedimentation pattern and lithofacies with Dionisos or coeval sedimentological tools (Granjeon & Joseph 1999), would have introduced further controls on lateral changes in sand v. shale ratios, which are likely to occur in the foreland autochthon. This would probably have accounted for the development of lateral permeability barriers in the foreland, which could

indeed preclude or at least delay the lateral escape of compaction water, and help generate overpressure in the Oligocene reservoirs in the foreland autochthon before the complete structural closure of El Furrial. (2) The Oligocene reservoirs of El Furrial trend behaved as a relatively closed system (0 value for the velocity of the fluids) until the onset of the foreland flexure development. Seemingly, these reservoirs have been again totally isolated (closed systems) since the time an effective structural closure had developed on both sides of this anticline. Between these two stages, the Ceres results indicate a short episode of active foreland-directed fluid flow in El Furrial reservoirs. This squeegee episode (e.g. Machel & Cavell 1999) occurred when the reservoirs were still connected with the foreland, in the footwall of active thrusts. Alternatively, vertical escape of El Furrial fluids toward the Pirital allochthon was apparently possible at the time Oligocene series of both compartments were still connected (Figs 2 & 3a). Further evidence that the Pirital thrust was episodically a conduit accounting for a vertical escape of the fluids is attested by the occurrence of (1) HC accumulations (mostly heavy biodegradated oil) in the Pliocene sedimentary infill of the Morichito thrust-top basin, which result from a remigration from deeper reservoirs, and (2) hydrothermal circulations and lateral escape of hot fluids along the basal unconformity of the Morichito basin, as indicated by hot springs and high temperature (Th) in the fluid inclusions of quartz cements within the Pliocene Morichito sandstones (BordasLe Floch 1999).

Topography-driven fluid flow v. tectonically induced squeegee episode of fluid expulsion in FTB Apart from the poor description of lateral variations in the sand v. shale ratios of Cenozoic series, other limitations of the Ceres basin model result from its current lack of horizontal compaction. As it stands, the values of the overpressures computed by Ceres are dominantly controlled by the hydraulic heads (i.e. the topography of the hinterland) and by the capillary pressure barriers. However, as discussed below, one of the major processes operating in both modern accretionary wedges and active FTB is the layer parallel shortening (LPS), which stimulates the pressure-solution mechanisms, inducing lateral changes in the compaction, decrease in porosity and permeability, and coeval increases in the seismic velocities. Most likely, LPS contributes a lot to the development of overpressures and tectonically controlled squeegee episode of forelandward expulsion of compaction water (Nieuwland et al.

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Fig. 2. 2D fluid flow and pore-fluid pressure modelling along the same Eastern Venezuelan transect. Green horizon ¼ Querecual source rock; underlying red unit ¼ Barranquin siliciclastic aquifer; overlying red unit ¼ Naricual Oligocene reservoir. (a) Top section illustrates a geometric configuration which prevented the building of overpressures in the El Furrial reservoirs (white dot) because of the lack of permeability barriers between the hanging wall and the footwall of the Pirital thrust. (b) Bottom section outlines the real configuration of the thrust pile, with an intervening seal made up of Carapita shale between the Barranquin units of the hanging wall and the Naricual of the footwall of the Pirital thrust, this later configuration allowing the building of overpressures in the El Furrial trend. (c) Depth v. pore-fluid pressure plot from wells of the El Furrial trend.

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2000). Generally first host-rock buffered fluids are squeezed out, then chemical compaction forms LPS stylolites. Later non-equilibrium fluids circulate through reopened structures. Palaeo-stress evolution reveals that the motor must be hydrofracturing. This deformation progrades through the foreland as a caterpillar-movement (Ferket et al. 2004). Unlike offshore accretionary wedges, FTB are not only characterized by fluid flow controlled by lateral permeability barriers but also by the topography-driven, gavitational flow of meteoric water, which operates from the positive relief of the hinterland towards the adjacent low lands and is mostly confined to the shallow horizons of the foreland, that is above the compaction-induced permeability barrier that has been previously described. The main differences observed in the results of Ceres modelling applied in Venezuela (a still currently active orogen), and the Canadian Rockies (where Laramide-related tectonic compaction stopped at the end of the Eocene, about 60 Ma) relate to the timing of squeegee fluid escape, which is Miocene in Venezuela (Fig. 3a), against Late Cretaceous in Canada (Fig. 3b1). Both systems are still affected by a gravitationally-driven, shallower flow of meteoric water, restricted to the Miocene flexural sequence in Venezuela and to the Cretaceous flexural sequence in Canada; that is, much shallower than the Oligocene sandstone reservoirs of El Furrial and the Mississippian carbonate reservoirs of the Rockies (Fig. 3b2). Although it would not affect the results of these Venezuelan and Canadian models, it is important to notice that sea-level changes and palaeo-bathymetry are also important in controlling the development of overpressures in some foreland basins and adjacent foothills, as described in Albania by Vilasi et al. (2009).

Meteoric v. basinal signature of circulating fluids Although large geochemical databases are now available to document the current distribution of meteoric v. saline waters in FTB, that is in the Canadian Rockies and in Taiwan, it is not always easy to characterize the origin of palaeo-fluids when fluid inclusions in diagenetic cements constitute the unique direct record left from these palaeocirculations. For instance, early studies made on fluid inclusions from cemented hydraulic fractures along the sole thrust of the Sicilian allochthon evidenced low salinities for the palaeo-fluids, these low salinity values being then interpreted as evidence for a meteoric signature (Guilhaumou et al. 1994;

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Larroque et al. 1996). Further d18O studies made by De Wever (2008) argue against this hypothesis, corroborated by other geochemical controls that rather suggest the smectite –illite transformation in Late Cretaceous to Palaeogene shale layers to be the main process accounting for the expulsion of such low salinity water. Similar processes have also been described for modern accretionary wedges (Vrolijk 1987). The same debate also occurred for the nature of the palaeo-fluids (either meteoric or basinal) accounting for quartz-cementation in the El Furrial Oligocene reservoir in Eastern Venezuela. Because no formation water was yet available from the numerous wells located in the El Furrial trend, basically because they did not reach the oil–water contact, an attempt was made to derive palaeosalinities of the formation waters by measuring Tm (melting temperature of the last ice crystal) in fluid inclusions (Bordas 1999; Roure et al. 2003). However, it turned out that the fluid inclusions were too small (less than 3 mm), thus precluding any accurate measurements of Tm. Further d18O studies made on the same cements ultimately evidenced the basinal, not meteoric signature of the palaeo-fluids, implying that the gravity-driven flow of meteoric water never went deeper than the regional lower Miocene, intra-Carapita seals (Schneider 2003).

Allochthon/foreland mechanical coupling, and further incidence of layer parallel shortening on reservoir damaging and pore-fluid pressure cyclicity In many FTB, LPS imprint has been mapped using the magnetic susceptibility anisotropy (AMS). AMS studies have been proven successful in deciphering the strain acquisition mechanisms for both sandstone (e.g. Saint-Bezar et al. 2002) and limestone (e.g. Evans et al. 2003), allowing the description of a general AMS fabric acquisition path related to strain. During the pre-folding LPS, AMS fabric evolves from a sedimentary fabric to intermediate and tectonic fabrics (Averbuch et al. 1992; Aubourg et al. 1997; Aubourg 1999; Evans et al. 2003). Both in extensional (e.g. Mattei et al. 1997) and compressional strain regimes (e.g. Frizon de Lamotte et al. 1997, 2002), most of the studies emphasized that the magnetic fabrics are generally acquired in response to pre-folding deformation. As an example, the AMS fabric recorded in the FTB of the Rocky Mountain along the Flathead Valley –Waterton dam transect (Fermor & Moffat 1992), shows this very classic pattern of fabric evolution in the sandstone reservoir levels, from the sedimentary magnetic fabric in the undeformed foreland basin to

94 F. ROURE ET AL. Fig. 4. Lateral evolution of AMS fabric along the Rocky Mountains transect (Fermor & Moffat 1992). (a) Classic pattern of evolution of magnetic fabric with increasing deformation (Averbuch et al. 1992); (b) Evolution of the magnetic ellipsoid shape across the belt; (c) Evolution of the magnetic fabric from the foreland to the Main Range.

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more evolved intermediate and even tectonic fabric within the belt (Fig. 4). The same pattern of magnetic fabric evolution has also been documented in Eastern Venezuela (Roure et al. 2003), where integration of various datasets ranging from seismic profiles to thin sections, analytical work and modelling also allowed for the appraisal of the quality of sub-thrust Oligocene sandstone reservoirs of the El Furrial trend. Their porosity –permeability evolution results from mechanical and chemical compaction, both processes interacting in response to sedimentary burial, horizontal tectonic stress and temperature. Actually, the main episode of sandstone reservoirs deterioration occurred in the footwall of frontal thrusts at the time of their nucleation, when the evolving thrust belt and its foreland were mechanically strongly coupled (Fig. 5). The related build-up of horizontal tectonic stresses in the foreland induced LPS at reservoir scales, involving pressure-solution at detrital grain contacts, causing in situ mobilization of silica, rapid reservoir cementation by quartz-overgrowth and coeval porosity and permeability reductions (Roure et al. 2003, 2005b). The age and duration of such quartz-cementation episodes can be roughly determined by combining micro-thermometric fluid inclusion studies with 1D and 2D basin modelling. In the case of the Oligocene El Furrial sandstone of Eastern Venezuela, homogenization temperatures (Th) in quartz-overgrowth cements reflect a very narrow temperature range (110 –130 8C), whereas the current reservoir temperature exceeds 160 8C. When plotted on burial/temperature v. time curves derived from 1D or 2D basin models calibrated against bottom hole temperatures (BHT) and maturity rank of the organic matter, it becomes obvious that cementation occurred during a very narrow time interval, no longer than a few million years, when the reservoir was not yet incorporated into the orogenic wedge (Fig. 5; Roure et al. 2003, 2005b). Such techniques of combined microthermometry and basin modelling can also be used for dating any other diagenetic episodes, provided the reservoir was in thermal equilibrium with the overburden at the time of cementation (without advection of hot fluids). Moreover, forward diagenetic modelling at reservoir scales can benefit from output data from basin modelling such as reservoir temperature, length of the diagenetic episode and, in the case of diagenesis in an open system, fluid composition and velocities. For the quantification of fluid –rock interactions in the pore space of a reservoir or along open fractures transecting it, information on these parameters is indeed required. Furthermore, the composition of the fluids involved and the kinetic parameters,

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which control the growth or dissolution of various minerals present in the system, must be known.

Error bars in palaeo-burial reconstructions related to lateral and temporal variations in the heat flow How to account for high maturities in the Serran˜a (Eastern Venezuelan transect)? Earlier in the paper we described the overall results of the integrated workflow applied for fluid flow modelling and HC appraisal only for the southern part of the Eastern Venezuelan transect. In fact, other complications occur further to the north along this transect. The main problem is to account for the very high maturity ranks measured for the Cretaceous Querecual source rocks in the Serran˜a del Interior, north of the Pirital thrust and its Morichito piggyback basin, where basically all the Cenozoic series have been removed by erosion (Fig. 1). As mentioned in the introduction, if they are constrained only by maturity ranks of the organic matter (Tmax and Ro), basin modelling tools cannot solve directly the two unknowns of this geological problem, that is, the maximum palaeotemperature and palaeo-burial reached by the Querecual source rocks. Indeed, two extreme hypotheses can be proposed to achieve the same results and match observed maturities: Hypothesis 1 would consider that the Serran˜a was located at the place of an early Miocene foredeep basin, which became subsequently eroded, the observed maturities entirely resulting from the early Miocene burial of the Cretaceous source rocks beneath the foreland deposits, thus predating their tectonic accretion within the thrust wedge; Hypothesis 2 would instead consider that the Serran˜a was located in the distal part of the former passive margin, not far from the ocean – continent transition, thus implying a higher heat flow regime than in the adjacent foreland domain which developed on top of the thermally equilibrated cratonic lithosphere, and a shallower sedimentary burial before tectonic accretion. In fact, both mechanisms are likely to contribute to part of the observed maturities, and in the lack of other controls (such as palaeo-barometers or age constraints), several solutions can be proposed by the model to match the data, mixing to various degrees the unknown thickness of the early Miocene series prior to their erosion, compensated by a given heat flow during the early, pre-orogenic stages of the transect evolution.

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(a)

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Burial curve and thermal - diagenetic evolution of the Miocene Oficina Reservoirs in the foreland prospects F End Oligocene

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Fig. 5. (a) Quartz-overgrowth in the Oligocene Naricual reservoir of El Furrial; (b) Burial/temperature v. time curve of the Oligocene Naricual reservoir of the El Furrial trend; (c) Histogram of Th measured in the fluid inclusions of quartz-overgrowth of the same Naricual reservoir (modified from Roure et al. 2003, 2005b).

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2

BASIN MODELLING TOOLS FOR HYDROCARBON AND RESERVOIR PREDICTION

Roll-back of subduction, slap detachment and corner flow of the mantle in the hinterland: their incidence on lateral and temporal changes in the thermicity of FTB Similar spatial and temporal changes in the basal heat flow have been documented in the Southern Apennines, where the various Cretaceous platform units involved in the Plio-Quaternary deformations were all derived from the former passive margin of Apulia (Casero et al. 1991; Mosca et al. 2004; Sciamanna et al. 2004). As such, the Apulian Platform in the foreland and sub-thrust units, and the Apenninic Platform units at the top of the allochthon, were both affected by the same basal heat flow during the Palaeogene and Miocene. However, they are currently located in two very distinct geodynamic environments, that is the cold thermal regime associated with the foredeep and outer portion of the thrust belt in the east (both developed on top of the thick, thermally equilibrated Apulian lithosphere), and the hot peri-Tyrrhenian province, where the lithosphere has been strongly attenuated during the PlioQuaternary opening of the Tyrrhenian Sea (Fig. 6). As in the Southern Apennines, hot thermal regimes can develop in the hinterland of fold and thrust belts during or after the late stages of compression and foothills emplacement, in association with an asthenospheric rise. Roll-back of the subduction and slab detachment have thus been advocated as the two main processes accounting for back-arc extension, resulting in the lateral changes of lithosphere thickness and basal heat flow observed beneath the hinterland of the Carpathians (Pannonian Basin) and the Maghrebides– Apennines system as a whole (Algerian/Provenc¸al and Tyrrhenian basins), respectively. Corner flow of the asthenosphere induced by the subduction of the Pacific is advocated as the main mechanism for post-Laramide development of metamorphic core complexes, crustal extension and anomalous topography in the North American Cordilleran domain (Fig. 7; Basin and Range, Canadian Rockies, Mexican Sierras; Vanderhaeghe et al. 2003; Teyssier et al. 2005; Hardebol et al. 2007, 2009), as well as in the Altiplano in South America.

Timing of unroofing and hydrocarbon prediction beneath crystalline basement and ophiolitic units (input of AFT) Uncertainties in thermal modelling can be drastically reduced when the timing of unroofing of the allochthon can be reasonably constrained by low thermal geochronology (e.g. apatite fission tracks, AFT), as discussed below with two new case studies.

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AFT data provide absolute age–temperature paths, which can be in turn converted to palaeo-depths if assuming a palaeo-heat-flow. Conversely, they can also provide constrains on the palaeo-heat-flow when independent depth estimates can be made.

Impact of the crystalline basement of the Garzon Massif on the thermicity of footwall strata In the Middle Magdalena Basin in Colombia, the Cretaceous source rocks of the La Luna Formation have been locally overthrusted by the crystalline basement of the Garzon Massif, which constitutes the southernmost extent of the Eastern Cordillera (Fig. 8). Although the Garzon Massif is currently bald of Neogene sediments, it was part of the initial Late Cretaceous to early Miocene foredeep basin that extended east of the Central Cordillera, prior to the development of late Miocene to PlioQuaternary foreland basement uplifts and basin inversions which characterize the northwestern part of the South American craton (i.e. Merida Andes and Sierra de Perija in Western Venezuela and Eastern Cordillera in Colombia). Serial structural transects have been compiled from surface and subsurface data across the Magdalena Basin and adjacent Garzon Massif, and subsequently modelled with Thrustpack (coupled 2D kinematic and thermal modelling) in an attempt to check the 3D distribution of oil kitchens and drainage areas. Although well data allowed calibration of the heat flow in the Magdalena Basin itself, where no major erosion occurred, AFT data obtained by van der Wiel and Andriessen (van der Wiel 1991; van der Wiel & Andriessen 1991) from surface samples of the Garzon Massif were very critical to actually constrain the timing of the exhumation of this unit above the partial annealing zone (i.e. when the current topographic surface of the crystalline allochthon crossed the 110 8C isotherm). According to AFT, the Garzon Massif has been uplifted and eroded since about 6 million years ago, with a cumulative erosion of Mesozoic to Miocene series amounting to about 6 km. Results of thermal modelling are shown in both maps and sections, in order to better understand the incidence of lateral and vertical heterogeneities in the conductive properties of the rocks (Fig. 8). Surprisingly enough, there is a delay in the maturation of Cretaceous source rocks beneath the Garzon Massif compared to adjacent horsts. The high conductivities in the basement helped to (1) mature the source rocks faster above the palaeo-horsts than in adjacent grabens, and (2) drive the heat quickly toward the surface across the basement allochthon, thus keeping the maturity rank of underlying

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Fig. 6. (a) Structural and lithospheric-scale sections of the Southern Apennines (modified after Carminati et al. 2004), outlining the current crustal and lithosphere thickness, and lateral variations observed in the heat flow (changing from one HFU (Heat Flow Unit) in the Adriatic foreland with thick lithosphere, to up to three HFU in the Tyrrhenian side of the section, where thin lithosphere occurs). (b) Result of 2D thermal modelling along the same transect, outlining the current maturity ranks of the organic matter in Mesozoic platform carbonate units for both the underthrust Apulian Platform (lowermost unit of the tectonic pile) and the far-travelled Apenninic Platform (uppermost unit of the tectonic pile).

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BASIN MODELLING TOOLS FOR HYDROCARBON AND RESERVOIR PREDICTION 99

Fig. 7. Erosional profile along the Banff–Calgary transect, recording the effect of post-Laramide asthenospheric rise and related thermal doming and unroofing of the former orogen. (a) present distribution of vitrinite reflectance (Ro) data; (b) thickness of eroded sediments derived from 1D modelling of selected wells.

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Fig. 8. Serial 2D result sections and maps derived from forward kinematic and petroleum modelling in the Middle Magdalena basin, outlining the effects of high conductivities of the Garzon crystalline allochthon on the maturity pattern of underthrust series (after Roure et al. 2005a).

sediments. This maturity rank indeed displays lower values beneath the Garzon Massif.

Impact of the Mirdita Ophiolite on the thermicity of footwall strata Well-known petroleum provinces extend in the foothills of the Outer Albanides and in the Arabian foreland of the Oman Range, two orogens which

are famous by the widely exposed ophiolitic units of their hinterlands, that is the Mirdita Ophiolite in Albania, and the Semail Ophiolite in the United Arab Emirates and Oman, respectively. However, companies have been reluctant until now to explore close or even beneath these palaeo-oceanic units, because of the uncertainties on the maturity ranks reached by the organic matter in the footwall units. For instance, most ophiolitic belts are

BASIN MODELLING TOOLS FOR HYDROCARBON AND RESERVOIR PREDICTION

underlain by a metamorphic sole made up of high pressure blueschists, amphibolite or greenschists, the age of this metamorphism being commonly much older than the last episodes of thrusting. Unexpectedly, and despite being dominantly made up of peridotite and gabbros, ophiolites can provide good lithotypes for AFT dating when they host oceanic plagiogranites, whose study provides a direct control on the timing of exhumation. In Albania, HC exploration is currently focused on oil-bearing Late Cretaceous to Paleocene carbonate turbidites of the Ionian Basin near the Vlora– Elbasan lineament, as well as on biogenic gas and thermogenic condensates hosted in Neogene sandstone reservoirs of the Peri-Adriatic Depression (Fig. 9). Alternatively, seismic imagery has also identified deep Mesozoic plays in sub-thrust duplexes involving the Mesozoic platform-to-basin transition, beneath the shallower Cretaceous platform allochthon of the Kruja thrust sheet, just west of the Mirdita Ophiolite. Furthermore, oil seeps are common in the Kruja Zone, and Thrustpack and Ceres modelling still predict the occurrence of active oil and gas generation along the eastern border of the Peri-Adriatic Depression and Ionian Basin, at the boundary between the Outer and Inner Albanides (Vilasi et al. 2009). The main challenge for the exploration in Albania remains to predict the real HC potential of sub-thrust platform and basinal units which are currently still stacked in the hinterland, east of the Kruja Zone, beneath the ophiolite, which constitutes the topmost unit of the tectonic pile. The work done recently by Muceku et al. (2007) in Albania demonstrates that the Mirdita Ophiolite, which was first obducted during the Mesozoic (post-obduction Cretaceous piggyback basin), was already deeply eroded at the time of its final thrusting on top of the Outer Albanides during the Neogene. Thus, the Mirdita Ophiolite unit had only a very limited impact on the burial and thermal evolution of underlying source rocks. The AFT ages obtained by Muceku et al. have been plotted on a regional structural transect across the Albanides (Fig. 9). The AFT data provide a Palaeogene age for the unroofing, demonstrating that the Mirdita unit was already at temperatures lower than 110 8C in late Oligocene and early Miocene times, when the Mesozoic and Palaeogene units of the Ionian Basin and Peri-Adriatic Depression became underthrust beneath the allochthon. These data would also provide an invaluable control when modelling further the maximum palaeo-burial and palaeo-temperatures reached by the lower plate at the time of the Mirdita emplacement over the Outer Albanides. Seemingly, the young AFT ages evidenced by Muceku east of the Mirdita Ophiolite, in the vicinity

101

of a tectonic window already described by Collaku et al. (1990), provide another direct chronological control on the timing of underplating or out-ofsequence duplexing in the lower plate. This AFT age indeed records the time when the sole thrust of the allochthon was refolded by the development of sub-thrust anticlines in the lower plate, the latter being coeval with the age of the deformation front in the foothills. As already evidenced for the maturity ranks of the Cretaceous source rocks beneath the Garzon Massif in Colombia it is not unlikely that high thermal conductivities in fractured peridotites and volcanic rocks could have induced a rapid heat transport toward the surface, thus resulting in a delay in the maturation of underlying units, an effect similar to the heat pumping of salt diapirs, which is discussed below.

Incidence of salt redistribution on further hydrocarbon prediction Recent oil discoveries have been made in the deep offshore of passive margins in the US part of the Gulf of Mexico and West Atlantic margin in Brazil, beneath thick salt accumulations. Surprisingly enough, hydrocarbon fluids are still present in these ultra-deep reservoirs located in the distal portions of passive margins, close to the continent– ocean transition. As evidenced in the Colombian study, high conductivities of the salt bodies are likely to efficiently drive out the heat to the surface, and to induce a relative delay of source rock maturation beneath the salt, compared to what would be expected beneath a similar thickness of less conductive siliciclastics blanketing the sediments below (O’Brien & Lerche 1987). As in passive margins, the remobilization and spatial redistribution of salt in fold and thrust belts make it challenging to calibrate the thermal models, because of the strong discrepancy observed between (1) maturity records of the organic matter measured in the wells, which were eventually achieved before the regional redistribution of the salt, and (2) the current distribution of BHT and geotherms, which accounts for the present salt distribution. The same problem actually impacts geothermal gradients measured at the crest of salt diapirs, as they cannot be extrapolated laterally to adjacent synclines, the latter being most likely devoid of salt. In FTB a good example of such salt-related problem is provided by the Salt Range and Potwar Basin in Pakistan, where the Infra-Cambrian Salt has been greatly redistributed during the Miocene. The salt series probably was relatively isopachous at the time of deposition, and remained undeformed until the end of the deposition of the passive margin sequences, from the lower Palaeozoic to the Eocene, all these

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Fig. 9. Structural section across the Inner Albanides, with plot of AFT data obtained by B. Muceku et al. (2007). Location map (after Albpetrol 1995). (a) Structural and restored section across the Peri-Adriatic Depression and Kruja foothills, west of the Mirdita Ophiolite, based on seismic interpretation and Locace restoration. Notice the lateral thinning of Mesozoic carbonates from the Kruja Platform in the east towards the Ionian Basin to the west. (b) Eastern continuation of the same transect across the Mirdita Ophiolite, and current distribution of AFT data. Notice that tectonic duplexing in the lower plate post-dates the thrust emplacement of the Kruja units and main erosion phase of the ophiolite.

BASIN MODELLING TOOLS FOR HYDROCARBON AND RESERVOIR PREDICTION

layers being isopachous in seismic profiles, with no record of syndepositional halokinesis. The first compressional episode was synchronous with the deposition of the Neogene Siwalik molasse, resulting in thin-skinned tectonics. This early deformation episode accounts for the thrust emplacement of the Salt Range at the southernmost limit of the former salt basin during the late Miocene, with up to 20 km of southward displacement of the allochthon, and for the local development of salt pillows within the Potwar Basin further north (Fig. 10). The structural style associated with the Plio-Quaternary deformation was instead dominantly thick-skinned, with the development of pop-up structures and fish tails in the sedimentary cover, accommodating the local transpressional inversion of underlying infra-salt normal faults and grabens (Fig. 10; Roure 2008). This tectonic agenda is consistent with the maturity rank (Tmax) of the organic matter in the infraCambrian series of the Salt Range, which are indeed still immature because this frontal structure behaved as a growth structure during the late Miocene. Early tectonic accretion and uplift prevented this part of the basin becoming deeply buried beneath the Siwalik series. It agrees also with the magneto-stratigraphic dating of growth strata around the surface anticlines of the Potwar Basin, which are Pliocene–Quaternary in age (Burbank & Johnson 1982; Burbank 1983; Burbank & Beck 1989), thus younger than the c. 10 Ma old emplacement of the Salt Range (Zeilinger et al. 2007). Both 1D (Genex) and 2D (Thrustpack) thermal modelling have been performed on various regional transects crossing both the Potwar Basin and the Salt Range, dominantly oil, province. Although infraCambrian source rocks amounting up to 20% of TOC (Total Organic Carbon) occur in the Salt Range, where they are still immature, oil-source rocks correlations demonstrate that most if not all of the oils discovered in the Potwar Basin derive from younger Palaeogene sources associated with the Paleocene Patala Shale and possibly also locally with Eocene carbonates. The timing of petroleum generation can easily be derived from geological observations. Because the Patala shale is still immature or at the onset of the oil window in the cored Pliocene–Quaternary anticlines, oil generation is assumed to have occurred in adjacent synclines, which recorded increasing burial during the growth of the anticlines, oil migration and trapping being obviously post-Miocene. This latter knowledge is important when addressing the thermal modelling, because all the BHT data come from anomalous points (anticlines), where the presently high geothermal gradient only relates to the recent thickening of the salt layer in the core of Pliocene–Quaternary anticlines. If misused by direct

103

lateral extrapolation over the entire basin, these high heat flow values would result in the prediction of a late gas potential (Fig. 10; overmaturation of the Paleocene source rocks). Instead, calibration of fictive 1D wells or 2D Thrustpack modelling is more accurate when using solely the maturity ranks of the Patala Shale measured in the same wells, because it relates to frozen maturities achieved prior to the tectonic uplift, at a time the geothermal gradients were still low and equal over the entire basin (i.e. before the salt remobilization).

Use of hydrocarbon-bearing fluid inclusions in palaeo-burial reconstructions Th (homogenization temperature) measurements in syngenetic fluid inclusions in minerals are mainly used in reservoir studies to estimate the minimal temperature of diagenetic fluids at the time of cementation of fractures in carbonate reservoirs and of development of quartz-overgrowth in sandstone reservoirs. Therefore this temperature should be corrected by a factor relative to the composition of the fluid and the pressure at time of fluid entrapment (Roedder 1984). This correction may be small and then is often neglected in basin modelling of petroleum occurrences because of low salinities aqueous systems (0– 3 wt%) with high CH4 in solution and relatively low pressure values (300 bars) attained in sedimentary basins. In foothills areas, using Th data only provides valuable information for calibrating petroleum modelling at different scales when pressure is high and tectonically dependent and basinal fluids are involved. However, the minimum palaeo-temperature reached by a given sample does not tell directly when this temperature was reached, nor at which palaeo-burial, making the pressure estimate and then the correction factor unknown, error bars in temperature being likely to exceed several tens of degrees Celsius. Fortunately, in some cases it is possible to get accurate constraints for calibrating a basin model. Because of great immiscibility of oil and aqueous phases, aqueous inclusions can develop synchronously (i.e. at the same pressure and temperature) with hydrocarbon-bearing inclusions,within the same fluid inclusion generation in cements, thus providing a means for solving both the palaeo-temperature and palaeo-pressure of the fluids circulating in the studied reservoir at the time of cementation. The technique applied to derive temperature (T) and pressure (P) values from these two types of fluid inclusions in the same set relies on the different thermodynamic properties of the two fluids. PT isochoric modelling (PT evolution when keeping the volume unchanged) can be addressed, provided density and composition of aqueous and hydrocarbon

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Fig. 10. 2D thermal modelling in the Salt Range–Potwar Basin, outlining the effect of lateral and vertical salt redistribution on temperature profile and overall maturity distribution in potential source rocks. (a) Location map; (b) Temperatures v. burial curves of the Eocene carbonate reservoir for the low heat flow hypothesis (full line) and high heat flow hypothesis (dashed line), respectively; (c) Top section (low heat flow hypothesis) matches the maturity rates of the organic matter; Bottom section (high heat flow hypothesis) was built to match present BHT in available wells. In the latter hypothesis, the resulting maturities are erroneous, because BHT are strongly influenced by salt pillows beneath the anticlines, and are not representative of the regional thermicity.

BASIN MODELLING TOOLS FOR HYDROCARBON AND RESERVOIR PREDICTION

phases can be well defined individually by joint micro-thermometry and FTIR (Fourier Transform Infra-Red spectrometry) in situ analysis (Guilhaumou & Dumas 2005). For a specific composition and density, the intersection of the hydrocarbon isochore at the aqueous fluid homogenization temperature or in some case (low dissolved CH4) with the aqueous isochore (Thiery et al. 2006), provides an accurate estimate of both pressure and temperature at the time of fluid inclusion trapping and then T and P values of the natural system at the time of crystallization/cementation (Guilhaumou et al. 2004; Ferket et al. 2010). Two applications of this integrated technique are described below.

Palaeo-burial reconstruction of hydrothermal karsts Fluorite deposits occur in karstified platform carbonates of many FTB, for example in Baluchistan (Koh-i-Maran, in the Khirtar Range, Western Pakistan; Fig. 11a) and in Tunisia (Hamman Zriba, North –South Axis; Fig. 11b). Mesozoic carbonates hosting fluorite and MVT (Mississippi Valley Type) ore deposits are directly overlain by thick shale series. Early interpretations consider the latter as unconformable above a palaeo-emersion surface. When studied in detail however (Guilhaumou et al. 2000, 2004; Benchilla et al. 2003), these two sequences do not show any evidence of emersion, thus precluding the former assumption that these karsts were meteoric in origin. Integrated studies coupling petrography with the study of fluid inclusion and basin modelling have now demonstrated that both karst development and fluorite deposition occurred at the time these carbonates were deeply buried beneath the overlying seals, that is during a much younger episode of squeegee expulsion of basinal fluids at the onset of foothill development and regional tilting of the foreland basin, and not during an emersion event occurring during the passive margin stage. As a matter of fact, these fluorite deposits are stratiform, and located at the top of porous carbonates which probably helped channelizing the basinal fluids when flushed updip the foreland flexure. Assuming that in both cases the circulating fluids were thermally equilibrated with the overburden (which would not be the case if fluorite occurred in vertical conduits such as faults and fractures), and that the pore-fluid pressure was close to hydrostatic (no evidence of hydraulic fractures), T and P values derived from the crossing of the two isochores of aqueous and oil-bearing fluid inclusions hosted in the fluorite of these ore deposits can be considered as representative of the palaeo-temperature and palaeo-burial of the host carbonate reservoirs at the time of the squeegee episode of fluid circulation.

105

The use of fluid inclusions for constraining the architecture of the Laramide flexure in Eastern Mexico The Cretaceous platform carbonates of the Co´rdoba allochthon, in the southeasternmost part of the North American Cordillera in Mexico, are almost bald of any synflexural nor synorogenic sediments. Only limited outcrops document the gradual changes from shallow water Cenomanian carbonates towards deepwater Late Cretaceous–Paleocene turbidites. The initial thickness of these flysch deposits is unknown. Furthermore, seismic data document a presently east-dipping attitude of the crystalline basement beneath these allochthonous Mesozoic carbonates (Fig. 12a). The forelandward dip presently observed in the autochthon is quite surprising, because in other segments of the Cordillera, such as the Canadian Rockies, the underthrust foreland still preserves an overall west-dipping attitude, inherited from the former Laramide flexure. Only a few Tmax and Ro data were available in the allochthon due to the lack of organic-rich outcrops. Diagenetic quartz could ultimately be identified in cemented fractures of the carbonate and used as a palaeo-thermo-barometer, because this single mineral turned out to contain both sets of synchronous aqueous and hydrocarbon inclusions. Unexpectedly, the P –T path derived from isochores documents a few kilometres of unroofing of the Cretaceous carbonate platform, which is best explained by the post-Laramide erosional removal of a similar thickness of Late Cretaceous – Paleocene synflexural flysch (Fig. 12c, d). When projected to its pre-orogenic configuration, assuming an initial horizontal top surface at the onset of the Eocene Laramide thrust emplacement, the restoration of this currently missing flexural sequence in turn requires generation of a coeval space at basin scale to accommodate such sedimentary thickness at the top of the well-known carbonate sequence, which is best explained by assuming an initial westdipping configuration of the foreland (Fig. 12b).

Use of calcite twins for palaeo-burial reconstructions For many years calcite twins have been considered among the most common stress –strain markers in fold-and-thrust belts. Calcite twin analyses have been widely used to constrain both the structural and kinematic evolution of thrust belts, but also recently to derive differential stress magnitudes during deformational events. Attention has been particularly focused on the estimates of palaeodifferential stresses at the onset of folding (layer parallel shortening, LPS) and during late stage

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Fig. 11. Cross sections and PVT modelling of Baluchistan and Tunisia (modified after Benchilla et al. 2003). Left column ¼ Pakistani case study; Right column ¼ Tunisian case study. In both cases, a kinematic model is provided, including pre-inversion, inversion and present stages. PVT modelling from fluid inclusions in fluorite deposits exposed at the surface (bottom diagrams) indicate these ore deposits developed at the onset of basin inversion, when the host reservoirs where still deeply buried beneath efficient seals.

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Fig. 12. Fluid inclusions data and PVT modelling as constraints for palaeo-burial reconstructions along a regional transect across the Cordoba Platform (Eastern Mexico; modified after Roure et al. 2009a; Ferket et al. 2003, 2010). (a) Top section: Present architecture of the transect, with an east-dipping attitude of the basement. (b) Central section: Laramide deformation stage, the basement being restored to accommodate the 4.5 km of Late Cretaceous– Paleocene flexural sequence, which have been subsequently removed by erosion, but are required to account for the PVT modelling of fluid inclusions taken from cements at the top of the Cretaceous platform carbonates in the inner (western) part of the section. (c) Burial v. depth plot of Mesozoic carbonates in eroded anticlines (indicated by a white circle in the sections). (d) Bottom: results of the PVT modelling on fluid inclusions from Mesozoic carbonates.

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fold tightening. An important result is that at the scale of individual structures, differential stresses recorded by rocks were largely dependent on the palaeo-burial before folding and on subsequent erosional history (e.g. Lacombe 2001). A new method to estimate maximum palaeoburial and subsequent uplift by folding in foldand-thrust belts, based on calcite twin analysis, has recently been proposed (Lacombe et al. 2009). This method basically combines estimates of differential stresses related to LPS with the hypothesis that stress in the upper continental crust is primarily in frictional equilibrium (Townend & Zoback 2000; Lacombe 2007). In the Albanian foreland taken as a case study (Figs 9 & 13a), calcite twin analysis provides reliable constraints on both the early stages of the tectonic history of the thrust belt, including development of pre-folding vein systems currently observed in folded strata, and on the amount of maximum burial of the Cretaceous foreland rocks during flexural subsidence and on their subsequent uplift during Neogene folding.

Summary of the method for determining of palaeo-stress orientations, differential stress magnitudes and palaeo-burial using inversion of calcite twin data Mechanical e-twinning readily occurs in calcite deformed at low temperature (Fig. 13b). Calcite twinning requires a low critical resolved shear stress (CRSS) of 10 + 4 MPa which depends on grain size and internal twinning, and has only a small sensitivity to temperature, strain rate and confining pressure, therefore fulfilling most of the requirements for palaeo-piezometry (Lacombe 2007). Basically, the principle of the stress inversion technique used herein (Etchecopar 1984; refer to Lacombe 2001 for details) consists of finding the stress tensor that best fits the distribution of twinned and untwinned planes (the latter being those of potential e-twin planes that never experienced a resolved shear stress of sufficient magnitude to cause twinning). The orientations of the principal stresses are calculated, together with the stress ellipsoid shape ratio and the peak differential stress (s1–s3). It is to date the only technique that computes simultaneously principal stress orientations and differential stress magnitudes from a set of twin data, therefore allowing us to relate unambiguously differential stress magnitudes to a given stress orientation and stress regime. Numerous studies have demonstrated its potential to derive regionally significant stress patterns, even in polyphase tectonics settings (e.g. Lacombe et al. 1990; Rocher et al. 1996; and references therein).

The hypothesis of crustal frictional stress equilibrium implies uniform stress differences at a given depth and for a given stress regime; regardless of the ‘intensity’ of deformation, the style of deformation is probably simply a function of the strain rate. The strength of the continental crust down to the brittle–ductile transition is therefore primarily controlled by frictional sliding on well-oriented preexisting faults, with frictional coefficients of 0.6– 0.9 and under hydrostatic fluid pressure (Townend and Zoback 2000). One can therefore draw the curves of differential stress values as a function of depth in a crust in frictional equilibrium for both strike-slip (SS) and reverse faulting (R) stress regimes, with values of l [l ¼ Pf/rgz where Pf is the pore-fluid pressure, r the density of the overlying rocks, g the acceleration of gravity and z the depth] of 0.38 (hydrostatic) and 0 (dry) and for friction coefficient m values of 0.6 and 0.9 (Lacombe 2007, Fig. 13c). Peak differential stress values are estimated from calcite twin analysis, and the principle of the method consists in reporting these values on the above-mentioned curves to derive the probable range of depths at which twinning occurred. If differential stresses are related to LPS that reflects the onset of stress build-up in horizontal strata and was likely recorded at the maximum burial just before the onset of folding, this allows us to infer the probable maximum range of depths at which rock samples recorded LPS twinning strain before becoming uplifted by folding.

Geological setting of the Albanides The Albanides are a branch of the Alpine orogenic belt, which can be subdivided into an eastern internal zone and a western external zone (Mec¸o & Aliaj 2000; Nieuwland et al. 2001). The internal Albanides consist of thick-skinned thrust sheets with ophiolites in the Mirdita Zone (see the section above on the Impact of the Mirdita Ophiolite on the thermicity of footwall strata). The external Albanides comprise the Krasta –Cukali, the Kruja and the Ionian Zones (Velaj et al. 1999; Mec¸o & Aliaj 2000) (Fig. 9). During the Alpine orogeny, the Albanian foothills formed as a consequence of the deformation of the former eastern passive margin of Apulia; the external zones were overthrust during the Neogene (Roure et al. 2004). Tectonic loading applied by the hinterland (Mirdita Ophiolite, see above) and westward thrusting of basinal units of the Krasta Zone induced the progressive development of a wide flexural basin, which ultimately impacted the Outer Albanides lithosphere in late Oligocene times. Growth anticlines started to develop in late Oligocene–Aquitanian times in the Ionian Basin. This main episode of shortening ended before

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Fig. 13. Use of calcite twins in palaeo-burial reconstructions. (a) Geological section across the Saranda anticline (modified after Roure et al. 1995; see location in Fig. 9). (b) Example of twinned calcite crystal observed in vein. (c) LPS-related differential stress values determined from calcite twins reported on stress/depth curves built for a crust in frictional stress equilibrium (Lacombe 2007), and derived palaeo-burial values of Cretaceous limestones in Saranda. Labels 1 and 2 (e.g. AL05-1, AL26-2) refer to stress estimates obtained from subsets of twin data collected in grains of homogeneous sizes, while others were obtained from the whole twin dataset of the sample (for details, refer to Lacombe et al. 2009).

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deposition of Langhian –Serravallian clastic rocks. Collisional deformation probably reached the outermost parts of the Albanides by early to middle (?) Miocene times. A second episode of tectonic shortening is documented near the Vlora–Elbasan transfer zone and further north in the Peri-Adriatic Depression, where Pliocene out-of-sequence thrusts and backthrusts offset a pre-Messinian erosional surface. The Saranda anticline is the outermost fold of the southern external Albanides (Figs 9 & 13a). This anticline has an asymmetric structure with a subvertical eastern flank. Two main vein sets were identified in Cretaceous limestones. The first vein set, oriented c. N1408, likely developed in response to the flexure of the foreland in front of the advancing thrust sheets, contemporary with burial and possibly under high fluid pressures (Lacombe et al. 2009). The second set (II), oriented c. N0608, is closely associated with LPS stylolites and marks the trend of the regional compression. Advantage has been taken of the widespread occurrence of these prefolding vein sets to collect calcite twin data (Fig. 13b) and to characterize stress orientations and differential stress magnitudes related to LPS.

Palaeo-stresses and palaeo-burial in the Saranda anticline Calcite twinning from vein sets consistently recorded pre-folding, NE–SW-directed regional compression. Since LPS reflects the onset of stress build-up in horizontal strata, LPS-related twin strain was recorded at the maximum burial just before the onset of folding. As a result, reporting differential stress magnitudes on the stress –depth curves (Fig. 13c) reveals that in Saranda, the maximum burial of the Cretaceous limestones was about 1.5–5 km, with a mean weighted value of around 4 + 1 km (Fig. 13c). This c. 4 km maximum palaeo-burial value is consistent with independent palaeo-burial estimates from stratigraphy, maturity rank of organic matter, palaeo-temperature/palaeogeothermal gradients from fluid inclusions and predictions of kinematic modelling of the Albanian foreland (Lacombe et al. 2009).

Conclusions The new method for estimating maximum palaeoburial and subsequent uplift by folding in foldand-thrust belts, based on calcite twin analysis, basically combines estimates of LPS-related differential stresses with the hypothesis that crustal stress is in frictional equilibrium. The limits of this approach have been discussed in Lacombe et al. (2009). Palaeo-depth values inferred from

LPS-related differential stresses yield an upper bound for burial and constrain the amount of subsequent exhumation/vertical movement. Palaeoburial estimates from post-folding stress tensors may place additional constraints on the depth of rocks when folding ended, and, therefore, on the exhumation path of these rocks toward the surface. A major interest of this method is that it can potentially be carried out in any fold and thrust belt where twinned calcite occurs. In the absence of other palaeo-depth indicators, this method will provide valuable constraints on the amount of burial of foreland rocks during flexural subsidence and of their subsequent uplift during folding, thus leading to a better quantification of vertical movements in forelands, even where subsurface data are not sufficient to build a well-constrained geological section (e.g. Saranda). A way to reduce the range of uncertainties on palaeo-stress/palaeo-burial estimates is to combine calcite twinning palaeo-piezometry with the systematic analysis of fluid inclusions, which allows derivation of the pore pressure from the fluid density or inference of the value of the vertical stress assuming hydrostatic conditions (see section above on Use of hydrocarbon-bearing fluid inclusions in palaeo-burial reconstructions). Estimates of palaeoburial and, therefore, of palaeo-depth of deformation in fold-and-thrust belts from calcite twins should therefore be combined in the future with the systematic use of palaeo-thermometers such as vitrinite reflectance, illite crystallinity, or fluid (mixed hydrocarbon/aqueous) inclusions coupled with numerical modelling of the thermal evolution of tectonic units.

Overall conclusions Major errors can be made in predicting the palaeoburial, thermicity and hydrocarbon potential of foothill areas if the rates and amounts of erosion cannot be properly estimated, and when major lateral changes in crustal/lithospheric thicknesses occurred in the foreland and in the hinterland during either the initial, passive margin episodes, or younger post-orogenic stages of slab detachment and asthenospheric rise. Conversely, fission track data, fluid inclusions micro-thermometry,magneticfabric,diageneticstudies and calcite twin analyses become increasingly useful for petroleum exploration when moving from passive margins and foreland basins, where sedimentary burial is more or less continuous, toward foreland fold and thrust belts, where large amounts of sometimes localized erosion occurred. In such settings, more analytical work becomes required to decrease the error bars during basin modelling. In the future, pioneer analytical techniques such as the ‘clumped isotopes’ or D47 (based on the

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molecular combination of the isotopes of 13C and 18O in the CO2 molecule which occur at masses 47 to 49; Eiler 2007) and Raman spectrometry of the organic matter are likely to provide other sensitive palaeothermometers for a range of temperatures of direct interest for sedimentary basins. Whereas clumped isotope geochemistry is likely to measure the temperature of crystallization of calcite and dolomite cements when lower than 120 8C, provided no further burial increase occurred (Ghosh et al. 2006, 2007; Eiler 2007), HRTM (High-Resolution Transmission Electro-Microscopy) and Raman micro-spectrometry can be used for ranking the organic matter evolution between 330 and 650 8C (Beyssac et al. 2002; Lahfid 2008; Lahfid et al. 2008). Most case studies developed in this paper have shown that the same overall dewatering processes known from offshore accretionary wedges still operate in FTB (i.e. squeegee episodes of tectonically induced fluid flow associated with tectonic compaction and pressure-solution mechanisms). These LPS episodes thus dramatically control the overall reservoir quality in both carbonate and sandstone units of the allochthon as well as the underthrust foreland. In contrast, gravitational (topography-driven) fluid flow does not have much impact on the fluid flow history of most productive reservoirs, except for tar belts such as the Athabasca sandstones (Alberta) and Orinoco belt in Venezuela (Faja Petrolifera), and possibly also in Papua New Guinea, known for its very active hydrodynamic regime, because hydrocarbon accumulations are usually mostly confined beneath the regional permeability barriers developing during the flexural evolution of the foreland. Domenico Grigo provided helpful comments on the initial version of the manuscript. Carlo Doglioni provided a high resolution file for Figure 6.

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Roure, F., Swennen, R. et al. 2005b. Incidence and importance of tectonics and natural fluid migration on reservoir evolution in foreland fold-and-thrust belts. In: Brosse, E et al. (eds) Oil and Gas Science and Technology-Revue de l’IFP, 60, 67–106. Roure, F., Alzaga, H. et al. 2009a. Long lasting interactions between tectonic loading, unroofing, postrift thermal subsidence and sedimentary transfer along the Western margin of the Gulf of Mexico: Some insights from integrated quantitative studies. In: Bertotti, G., Frizon de Lamotte, D. & Teixell, A. (eds) Tectonics of Vertical Movements, Special Issue, Tectonophysics, in press. Roure, F., Cloetingh, S., Scheck-Wenderoth, M. & Ziegler, P. 2009b. Achievements and challenges in sedimentary basins dynamics. In: Cloetingh, S. & Negendank, J. (eds) New Frontiers in Integrated Solid Earth Sciences. Springer, 145– 233. Saint-Bezar, B., He´bert, R. L., Aubourg, C., Robion, P., Swennen, R. & Frizon De Lamotte, D. 2002. Magnetic fabric and petrographic investigation of hematite-bearing sandstones within ramp-related folds: examples from the South Atlas front (Morocco). Journal of Structural Geology, 24, 1507– 1520. Sassi, W. & Rudkiewicz, J. L. 2000. Computer modelling of petroleum systems along regional cross-sections in foreland fold-and-thrust belts. EAGE Conference, Malta 2000, extended abstract. Schneider, F. 2003. Basin modelling in complex area: examples from eastern Venezuelan and Canadian foothills. Oil and Gas Science and Technology, Revue de l’IFP, 58, 313–324. Schneider, F., Devoitine, H., Faille, I., Flauraud, E. & Willien, F. 2002. Ceres2D: a numerical prototype for HC potential evolution in complex areas. Oil and Gas Science and Technology, Revue de l’IFP, 57, 607– 619. Sciamanna, S., Sassi, W., Gambini, R., Rudkiewicz, J. L., Mosca, F. & Nicolai, C. 2004. Predicting hydrocarbon generation and expulsion in the Southern Apennines thrust belt by 2-D integrated structural and geochemical modelling. Part I- Structural and thermal evolution. In: Swennen, R., Roure, F. & Granath, J. (eds) Deformation, Fluid Flow and Reservoir Appraisal in Foreland Fold and Thrust Belts. American Association of Petroleum Geologists, Hedberg Series, 1, 51–68. Smith, T. 2008. Salt’s effect on petroleum systems. Geo Expro. Website: www.geoexpro.com/geoscience/salt Summa, L. L., Goodman, E. D., Richardson, M., Norton, I. O. & Green, A. R. 2003. Hydrocarbon systems of Northeastern Venezuela: plate through molecular-scale analysis of the genesis and evolution of the Eastern Venezuela Basin. Marine and Petroleum Geology, 20, 323– 349. Swennen, R., Roure, F. & Granath, J. (eds) 2004. Deformation, fluid flow and reservoir appraisal in

foreland fold and thrust belts. American Association of Petroleum Geologists, Hedberg Series, 1, 430. Teyssier, C., Ferre´, E., Whitney, D. L., Norlander, B., Vanderhaeghe, O. & Parkinson, D. 2005. Flow of partially molten crust and origin of extensional detachments during collapse of the Cordilleran orogen. In: Bruhn, D. & Burlini, L. (eds) High Strain Zones Structure and Physical Properties. Geological Society, London, Special Publication, 254, 39– 64. Thiery, R. 2006. Thermodynamic modelling of aqueous CH4-bearing fluid inclusions trapped in hydrocarbon rich environments. Chemical Geology, 227, 154– 164. Townend, J. & Zoback, M. D. 2000. How faulting keeps the crust strong. Geology, 28, 399–394. Trave´, A., Labaume, P. & Verge´s, J. 2007. Fluid systems in foreland fold-and-thrust belts: an overview from the Southern Pyre´ne´es. In: Lacombe, O., Lave´, J., Roure, F. & Verge´s, J. (eds) Thrust Belts and Foreland Basins. Frontiers in Earth Sciences, Springer, 93–116. Van Der Wiel, A. M. 1991. Uplift and volcanism of the SE Colombian Andes in relation to Neogene sedimentation of the Upper Magdalena Valley. PhD thesis, Wageningen, the Netherlands, 208. Van Der Wiel, A. M. & Andriessen, P. 1991. Precambrian to Recent thermotectonic history of the Garzon Massif (Eastern Cordillera of the Colombian Andes) as revealed by fission track analysis. In: van der Wiel, A. M. (ed.) Uplift and Volcanism of the SE Colombian Andes in Relation to Neogene Sedimentation of the Upper Magdalena Valley. PhD Thesis, Wageningen, the Netherlands. Vanderhaeghe, O., Medvedev, S., Fullsack, P., Beaumont, C. & Jamieson, R. A. 2003. Evolution of orogenic wedges and continental plateaus: insights from thermal–mechanical models with subduction basal boundary conditions. Geophysical Journal International, 153, 1 –25. Velaj, T., Davison, I., Serjani, A. & Alsop, I. 1999. Thrust tectonics and the role of evaporites in the Ionian Zone of the Albanides. American Association of Petroleum Geologists Bulletin, 83, 1408–1425. Vilasi, N., Malandain, J. et al. 2009. From outcrop and petrographic studies to basin-scale fluid flow modelling: the use of the Albanian natural laboratory for carbonate reservoir characterization. In: Cloetingh, S. A. P. L., Ziegler, P. A. & Bogaard, P. J. F. et al. (eds) TOPO-EUROPE: The Geoscience of Coupled Deep Earth-Surface Processes. Tectonophysics, 474, 367–392, doi: 10.1016/j.tecto.2009.01.033. Vrolijk, P. 1987. Tectonically driven fluid-flow in the Kodiak accretionary complex, Alaska. Geology, 15, 466–469. Zeilinger, G., Seward, D. & Burg, J. P. 2007. Exhumation across the Indus Suture Zone: a record of back sliding of the hanging wall. Terra Nova, 9, 425– 431.

Spontaneous fluid emissions in the Northern Apennines: geochemistry, structures and implications for the petroleum system ROSSELLA CAPOZZI* & VINCENZO PICOTTI Dipartimento di Scienze della Terra e Geologico-Ambientali, University of Bologna, Via Zamboni 67, 40127 Bologna, Italy *Corresponding author (e-mail: [email protected]) Abstract: Natural seeps in the Northern Apennines document a variability of fluids and reservoirs in terms of origin, age and evolution. Their spatial distribution appears controlled by the presence or absence of the tectonic overburden provided by the Ligurian nappe. The general trend of deepening of the Mesozoic basement toward the internal part of the thrust belt is reflected by the nature of the seeps, characterized by thermogenic methane and oil at the foothills, whereas the innermost seeps show occurrence of dry thermogenic gas suggesting overcooking of the residual oil. At the front of the Ligurian nappe, or in places never covered by it, the seepages are associated with biogenic methane related to bacterial degradation of the organic-rich intervals occurring in the Pliocene and Pleistocene marine succession. The coupling of geochemical and structural analysis allows reconstructing the tectono-thermal evolution of the belt, improving our knowledge on the processes acting within the reservoir and controlling important parameters of the petroleum system, such as the reservoir porosity and its modifications, and the migration patterns.

Hydrocarbon exploration methods traditionally include the study of different surface and nearsurface expressions of hydrocarbon leakage, as fluid seepages are known as indicators of hydrocarbon fields. In the last few decades, a number of cases have been reported in the literature, dealing with spontaneous fluid emissions described with different geophysical to geochemical techniques. Mud volcanoes and natural seepages occur in a wide range of geological and geodynamic settings, such as oceanic and continental thrust wedges, back- and fore-arc basins and passive margins. Moreover, the different sedimentary systems associated with these geodynamic contexts, play a major role in triggering fluid migration and mud diapirism, also governing the stability condition and morphologic evolution of lands and seafloors. The literature, concerning the identification of fluid migration in marine environments, shows a wide range of features, varying from hills and mounds to depressions and craters (e.g. Kaluza & Doyle 1996; Loncke, et al. 2004; Leo´n et al. 2006) that are linked to both seepage in shallow sediments and hydrocarbon-enriched fluid and gas which leaks from deeply buried sediments along the continental margins and slopes. On land exploration has been addressed to the nature of mud volcanism, with respect to degassing of deeply buried sediments (e.g. Dimitrov 2002 and references therein) or with respect to geological setting (e.g. Bonini 2007). The assessment of lithology of mud breccias (e.g. Akhmanov et al. 2003), and geochemical characters and isotopic signature of fluids and hydrocarbons,

has also been widely carried out (e.g. Capozzi & Picotti 2002; Kholodov 2002; Charlou et al. 2003; You et al. 2004; Picotti et al. 2007; Etiope et al. 2009). Some studies, in particular, were addressed to the geochemistry of the brines that allows definition of the main processes that occurred during burial, expulsion and migration of connate waters, and the possible contribution of meteoric waters to the system (e.g. White 1965; Hanor 1987, 1996, 2001; To´th 1996). The integration of information on the present bacterial activity and the thermal affinity of bacterial communities is locally useful for the characterization of the typology of the processes involved in fluid migration. In some papers, the geological setting and geometry of the reservoirs was investigated, highlighting structural and stratigraphic traps and their hydrological characteristics (Thrasher et al. 1996). According to White (1965), Hanor (1987) and Holysh & To´th (1996) waters of different provenance contribute to the ascending fluids and their geochemical characteristics can help in understanding their evolution during diagenesis and migration. The main challenge of our study is the evolutionary reconstruction of the main steps of the fluid expulsion/migration, based on the integration of various tectono-thermal and geochemical aspects. To achieve this goal we coupled geochemical study on hydrocarbons, saline waters and brines with structural interpretations of the subsurface geology, to fully characterize the tectono-thermal evolution of the petroleum system.

From: Goffey, G. P., Craig, J., Needham, T. & Scott, R. (eds) Hydrocarbons in Contractional Belts. Geological Society, London, Special Publications, 348, 115–135. DOI: 10.1144/SP348.7 0305-8719/10/$15.00 # The Geological Society of London 2010.

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Regional geological setting The Northern Apennines (Fig. 1) are a fold and thrust belt evolved during Tertiary times as a response to the subduction of the Adriatic plate under the European lithosphere. The flexuring of the foreland lithosphere since the Oligocene brought about deep subsiding foreland basins and thick foredeep successions, progressively incorporated within the thrust belt. A general SW dip of the crust characterizes the belt, with involvement of the Mesozoic source rock intervals to remarkable depth. The residual oceanic accretionary wedge, the so-called Ligurian unit, encroached onto the foredeep and travelled towards the foreland from the Oligocene to the early Pliocene as a surface nappe, overthrusting, as a rule, the youngest foredeep turbidites (Zattin et al. 2002). The recent tectonic activity overprints the thrust belt with high-angle normal faults cutting through most of the Apennines relief (Picotti et al. 2009). The shape of the mountain front was recently related to the activity of a deep thrust, deforming the whole stack of the nappes including upper Pleistocene to Holocene continental deposits (Picotti & Pazzaglia 2008).

Seeps sampling and analysis methods Cold seeps and mud volcanoes in the Northern Apennines are aligned approximately along two belts striking NW, located respectively near the main divide and along the foothills (Fig. 1). In the western sector, gas vents along the main divide lie close to the southern limit of the Ligurian unit (Fig. 1). Borgia et al. (1986) discussed the origin of many spontaneous fluid emissions mainly on the base of the isotopic composition of gases. The gases have been characterized as thermogenic, with d13C values higher than 250 ppt. In the same western sector, on the other hand, the foothill emissions, located at the northern border of the Ligurian unit appear mainly characterized by mixed gas (d13C 260, 250 ppt). Biogenic gas is considered as indigenous in the Plio-Pleistocene successions occurring at the foothills and adjacent Po Plain (Mattavelli et al. 1983; Borgia et al. 1986). In the Apennine foothills near Forlı`, which were never covered by the Ligurian unit, apart from a few exceptions (see Fig. 1), biogenic methane emissions occur within deformed upper Neogene foredeep units, which represent an uplifted portion of the main hydrocarbon system exploited in the eastern Po Plain. In some spontaneous fluid emissions, along the Northern Apennine foothills, the natural gas was characterized as mixed thermogenic and biogenic methane (Mattavelli et al. 1983; Borgia et al.

1986). This suggests the presence of a transitional boundary between a region with deep burial of the organic matter responsible for the thermogenic methane and a shallow buried region, where the bacterial fermentation of organic matter in the foredeep succession is still possible. We observe this boundary along the northeastern tip of the Ligurian nappe, that is therefore considered as the main origin of the overburden. We addressed our work on selected spontaneous surface seeps along the foothill belt, bearing cold saline waters associated mainly with gas and condensate, locally in the form of mud volcanoes. We also studied the Porretta thermal springs that, on the other hand, bear gas and low saline warm waters, as it is typical along the southern border of the Ligurian unit, near the main divide of the Northern Apennines. The studied emissions were characterized for their ion content, analysed at the laboratory of our department by means of ion chromatography and atomic absorption spectroscopy. The isotope analysis on waters was performed using a mass spectrometer at the CNR laboratory, Istituto di Geoscienze e Georisorse (Pisa). The isotope analysis on methane was performed at the ENI-Agip Division laboratory at San Donato Milanese. The depth interpretation of the structures was obtained by integrating the surface geology with subsurface data, such as wells and seismic lines.

Salsomaggiore Local geological setting In the area of Salsomaggiore, located between Parma and Piacenza provinces in the Emilia Region (Figs 1 & 2), spontaneous fluid emissions and drilled wells are very rich in high saline water, with minor methane and oil. The chemical composition of brines shows a high content of major ions that is unique among the known cold vents of Italy. The reservoir is a lower Miocene foredeep succession of turbiditic sandstones and sands folded during a middle Miocene thrusting event (Fig. 2, see Picotti et al. 2007). The sealing was subsequently provided by syntectonic hemipelagic marls. Following the exposure and abrasion of the thrust-top anticline, several chemioherms spread during the Tortonian (Conti et al. 2007), documenting active fluid leaking. A renewed subsidence pulse during the Messinian brought about the drowning of the anticline, that was covered with a huge olistostrome, detached from the tip of the Ligurian nappe and reaching the base of slope on the footwall of the thrust (see Fig. 2). The latter acted as seal until the Pliocene, when a reactivation of the structure led to the final erosional unroofing (see Fig. 2).

8°

10°

12°

45° PO RIVER

A FERRARA

PARMA REGGIO E.

B

MODENA BOLOGNA

C 44°

1 2 3 4 5 6 7 8

RAVENNA

D FORLI'

E wa t

er

0

km

GEOCHEMISTRY AND STRUCTURES OF APENNINES SEEPS

PIACENZA

div

ide

50 FIRENZE

117

Fig. 1. Geological sketch map and locations of the fluid emissions of the Northern Apennines alongside the Po valley (modified after Borgia et al. 1986). Legend: 1, Continental Quaternary; 2, marine to continental Plio-Pleistocene foredeep units at the foothills; 3, Miocene foredeep units; 4, Ligurian –Epiligurian units; 5, spontaneous fluid emissions and 6, oil and gas from surficial drilled wells; 7, thrust front in the subsurface; 8, traces of the sections: A: Salsomaggiore; B: Regnano; C: Castel San Pietro; D: Castrocaro; E: Porretta.

118 R. CAPOZZI & V. PICOTTI Fig. 2. Geological sketch map and cross sections along dip and strike of the Northern Apennines foothills in the area of Salsomaggiore (see Fig. 1 for location). Modified after Picotti et al. (2007). Purple colour indicates the main fluid migration pathway.

GEOCHEMISTRY AND STRUCTURES OF APENNINES SEEPS

Fluids When compared to seawater, the Salsomaggiore brines show a remarkable increase of chloride and calcium (Table 1), whereas Mg and SO4 are depleted or absent, respectively; however, H2S does not occur. The content of iron and NH4 is significant (see Table 1) as well as iodine (54 mg/L) and bromine (up to 245 mg/L). The ion content of the Salsomaggiore waters fit the range of values of ‘membrane-concentrate saline water’ and matches a depth of origin between 1000 and 4000 m (White 1965). Within connate saline waters, the increase of Na and Cl ions becomes significant in the case of water coming from intermediate depth, deeper than 700 m. Ca increases with depth and formation age, likely in association with sulphate reduction, due to bacterial activity (White 1965). The concentration of the different ions depends on membrane-filtering mechanisms (hyperfiltration) that, as described in the literature (e.g. White 1965; Hanor 1987), could account for an increase of water salinity with depth to up to ten times that of seawater. These changes in interstitial water chemistry reflect their interaction with solid phases during burial and with time. When subsurface waters move along steep hydraulic gradients through shales or clays, the matrix can act like a reverse-osmosis membrane, retarding the movement of larger molecules. A ten-mineral phase system, which reacts to a chloride enrichment, is most suitable to explain the interactions between water and solid phases within the Salsomaggiore siliciclastic reservoir (Hanor 1996). In this case, we assume that the system developed in the absence of CO2, as normal under subsurface conditions. In fact, the d2H and d18O values provide evidence that the analysed brines never mixed with meteoric water, documenting a system closed to the supply of CO2 from the surface. In the absence of CO2, calcite and dolomite no longer interact with the system and the alkalinity does not change. With high chloride content, quartz is mobilized: however, Si in the brines increases slowly, because it is incorporated into other mineral phases. In this system, anorthite partially dissolves and the calcium concentration increases in the solution, whereas K and Na do not increase because they are involved in the precipitation of K-feldspar and albite (Hanor 1996). This mass transfer between mineral phases can locally increase the porosity of the reservoir, as could be the case for Salsomaggiore. As sulphate is totally depleted, we would expect the occurrence of bacterially mediated sulphide. However, as sulphide no longer occurs, a possible subsurface equilibrium could have been reached. At the measured values of pH and alkalinity, a

119

relatively moderate sulphate reduction process probably occurred in the system and prevented the precipitation of calcium carbonate (Morse 2003). The total removal of H2S can be ascribed to the very high Fe content which can precipitate, in the measured range of pH and Eh, mainly as marcasite (Seemann 1987). NH4, bromine and iodine are likely due to the in situ reduction of abundant organic matter, even if we cannot establish the source of this from the water composition. However, the high NH4, bromine and iodine concentrations could only be supplied to the brine by the oil that characterizes the Salsomaggiore anticline. The hyperfiltration mechanism hypothesized to explain the high ion contents in the Salsomaggiore brines, is probably the more suitable mechanism that can account for the enrichment in Cl2, and for isotopic composition of 18O and 2H (Fig. 3 and Table 2). Isotopic measurements of brines in sedimentary basins typically show a trend toward d18O enrichment with minor d2H enrichment (Clayton 1966; Clark & Fritz 1997). d2H enrichment in basinal brines can be partially explained by dehydration of clay minerals, but can also be generated through exchange with H2S or hydrocarbons that, in turn, can impart a positive shift in d18OH2O (Clark & Fritz 1997). Horita (2005) documents that water –rock interaction at temperatures ,100 8C mainly leads to a depletion in d18OH2O, whereas at T .100 8C the interaction brings to an increase of the d18OH2O, a result similar to the hyperfiltration mechanism. Since the Salsomaggiore brines are deriving from a reservoir that never experienced T . 100 8C (see later), the recorded high values of d18OH2O can more likely be interpreted as the result of a hyperfiltration mechanism. The Salsomaggiore oil compositional characters were grouped by Riva et al. (1986), together with other oils of the region, including the well-known Cortemaggiore field (Pieri 1992), into a so-called Cortemaggiore group. The Cortemaggiore group shows similar geochemical trends, but includes oils with different thermal evolutionary stage (Riva et al. 1986). Both Salsomaggiore and Cortemaggiore oils indicate an oxic environment of deposition of marine/deltaic siliciclastic source rocks and include 18a(H)oleanane that derives from plants occurring since the Late Cretaceous and blooming in the Tertiary (Riva et al. 1986). In literature, the Cortemaggiore group is considered as sourced by the same Tertiary foredeep unit that acts as reservoir (Riva et al. 1986). This interpretation is questioned by the absence of organic-rich intervals in the Oligo-Miocene succession, that never reach a Total Organic Carbon (TOC) of .0.5%. Furthermore, the thermal maturity of the Salsomaggiore oil, with a Vetrinite reflectance

120

Table 1. Physical properties and major ion composition of the analysed saline waters and brines T (8C)

pH

Eh (mV)

Cond. (mS/cm)

Salsomaggiore Regnano Castel S. Pietro 1 Castel S. Pietro 2 Castrocaro Porretta Bove Porretta Puzzola Seawater

17.8 12.2 15.2 13.4 16.7 32.3 21.5

6.9 8.2 7.54 7.48 6.75 7.5 7.63

2103 299 251 269 2372 2260 2315

173 22 54 54.2 58.9 8.4 4

Ca2þ (ppm)

Mg2þ (ppm)

Naþ (ppm)

5851.7 1556.48 60900 58.12 86.34 6210 247.7 676 10060 264.7 641 10000 885.77 206.72 15308.8 40.08 8.51 2114 44.09 7.54 988.08 400 1272 10560

Kþ (ppm) 269 22.82 145.1 167.1 115.34 84.85 37.15 380

NHþ 4 (ppm)

NO2 2 (ppm)

95006 90 170.8 – 135 8875 2 2928 7.09 0.8 20590 2 512.4 24.81 – 20590 0 512.4 27.47 – 26595 130.16 610.2 61.134 31 2836.8 5.3 1311.5 4.43 17 1365.2 26 884.8 2.48 7 18980 2649 140 0.23 0.07

– ,0.1 – – ,0.1 ,0.1 ,0.1 0.16

Cl2 (ppm)

SO24 – (ppm)

HCO2 3 (ppm)

NO23 – (ppm)

H2S SiO2 Fe3þ (ppm) (ppm) (ppm) – 2 0 0 .5 0 1 0

67 11.53

20.3 3.4

0.02

24.5 19.04 7

R. CAPOZZI & V. PICOTTI

Samples

GEOCHEMISTRY AND STRUCTURES OF APENNINES SEEPS

121

20,0 10,0

GMWL

0,0

δ 2H

– 10,00

– 5,00

0,00

5,00

–10,0

10,00

15,00

Salsomaggiore Regnano

–20,0

Castrocaro –30,0

Porretta

–40,0

Seawater

–50,0 –60,0

δ 18O Fig. 3. d2H v. d18O of the saline waters and brines. The Salsomaggiore and Regnano isotope composition falls in the field of formation water without meteoric mixing. The more positive isotope values in the Salsomaggiore brines document hyperfiltration processes. Castrocaro brines show mixing with meteoric water and Porretta waters are almost entirely of meteoric origin. GMWL: Global Meteoric Water Line.

Ro% .1.1, is higher than that of the Burdigalian reservoir (Picotti et al. 2007), documenting a migration of the oil from a deeper source. The low thermal evolution of this reservoir is confirmed by apatite fission track data (Zattin, pers. comm.) that show the reservoir never entered the partial annealing zone (i.e. it never passed the 70 8C isotherm). Table 2. d18O and d2H composition of the analysed saline waters and brines Samples Well No. 20 Salsomaggiore Well No. 21 Salsomaggiore Well No. 25 Salsomaggiore Well No. 27 Salsomaggiore Well Agip No. 93 Salsomaggiore Regnano 20 Regnano 2B Regnano 9A Regnano 9B Nirano (Minissale et al. 2000) Porretta (Minissale et al. 2000) La Bolga Well, Castrocaro Local meteoric water

d18O (V-SMOW)

d2H (V-SMOW)

11.45

213.3

12.63

211.7

10.57

29.1

11.05

219.7

9.14

29.2

3.78 2.70 4.28 1.42 5.50

220.7 220.8 219.5 227.1 4.0

27.90

251.1

24.00

241.8

28.00

254.0

The thermal evolutionary stage of Salsomaggiore is late mature, as is the case of the oil of the Cortemaggiore field, hosted in a younger Tortonian reservoir. In contrast, other oils, hosted in Langhian to Serravallian reservoirs and belonging to the Cortemaggiore group, yield a lower thermal maturity (peak Ro ¼ 0.7 –0.8, Riva et al. 1986). The Salsomaggiore oil, as well as the Cortemaggiore oil, are probably migrating from a source deeper than the reservoir. Finally, the geochemical and stable isotope composition of the methane associated with oil in Cortemaggiore (Mattavelli et al. 1983, see Fig. 4 and Table 3) is well within the field of thermogenic gas, adopting the natural gas characterization proposed by Whiticar (1994). All this evidence indicates an unlikely OligoMiocene age for the source, that is still unknown but is deeper, and possibly of Cretaceous age (see discussion in Picotti et al. 2007).

A synthesis of the main evolutionary trends of the petroleum system We prepared a sketch of the history and processes acting at Salsomaggiore, depicting the main components of the petroleum system, namely source and reservoir. The diagram (Fig. 5) shows the main steps of the tectono-thermal evolution of the volume of rocks involved in the analysis, which include the Salsomaggiore anticline and the surroundings to the south and east of the anticline (see Fig. 2). We reconstruct the burial and exhumation history of the source and reservoirs, based on the cross

122

R. CAPOZZI & V. PICOTTI

sections of Figure 2. As the volume of rock within the drainage area of the Salsomaggiore anticline also includes sectors structurally depressed, we used these maximum burial values for tracking the hydrocarbon history in the diagram of Figure 5, whereas for the reservoir we considered the thrust top. As the geothermal gradient of the Miocene foreland basin is not known, we adopted the gradient of the present-day foreland basin, the Po Plain, as revealed by the numerous hydrocarbon wells (e.g. Pasquale et al. 2008), which is low (around 238/km), as expected in a convergent setting. The figure shows a deep source, assumed as Cretaceous as working hypothesis, that almost reached the oil window during the subsidence phase, that is described by two curves, a first low rate, followed by the second segment (the foredeep stage), characterized by important sedimentary and final tectonic burial, provided by the Ligurian unit. Subsidence culminates with expulsion and migration of oil and thermogenic methane towards the newly formed anticline during the middle Miocene. At the same time, waters in the reservoir underwent concentration. After a phase of leakage during the Tortonian, the new subsidence phase provided an effective sealing, whereas the geochemistry of the fluids did not vary significantly. The last thrusting event, however, is responsible for the disruption of the previous equilibrium and led to fluids migration toward the present-day culmination (see Fig. 2).

Fig. 4. Isotopic composition of gas samples. Diagram after Schoell (1983). Primary gases: B, bacterial; M, mixing of thermogenic and bacterial gases; T, thermogenic associated; TT, non-associated deep dry gases.

Table 3. Chemical and isotopic composition of gas samples Samples

%C1

94.76 Cortemaggiore 37D (Mattavelli et al. 1983) Regnano 96.77 background activity 93.07 Regnano paroxismal event Castel San Pietro 45.3 Ca’ Zini 2 Sassuno 90.6 (Minissale et al. 2000) La Bolga well 91.68 Castrocaro 98.76 Porretta (Minissale et al. 2000)

%C2

%C3

%nC4

%CO2

d13C1

d13C2

d13C3

3.64

1.09

0.64

na

240.5

na

na

0.38

0.01

0.01

2.53

244.7

221.7

0.67

nd

nd

2.32

246.26 222.17

nd

nd

nd

nd

251.35

nd

1.44

0.17

1.72

243.8

nd

nd

8.32 0.36

4.4 nd

0.420 0.067 0.011

na, not analysed, nd, not detectable

54.7

d13C4 d13CCO2

d2D

na

2156

20.07

2149

nd

17.36

nd

nd

nd

241.08

nd

nd

nd

nd

nd

na

275.5

nd

nd

nd

219.2

2171

230.5

nd

nd

nd

na

na

na

29.57 220.4

GEOCHEMISTRY AND STRUCTURES OF APENNINES SEEPS

123

SALSOMAGGIORE PETROLEUM SYSTEM

km

100°

NE -thr ust ing

150°

ex p

r

uls

tte

6

4 Ma

ion

ma

5

su for bs el id an en d ce

10°C

g tin us d thr n an g NW ratio shin g rwa i m te wa

an

ic

an

org

16 Ma

Present

7 Ma

igr ati on

20 Ma

fluid leaking

12 Ma

on rati ent onc

c ce en sid sub nd ela for

of

4

on

3

ati tur

2

20 Ma

ma

1

?mid K

dm

0

T °C

chemioherms

7 16 Ma

8

oil and Th methane generation

Fig. 5. Tectono-thermal evolution of the Salsomaggiore petroleum system: blue colour for the source/hydrocarbons and orange colour for the reservoir.

Regnano mud volcano Local geological setting Located at the foothills of Reggio Emilia, the Regnano mud volcano spreads over an area of 0.5 km2, and consists of mud breccias and mudflows. The Regnano mud volcano is the main feature generated by spontaneous fluid seep within a wider area occupied by minor seepages (see Figs 1 & 6, and Capozzi & Picotti 2002). The Regnano fluid vent includes mainly saline waters and gas, whereas during the observed paroxysmal event, which occurred in March 1999, emission of condensates and oil was observed, together with remarkable quantities of mud and water. From a structural point of view (Fig. 6), the mud volcanoes are aligned along some high-angle normal faults. These latter are widespread in the foothills of the Northern Apennines (e.g. Bertotti et al. 1997; Picotti et al. 2007, 2009), are active and are interpreted as the upper carapace extension of a larger ramp fold (Picotti & Pazzaglia 2008). Similarly to Salsomaggiore, the Ligurian unit with its sedimentary cover, the so-called Epiligurian unit, overly the turbiditic succession deposited on the foredeep. In the case of Regnano, the sandstone– mudstone foredeep succession is Tortonian in age, and the deeper thrust, deforming it and visible in the cross section of Figure 6, is late Tortonian to Messinian in age. The presence of microfossils in the extruded mud confirms that the latter forms from mixing of connate waters, leaking from the Tortonian

reservoir throughout the fault, and the sedimentary rocks of the Epiligurian deposits, which deposited on top of the nappe and avoided a severe compaction history (Capozzi & Picotti 2002).

Fluids In the case of the Regnano mud volcano, the saline water has low temperature, indicating an equilibration with the air temperature. The pH averages at about 7.8, and Eh varies from 299 to 2286. Chlorinity is up to 10 000 ppm, less than the 18 980 ppm typical of seawater (Table 1). The small vents show dilution with meteoric water during autumnal rainfall. The Regnano mud volcano samples show that the Ca2þ/Cl2, Mg2þ/Cl2and SO24 – /Cl2 ratios are lower, while Naþ/Cl2 and HCO32/Cl2 ratios are higher than in normal seawater. This chemical composition can be interpreted as a result of membranefiltering processes occurring on the formation water (White 1965), as the isotopic signature, reported in Figure 3 and Table 2, indicates the absence of meteoric mixing. This interpretation could be supported by a comparison with the isotopic composition of the Salsomaggiore brines. At Regnano isotopic values have been correlated to membranefiltered formation waters (Capozzi & Picotti 2002). In this case the filtrate water is expected to be depleted in heavier isotopes and, in fact, even if the reservoir of both Salsomaggiore and Regnano water is constituted by Tertiary siliciclastic sediments, in the Regnano case filtration through the

124 R. CAPOZZI & V. PICOTTI Fig. 6. Geological sketch map and cross sections along dip of the Northern Apennines foothills in the area of Regnano (see Fig. 1 for location). Modified after Capozzi & Picotti (2002). Purple colour indicates the main reconstructed fluid migration pathway.

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fault zone crossing the topmost Ligurian unit is clearly testified by chloride and isotope depletion. The H2S content (about 2 mg L21), the depletion of the SO24 – /Cl2 value and the increase in HCO2 3 in the water, are all consistent with a current sulphate reduction which is a process where bacteria obtain energy for metabolism from oxidation of organic compounds. The generalized reaction (Friedman et al. 1992, p. 102) for sulphate reduction  is SO2 4 þ 2Corganic þ 2H2 O ¼ H2 S þ 2HCO3 . Carbonate equilibria control the amount of dissolved calcium and magnesium in solution. The presence of dissolved constituents at relatively high concentrations (i.e. sodium and bicarbonate concentrations) can control the amount of calcium and magnesium in solution through the precipitation of carbonate minerals. Microbiological analysis of the water and the mud revealed a population of active sulphatereducing bacteria, with an incubation temperature of 35 8C, that colonizes the seeps. Bacterial cultures had up to 7  103 cell/g in the water and .1  104 cell/g in the mud. Cells of active sulphatereducing bacteria with a higher incubation temperature (60 8C) have also been observed. The depth of hydrocarbon source must be over 6000 m because of the clear thermogenic isotopic signature of the methane (Table 3). The migration of fluids in the Regnano system up to the surface occurs via a normal fault (Capozzi &

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Picotti 2002), capable of hydraulically connecting a deep source of fluids to the surface. The present migration through the disrupted seal of the Ligurian unit allows the water filtering by clay membranes and the mixed fluid ascends slowly and rises up to the base of the Epiligurian sedimentary succession, around 1 km below the surface (Fig. 6). Here, the higher permeability of this latter causes a decrease in pressure, allowing accumulation of a pool, where bacterial degradation occurs. While the normal fault system in the Epiligurian unit provides a continuous leakage of water and methane, muds are usually extruded only during paroxysmal events (Capozzi & Picotti 2002).

A synthesis of the main evolutionary trends of the petroleum system The time–temperature path we prepared for the discussion of the expulsion/migration trend of the Regnano fluids (Fig. 7) shows again a two-step history of subsidence, with a first low rate, followed by the foredeep formation and the final tectonic burial. With respect to the previous example of Salsomaggiore, the time is shifted forward by some few million years, due to the younging of the pair foredeep evolution–deformation, described earlier. The formation of the thrust –ramp anticline for the fluid trapping went on throughout the late Tortonian to the earliest Pliocene, whereas the last

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step of fluid migration occurred only after the formation of the main normal fault system, that occurred later than 1 Ma, as documented by the presence of lower Pleistocene deposits at the hanging wall of the fault (see Capozzi & Picotti 2002). The process of membrane filtering continues to take place, due to the leaking of fluid along the shaly and muddy Ligurian to Epiligurian units.

Castel San Pietro Local geological setting As documented in the previous cases, along the foothills, the Ligurian nappe wedges towards the NE. East of Bologna, the nappe also thins to the east, due to the structural culmination of the Romagna (see Zattin et al. 2002) and the nappe base crops out, putting into contact the two realms (the so-called Sillaro Line, see Zattin et al. 2002, and Fig. 1). To describe this transition, we sampled fluids at Castel San Pietro across the tip of the Ligurian wedge (see section of Fig. 8). The section shows fluids leaking from the Tortonian foredeep because of the presence of small, high-angle, active normal faults, giving origin to a mud volcano at Sassuno (see Minissale et al. 2000). There is a striking similarity of this mud volcano with the above-described Regnano volcano. To the NE, the Ligurian wedge thins and it is covered by the Pleistocene marine deposits, mainly mudstones, bearing lenses of sands, deposited after the stopping of its advancement. These sands host fluids, locally leaking at surface and exploited by wells along the western side of the Sillaro valley, drilled for supplying the Castel San Pietro spa. We sampled two wells, reaching the depth of 523 m (Ca’ Zini 1 and 2, Fig. 8) and located close to three deeper wells drilled for hydrocarbon exploration at the end of the 1960s. The main mineralized sandy horizon is located between 451 and 504 m below the surface.

Fluids The average temperature of the water ranges between 13.5 and 16 8C. Table 1 reports major ions content in the Castel San Pietro water. Brine composition also includes 125 mg/l of bromide which doubles the value of marine water. Bromide and NO2 3 can be explained with the presence of a relatively high content of organic matter. The composition of brines in the Ca’ Zini 1 and 2 wells shows minor differences with respect to the seawater, as reported in Table 1. We can observe a small enrichment in chloride coupled with a decrease in Ca and Mg which, in turn, maintain the typical marine ratio of 1/3. These values can be explained with cation exchange, a reaction in

which the calcium and magnesium in the water are exchanged for sodium that was adsorbed to aquifer solids such as clay minerals, resulting in higher sodium concentrations and decreased calcium and magnesium concentrations. These values when compared with the HCO2 3 content, which is four times that of marine water, and with the SO22 4 which, on the contrary, is absent, suggest a sulphate reduction process mediated by sulphur-reducing bacteria. On the other hand, also H2S does not occur in the solution, then sulphur has to be precipitated in a mineral form (Seemann 1987). All the reported analytical results also indicate the absence of dilution due to meteoric water. The complete assessment of fluid in Castel San Pietro includes gas composition that, in Ca’ Zini 2 Well is composed by over 50% of CO2 and the rest by methane (Table 3). The composition of these two gases is related to the methanogenic process mediated by different bacteria consortia. At first, the anaerobic bacteria population induces fermentation of complex organic compounds, producing acetate and by-products as CO2 and H2. These products support methanogenesis following the reaction: CH3 COOH ! CH4 þ CO2 (acetate fermentation) Methanogenic bacteria utilize acetate as a methanogenic substrate in the presence of hydrogen; both Methanosarcina barkeri and Methanobacterium thermoautotrophicum rapidly convert acetate to methane (Zeikus et al. 1975). Other bacterial populations exploit hydrogen to reduce CO2 by the following equations: CO2 þ 4H2 ! CH4 þ 2H2 O or HCO 3 þ 4H2 ! CH4 þ 2H2 O þ OH (CO2 reduction): The composition of the gas sampled in Castel San Pietro can be interpreted as the acetate fermentation phase and, possibly, to incipient methanogenesis via CO2 reduction, probably testifying that this gas is now generating and is not yet evolved towards the character of the biogenic methane typical of the late Neogene reservoir of the eastern Po Plain. The isotopic composition of the gas, reported in Table 3, depends on the reaction pathway, and 13 C fractionation between CO2 and CH4 is different if the pathway is acetate fermentation or CO2 reduction (Clark & Fritz 1997). Biogenic methane could be highly depleted in 13C if it derives from CO2 reduction rather than from acetate. A useful

GEOCHEMISTRY AND STRUCTURES OF APENNINES SEEPS Fig. 8. Geological sketch map and cross section along dip of the Northern Apennines foothills in the area of Castel San Pietro (see Fig. 1 for location). Purple colour indicates the main fluid migration pathway. Note that the location of Ca` Zini 1 and 2 corresponds to that of Castel S. Pietro 3 well.

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tool to determine the dominant process occurring in methanogenesis is the fractionation factor: a13 CCH4 CO2 ¼ d13 CCH4 þ 1000=d13 CCO2 þ 1000: In the Ca’ Zini sample this factor is 0.989. When the fractionation factor is less than about 0.935 the methane generation is due to CO2 reduction, whereas when this factor is greater than 0.95 the dominant process is the acetate fermentation (Clark & Fritz 1997). Adopting the diagram of Whiticar (1994) based on a different ratio: a13 CCO2 CH4 ¼ d13 CCO2 þ 1000=d13 CCH4 þ 1000 the fractionation factor is 1.0108, that falls within the field of methane oxidation. Therefore, we cannot discriminate between the two processes, but further studies are needed for the use of methane– carbonate fractionation factors. It is worth noting that the absence of SO22 4 prevents the possibility of present-day oxidative processes. Both interpretations point to biogenic processes at shallow depth for this gas generation. Lithologic and stratigraphic information on the drilled wells indicates that hydrocarbon-bearing layers belong to a marine Quaternary succession. This reservoir is younger than the reservoirs drilled in the Romagna region, which are mainly made up of turbiditic successions of Pliocene age with intervening sapropel intervals, but it is more similar to pools exploited in Pleistocene sediments in the Northern Adriatic Sea. We explored the mud volcano at Sassuno, but it is now quite exhausted and it was not possible to sample fluids not contaminated with surface water and atmosphere. Therefore, we provide the data collected by Minissale et al. (2000) during the last period of activity (Fig. 4 and Table 3). The gas composition and its isotopic signature suggest a clear similarity with the emissions of the Regnano mud volcano (Capozzi & Picotti 2002). In fact, also in this case the active normal faults, rooted at depth, are very effective to provide leakage of thermogenic methane from deep sources located in the Tertiary reservoir below the Ligurian nappe (Capozzi & Picotti 2006). Filtration mechanisms through the fault system, however, cannot be further investigated.

Castrocaro Local geological setting Toward the SE of the Sillaro Line, at the Apennine foothills near Forlı`, the Ligurian unit is mostly absent (Fig. 1). Here, several natural seepages occur, locally exploited in small spas; the most

important of them is located in Castrocaro village, where a significant reservoir has been drilled and exploited. The wells are drilled at about 100 m of depth within the Pliocene calcarenites (the so-called Spungone, see Capozzi & Picotti 2003). The structure of the Castrocaro field is shown in the cross section of Figure 9 (see Fig. 1 for location), where a thrust deforms the Miocene foredeep succession, allowing the Pliocene to onlap the thrust top with shallow water calcarenites. The Pliocene foredeep succession, bearing some intervals of organic-rich laminites (sapropels in Fig. 9), is present on the footwall block, currently buried to the north of the thrust top. The post-thrust succession, consisting of mudstones evenly dipping to the north, documents a late event of large wavelength folding, associated with a deeper thrust, as described by Picotti & Pazzaglia (2008).

Fluids The fluids consist of saline waters and gas. To discuss the characters of the water chemistry of Castrocaro, we first have to take into account the isotopic analysis (Fig. 3 and Table 2) that indicates a significant mixing with meteoric water. Notwithstanding this dilution, the salinity of the water is higher than that of seawater (Table 1). The Mg2þ ion is highly depleted whereas Ca2þ doubles the value of the seawater and HCO2 3 is also present in a higher proportion. The concentration of these latter ions probably depends on the lithology of the reservoir which is constituted by calcarenites. The dissolution of carbonate is also facilitated by the acidic pH due to the relevant H2S content. The occurrence of high concentration of H2S parallels the depletion of SO22 4 via sulphate reduction processes. However, the activity of sulphatereducing bacteria that mediate this process does not occur at present. The gas that leaks together with the saline water in Castrocaro is about 90% methane and the rest is CO2. The isotopic characterization provided a d13C (CH4) in units per mill Vienna Pee Dee Belemnite standard (VPDB) of 275, that account for a biogenic origin of this gas (Fig. 4 & Table 3). This biogenic methane occurs within deformed upper Neogene units, which represent an uplifted portion of the main hydrocarbon system exploited in the eastern Po Plain. In this reservoir, the biogenic methane is likely supplied by turbidites successions and by a sapropel-bearing interval of middle Pliocene age and of about 60 m in thickness. The high concentration of H2S in the saline waters probably derives from this latter sapropel-bearing interval because within the sapropel outcrops at the foothills, sulphides precipitated as pyrite framboids, as clearly documented by Capozzi et al. (2006). In

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the subsurface, then, sulphides could still be significant compounds in the interstitial water. On the other hand, meteoric water is provided by infiltration in the same calcarenitic reservoir outcropping south of the exploited site. The Pliocene and Pleistocene succession of the foothills dips to the north and acts as an effective carrier for gas and formation water generated in the adjacent Po Plain (NE) (Fig. 9). The location of the natural seepages appears controlled mainly by the presence of good-quality reservoir, such as the calcarenites. Usually, in this sector, seepages are associated with shallow permeable carrier beds more or less sealed by Pliocene to Pleistocene mudstones.

A synthesis of the main evolutionary trends of the petroleum system The history of the petroleum system in Castrocaro is much simpler than the previous examples, because of the presence of biogenic methane in a fully Pliocene system. Therefore, the main steps are shown by Figure 10, where a simple shallow burial for both source and reservoir is provided. Note the importance of the northern tilting, acquired only in the last 1 Ma, to provide the carrier for charging the reservoir. Again, in Figure 9, one can appreciate the role of the meteoric waters percolating within the reservoir from the outcrop, and finally mixing within the reservoir. This meteoric water displaced the natural gas and maintained pressured the reservoir, forming a small gas field (Marzeno gas field)

located some 8 km to the NW and located at the same culmination of the La Bolga well (Fig. 9).

Porretta Local geological setting At Porretta (see Fig. 1 for location), there is a sharp geological boundary between the northern hilly side, characterized by the presence of the Ligurian nappe, and the southern mountainous portion, where the intensely deformed Tertiary foredeep succession crops out (Fig. 11). This latter consists of interbedded hard sandstones and shaly mudstone of lower Miocene age. To separate the two realms, a normal fault system occurs, disrupting the previous thrust belt. As shown in Figure 11, the normal faults, with local antithetic structures, produce significant stratigraphic separations. They are considered active (Picotti & Pazzaglia 2008) and are associated with a significant jump on the apatite fission track ages: from 3 to 6 Ma to the south (Tertiary foredeep succession) against around 13 Ma in the north (Ligurian nappe) (Ventura et al. 2001). The Tertiary succession represents the main reservoir in the subsurface, its porosity being mainly secondary and having originated from the pervasive tectonization along the major fault zones (Gargini et al. 2008). Of course, even the Mesozoic carbonates, at depth, could act as an aquifer. The olistostrome atop the Tertiary succession (the so-called Sestola– Vidiciatico unit, see Fig. 11) act as aquitard, whereas the overlying

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Ligurian unit acts as real aquiclude, interrupted only along the main extensional faults. Note the anticline folding the Tertiary on the north of the main fault, sealed by the Ligurian and culminating to the SE of the section, where the Tertiary succession outcrops (Castiglione dei Pepoli anticline, see Fig. 11). This latter anticline and the extensional faults following its trend are the best candidate for the charging of meteoric water within the reservoir.

Fluids The fluid emission along the main divide of the Northern Apennines has higher temperature than that at the foothills. In Porretta the two main springs have an average temperature of 21 8C (Puzzola) and 35 8C (Bove). The fluids consist of low saline waters and gas. The isotopic analysis (Fig. 3 and Table 2) indicates that meteoric water is dominant, and gives the first indications to interpret the water chemistry in Porretta. Nevertheless, a mixing with saline waters (Table 1), trapped within a reservoir in the subsurface, is indicated by the ion content. The amount of chloride in absence of halite dissolution, as in the case of the Porretta succession (Table 4), can be interpreted taking into account that in most common sedimentary silicates and carbonates chloride behaves conservatively and its concentration is controlled by physical processes such as advection or dispersion (Hanor 2001). On the other hand, a slight sodium excess with respect to the stoichiometric ratio of seawater (Table 4) can be ascribed to a filtering mechanisms through cation-exchanging clay minerals in the upper part of the migration pathway. Variations in ion composition also concern Mg2þ, which is totally removed from the solution, 2 HCO 3 , which significantly increases, and SO4 that shows a decrease not balanced by the Table 4. Ionic content of the Porretta waters, normalized with respect to Cl2 Ratio

Seawater Cl218 980 ppm

Porretta (Bove) Cl22387 ppm

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Ca2þ/Cl2 Mg2þ/Cl2 Naþ/Cl2 Kþ/Cl2 2 HCO 3 /Cl 2 2 SO4 /Cl SiO2/Cl2

0.021 0.067 0.53 0.02 0.0074 0.14 0.00037

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occurrence of sulphides. The filtration mechanism that can account for Na increase can also be responsible for Mg removal, even if this component could precipitate via bacterially mediated processes. In fact, the Porretta fluid vent releases thermogenic methane, which likely undergoes anaerobic methane oxidation (AOM) within the reservoir. This second interpretation could be supported by the presence of high HCO 3 concentration which is known to derive from bacterial metabolism (Douglas 2005) and is directly associated with the decrease in SO2 4 due to sulphate-reducing bacterial activity according to the overall reaction: 3 þ HS þ H2 O CH4 þ SO2 4 ! HCO

This process is probably favoured by the surface water that can supply CO2 to the reservoir system. In fact, in a system closed to CO2, as described for Salsomaggiore, alkalinity cannot increase. The related sulphides by-product that we expect to form from this reaction, is known to easily precipitate in presence of iron. When compared to Regnano, a similar reaction can be inferred, even if in that case the H2S occurs in solution. The comparison can be done also concerning the incubation temperature of about 35 8C for bacteria consortia. Furthermore, in the Porretta aqueous solution, a relatively high enrichment in SiO2 can support the hypothesis of a dominant bacterially mediated process. Bacterial activity can produce soluble organic acids which have been found in a variety of oilfield formation waters. The presence of soluble organic acids increases the dissolution rate and solubility of feldspars, quartz and other minerals under natural anoxic conditions, and has also been experimentally tested in laboratory (Ullman et al. 1996). The classification of the methane of the Porretta site falls in the field of mixing of deep dry gases with gas associated with oil (Schoell 1983) (Fig. 4 and Table 3). We interpret that this isotopic composition could be derived from a thermal alteration in an overmature environment, as the vitrinite reflectance of the reservoir rocks is over 1.2% (Ventura et al. 2001), where thermogenic gas from oil cracking is still associated with residual occurrence of petroleum at depth. Present migration of saline waters and gas in this system is likely due to the enhanced displacement because of surface water circulation in a fractured system at depths up to 1000 m.

Conclusions Spontaneous seeps are an open window onto the various processes controlling the evolution of the petroleum system. This study, based on some cases distributed along the chain, led us to identify

GEOCHEMISTRY AND STRUCTURES OF APENNINES SEEPS

an extreme variety of natural settings, where fluids leak out from reservoirs of different ages, which are deformed at different times and as a consequence of different stress regimes (compressional or late-orogenic extensional). At Salsomaggiore, a deep source of oil charged the Burdigalian reservoir during the first phase of subsidence and thrusting in the middle Miocene. In the Late Miocene the same source charged the subsequent structure of Cortemaggiore. The Pliocene and Pleistocene remobilization of the Salsomaggiore structure produced the dismigration of the fluids towards the newly formed and eroded culmination. The early charging of the reservoir allowed the foredeep sandstone to escape compaction and maintain a good primary porosity. A similar deep source is required to explain the thermogenic methane of Regnano. Here, the seeps leak along an active normal fault, suggesting a role of fractures in increasing the permeability of the reservoir. However, we cannot rule out the occurrence of primary porosity, especially in the early charged structures, even though in this case study the tectonic overburden appears larger than that of the Salsomaggiore culmination. The fractured muddy and shaly units, capping the reservoir, are actively filtering the saline waters during the upward migration. Regnano and Castel San Pietro share the same situation, when dealing with seeps leaking throughout the Ligurian shales, such as the Sassuno mud volcano. At the tip of the Ligurian nappe in the Castel San Pietro case, we documented seeps from a Pleistocene succession covering and sealing the Ligurian nappe. This system is completely different from the previous one, in that biogenic methane is produced by organic-rich layers interbedded in the Pliocene to Pleistocene succession of the adjacent Po Plain. This last setting is typical of the region to the east of the Sillaro Line, where the Ligurian nappe never reached the foothills, and the petroleum system is similar to Castrocaro, even with reservoirs of different age. Because of the recent NE tilting of the entire succession at the mountain front (see Picotti & Pazzaglia 2008), both Castel San Pietro and Castrocaro share the same geometry of the carriers, driving connate waters and biogenic methane towards the foothills. In these latter cases the reservoirs bear a good primary porosity because of the scarce compaction and the early diagenesis, as is the case with the Pliocene Spungone calcarenites of Castrocaro. Finally, Porretta seeps display the features of a petroleum system close to the end of the oil window, that was expected for such an internal setting on the thrust belt. In this setting, paradigmatic for the entire inner belt of seeps, shown in Figure 1, thermogenic methane and remarkable

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mixing with meteoric waters characterize the fluids, that circulate along the damaged fault zones, documenting the presence of residual pools of methane and subordinate oil at depth. The Northern Apennines are usually considered a single petroleum province (e.g. Lindquist 1999), or as a triple system, with biogenic gas, thermogenic gas and oil belonging to different geodynamic settings, from the shallower and external (Po Plain) to the deeper and internal provinces (Bertello et al. 2008). Our study documents that the Tertiary foredeep system in the Apennine fold and thrust belt is anything but a homogeneous petroleum system. The study of spontaneous seeps allows us to clarify the various tectono-thermal histories occurring on the Northern Apennines, improving our knowledge of the spatial variability of the different oil and gas accumulations, with clear impact on the regional exploration strategy. Collaboration with ENI-Agip’s division for isotope analysis on methane and microbiological analysis on water, and with Edison S.p.A. for wells and seismic data, was fundamental for the achievement of these results. The thoughtful comments of the anonymous reviewers were highly appreciated. Funding by MIUR - PRIN 2006 is gratefully acknowledged.

References Akhmanov, G. G., Premoli Silva, I., Erba, E. & Cita, M. B. 2003. Sedimentary succession and evolution of the Mediterranean Ridge western sector as derived from lithology of mud breccia clasts. Marine Geology, 195, 277– 299. Bertello, F., Fantoni, R. & Franciosi, F. 2008. Exploration Country Focus: Italy. Search and Discovery Article #10165 (2008), Posted 16 October 2008. World Wide Web Address: http://www. searchanddiscovery.net/documents/2008/08158bertello/ index.htm Bertotti, G., Capozzi, R. & Picotti, V. 1997. Extension controls Quaternary tectonics, geomorphology and sedimentation of the N Apennines foothills and adjacent Po Plain (Italy). Tectonophysics, 282, 291–301. Bonini, M. 2007. Interrelations of mud volcanism, fluid venting, and thrust– anticline folding: examples from the external northern Apennines (Emilia–Romagna, Italy). Journal of Geophysical Research, 112, 10.1029/2006JB004859. Borgia, G. C., Elmi, C. & Martelli, G. 1986. Hydrocarbons in the Tuscan–Emilian Apennines: origin and characters of mineralization. Memorie Societa` Geologica Italiana, 31, 255–266. Capozzi, R. & Picotti, V. 2002. Fluid migration and origin of a mud volcano in the Northern Apennines (Italy): the role of deeply rooted normal faults. Terra Nova, 14, 363–370. Capozzi, R. & Picotti, V. 2003. Pliocene sequence stratigraphy, climatic trends and sapropel formation in the Northern Apennines (Italy). Palaeogeography, Palaeoclimatology, Palaeoecology, 190, 349 –371.

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Capozzi, R. & Picotti, V. 2006. Genesis of cold seeps and mud volcanoes of the Northern Apennine foothills. In: Fluid Seepages/Mud Volcanism in the Mediterranean and Adjacent Domains. Commission Internationale pour l’Exploration Scientifique de la Mer Workshop Monographs 29, 59– 64. Capozzi, R., Dinelli, E., Negri, A. & Picotti, V. 2006. Productivity-generated annual laminae in MidPliocene sapropels deposited during precessionally forced periods of warmer Mediterranean climate. Palaeogeography, Palaeoclimatology, Palaeoecology, 235, 208– 222. Charlou, J. L., Donval, J. P., Zitter, T., Roy, N., JeanBaptiste, P., Foucher, J. P., Woodside, J. & MEDINAUT SCIENTIFIC PARTY 2003. Evidence of methane venting and geochemistry of brines on mud volcanoes of the eastern Mediterranean Sea. Deep-Sea Research I, 50, 941–958. Clark, I. D. & Fritz, P. 1997. Environmental Isotopes in Hydrogeology. Lewis Publishers, Boca Raton, New York. Clayton, R. N., Friedman, I., Graff, D. L., Mayeda, T. K., Meents, W. F. & Shimp, N. F. 1966. The origin of saline formation water, 1. Isotopic composition. Journal of Geophysical Research, 71, 3869– 3882. Conti, S., Artoni, A. & Piola, G. 2007. Seep-carbonates in a thrust-related anticline at the leading edge of an orogenic wedge: the case of the middle–late Miocene Salsomaggiore Ridge (Northern Apennines, Italy). Sedimentary Geology, 199, 233–251. Dimitrov, L. I. 2002. Mud volcanoes – the most important pathway for degassing deeply buried sediments. Earth-Science Reviews, 59, 49–76. Douglas, S. 2005. Mineralogical footprints of microbial life. American Journal of Science, 305, 503– 525. Etiope, G., Feyzullayev, A. & Baciu, C. L. 2009. Terrestrial methane seeps and mud volcanoes: A global perspective of gas origin. Marine and Petroleum Geology, 26, 333– 344. Friedman, G. M., Sanders, J. E. & Kopaska-Merkel, D. C. 1992. Principles of Sedimentary Deposits: Stratigraphy and Sedimentology. Macmillan Publishing Company, New York. Gargini, A., Vincenzi, V., Piccinini, L., Zuppi, G. M. & Canuti, P. 2008. Groundwater flow systems in turbidites of the northern Apennines (Italy): natural discharge and high speed railway tunnel drainage. Hydrogeology Journal, 16, 1577– 1599. Hanor, J. S. 1987. Origin and migration of subsurface sedimentary brines. Society Economic Paleontology Mineralogy Short Course, 21, 1– 247. Hanor, J. S. 1996. Variations in chloride as a driving force in siliciclastic diagenesis. In: Crossey, L. J., Loucks, R. & Totten, M. W. (eds) Siliciclastic Diagenesis and Fluid Flow: Concepts and Applications. Society Economic Paleontology Mineralogy Special Publication, 55, 3 –12. Hanor, J. S. 2001. Reactive transport involving rockbuffered fluids of varying salinity. Geochimica et Cosmochimica Acta, 65, 3721– 3732. Holysh, S. & To´th, J. 1996. Flow of formation waters: likely cause for poor definition of soil gas anomalies over oil fields in East –Central Alberta, Canada. In: Schumacher, D. & Abrams, M. A. (eds)

Hydrocarbon Migration and its Near-Surface Expression. American Association of Petroleum Geologists, Memoir, 66, 255–277. Horita, J. 2005. Saline waters. In: Aggarwal, P. K., Gat, J. R. & Froelich, K. F. O. (eds) Isotopes In the Water Cycles. Past, Present, and Future of a Developing Science. Springer, 271– 288. Kaluza, M. J. & Doyle, E. H. 1996. Detecting fluid migration in shallow sediments: continental slope environment, Gulf of Mexico. In: Schumacher, D. & Abrams, M. A. (eds) Hydrocarbon Migration and its Near-Surface Expression. American Association of Petroleum Geologists, Memoir, 66, 15– 26. Kholodov, V. N. 2002. Mud volcanoes: distribution regularities and genesis (Communication 2. geological– geochemical peculiarities and formation model). Lithology and Mineral Resources, 37, 293– 309. Leo´n, R., Somoza, L., Medialdea, T., Maestro, A., Dı´az-Del-Rı´o, V. & Del Carmen Ferna´ndezPuga, M. 2006. Classification of sea-floor features associated with methane seeps along the Gulf of Ca´diz continental margin. Deep-Sea Research II, 53, 1464– 1481. Lindquist, S. J. 1999. Petroleum systems of the Po Basin Province of Northern Italy and the Northern Adriatic Sea: Porto Garibaldi (Biogenic), Meride/Riva di Solto (Thermal), and Marnoso Arenacea (Thermal). On-Line Report, USGS Open-File Report 99–50-M. Loncke, L., Mascle, J. & FANIL SCIENTIFIC PARTIES 2004. Mud volcanoes, gas chimneys, pockmarks and mounds in the Nile deep-sea fan (Eastern Mediterranean): geophysical evidences. Marine and Petroleum Geology, 21, 669–689. Mattavelli, L., Ricchiuto, T., Grignani, D. & Schoell, M. 1983. Geochemistry and habitat of natural gases in Po basin, Northern Italy. American Association of Petroleum Geologists Bulletin, 67, 2239– 2254. Minissale, A., Magro, G., Martinelli, G., Vaselli, O. & Tassi, G. F. 2000. Fluid geochemical transect in the Northern Apennines (central– northern Italy): fluid genesis and migration and tectonic implications. Tectonophysics, Berlin, Heidelberg, New York, 319, 199– 222. Morse, J. W. 2003. Formation and diagenesis of carbonate sediments. Treatise on Geochemistry, 7, 67–85. Pasquale, V., Chiozzi, P., Gola, G. & Verdoya, M. 2008. Depth– time correction of petroleum bottomhole temperatures in the Po Plain, Italy. Geophysics, 73, E187–E196. Picotti, V. & Pazzaglia, F. J. 2008. A new active tectonic model for the construction of the Northern Apennines mountain front near Bologna (Italy). Journal of Geophysical Research, 113, B08412, 10.1029/2007JB005307. Picotti, V., Capozzi, R., Bertozzi, G., Mosca, F., Sitta, A. & Tornaghi, M. 2007. The Miocene petroleum system of the Northern Apennines in the central Po Plain (Italy). In: Lacombe, O., Lave´, J., Roure, F. & & Verge`s, J. (eds) Thrust Belts and Foreland Basins. Frontiers in Earth Sciences Special Volumes, Springer-Verlag, Berlin, Heidelberg, New York, 117–131. Picotti, V., Ponza, A. & Pazzaglia, F. J. 2009. Topographic expression of active faults in the foothills of

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Thrust belt architecture of the central and southern Western Foothills of Taiwan FERNANDO A. RODRIGUEZ-ROA1 & DAVID V. WILTSCHKO2* 1

Conoco-Phillips Co., 600 North Dairy Ashford – DU 3076, Houston, TX 77079

2

Texas A&M University, Department of Geology and Geophysics, MS 3115, College Station, TX 77843 – 3115 *Corresponding author (e-mail: [email protected]) Abstract: An internally consistent 3D structural model for the central and southern Western Foothills Fold and Thrust Belt (WFFTB) was constructed from serial balanced cross sections. The level of exposure, thrust sheet thickness and degree of internal complexity observed within the WFFTB are influenced by the presence of pre-existing normal faults. Most of the faults of the Western Foothills started their activity before the deposition of the Cholan Formation (c. 3.5 Ma). Out-of-sequence faulting is common and may be due to localized erosion and fault inversion. Basement appears to be more significantly involved towards the south where a new structure, the subYuching uplift, has been identified. The estimated aggregate displacement on WFFTB thrusts is uniformly about 40 km on the central segment cross sections, even in the region of greatest basement involvement. Total thrust displacement starts to decrease on the southernmost cross sections, which may be coincident with the transition from collision in the north to accretion. The restored position of the pre-existing normal faults places them as far east as the present-day Coastal Range. The WFFTB rocks must have been stripped off the Eurasian margin before significant burial could take place.

The role of pre-existing extensional structures as modifiers of the structural architecture of collisional orogens has been well documented (e.g. Gillcrist et al. 1987; Butler 1989; de Graciansky et al. 1989; Williams et al. 1989; Coward 1994; Butler 1997). Pre-existing extensional structures have been widely recognized in both offshore and onshore Taiwan (e.g. Sun 1985; Letouzey 1990; Yang et al. 1991; Huang et al. 1992; Huang et al. 1993; Sibuet & Hsu 1997; Lin & Watts 2002; Lin et al. 2003 Fig. 1). These basins are incorporated into thrusts within the Western Foothills Fold and Thrust Belt (WFFTB) (e.g. Lee et al. 2002; Mouthereau & Lacombe 2006) and the Hsueshan Range (Clark et al. 1993). Lee et al. (2002) point out that due to the inversion of pre-existing oblique structures, the Taiwan fold and thrust belt is three dimensional (3D) and must be approached in that manner. Our goal is to construct an internally consistent 3D model for the main structures in the WFFTB. With this model, it is then possible to assess the extent to which pre-existing structures affected the distribution and timing of structures.

Geological background Plate tectonic setting The active Taiwan orogen is the result of the oblique collision of the Philippine Sea and the Eurasian

plates (e.g. Suppe 1984; Barrier & Angelier 1986; Ho 1986; Tsai 1986; Teng 1990; Dadson et al. 2003). The island of Taiwan lies above both the east-dipping Manila and the north-dipping Ryukyu subduction zones, respectively (Fig. 1). Somewhere beneath Taiwan the subduction polarity flips (e.g. Chiai 1972; Suppe 1984; Tsai 1986; Chemenda et al. 1997; Wu et al. 1997; Lallemand et al. 2001). Although the onset of collision is generally attributed to the uppermost Miocene (e.g. Suppe 1981, 1984; Ho 1986; Lin et al. 2003; Simoes et al. 2007), some workers instead argue for an earlier onset (e.g. Tensi et al. 2006). The collision may have been diachronous (Chang & Chi 1983; Huang 1984; Delcaillau et al. 1994). From thermochronological evidence Lee et al. (2006) suggest that the exhumation history of Taiwan may be separated into two stages. The first stage started at 6 Ma and continued until about 1.5 to 1 Ma at a low uplift rate ,1 mm a21. A second high uplift rate (4– 16 mm a21) stage started at about 1.5 to 1 Ma until the present day (Lin 2002; Lee et al. 2006). At present the Philippine Sea Plate is moving NW (3068), towards the N055–0708 trending Eurasian plate margin (Seno 1977; Seno et al. 1993; Teng 1990; Yu et al. 1997). The main implication of this oblique collision is that the arc –continent collision has migrated southward through time. The collision has propagated from north to south at rates from 60 to 90 mm a21 (Suppe 1984; Barrier

From: Goffey, G. P., Craig, J., Needham, T. & Scott, R. (eds) Hydrocarbons in Contractional Belts. Geological Society, London, Special Publications, 348, 137–168. DOI: 10.1144/SP348.8 0305-8719/10/$15.00 # The Geological Society of London 2010.

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Fig. 1. Tectonic context of Taiwan and main tectonic features. Basins from Sibuet & Hsu (1997), Lin et al. (2005) and Yang et al. (2006). Offshore faults from Yang et al. (2006). Onshore tectonic features from Ho (1988) and Sibuet & Hsu (1997).

& Angelier 1986; Malavieille et al. 2002). Present deformation may be characterized as subduction and accretionary prism growth above the Manila Trench to the south of the island of Taiwan,

incipient collision in southernmost Taiwan, active collision in the south –central portion of the Island, and reduced tectonism in the north (e.g. Teng 1990; Lallemand & Tsien 1997). In the eastern Central

THRUST BELT ARCHITECTURE IN TAIWAN

Range (Fig. 1), the maximum local rates of uplift are 36– 42 mm a21 during the last decade. These are maximum rates based on levelling measurements (Liu 1995). The average rate of uplift now in the Central Range is 3–6 mm a21 based on stream erosion rates and thermochronometry (Dadson et al. 2003); GPS uplift rates are of the order of a centimetre per year (Wiltschko et al. 2002).

Regional geology Taiwan may be divided into five main geological provinces (Ho 1986, 1988, Fig. 1). From west to east, in central and southern Taiwan these are the Coastal Plain, Western Foothills, Slate Belt, Central Range and Coastal Range. The littledeformed Coastal Plain province is characterized by both low topographic relief and mainly Quaternary deposits. The western boundary of the Western Foothills is either an emergent or blind thrust at the leading edge of the collision. The Western Foothills overall is a NW verging, largely NE– SW trending fold and thrust belt that averages 50 km wide and more than 380 km long (Fig. 1). This fold and thrust belt contains folded and thrusted precollisional continental shelf strata underlying westward prograding synorogenic sediments. The latter clastic rocks were deposited in a rapidly subsiding foreland basin that deepen and accumulated more sediment to the south (e.g. Teng 1990; Mouthereau et al. 2001b; Lin et al. 2003; Simoes & Avouac 2006). Unmetamorphosed Miocene and younger rocks are involved in the Western Foothills structures. The boundary between the Western Foothills and the Slate Belt to the east is the Tulungwan and Chaochou thrust faults. The exposed Eocene to Oligocene low-grade metasediments and underlying rocks of the Slate Belt have been commonly referred as the basement of the Western Foothills. The Slate Belt– Central Range boundary is also marked by thrusts that may have moved obliquely. The Central Range, or metamorphic core of the orogen, is composed of preTertiary highly folded and foliated metamorphic rocks including gneisses, schists and some marble. The eastern boundary of the Central Range is the Longitudinal Valley that is considered to be the collapsing fore-arc basin along the Eurasian –Philippine Sea Plate boundary (e.g. Lundberg et al. 1997). The Coastal Range is a remnant Neogene island arc that is colliding with the Eurasian margin. It is a northern extinct extension of the Luzon magmatic arc.

Pre-existing normal fault-bounded basins The Taiwan collision is placing thrusted shelf rocks back up onto the rifted margin. The existence of

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Cenozoic rift basins has been well documented in the continental shelf, offshore and onshore Taiwan (Chow et al. 1991; Hsiao et al. 1991; Yang et al. 1991; Huang et al. 1993; Lee et al. 1993; Shen et al. 1996; Tzeng et al. 1996; Sibuet & Hsu 1997; Lin et al. 2003). These basins were active during the Tertiary and the age of initiation of the basins youngs to the SE (e.g. Sibuet & Hsu 1997; Tensi et al. 2006). Lin et al. (2003) summarize the extensional history of basins marginal to Taiwan. During the lower to middle Eocene lithospheric extension occurred simultaneously in discrete rift belts. The syn-rift record is now exposed in the Backbone and Hsuehshan Ranges in Taiwan and represents sediments deposited in the outer part of the margin. From late Eocene to early Oligocene times the focus of rifting moved to the present-day ocean –continent boundary on southern Taiwan, leading to continental rupture and initial sea-floor spreading. Oligocene thermal uplift was followed by rapid, early post-breakup subsidence accompanied by normal faulting from the late Oligocene to middle Miocene. During the early to middle Miocene, the entire margin experienced broad thermal subsidence. During the middle Miocene rifting started again, with higher amounts of extension in the outer margin; this lasted until the late Miocene (c. 6.5 Ma). Synsedimentary growth faults trend nearly east–west, in the offshore Tainan and Taihsi basins. The trends of these rift basins and normal faults within them are not the same. For instance, the Tainan basin is characterized by internal east – west normal faults that are oblique to the N –NW to S –SE opening direction of the basin. Especially in northern Taiwan, the normal faults trend around 060 to nearly east –west, whereas the basin margins mimic the Eurasian coastline (Fig. 2). In the southern half of the Taiwan fold–thrust belt, the trends of the shelf basins and the foothills diverge and as a result the pre-existing normal faults are even more oblique to thrust fault trends. Rifting occurred from early Oligocene to early middle Miocene. The Taishi basin to the north was subjected to extension during the Eocene with weak activity also during the Oligocene and perhaps as early as the Paleocene. The Nanjihato basin is a NE to SW trending half-graben that formed between the early Paleocene and late Eocene. Synsedimentary growth faults trend nearly east –west, as in the offshore Tainan and Taishi basins (Lu et al. 1991). Peak rifting occurred during the middle Eocene to Oligocene and perhaps as early as the early Miocene (Lu & Hsu¨ 1992). Onshore, the Hsuehshan trough has been recognized previously by Ho (1986) as a pre-existing large basin that has been incorporated into the Taiwan orogen.

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THRUST BELT ARCHITECTURE IN TAIWAN

How important these pre-existing basins are to the subsequent development of the WFFTB is controversial. Some workers suggest that the thrust belt is a fairly typical forward breaking thin-skin thrust belt above a largely planar, east-dipping detachment (i.e. Suppe 1984; Dahlen & Barr 1989; Hickman et al. 2002). Other researchers propose that many structures in the Western Foothills both involve pre-Miocene basement and have been inverted (Hung et al. 1999; Mouthereau et al. 2001a, b; Lacombe & Mouthereau 2002; Lee et al. 2002; Mouthereau et al. 2002; Lacombe et al. 2003; Rodriguez-Roa & Wiltschko 2006; Mouthereau & Lacombe 2006). The extent of inversion has not been described in a systematic way. We provide further constraints on the location of pre-existing extensional faults, and through a series of internally consistent serially balanced cross sections show their configuration before thrusting began.

Western Foothills Fold and Thrust Belt We focus on the fold and thrust belt of the central and southern portion of the Western Foothills. The structures observed at the surface are wide synclines and narrow anticlines on thrust faults. The vergence of the structures is largely westward and the fold axes and fault traces generally trend N10–20E (Fig. 3). Oblique-trending high-angle transverse faults are also common (e.g. Deffontaines et al. 1997; Fig. 3).

Basis for cross sections In order to understand the geometry and kinematics of the WFFTB, 13 serial balanced cross sections (Figs 5–17) were constructed from the leading edge of the Slate Belt to the undeformed foreland (Fig. 3). These cross sections were sequentially restored and balanced using 2DMovew. Our procedure was to use kinematic modelling first, where particles are restrained to motion parallel to either fault or other bedding surfaces. Complex areas were restored by using line length unfolding and area preservation. Finally, we sequentially restored every fault, backward and forward to test the accuracy of the modelled geometries. We did not include decompaction or isostatic adjustments. Depth of detachment. A necessary starting point for any cross section is the depth to detachment. In Taiwan, the regional depth to detachment has been placed at several different levels: c. 6 km in the north (Yue et al. 2005); 12–15 km (Mouthereau et al. 2001b, 2002); c. 7 –12 km (Yang et al. 2007); c. 12 km (Hung et al. 1999; Hickman et al. 2002); c. 10 km in central Taiwan (Carena et al. 2002). The main criteria used by previous workers is largely geometric, although Carena et al. (2002)

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and Mouthereau et al. (2002) used low-magnitude focal mechanisms to place the main detachment between 10 and 15 km below the Western Foothills at the western edge of the Slate Belt. Based on geometric constructions and reported focal mechanisms we place the main detachment zone at a depth between 10 and 15 km. Western Foothills stratigraphy. An accurate stratigraphic frame for the Western Foothills is difficult to establish because the syntectonic strata change facies both from north to south and east to west and the stratigraphic nomenclature varies over short distances. Moreover, chronostratigraphic control is not of sufficient resolution in key areas. Additionally, the structural complexity of the Western Foothills makes the stratigraphic relationships more complicated. To overcome the stratigraphic nomenclature problem, a lithostratigraphic approach was adopted. Lithological units are the basis of surface geological maps because they have characteristic topographic expressions and are otherwise more straightforward to map. Correlating lithological units avoided the plethora of nomenclature conflicts from north to south. Our correlation (Table 1) is based on the one from Tensi et al. (2006), who incorporated some age control. The broad pattern of sedimentation is well known. The Eocene to Miocene section is interpreted to begin with rift related to passive margin deposits followed by synorogenic clastics since the upper Miocene (Chou 1973, 1980; Ho 1988). Most of Pliocene and younger sediments are shaly sandstones or sandy shales with alternating sandstones. Some of these shales correspond to regional detachment surfaces like the Chinsui shale (Table 1). We used the Chinsui shale and the equivalent formations as the regional datum for restoration, because this marker can be followed on maps regionally and is present throughout the WFFTB. The base geological maps used are the No. 5 Chia-I, and No. 6 Tai-Nan by the Chinese Petroleum Company (CPC 1986, 1989), and the maps by Chiu (1975) and Chang (1971). We incorporate data from published wells (Fig. 3) and seismic data (Chang 1971; Hung et al. 1999; Huang et al. 2004; Yue et al. 2005; Yang et al. 2007) The most important wells incorporated were PKS-1 (Chang 1971), KTL-3 (Chang et al. 1998), MLN-1, HM-3 (Suppe & Namson 1979) and TSK-1 (Chiu 1975). Published thermochronological data were used to constrain the thermal history implied by the kinematics. The data were detrital apatite and zircon fission track ages, from Liu et al. (2001) and Fuller (2002) (Fig. 3). Specifically, the thermal data may be used to infer both the maximum burial depth and the time since exhumation through the partial annealing zone for each mineral. For example,

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Fig. 3. WFFTB geological map showing cross section lines, GPS velocities, key wells and geological areas defined in the text. Compiled from Chinese Petroleum Company (CPC 1986, 1989), Chang (1971) and Chiu (1975). See Figure 4 for index to numbered structures. Points a–d are locations referred to in the text.

assuming constant cooling rates within the range of 1–100 8C/Ma, the effective track retention temperatures have been estimated at 135 + 20 8C for apatite (Wagner & Reimer 1972; Liu et al. 2001), and 235 + 20 8C for zircon (Liu et al. 2001). Based solely on the annealing properties of apatite, the partially reset samples would have had to reach temperatures of the order of 90–100 8C, while the unreset samples had to stay below

+60–80 8C (Fuller 2002; Fuller et al. 2006). Additionally, muscovite Ar40/Ar39 ages and partially reset biotite Ar40/Ar39 ages in rocks from the Central Range indicate that the maximum temperatures experienced by these rocks are between 350 and 400 8C. These temperatures are consistent with the metamorphic greenschist facies conditions observed in the metamorphic pre-Tertiary basement rock of Taiwan (Liou & Ernst 1984).

THRUST BELT ARCHITECTURE IN TAIWAN 120°30’

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Fig. 4. Simplified fault map of the WFFTB showing major structures, cross section lines and geological areas discussed in text. List of numbered faults on right is a key to the faults labelled on both Figure 3 and the cross sections.

Assuming an average surface temperature of 15 8C, and a geothermal gradient of about 20– 25 8C, we estimate that the apatite fission track (AFT) data constrain partially reset samples to a maximum burial depth of 3–4 km. With the same assumption, partially reset zircon fission track (ZFT) samples imply a maximum burial depth of 6–7 km. The Ar40/Ar39 ages from rocks of the Central Range imply a maximum depth of burial of 14 to 16 km. The AFT samples in the Western Foothills (Fig. 3) show that while none of the Miocene age samples are reset, they do show different degrees

of annealing. The degree of annealing may be correlated to stratigraphic position and/or to the position in fault blocks with different thermal histories. Additionally, Ar40/Ar39 ages in rocks from the Central Range constrain the geometry of faults and the amounts of vertical uplift. Large uplift on high-angle crustal-scale faults beneath the Slate Belt would have reset Ar40/Ar39 thermochronometers. We therefore speculate that the depth of the detachment beneath the Central Range is about 19 –21 km. This depth is constrained by maximum depth of burial (14–16 km) of the outcropping rocks with partially reset Ar40/Ar39 samples

144 F. A. RODRIGUEZ-ROA & D. V. WILTSCHKO Fig. 5. Section A–A0 . See Figure 3 for location. Numbered structures on section are defined on Figure 4. Primed numbers refer to the restored positions of the respective faults on the deformed section. Extensional fault associated with Peikang High interpreted from seismic lines of Chang (1971) and PKS-1 well data. C, Changhua thrust (intersected by PKS-1 well). Chi-Chi Earthquake hypocentre (star) from Kao et al. (2000). Restored section comments: (a) This normal fault is inferred from thickening in Miocene section and is consistent with the structural style observed toward the south. This is the shallowest position estimated from restoration. This block may have been deeper because the pre-Miocene sediments are little metamorphosed. (b) This detachment is interpreted as being the fault whose shallowest segment will be reactivated during the Chi-Chi earthquake. Deformed section comments: (c) This detachment, interpreted at about 13 km depth, is consistent with focal mechanisms. (d) This small sub-thrust anticline is based on growth strata (mainly onlaps) interpreted by Chang (1971).

THRUST BELT ARCHITECTURE IN TAIWAN Fig. 6. Section B– B0 . See Figure 3 for location. Numbered structures on section are defined on Figure 4. Primed numbers refer to the restored positions of the respective faults on the deformed section. PH, Peikang High; HR, Hsueshan Range; C, Changhua thrust; Chi-Chi earthquake hypocentre (star) from Kao et al. (2000). (a) normal fault inferred from thickening in Miocene section.

145

146 F. A. RODRIGUEZ-ROA & D. V. WILTSCHKO Fig. 7. Section C–C0 . See Figure 3 for location. Numbered structures on section are defined on Figure 4. Primed numbers refer to the restored positions of the respective faults on the deformed section. PH, Peikang High; HR, Hsueshan Range. C, Changhua thrust, Chi-Chi Earthquake hypocentre (star) from Kao et al. (2000). (a) and (b) normal faults inferred from thickening in Miocene section.

THRUST BELT ARCHITECTURE IN TAIWAN Fig. 8. Section D–D0 . See Figure 3 for location. Numbered structures on section are defined on Figure 4. Primed numbers refer to the restored positions of the respective faults on the deformed section. PH, Peikang High. D, Extensional fault associated with Peikang High, interpreted here from the change of thicknesses between the Peikang High Cenozoic rocks and the thrust belt Cenozoic rocks. (a) Normal fault inferred from thickening in Miocene section. (b) We choose to interpret a normal fault here to preserve the structural style interpreted in sections to the north. 147

148 F. A. RODRIGUEZ-ROA & D. V. WILTSCHKO

Fig. 9. Section E –E0 . See Figure 3 for location. Numbered structures on section are defined on Figure 4. Primed numbers refer to the restored positions of the respective faults on the deformed section. PH, Peikang High; HR, Hsueshan Range. (b) and (c), Faults interpreted here to be consistent with the structural style of adjacent cross sections. (d) and (d0 ), point that shows the restored depth of the lower Miocene sediments and implies the thickening of Miocene sedimentary section.

THRUST BELT ARCHITECTURE IN TAIWAN Fig. 10. Section F–F0 . See Figure 3 for location. Numbered structures on section are defined on Figure 4. Primed numbers refer to the restored positions of the respective faults on the deformed section. PH, Peikang High. E, Sub-Yuching fault. (a) Normal fault inferred here from thickening in Miocene section. (b) normal fault placed here to preserve the structural style interpreted in sections to the north to be consistent with the structural style. (c) This point is the maximum restored depth of the lower Miocene sediments and implies the thickening of the Miocene sedimentary section. 149

150 F. A. RODRIGUEZ-ROA & D. V. WILTSCHKO Fig. 11. Section G– G0 . See Figure 3 for location. Numbered structures on section are defined on Figure 4. Primed numbers refer to the restored positions of the respective faults on the deformed section. PH, Peikang High. E, Sub-Yuching fault. F, Hsiamei anticline. (a) normal fault here inferred from thickening in Miocene section. (b) normal fault required here to preserve the structural style interpreted in sections to the north. (c) This point shows the maximum restored depth of the lower Miocene sediments and implies the thickening of the Miocene sedimentary section. See text for details.

THRUST BELT ARCHITECTURE IN TAIWAN Fig. 12. Section H– H0 . See Figure 3 for location. Numbered structures on section are defined on Figure 4. Primed numbers refer to the restored positions of the respective faults on the deformed section. PH, Peikang High. Numbers on section are the following: K, Kuantzuling– Nanliao anticline; E, Sub-Yuching fault. (a) normal fault here inferred from thickening in Miocene section. (b) normal fault here to preserve the structural style interpreted in sections to the north.

151

152 F. A. RODRIGUEZ-ROA & D. V. WILTSCHKO Fig. 13. Section I–I0 . See Figure 3 for location. Numbered structures on section are defined on Figure 4. Primed numbers refer to the restored positions of the respective faults on the deformed section. PH, Peikang High. E, Sub-Yuching fault. (a) normal fault inferred from thickening in Miocene section.

THRUST BELT ARCHITECTURE IN TAIWAN Fig. 14. Section J–J0 . See Figure 3 for location. Numbered structures on section are defined on Figure 4. Primed numbers refer to the restored positions of the respective faults on the deformed section. PH, Peikang High. E, Sub-Yuching fault. (a) and (b) normal faults inferred from thickening in Miocene section.

153

154 F. A. RODRIGUEZ-ROA & D. V. WILTSCHKO Fig. 15. Section K– K0 . See Figure 3 for location. Numbered structures on section are defined on Figure 4. Primed numbers refer to the restored positions of the respective faults on the deformed section. PH, Peikang High. E, Sub-Yuching fault. (a) and (b) Normal faults inferred from thickening in Miocene section.

THRUST BELT ARCHITECTURE IN TAIWAN Fig. 16. Section L –L0 . See Figure 3 for location. Numbered structures on section are defined on Figure 4. Primed numbers refer to the restored positions of the respective faults on the deformed section. E, Sub-Yuching fault. See points (a), (b) in Figure 15 for explanation. (c) Partially reset AFT sample (5.5 Ma), from Fuller (2002). (d) Partially reset AFT sample (3.3 Ma), from Fuller (2002). See text for details.

155

156 F. A. RODRIGUEZ-ROA & D. V. WILTSCHKO Fig. 17. Section M– M0 . See Figure 3 for location. Numbered structures on section are defined on Figure 4. Primed numbers refer to the restored positions of the respective faults on the deformed section. PH, Peikang High. E, Sub-Yuching fault. (a) normal faults inferred from thickening in Miocene section.

Table 1. Stratigraphy of the central and southern Western Foothills of Taiwan

Epoch Quaternary

Chiu H.T., 1975 Kuohshing Area

Lingkou Huoyensahan Huoyensahan

0.55+/-0.15 Toukoshan

Liushang

Liushang

Liushang

Huoyenshan

Toukoshan

0.88+/-0.1

Hsiangshan

Hsiangshan

Erhchungchi Erhchungchi

1.02+/-0.5

Yunshing Kanhshaliao

0.3+/-0.1 2+/-0.2

Chingmien

Erhchungchi

Transition Zone Toukoshan

Gutingkeng Upper Part

Cholan

Cholan Yunsuichi

Chinsui

Oligocene

23+/-1 23.8+/-1 23.8 24.75+/0.25 32+/-2 33.7

Eocene

55.8

Paleocene

66.5

After Tensi et al. (2006)

Liuchungchi Cholan

Peiliao

Liuchungchi

Chutouchi

Chutouchi

Yunsuichi

Maopu

Maopu

Maopu

Aliaochiao

Aliaochiao

Cholan Member Chutouchi Maopu

Chinsui Niaotsui

Shihlioufen

Lower Member Gutingkeng Lower Part

Aliaochiao

Chinsui

Chinsui

Nanshihlun

Yutenping Shihlioufen

Yensuikeng

Kueichulin

Kaitzuliao

Kueichulin

Kueichulin

Aliaochiao

Yensuikeng

Yensuikeng

Yensuikeng

Tangenshan

Tangenshan

Tangenshan

Chunglun Kuantaoshan

Kuantaoshan

Nanchuang Shangfuchi Tungkeng

14.6+/-1

20+/-1

Chinsui Yutenping

shihliufen

10+/-1 12.5+/-1

17+/-1

Chinsui

Yutenping

8.6+/-0.5

Miocene

Peiliao Lower Member Upper Member

3+/-0.5

9+/-1

Upper Member

Peiliao

2.8+/-0.5

3.5+/-0.5 3.7+/-0.5 4+/-0.8 5.2+/-0.2 5.33 5.3+/5.3 5.5+/-0.5 7+/-1.5

2nd Member 1st Member

Hsiangshan Facies

Liuchungchi Cholan

Cholan

Pliocene

Map CPC, Yuching Sc

Lateritic Terrace Deposit 0.4+/10.1

Pleistocene

Chang 1970 Taichung Basin Age M.a.

Tangenshan

Shangfuchi

Nanchuang

Tungkeng

Kuantaoshan

Tangenshan

Changchikeng

Shangfuchi

Hunghuatzu Shamin

Tungkeng

Kuanyinshan Kuanyinshan Kuanyinshan Kuanyinshan Kuanyinshan Kuanyinshan Nankang

Wushan

Changchikeng Nanchuang Fm

Talu

Talu

Talu

Talu

Talu

Talu

Talu

Peiliao

Peiliao

Peiliao

Peiliao

Peiliao

Peiliao

Shihti Taliao

Shihti Taliao

Shihti Taliao

Shihti

Shihti Piling

Shihti

Mushan

Mushan

Mushan Wuchihshan

Wichihshan Wichihshan

Mushan Wichihshan Wichihshan

Mushan

Hunghuatzu Shamin

Nanchuang Fm

Changchikeng Hunghuatzu Shamin

Nanchuang Fm

Changchikeng Hunghuatzu Shamin

Kuanyinshan Mb

Peiliao Shihti Taliao

Nanchuang Fm

Nankang (Nanko) Tsuoho (Sogo) Fm

Shuichangliu (Suityoryu Fm)

Wichihshan Paileng (Hakurei Fm)

Nankang Fm

Nankang Fm

Nankang Fm

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F. A. RODRIGUEZ-ROA & D. V. WILTSCHKO

from the Central Range plus an additional 5 km of post-Pliocene sediments. We projected the restored Chinsui shale, because this unit would have been the top of the thrust sheet at the time of initial deformation in late Pliocene– Early Pleistocene (Mouthereau et al. 2001b). A regional detachment surface between 10 and 15 km depth below the Western Foothills, must be the updip extension of the regional detachment we estimate beneath the Central Range. The kinematic model was integrated with geodetic data. We used the 3D GPS velocities of Wiltschko et al. (2002), supplemented by horizontal velocity vectors from Yu et al. (1997). We correlated the current motion with accumulated displacement by projecting current GPS velocity vectors on our cross sections.

Description of structures and balanced cross sections We group the structures in three main areas for convenience of discussion. These three areas are defined based on the average orientation of structures, topographic relief and the amount of shortening on the main thrusts. From north to south they are the Taichung, Meishan and Yuching areas (Figs 3 & 4). The Taichung– Meishan area boundary (about section D –D0 ) is marked by a change in structural trend and a widening to the south of the WFFTB. The Meishan –Yuching area boundary (about section H –H0 ) is marked by a reduction of surface exposures of Miocene rocks and a broad bend in the trend of the Tulungwan fault.

Taichung area The Taichung area is characterized by a thickening of the upper Pliocene and Pleistocene units compared with the thickness observed toward the south. The Changhua thrust constitutes the deformation front. The maximum displacement estimated for this thrust is 2.2 km along section A –A0 (Fig. 5), decreasing toward the south. The axial traces of the Tatushan and Pakuahsan anticlines are arcuate and follow the shape of the Peikang High. The equivalent structure in the Meishan Area is the Meilin anticline (cross section D –D0 , Fig. 8). Little to no deformation propagates west of this group of structures into the Coastal Plain. The Chelungpu thrust is the emergent frontal thrust in the Taichung area. The Chelungpu thrust sheet is bound to the east by Shuangtung thrust system; its footwall is the Coastal Plain. The Chelungpu thrust ruptured during the Chi-Chi earthquake (M ¼ 7.6) on 21 September 1999 (Ma et al. 1999; Chang et al. 2000). The monocline on its

hanging wall is a fault-bend fold that in map view shows a flat on ramp detachment along the Chinsui shale. This Chinsui shale detachment crops out parallel to the Chelungpu thrust (Fig. 3). In addition, the Chelungpu thrust has at least another detachment in a lower stratigraphic level, probably the Talu shale. The latter is suggested by the presence of the Kueichulin Formation that crops out on the northern portion of the Chelungpu thrust (Chang 1971) and, therefore, must be restored to a lower stratigraphic position than the Chinsui shale. The Chelungpu thrust loses displacement south of the Choshui River (point b, Fig. 3) where its hanging wall syncline plunges toward the south (closure around 238450 ; CPC 1986) at the Chinsui shale level. About 20 km of displacement is transferred from the Chelungpu and Schiangliu thrusts to the Tachienshan thrust in front of the Peikang High. The Chelungpu thrust is listric, dipping up to 458 on its shallower portions. The maximum displacement estimated for the Chelungpu thrust is 15 km in section A– A0 , decreasing to 13 km in section B –B0 , 11.6 km section C–C0 and 8 km in section D –D0 . In a similar manner to both Mouthereau et al. (2001a) and Yue et al. (2005), we call on a deeper fault, here called Chi-Chi fault, on which slip took place during the Chi-Chi earthquake. The Chi-Chi fault links with the Chelungpu thrust at depth and can be interpreted as an inverted extensional fault. We base our interpretation on the following. First, the focal depth for the Chi-Chi earthquake (10 km +0.5 km depth; Kao et al. 2000) is deeper than the Chelungpu thrust and must be interpreted as a different fault. Secondly, there are well-documented Palaeogene extensional faults along Taiwan that have been inverted, as discussed above. Consequently, the existence of normal faults beneath the Western Foothills is not only possible but likely. Thirdly, the surface expression of the Chi-Chi subsurface rupture occurred along the Chelungpu thrust. Since the Chi-Chi fault and Chelungpu thrust must be two different faults, we suggest that the Chi-Chi fault and the Chelungpu thrust join in the subsurface. This interpretation is reflected in sections A-A0 to C-C0 (Figs 5–7). The Chi-Chi fault must have a small cumulative displacement in order to not create observable surface deformation such as second order folding in the Chelungpu thrust sheet (Figs 3 & 5 –8). To honour this constraint, we gave the Chi-Chi fault a maximum displacement of 600 m. Toward the south, the Chelungpu thrust seems to be truncated by the Tachienshan thrust (Figs 8 & 9). The Schuangtung thrust exposes rocks as old as Oligocene within the Tsukeng anticline and Eocene rocks Schuangtung thrust sheet (Fig. 3). These Eocene and Oligocene rocks are considered basement rocks from the Slate Belt. The Eocene and

THRUST BELT ARCHITECTURE IN TAIWAN

Oligocene rocks have incipient metamorphism, as can be inferred by the presence of slaty shales and arkosic sandstones with small amounts of muscovite (e.g. Chiu 1975). One remarkable feature of the Schuangtung thrust is the large change in stratigraphic thickness across it. The Oligocene units within the upper plate are at least 1.7 km thick or almost double the thickness found in the PKS-1 well (about 0.9 km, Fig. 5). The north–south Tahengpingshan syncline and its anticline pair are the main structures associated with the Schuangtung thrust. Minor splays cut and refold the Tahenspinshan syncline in several segments. We modelled the east limb of the Tahengpingshan syncline as the front-limb of the frontalmost anticline of the Slate Belt. This anticline is broken on its backlimb by the out-of-sequence Schuiangliu (Tulungwan) fault that gains displacement from this point toward the south. The exposed rocks on the hanging wall of the Schuangtung thrust are the oldest and most thermally mature rocks of the WFFTB, showing evidence of very low grade metamorphic conditions. The displacement on the Schuangtung thrust varies from 20.7 km in section A –A0 (Fig. 5), in the north, to a maximum in section B–B0 of 22.8 km (Fig. 6). The displacement decreases toward the south to 14.5 km in section C –C0 (Fig. 7), and 10.5 km in section D –D0 (Fig. 7). The Tulungwan fault carries a number of lowgrade metamorphosed hinterland imbricates and is the boundary between the Hsueshan Range portion of the Slate Belt and the Western Foothills. This fault system may be interpreted as a low-angle thrust. It begins as an out-of-sequence thrust that breaks the backlimb of a basement-cored anticline at 248100 (around point a in Fig. 3). At this point the Paileng Fm (Eocene) underlies sedimentary rocks from the foothills Neogene sedimentary cover. The complete sequence from Eocene to Oligo-Miocene foothills sediments is present there. The throw is less than 100 m in most of the northern area because the fault juxtaposes the Shithi Formation against the Peiliao Formation omitting the Peiliao Formation, which is about 100 m thick. The Tulungwan fault increases displacement from this northern tip to the south. In the segment between 238420 and 238330 there is a change in the strike of this fault from N10E to N30W. This change in orientation can be interpreted as either an oblique ramp or a relay ramp where displacement is transferred from the Tulungwan fault to the Chaochou thrust. The position of this oblique feature also corresponds in latitude with both the approximate northern extent of the Peikang High and the southern extension of the Hsuehshan Range (Mouthereau et al. 1999). From this oblique ramp segment towards to the south, this fault (or its equivalent) is often called the Chaochou thrust.

159

The Meishan area South of section D –D0 the WFFTB widens and the trend of the main structures changes from approximately north –south to NNE–SSW. In addition, the topographic elevations increase and the level of exhumation exposes older rocks (up to Miocene) on the leading edge of structures. The main structures in the Meishan Area are the Meilin anticline, and the Tachienshan, Luku and Tulungwan – Chaochou thrusts. South of the Choshui River (point b on Fig. 3), the Meilin anticline occupies an equivalent structural position to that of the Pakuashan anticline. However, based on both well data and previous interpretations (Suppe & Namson 1979) we interpret this anticline as a fault-propagation fold. The best geometric fit is obtained if the anticline grows in two stages. The first stage of growth is by faultpropagation folding with 4 km of displacement and the second stage is by fault-bend folding with 1.4 km of displacement. The southern part of the Meilin anticline ends abruptly against a transverse fault we interpreted as a transfer fault because there is a change in vergence of the equivalent structure toward the south. The Tachienshan thrust is the frontal emergent thrust in the Meishan area (Fig. 3). From the Choshui River (point b, Fig. 3), this fault progressively increases displacement towards the south (Figs 7–12). The Pingchi fault occupies an equivalent structural position to the Tachienshan thrust from the Yuching area to the south. The Tachienshan thrust can be interpreted either as an out-ofsequence thrust, or an out-of-sequence reactivation of an existing thrust. This out-of-sequence interpretation is inferred from balanced cross sections and from the geological map in which we observe that the fault trace becomes significantly displaced towards the west. In addition, the Tachienshan thrust cuts both the Chelungpu thrust and the Chukou and Lunhou thrusts in the northern and southern parts of the Meishan area, respectively. The increase in displacement on the Tachienshan thrust coincides with the decreasing of displacement in the Chelungpu thrust in the Taichung area. The area of maximum displacement along the Tachienshan thrust coincides with its closest position to the Peikang High. The exhumation level of the hanging wall of the Tachienshan thrust remains constant south of the Choshui River. No rocks older than Miocene (Nankang and Nanchuang Formations and equivalents) are currently exposed. Moreover, AFT data show unreset samples of apatite for this area. The AFT results suggest that these rocks were never buried deeply enough to anneal the apatite fission tracks. Previous interpretations involve, or imply, stacks of imbricate thrust sheets

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with Miocene rocks coming from depths of 6–8 km (Hung et al. 1999; Yang et al. 2007). We instead interpret the Tachienshan thrust as an inverted fault with a footwall shortcut. We propose that the Tachienshan thrust evolved in two stages. First, after the deposition of the Chinsui shale in the Upper Pleistocene, motion on the Tachienshan thrust generated enough relief to restrict the space available for Plio-Pleistocene sediments (Cholan Fm, Table 1). As a result, the Plio-Pleistocene sediment thickness was not sufficient to reset the AFT samples. Second, the Tachienshan thrust was reactivated in an out-of-sequence manner that is synchronous with the deposition of the Upper Pleistocene and Quaternary sediments (Toukoshan Fm. and younger sediments, Table 1). Even though the velocity vector orientations from GPS are parallel to faults in most cases on the Western Foothills, there is a zone of anomalous velocities on the hanging wall of the Tachienshan thrust (Fig. 3). The vertical component in the anomalous velocity zone, indicates a downward direction of displacement (sections E –E0 to H –H0 , Figs 9– 12) with magnitudes up to 70 +7 mm. South of 238220 the Tachienshan thrust has an oblique ramp and decreases in displacement. The latter oblique ramp seems to be cutting the Chukou and Lunhou thrusts in an out-of-sequence relationship. We treat the Fenghuachan and Luku thrusts as one because, (1) they have an equivalent structural position and (2) their surface trace is aligned on-strike in map view. There is a large change in stratigraphic thickness across the Fenghuachan– Luku thrust. This change in thickness is evident on cross section E –E0 (Fig. 9), where the restored position of the topography in the hanging wall of the Luku thrust implies a thicker Miocene section (at least 3 km) than that encountered in the MLN-1 well (about 0.6 km). The displacement of the Luku thrust increases towards the south (Figs 8–11). South of section G –G0 (Fig. 11), the Luku thrust seems to be cut by the Tachienshan thrust. This can be inferred from both the Luku fault’s displacement profile (Fig. 19), and the fact that the Luku thrust truncates against the southern oblique ramp of the Tachienshan thrust (Fig. 18). The Tulungwan or Chaochou thrust seems to be increasing in displacement from south of the Taichung area to the Meishan area. From the Meishan area to the south, the Chaochou thrust decreases in displacement. This decrease in displacement and strain intensity has been inferred by Wiltschko et al. (2010) in southern Taiwan based on: (1) the absence of rotational strain indicators such as sigmoidal quartz fringes, (2) the metamorphic grade of the Slate Belt and (3) the intensity of strain in the outcropping rocks on the hanging wall the

Tulungwan –Chaochou thrust decreases toward its south portion, as well as the amount of active uplift.

Yuching area South of about section J–J0 , the age of exposed rocks decreases and the WFFB– Slate Belt boundary starts a broad turn towards the south (Figs 14– 17). In the Yuching area, the Tachienshan thrust loses displacement and the dominant structures are the Chukou and Lunhou thrusts and their associated structures, the Yuching and Tingpinglin synclines, which are interpreted as fault-bend folds. At the south end of the Yuching area the displacement along the Lunhou thrust dies out and the displacement along the Chukou thrust decreases to the south. The remaining displacement along the Chukou thrust is transferred to the hinterland through a lateral ramp. At the northern part of the Yuching area, around 238220 , the Tachienshan thrust cuts the Chukou and Lunhou thrusts in an out-of-sequence relationship based on the following. (1) The geological map (Fig. 18) indicates that the Tachienshan thrust truncates the Chukou and Lunhou thrusts, (2) displacement profiles (Fig. 19) show a truncation in the displacement of the Chukou and Lunhou thrusts against the Tachienshan thrust; (3) it is not possible to continue the Chukou and Lunhou thrusts from section H –H0 to G– G0 (Figs 11 & 12) due to changes in stratigraphic thicknesses and structural style; (4) juxtaposition of units across the Tachienshan thrust shows a reverse fault relationship along most of its trace (i.e. Fig. 18c, points b and c), except in the intersection between the Chukou and Lunhou thrusts, where it shows a normal fault juxtaposition relationship (Fig. 18c, point b). By comparing the present elevations of the Chinsui shale (or equivalents) we found that: (1) the Yuching and Tingpinglin synclines are uplifted with respect to the regional level and (2) the footwall of the Chukou thrust is, in general, a wide panel dipping toward the west. This dipping panel can be described as a frontal monocline along the footwall of the Chukou thrust. To explain the regional uplift of the Yuching and Tingpinglin synclines and the frontal monocline on the footwall of the Chukou thrust, we propose the presence of a basement-cored anticline that detached at 10–13 km (sections G –G0 to M–M0 , Figs 11–17). We will call this structure the sub-Yuching uplift. It is modelled as a fault-bend fold with a maximum displacement of 11.5 km in section J –J0 (Fig. 14). The displacement along the sub-Yuching fault decreases both north and south (Fig. 19). The forelimb of the sub-Yuching structure explains the frontal monocline located in the footwall of the Chukou thrust. There are minor folds associated with this forelimb. These minor folds are interpreted as part of an intercutaneous wedge resting

THRUST BELT ARCHITECTURE IN TAIWAN

161

Fig. 18. Examples of normal and inverted fault in the Western Foothills. (a) Location map. (b) Example of a transported normal fault on the Schuangtung thrust. d, detail of out-of-sequence relationship. (c) Example of the out-of-sequence relationship on the Tachienshan thrust truncating the Chukou and Lunhou thrusts. Letters in (c) show an inverted fault example along the Tachienshan thrust. Points b and c show a reverse fault relationship across the Tachienshan thrust. Point a shows a normal fault relationship across the Tachienshan thrust. See text for details.

above and linked to the roof thrust of this wedge (e.g. section J –J0 , Fig. 14) The wedge transfers displacement from the sub-Yuching fault toward the frontlimb. The sub-Yuching uplift also explains the elevated position of the Yuching and Tingpinglin synclines, because these two structures rest over the wide and flat crest of the sub-Yuching uplift. An alternative interpretation to explain this regional uplift is the presence of an antiformal stack of sub-thrust duplexes beneath the Yuching and Tingpinglin synclines. However, if this were the case, a stack of duplexes might cause interference folding, reflecting the structural thickening underneath. Also if the Chukou and Lunhou thrusts are stacked thrusts (Hickman et al. 2002) it would imply more vertical uplift (about 8 km) than that inferred from the fission track data (Fig. 3). Finally, a stack of thrusts would imply larger horizontal displacements. This is not consistent with the fact that displacement decreases to the south along the Chukou and Lunhou thrusts.

The Pingchi thrust is the eastern boundary of the Tingpinglin syncline. The Pingchi thrust sheet is characterized by the wide Chiahsien syncline. The Miocene in this sheet is thicker than that in the Chukou and Lunhou thrusts. This thickening is interpreted to be a consequence of being on the hanging wall of a normal fault. The eastern part of the Chiahsien syncline is cut by the Chishan thrust. The rocks within the Chishan thrust sheet also show stratigraphic thickening and partially reset AFT ages (Fig. 3). The thermal maturity of this rock is due to its location in a deeper part of the basin subjected to more overburden (Fuller 2002). Towards the south of the Chukou thrust lateral ramp this faults gains displacement.

Three-dimensional model A 3D model was constructed based on 13 balanced cross sections and three additional transverse sections in order to ensure that the structures as drawn

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Fig. 19. Fault displacement v. distance along strike. For location of distance profile see Figure 3 (dashed line). See text for details.

represent an internally consistent model for fault geometry (Fig. 20). Due to stratigraphic complexity in the WFFTB we decided to limit our model to the major thrust sheets, namely, the Chukou, Lunhou, Pingchi, Chishan and Chaochou thrust sheets.

Discussion The structural style of the WFFTB is well characterized as a thrust faulted and inverted basin (Lee et al. 2002; Mouthereau & Lacombe 2006; RodriguezRoa & Wiltschko 2006). The presence of Cenozoic rift basins has been well documented in the continental shelf offshore and onshore Taiwan (Sibuet & Hsu 1997; Lin et al. 2003). In addition, the geometry and magnitude of normal faults has been extensively documented beneath the Coastal Plain from seismic profiles (Yu & Chou 2001; Chou & Yu 2002; Lin et al. 2003). Some of these normal faults should be expected to be reactivated during the collision and some inverted normal faults have

been shown in seismic profiles (fig. 8 in Chou & Yu 2002). Most dramatically, the Hsuehshan range is commonly interpreted as an inverted graben (Clark et al. 1993). We interpreted some reverse faults as inverted normal faults based on the presence of noticeable differences in stratigraphic thicknesses across the fault. One example of this is the difference of thicknesses on the Miocene units across the Tachienshan and Pingchi thrust faults. Normal faults were placed in areas where changes in stratigraphic thicknesses required the existence of a normal. A set of normal faults is placed in front of the Peikang High because a stratigraphic expansion is reported between this location and the Western Foothills (Ho 1971; Sun 1985; Shaw 1996). The remaining normal faults are usually placed in intermediate positions between the foreland and the restored normal faults located towards the hinterland. Most of these faults are clearly interpretative and placed in order to continue the sub-thrust structural style where no data exist.

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Fig. 20. 3D structural model looking from the south. See Figure 3 for location and Figure 4 for a key to the faults.

Pre-existing normal faults trend around 060 to nearly east– west, whereas the basin margins mimic the Eurasian coastline. The interaction between forming thrusts and pre-existing features oriented oblique to the thrust vergence permits motion on lateral and oblique ramps. This interaction is seen in oblique and lateral ramps along some thrusts, where there is indication of the existence of inverted faults in map view. One example is the southern oblique ramp of the Tachienshan thrust. In this segment both normal and reverse fault age relationships are found (Fig. 18c normal at point a, reverse at points b and c). A similar situation where a normal fault is transported on a thrust sheet as a footwall shortcut is found in the Schuangtung thrust sheet (Figs 7 & 18b). The average thrust displacement estimated for the WFFTB is about 40 km consistently from section A-A0 to J –J0 . As one thrust ends one or more take up the displacement such that the total displacement is nearly constant. South of section J–J0 , displacement begins to decrease. Interestingly, the decrease does not start where the region of most basement involvement begins, namely, the region of our proposed sub-Yuching thrust. Basement involvement does not seem to greatly influence the overall displacement of the WFFTB. The decrease in displacement to the south may be attributed to the transition from collision to accretion in the southernmost cross sections. In terms of steady state, the WFFTB seems to have reached constant

displacement by section J– J0 . This line of section roughly corresponds to the southern limit of reset zircon ages of Willett et al. (2003) but is well north of both the north end of the Pingtung Valley and the northwestward bend in the trend of the Chaochou fault, both of which have been taken as marking the transition from accretion to collision (e.g. Lallemand et al. 2001). The restored position of the normal faults found in the cross sections moves the easternmost interpreted normal faults about 40 km towards the east. Consequently, the current surface trace of the Tuluwang –Chaochou thrust originated below the present position of the Coastal Range (Fig. 21). The restored position of the normal faults to their pre-transported position involves mainly horizontal displacement. Significant burial would violate the thermochronologic constraint that the currently exposed Central Range rocks were never hotter than 400 8C during the Tertiary. The evidence is the lack of reset high-temperature thermochronometers (Ar40/Ar39 hornblende and muscovite) and the presence of partially reset biotite Ar40/Ar39 ages. If instead the Central Range were deeply exhumed, rocks would have originally been at 20 –25 km (500 –600 8C, Willett et al. 2003). Those amounts of vertical uplift are not shown by thermochronologic data (Willett et al. 2003). Instead, thermochronologic data plus our sections suggest that the thrust sheets now found in the WFFTB were stripped off basement before it was

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Fig. 21. Geological map of central and southern Taiwan showing the restored position of normal faults, estimated from balanced cross sections.

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deeply buried. The remnant basins from which some of the thrusts emerged may have been subsequently incorporated into the Central Range at depth. Gourley et al. (2007) suggest that the basal detachment in the northern Central Range is as deep as 35 km but shallows rapidly on the west side. The steepening may be due to addition of Eurasian margin upper crustal rocks, stripped of much of the sedimentary cover that now makes up the WFFTB. A new structure, the sub-Yuching uplift, is required in the southern WFFTB to account for surface and known subsurface features. The subYuching thrust has been drawn as a low-angle thrust with a detachment located at c. 13 km depth. This depth has been carefully modelled to obtain a structure with the observed dimension of the front-limb and the width and vertical relief of the anticlinal crest. This regional low-angle thrust lies below the Yuching and Tingpinglin synclines to account for the elevation of these structures over their regional level to the north and south. Its front-limb explains a long monocline found in the southern foothills on the footwall of the Chukou thrust. This structure is compatible with both fission track data, which imply about 3 km of vertical uplift, and focal mechanisms. Because the basement rocks are not exposed, it is difficult to supply an explanation for why the subYuching structure exists. As noted above, this structure is on strike with the location commonly taken as the zone of transition between accretion to the south and the Eurasian–Philippine Sea plate collision to the north (Teng 1990; Teng et al. 2000; Lallemand et al. 2001). This is also the projection of the Manila Trench onto and under Taiwan. Given that this is an anomalous structure, it is not likely that it represents a stage through which more northern portions of the WFFTB have passed. Trading space for time might not hold for the structures of the WFFTB in other ways. There is evidence of inverted faults that were not developed in sequence. Therefore changes in the structural style along the strike of the Western Foothills are common and it is difficult to extrapolate a cross section in space and time. The timing for the Western Foothills of Taiwan is constrained by unconformities and thermal data. The three main unconformities are at the upper Miocene (Tangenshan Formation and equivalents), upper Pliocene (Cholan Formation and equivalents) and Middle Pleistocene (Toukoshan Formation and equivalents). From the cross section restoration and thermal data we can infer at least two cooling events. Non-reset AFT ages indicate that the rocks were never buried more than 3– 4 km, whereas partially reset samples were at one time deeper (Fuller 2002). We interpret that the Cholan Formation and equivalents were partially or not deposited over the Chinsui Formation. The

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reason for non-deposition was lack of accommodation space due to uplift on thrust ramps. A possible interpretation, therefore, is that the Tachienshan, Schuangtung and Pingchi thrusts started moving between 6 and 3 Ma and were reactivated from 1.5 Ma to the present day, as indicated by the deposition of molasse sediments from the Upper Pleistocene to present. This interpretation fits with the thermal data and our kinematics interpretation.

Conclusions (1)

(2)

(3)

(4)

(5)

(6)

The structural style of the Western Foothills Fold and Thrust Belt (WFFTB) is well described as a series of thrusted and inverted basins. Significant thickening of strata across thrust faults places constraints on the depth and location of pre-existing basins. The restored position of the pre-existing normal faults places them beneath the Central and Coast Ranges. Our reconstructions plus published thermochronologic data show that the WFFTB rocks must have been stripped off the Eurasian margin before significant burial could take place. The presence of basement highs like the Peikang High modifies the geometry and timing of thrusting in the Western Foothills. The axial trace geometry of the Neilin and Pakuashan anticlines, as well as the outof-sequence reactivation of the Tachienshan thrust are interpreted as related to the Peikang High. A new regional structure, the sub-Yuching uplift, is identified in the southern Western Foothills of Taiwan. This structure explains the regional uplift of the Yuching and Tingpinglin synclines, as well as the monocline along the footwall of the Chukou thrust. The centre of the structure lies roughly at the southern limit of reset zircon fission track ages and is on-line with the northern projection of the Manila Trench. The Tachienshan, Schuangtung and Pingchi thrusts were active before the deposition of the Cholan Fm. These faults were reactivated from about 1.5 Ma to the present day, as indicated by the deposition of molasse sediments from the Upper Pleistocene to present. The total thrust displacement is constant at 40 km for most of the WFFTB, even in the region of greatest basement involvement. Displacement starts to decrease south of the region of greatest basement involvement, which may be coincident with the transition from collision in the north to accretion in southernmost Taiwan.

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We would like to thank NSF grant EAR-99009638, the Conoco-Phillips Company and especially Peter Hennings, as well as the M. T. Halbouty endowment for support of this project. Midland Valley provided academic licences for the 2DMove software, for which we are grateful. F. Mouthereau and an anonymous reviewer substantially improved the manuscript.

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Deepwater folding and thrusting offshore NW Borneo, SE Asia S. HESSE1, S. BACK1* & D. FRANKE2 1

Geological Institute, RWTH Aachen University, Wu¨llnerstr. 2, D-52056 Aachen, Germany 2

Federal Institute for Geosciences and Natural Resources (BGR), Stilleweg 2, D-30655 Hannover, Germany *Corresponding author (e-mail: [email protected])

Abstract: The slope and deepwater portion of the offshore NW Borneo continental margin hosts a number of proven hydrocarbon accumulations. Reprocessed and post-stack depth-migrated regional 2D seismic data reveal the occurrence of an extensive series of deepwater folds located at the leading edges of imbricate thrusts. Typical thrust-top folds include (1) anticlines characterized by large interlimb angles that lack a sea-floor expression; (2) anticlines with medium interlimb angles that show a clear sea-floor expression and normal faulting in the crest; and (3) anticlines with small to medium interlimb angles, a clear sea-floor expression and intensive crestal faulting associated with partial crestal failure. The different fold types occur at specific locations within the fold– thrust system, the widest and youngest anticlines near the present-day thrust front, and the narrowest and oldest folds in the most landward parts of the fold –thrust belt. Geometric restoration of the deepwater fold –thrust system along six regional shelf-to-basin cross sections provides incremental measurements of fault- and fold-related shortening for the time between the Miocene and present day in deepwater NW Borneo. Across the study area, the main thrust and fold activity appears to be largely of Pliocene–Holocene age. An apparent maximum of both incremental and total shortening is located in the central part of the study area. This location coincides with the maximum width of the fold–thrust belt and the preferential location for the development of the most recent deepwater anticlines.

Deepwater fold–thrust belts develop at both active and passive continental margins. At several locations they are the focus of hydrocarbon exploration, with thrust hanging wall anticlines representing the prime trapping structures (e.g. Morgan 2003; Ingram et al. 2004). At the continental margin of NW Borneo (Fig. 1), two key mechanisms have been proposed as the main controlling factors for deepwater thrusting and folding: (1) basementdriven crustal shortening; and (2) gravity-related delta tectonics. Based on regional two-dimensional (2D) reflection seismic data, Hinz et al. (1989) interpreted the deepwater tectonics offshore NW Borneo as reflecting a continent–continent collision, and considered that regional compression induced major thrusting that continued until today. In contrast, Hazebroek & Tan (1993) draw attention to the similarities of structures with thrusts at the toe of the Niger Delta, suggesting that deepwater folding and thrusting offshore NW Borneo primarily developed in response to gravitational delta tectonics. Between these two end-member interpretations, Ingram et al. (2004), Morley & Leong (2008) and Hesse et al. (2009) suggested that the compressional deformation in the deepwater area offshore NW Borneo was influenced by both deep-seated crustal shortening and an overburden-restricted, gravity-related toe thrusting.

This study presents over 1700 km of post-stack depth-migrated, 2D multichannel seismic data (Fig. 1b) that image in total over 50 folds in the compressional domain of the NW Borneo deepwater fold– thrust belt. The analysis of the depth-migrated data allows the determination of true subsurface geometries of faults, fault-related folds and associated synkinematic deposits. Regional structural and stratigraphic interpretations of the deepwater dataset are used as input for a step-wise geometric retro-deformation of the deepwater thrusts and folds encountered offshore NW Borneo, providing quantitative constraints for the shortening of the fold– thrust belt between the Miocene and present day. The results of the incremental tectonic restoration are used for the delineation of the relation between variable fold styles and lateral variations in tectonic shortening, taking modifications by external parameters such as basement configuration or variable sediment input into account. The observations presented might help to identify future exploration targets in deepwater fold–thrust belts.

Geological framework The principal features of the geological evolution of northern Borneo are summarized in Liechti et al.

From: Goffey, G. P., Craig, J., Needham, T. & Scott, R. (eds) Hydrocarbons in Contractional Belts. Geological Society, London, Special Publications, 348, 169–185. DOI: 10.1144/SP348.9 0305-8719/10/$15.00 # The Geological Society of London 2010.

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Fig. 1. (a) Regional location map of the NW Borneo study area (black rectangle) in South East Asia. (b) Structural map showing the key tectonic elements of NW Borneo and the location of the 2D regional seismic data analysed in this study. Bathymetric contours are drawn at 500 m intervals. Bold black line indicates location of cross section A– A0 . (c) Regional geological cross section across NW Borneo along line A–A0 . The section is based on offshore seismic reflection data (Sandal 1996; Van Rensbergen & Morley 2000), onshore seismic reflection and well data (Back et al. 2008), geological maps (Sandal 1996; Back et al. 2001, 2005) and models for the tectonic development of northern Borneo (e.g. James 1984; Morley et al. 2003; Morley & Back 2008).

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(1960), Wilford (1961), Hamilton (1979), Hinz et al. (1989), Hutchison (1996a, b), Milsom et al. (1997), Petronas (1999), Hall & Wilson (2000), Hutchison et al. (2000), Morley et al. (2003), Hall et al. (2008) and Morley & Back (2008). Northern and Central Borneo were built from the Mesozoic to Recent and record a complex plate tectonic history involving oceanic and continental crust. Today, NW Borneo is a mountainous region (Fig. 1) exposing Cretaceous –Eocene deepwater clastic deposits that are thrusted, folded and locally metamorphosed to phyllites (Crocker and Rajang Groups exposed in the Crocker Mountain Range; for detailed description see Hutchison 1996a, b), and Eocene to lower Miocene sand-rich turbidites (Crocker Formation; for a detailed description see Van Hattum et al. 2006). These Crocker sediments are commonly interpreted to represent deepwater units in or adjacent to an accretionary prism. Several authors proposed that NW Borneo is underlain largely by Mesozoic ophiolitic rocks (e.g. Hutchison 1996a; Hall & Wilson 2000), mainly due to the fact that wherever deeper basement rocks are exposed they are dominantly ultrabasic to basic rocks and their probable metamorphosed equivalents (see e.g. Hutchison 1996a; Hall et al. 2008). During Oligocene –early Miocene times, the South China Sea opened by sea-floor spreading. Sea-floor spreading anomalies in the South China Sea oceanic basin range from magnetic anomaly 11 (c. 32 Ma) to anomaly 5c (c. 16 Ma; Taylor & Hayes 1983; Briais et al. 1993). This contrasts with the work of Barckhausen & Roeser (2004), which identifies the same anomalies differently providing an age of 20.5 Ma for the termination of sea-floor spreading (based on their magnetic anomaly 6A1). With the opening of the South China Sea, the thinned continental crust of the Dangerous Grounds region was rifted away from the southern margin of China (e.g. Holloway 1981; Hinz & Schlu¨ter 1985; Taylor & Hayes 1983; Briais et al. 1993). On the southeastern side of the Dangerous Grounds, Hinz & Schlu¨ter (1985) interpreted an older region of oceanic crust, the Proto-South China Sea. During the Palaeogene, this Proto-South China Sea closed most likely due to a SE-directed subduction beneath NW Borneo. Following complete subduction of the Proto-South China Sea oceanic crust, continental crust of the Dangerous Grounds region was partially subducted beneath the Crocker Formation basin of NW Borneo in the latest early Miocene before its buoyancy locked the system (James 1984; Levell 1987; Hazebroek & Tan 1993; Hutchison 1996a, b; Hall 1996; Sandal 1996; Milsom et al. 1997). Subsequently, northern Borneo experienced significant contractional deformation as documented onshore in folded sedimentary units of late early Miocene to middle Miocene ages (e.g. Sandal

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1996; Back et al. 2001; Morley et al. 2003; Back et al. 2005, 2008) as well as offshore in folded and thrusted middle Miocene to present-day shelf and slope sequences (e.g. Levell 1987; Hinz et al. 1989; Hazebroek & Tan 1993; Morley et al. 2003; Ingram et al. 2004). On the shallow NW Borneo shelf, late Neogene contraction coincided with the development of major synsedimentary normal faults in the up to 10-km-thick late Neogene deltaic overburden (Fig. 1c). This ‘thin-skinned’ extensional deformation was superimposed on deep-seated compressional structures and generated, particularly in the southern part of the NW Borneo shelf, a multitude of complex tectonic features influenced by both extensional and compressional tectonics (Morley et al. 2003). The interference of extensional and compressional structures mainly affected the sedimentary succession landwards of the present-day shelf break, beyond which a purely compressional fold– thrust belt developed (Figs 2–4). Recent tectonic deformation in the deepwater thrust-belt is documented by the prominent sea-floor expression of several thrust hanging wall anticlines, with the youngest structures forming at the thin, distal part of the sediment wedge near the NW Borneo Trough (Figs 2–4; Ingram et al. 2004; Gee et al. 2007; Morley 2007; Hesse et al. 2009).

Seismic data The seismic data of this study were acquired by the Federal Institute for Geosciences and Natural Resources (BGR) in 1986. The data presented were recently reprocessed and depth-migrated. The pre-stack processing included trace editing, CMP-sort (25 m spacing), predictive deconvolution, amplitude correction for spherical divergence based on stacking velocities, normal move-out correction and muting. Multiple attenuation was performed by a Radon velocity filtering technique that was combined with an inner trace mute providing sufficient results. Stacking velocities were picked at regular intervals of 1.5 km along every line. After the normal move-out correction the seismic data were stacked. The velocity field for the post-stack depth migration was estimated by a step-by-step approach based on the stacking velocities determined by semblance analysis. The velocity information was converted into interval velocities using a smoothed-gradient algorithm. The final depthmigration algorithm used was an implicit finite difference (FD) migration code. FD time-migration was run for quality control of the post-stack depth migration. All data presented in this paper are displayed with no additional gain applied and with a mute above the sea-bottom.

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Fig. 2. NW– SE oriented, depth-migrated seismic sections BGR86-12 and BGR86-14 of the northern study area and line drawing showing the main tectonic and stratigraphic features. The seismic sections cross the shelf, the slope and the NW Borneo Trough. At the landward edge of section BGR86-12, a high velocity body is observed that masks the seismic signal of the underlying strata. Different colours indicate seismic units U1, U2, U3, U4 and U5. For section location see Figure 1b. Studied anticlines labelled A to G plus corresponding seismic section number and interlimb angle measured.

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Fig. 3. NW–SE oriented, depth-migrated seismic sections BGR86-18 and BGR86-20 (location of the seismic lines shown in Fig. 1b), and line drawing illustrating the main tectonic and stratigraphic features of the central part of the study area. The seismic lines image the shelf, slope and foot of slope and the eastern edge of the NW Borneo Trough. Colours and labelling as in Figure 2.

Cross section geometry The study area is characterized by a wide variety of slope geometries and structural styles (Figs 2 –4). The north (lines BGR86-12, BGR86-14; Fig. 2)

exhibits a multitude of steep, SE dipping thrustrelated fold-anticlines, of which the youngest, most westward located anticlines exhibit a clear seafloor expression. The anticlines are closely spaced (average distance 2 to 5 km) and asymmetric.

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Fig. 4. NW–SE oriented, depth-migrated seismic sections BGR86-22 and BGR86-24 of the southern part of the study area and line drawings showing the main tectonic and stratigraphic features. For section location see Figure 1b. Colours and labelling as in Figure 2.

Individual anticlines show an oversteepened forelimb that developed above the upper tip of an associated thrust fault. The observed thrusts range in dip from 308 near the NW Borneo Trough to 608 a few tens of kilometres landward. Near the presentday deformation front, the thrust faults sole into a gently dipping detachment (average dip 3.88,

maximum dip 78) that is imaged at depths between 5 and 8 km. High amplitude reflector patches further east can possibly be interpreted as a detachment continuation to depths of c. 12 km below the NW Borneo shelf (Hesse et al. 2009); a connection between the thrust roots and these reflector patches cannot be proven due to the deterioration of the

DEEPWATER FOLDING AND THRUSTING OFFSHORE NW BORNEO

seismic signal with depth (Fig. 2). Significant parts of the upper slope of the northern study area contain the ‘Major Thrust Sheet’ previously defined by Hinz et al. (1989). New refraction seismic data document a high velocity body at the basinward edge of this feature (Franke et al. 2008; Fig. 2), indicating the presence of a large carbonate body encased in siliciclastics, or a thrust block built of Palaeogene Crocker sediments. In the central and southern parts of the study area (e.g. BGR86-18, BGR86-20, lines BGR86-22, BGR86-24; Figs 3 & 4), up to eight successive NE–SW trending thrust-related fold-anticlines were observed. As in the northern part of the study area, the anticlines are mostly asymmetric and exhibit steep forelimbs above the upper tip of a basal thrust fault. However, the southern anticlines are wider spaced (distance 4 to 15 km) and exhibit less surface expression (Fig. 4). The associated thrust faults dip towards the SE and range in dip from 158 near the deformation front to 408 below the presentday shelf edge. As in the northern study area, the downward continuation of the thrusts towards the southeastwards dipping basal detachment remains speculative due to insufficient seismic resolution. For a regional stratigraphic subdivision, five seismic units were mapped (U1 to U5 from old to young) that are separated by five marker reflections of high amplitude and lateral continuity (Figs 2–4). Due to the lack of deepwater well data, we followed chronostratigraphic levels defined by Hinz et al. (1989) that provide an approximate late Pliocene to recent age for stratigraphic unit U5, an early to late Pliocene age for stratigraphic unit U4, a latest Miocene to early Pliocene age for stratigraphic unit U3, and an undifferentiated Miocene age for stratigraphic unit U2. A comparison of this regional stratigraphic framework with published data of deepwater Sabah (e.g. Petronas 1999; Ingram et al. 2004) and adjacent areas of Brunei Darussalam (Sandal 1996; Van Rensbergen & Morley 2000; Saller & Blake 2003; Gee et al. 2007; Morley 2007; Morley & Back 2008; Back et al. 2008) documents that the horizon levels defined in this study fit well into a margin-scale geological framework. The lowermost marker reflection is the top of the acoustic basement, which occurs at depths between 5 (NW) and 13 km (SE). The basement reflection is positive, of high continuity and shows prominent amplitudes particularly in depths less than 7 km (Figs 3 & 4). The acoustic substratum (unit U1) below probably comprises systems of horsts, tilted blocks and graben structures. The basement grabens either show onlap fill with laterally continuous, parallel to subparallel reflections of moderate to high amplitudes, or a complex, nearly transparent fill pattern (Figs 2 –4). The horst structures are commonly overlain by laterally continuous, parallel to

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subparallel reflectors with high amplitudes. The seismic response of the top of unit U1 deteriorates with increasing depth towards the eastern part of the study area, and only segments of it exist below 8 km depth. Above the acoustic basement, seismic unit U2 defines a zone dominated by seismic noise, and only locally subparallel reflections of low to moderate amplitudes occur (Figs 2–4). The top of unit U2 is an amplitude peak of relatively high lateral continuity, mapped at depths between 3 and 8 km. In the western part of the study area, seismic unit U2 onlaps onto and terminates against the top of the acoustic basement (unit U1). Succeeding seismic unit U3 is characterized by laterally continuous, subparallel to parallel reflections of low to medium amplitudes in the NW Borneo Trough, and by higher amplitudes and a wider reflector spacing on the NW Borneo slope (Figs 2–4). Throughout the survey area, unit U3 shows an upward increase in amplitude and frequency. The top of the unit is defined by a continuous seismic reflector of medium to high positive amplitudes located at depths of 2 to 5 km. Seismic unit U4 is characterized by parallel to divergent reflections of moderate to high amplitudes, moderate to high frequency and high lateral reflection continuity (Figs 2 –4). Internally, unit U4 exhibits alternating intervals of moderate amplitudes and lower frequency and high amplitude zones with dense reflector spacing. The top of unit U4 is mapped on a positive reflection of high continuity, located in depths of 4 km near the NW Borneo Trough and 1.8 km near the shelf edge. The present-day sea-bottom forms the top of unit U5, a unit characterized by parallel, subparallel, chaotic and divergent reflections of moderate to high amplitude with variable frequencies and high reflection continuity (Figs 2– 4). The typical slope and deepwater pattern of this seismic unit reflects growth strata preferentially stored on the backlimb sides of thrust anticlines associated with parallel, subparallel and divergent reflectors of medium to high amplitude and reflector spacing. In places, the stratified seismic facies is interbedded with chaotic seismic units that are interpreted as slumped mass accumulations. These deposits most likely originated from the unstable, oversteepened forelimbs of the respective thrust anticline landwards (also see McGilvery & Cook 2003; Gee et al. 2007). Particularly in the southern and central shelfal parts of the study area, unit U5 is affected by the development of major synsedimentary normal faults.

Anticline geometry across the slope The following paragraphs present a detailed description of the anticline geometries of deepwater NW

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Borneo including dip and interlimb angle measurements (Fig. 5), and the architecture of the associated sedimentary units, starting at the deformation front and successively moving landwards up to the present-day shelf. The typical fold styles encountered are (1) anticlines that are gently folded, lacking sea-floor expression, (2) anticlines with a clear seafloor expression that are internally affected by normal faults, and (3) anticlines that are internally affected by normal faults and on the flanks by mass movement deposits. The most distal, smoothly rounded anticlines A12, A14, B14, A18, A20, A22 (Figs 2– 4) that show no sea-floor expression represent the youngest generation of anticlines of the deepwater fold– thrust belt. These anticlines are generally symmetric with gently dipping limbs (1 to 58). The interlimb angles average 1758 (Fig. 5). Around this youngest fold generation, the sedimentary succession maintains its thickness and only the uppermost reflectors show a subtle stratal thinning towards the anticline crest. Most of these sub-Recent to Recent folds are covered by c. 200 m sediment. Anticlines C14, B18, B20, B22 (Figs 2–4) of the more proximal portions of the fold–thrust belt show a clear sea-floor expression, are affected by normal faults at their tops, and verge basinwards. These anticlines are characterized by interlimb angles between 152 and 1748 (Fig. 5), exhibiting short and steep forelimbs with dip angles between 10 and 208 (Figs 2– 4). Low reflectivity or blanketing zones usually characterize the forelimb domain. These zones are possibly caused by fluids or gas and a large number of narrow normal faults at the limit Crest Forelimb

of seismic resolution. The wider backlimbs are dipping landwards with angles averaging 138. Numerous planar to slightly curved, small-scale normal faults mark the crests of these anticlines. The majority of the anticlines located further landwards (B12 to E12, D14 to G14, C18 to E18, C20 to H20, C22 to H22, A24 to E24; Figs 2 –4) exhibit a clear sea-floor expression and are significantly affected by mass movement processes. The interlimb angles of these anticlines average 1408 (Fig. 5). The dips of the forelimbs average 258, with a general increase in dip in landward direction. A vast part of the relatively steeply dipping forelimbs is seismically transparent due to the deflection of the seismic signal. The backlimbs of the anticlines dip generally more gently than the forelimbs, at average 208. Similar to the more basinward anticlines, these anticlines are affected by numerous normal faults at the fold crests. Figures 3 and 4 image anticlines F18, I22 and F24, which are folds at the burial stage located in the immediate vicinity of the present-day shelf break. These anticlines are covered by a sedimentary overburden exceeding 500 m, and characterized by a weak response of the seismic signal, which can be partly related to limited reflection imaging of steeply inclined surfaces (Lynn & Deregowski 1981). The interlimb angles of the anticlines F18, I22 and F24 are around 1008 (Fig. 5). If set into a relative chronological order, the three different anticline types described above can be interpreted to document successive stages in the evolution of a deepwater fold in the NW Borneo fold–thrust belt: at the initial stage of fold development, the studied anticlines do not have a sea-floor

Backlimb Seafloor

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Fig. 5. Measured interlimb angles (in degrees) and schematic diagram illustrating relevant measured anticline angles including fold-interlimb angle, forelimb and backlimb dip. Anticline labels correspond to annotation on Figures 2– 4. Labelling is per section, that is, anticlines do not necessarily continue into adjacent sections.

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Structural restoration In order to evaluate the validity of the seismicbased structural and stratigraphic interpretation,

ar zo Trish e

(a)

backlimb) oversteepening, enabling slope failure. In cases where the fold-interlimb angles have passed a critical value of around 1308 (Fig. 5), slope degradation occurs. In the final stage of anticline development, the backlimb accommodation for sediments derived from the shelf is filled and the folds become deeply buried.

ne

expression. Consequently, surface processes do not affect folding. Further growth of the thrust hanging wall folds initiates the development of sea-floor relief. Commonly, thrust-related piggyback basins form in the backlimb domain, trapping incoming sediment. Depending on the effectiveness of these basins as local depocentres, sedimentary loading can accelerate mini-basin subsidence locally rotating the underlying thrusts. With ongoing contraction and syntectonic sedimentation, bending stresses produce arrays of normal faults at the anticline crests. Further anticline growth and synkinematic sedimentation result in forelimb (and locally

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original bed thickness total shortening

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Fig. 6. Schematic illustration of the geometric restoration approach used for this study. (a) Representative interpretation of a fault-related fold. Dashed circles mark the fault– horizon intersections on footwall and hanging wall sides. (b) Section after fault restoration balancing displacement of top horizon. (c) Flexural-slip unfolding of top horizon. Underlying surfaces are carried with the top surface along same movement vectors as it is deformed with the flexural-slip algorithm. (d) Target fold after removal of restored sedimentary unit followed by decompaction and isostatic correction.

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Pin 2 Section length 96 km (Pin 1 to Pin 2)

Fig. 7. (a) Structural restoration of depth-migrated seismic section BGR86-14 (northern part of the study area) using Pin 1 as fix point. Upper line drawing illustrates the present-day geological situation with a 78.5 km distance between the deepwater deformation front (Pin 1 at the upper tip of the youngest thrust fault) and the present-day shelf edge (Pin 2). The cartoon below shows the interpreted section after the complete fault restoration and unfolding of the top of unit U4, including decompaction and isostatic correction; restored cross section length between Pin 1 and

DEEPWATER FOLDING AND THRUSTING OFFSHORE NW BORNEO

reconstruct the tectonic evolution of the NW Borneo deepwater fold–thrust belt between the development of seismic units U2 and U5, and measure deformation associated with deepwater folding and thrusting, a series of 2D geometric restorations (unit-per-unit, from young to old) was carried out on seismic lines BGR86-12, BGR86-14, BGR86-18, BGR86-20, BGR86-22 and BGR86-24 Figs 2 –4). The retro-deformation work included (1) the fault-by-fault restoration of the thrust displacement (2) a fold-by-fold unfolding, (3) a unit-wide decompaction and (4) a unit-wide isostatic correction (Fig. 6). It was assumed that there was no significant transport of material into or out of the individual section planes. For fault restoration, a Trishear algorithm sensu Erslev (1991) was used in which fault displacement was accommodated by heterogeneous shear in a triangular zone radiating from the fault tip line (Fig. 6a). After each step of restoring a hanging wall horizon cut-off to its position before faulting, remnant folding of the target horizon was balanced using flexural-slip sensu Griffiths et al. (2002; see Fig. 6b). This two-step fault-restoration/unfolding approach was carried out systematically within each stratigraphic unit starting at the deepwater deformation front and progressively moving landwards. Fault displacement, throw, heave, fold-related shortening and total shortening was measured incrementally along each section. Complete fault restoration and unfolding within each stratigraphic unit was followed by a section-wide decompaction sensu Sclater & Christie (1980) and an isostatic correction sensu Burov & Diament (1992). Figure 7 shows two representative examples of the unit-per-unit restoration of the NW Borneo deepwater fold–thrust belt based on the geometric retro-deformation of seismic lines BGR86-14 (northern study area; Fig. 7a) and BGR86-24 (southern study area; Fig. 7b) with Pin 1 as fix point. Figure 8 summarizes the restoration results

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of all sections of this study documenting total shortening of each section as well as incremental shortening during times of the development of stratigraphic units U2, U3 and U4. The total shortening values measured across the c. 120-km-wide study area range from 8 to 13 km per section (Fig. 8). Low total shortening is observed in the very north (8 km; line BGR86-12) and at the very south of the study area (10 km; line BGR86-24). From south to north, total shortening increases from 10 km (line BGR86-24) to a maximum of 13 km at line BGR86-22. Shortening decreases 20 to 40 km northwards to 12 km (lines BGR86-20 and BGR86-18). At line BGR86-14, total shortening was determined to be 10.5 km, before reaching a minimum of 8 km at line BGR86-12. If broken into incremental steps, the spatial distribution of shortening remains non-uniform, but shortening increases through time (Fig. 8). The results of the fault restoration and unfolding of six regional shelf-to-basin sections across the NW Borneo deepwater fold–thrust belt offshore Sabah indicate a significant regional variation in both total and incremental shortening through time. We consider the observed shortening variations between the individual transects as valid, although absolute shortening values might carry an error of up to 15% taking into account seismicprocessing uncertainties (i.e. depth migration), horizon-interpretation errors (i.e. across-fault and fold correlations) or uncertainties during restoration (included in Fig. 8). Based on the stratigraphic approximation used for horizon and unit interpretation, we calculate a shortening of 1 to 3 km from each reconstructed section until early Pliocene times (base U3). Between the early and late Pliocene, incremental shortening varies throughout the study area ranging between 2 km and 4 km. Since the late Pliocene, the study area seems to have undergone a major shortening pulse recording shortening values of 5 to 7 km, with the maximum

Fig. 7. (Continued) Pin 2 is 83.5 km (5 km shortening). The third panel illustrates the section after retro-deformation, decompaction and isostatic correction of the top of unit U3; restored cross section length between Pins 1 and 2 is 86 km (2.5 km incremental shortening). The fourth cartoon shows section BGR86-14 after geometric restoration, decompaction and isostatic correction of the top of unit U2, finally restoring the cross section to a length of around 89 km between Pins 1 and 2 (3 km incremental shortening, 10.5 km total shortening). The change in basement dip (top unit U1) between the present-day and final restored geological situation is a result of the sequential isostatic compensation after each restoration step. Colours as in Figure 2. (b) Structural restoration of depth-migrated seismic section BGR86-24 (southern part of the study area) using Pin 1 as fix point. Top panel illustrating the present-day geological situation with an 86 km distance between the deepwater deformation front (Pin 1) and the present-day shelf edge (Pin 2). Second panel shows the interpreted section after complete retro-deformation, decompaction and isostatic correction. Fault restoration and unfolding has restored the distance between Pins 1 and 2 to 91.5 km (5.5 km shortening). The third cartoon shows the balanced section after retro-deformation, decompaction and isostatic correction of the top of unit U3; restored cross section length between Pins 1 and 2 is 93.5 km (2 km incremental shortening). The fourth panel shows section BGR86-24 after tectonic balancing, decompaction and isostatic correction of the top of unit U2; this finally restores the cross section to a length of around 96 km between Pins 1 and 2 (2.5 km incremental shortening, 10 km total shortening).

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Survey distance (km) Fig. 8. Shortening values calculated by geometric restoration for three incremental time steps: shortening recorded between the late Pliocene and recent is 5 to 7 km (green line, calculation based on restoration of base U5), shortening between the early Pliocene and late Pliocene is 2 to 4 km (blue line, calculation based on restoration of base U4), and shortening between latest Miocene times and the early Pliocene is between 1 and 3 km (red line, calculation based on restoration of base U3). The total shortening calculated for latest Miocene to Recent times ranges between 8 and 13 km (black line).

located in the central part of the study area between lines BGR86-22 and BGR86-18. Despite a large Pliocene to Recent sediment influx from the shelf to the continental slope (Sandal 1996; Petronas 1999; Van Rensbergen & Morley 2000), the stepped topography of the fold-belt is not levelled-out on the modern sea-floor (Figs 2– 4; also see detailed 3D sea-floor images of Gee et al. 2007; Morley 2007; Morley & Leong 2008), indicating that many folds and faults in the study area are active today.

Discussion Figure 9 is a synoptic plot of (1) the late Pliocene to Recent deepwater shortening in the study area

(Fig. 9a); (2) estimates of late Pliocene to recent shelfal extension offshore Sabah (Fig. 9b; extension data from Hesse et al. 2009); (3) the calculation of shortening unrelated to shelf extension, based on the subtraction of the late Pliocene to Recent shelfal extension values from the calculated late Pliocene to recent deepwater shortening values (Fig. 9c); (4) the observed quantity of deepwater thrusts per section (Fig. 9d); and (5) the landward change of the interlimb angles of the youngest folds along each seismic section, measured between the present-day deformation front and an arbitrary point located 20 km landwards, respectively (Fig. 9e). A comparison of Figures 9a and 9b documents an overall balance between deepwater shortening and shelfal extension in the very south

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Fig. 9. (a) Incremental late Pliocene to recent deepwater shortening values calculated by structural restoration. (b) Values for late Pliocene to recent shelfal extension from Hesse et al. 2009. (c) Surplus of compression calculated by subtracting shelfal extension (b) from calculated deepwater shortening (a). The late Pliocene to Recent compressional surplus can be interpreted as reflecting ongoing basement-driven compression, or large-scale gravitational sliding related to regional uplift of the Mt Kinabalu area (see Figs 1 & 10). (d) Total amount of interpreted deepwater thrusts per section. (e) Change of the interlimb angles of the youngest anticlines per section, measured between the present-day deformation front and an arbitrary location 20 km landward.

of the study area (line BGR86-24), suggesting that extension driven by shallow-water delta tectonics is transferred to the continental slope serving as primary control for deepwater shortening. However, shelfal extension significantly decreases towards the north, causing an imbalance between shallow-water shelf extension and deepwater compression (Fig. 9c). The resulting surplus of shortening in this part of the study area can be interpreted as indicating a northward increase of basement-driven shortening (sensu Hesse et al. 2009). In this case, the apparent decrease in deepwater shortening (Fig. 9a) and shortening unrelated to shelf extension (Fig. 9c) between lines BGR86-18 and BGR86-12 could stem from the lack of neo-tectonic shortening data from the folded and thrusted sediments of nearshore and onshore Sabah around Kota Kinabalu (Hamilton 1979; Tongkul 1997), or the incomplete tectonic balancing of possible faults and folds that might occur below the high velocity body of seismic line BGR86-12 (Fig. 2). A different cause for the imbalance between shallow-water shelf extension and deepwater compression in the northern study area (Fig. 9c) might be suggested by the presence of the 4095 m high granite peak of Mt Kinabalu within 50 km of the present-day Sabah coast, c. 150 to 200 km off the NW Borneo deepwater fold–thrust belt (Figs 1 & 10). If assuming a considerable late Neogene to recent uplift centred on the Mt Kinabalu region (Fig. 10), the offshore areas at the foot of this structure might have received compressional stress related to a regional, gravity-driven readjustment of the sedimentary overburden. The interference of such an

uplift-related compressional regime with the typical delta-toe folding and thrusting observed further south might be ultimately responsible for the complexity of the modern stress-field documented in both the deepwater and shelf areas offshore NW Borneo (Tingay et al. 2005; King et al. 2009). Apart from the restoration balance and imbalance discussed above, Figures 9d and 9e document significant variations in structural styles between the northern and southern parts of the study area, with the maximum of thrusts and associated folds in the very north of the study area, a location that coincides with the maximum along-line change in fold-interlimb angles. Deepwater folds and thrusts in the southern part are fewer, generally wider spaced and the along-line changes in interlimb angles are less (Fig. 9e). If extended further towards the south into the deepwater areas offshore Brunei Darussalam, the general southward decrease of anticline and thrust frequency is even more pronounced, with only three clear deepwater folds developed offshore the Baram Delta (Gee et al. 2007; Morley 2007). An explanation for these differences is that factors other than shortening contribute to the structural development of the studied deepwater folds and thrusts, such as intra-formational rock inhomogeneity, irregular basement topography or a pronounced lateral variation in sediment input. Concerning intraformational rock inhomogeneity, the high frequency of faults and folds in the north (Figs 2 & 9d) can be possibly interpreted as the geometric consequence of the presence of the high velocity rock body (carbonate body or upthrusted basement?) on the upper slope that acted as a rigid obstacle during folding

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Fig. 10. Map of the greater NW Borneo study area (for location see Fig. 1a), illustrating the region possibly influenced by late Neogene uplift centred on Mt Kinabalu (dashes circles, maximum radius shown is 200 km). Downslope shelf and onshore areas that record significant Neogene inversion (Morley et al. 2003) are indicated by red stripes. The spatial distribution of the inversion provinces of NW Borneo indicates a possible relation between the Mt Kinabalu uplift and downslope basin inversion; however, the amount of uplift-related downslope compression transferred into the NW Borneo deepwater fold–thrust belt cannot be quantified.

and thrusting. A similar approach that takes the local geological conditions into account can be used if prominent irregularities of the acoustic basement are present. For example, the occurrence of basement peaks on lines BGR86-14 (Fig. 2) and BGR86-22 (Fig. 4) seems to correspond with the

preferential appearance of the very recent, smoothly rounded thrust-front anticlines that have not yet developed a sea-floor relief. This coincidence suggests a relation between the occurrence of local basement highs and the preferred position of the initiation of deepwater folds. A third important

DEEPWATER FOLDING AND THRUSTING OFFSHORE NW BORNEO

factor influencing the structural style of the deepwater folds and thrusts is the variability of the influx of sediment into the fold–thrust belt. The southern part of the study area contains both on the shelf and on the continental slope the thickest Pliocene to Recent sediment accumulation (e.g. Fig. 4), while the northern study area accumulated significantly less material (e.g. Fig. 2). Consequently, the development of many folds of the southern study area was strongly affected by syntectonic sedimentation, as recorded by thick piggyback basin successions. If significantly varying through space and time, differential loading of these individual depocentres might exert a considerable influence on the development of laterally varying fold geometry, the preservation or degradation of individual anticlines, and possibly the lateral stress distribution within the deepwater wedge.

Conclusions Regional 2D seismic data of the deepwater NW Borneo fold–thrust belt have been interpreted and geometrically restored in order to investigate the development of deepwater folds and thrusts in response to crustal shortening. Key results of this study are noted below. Three different types of thrust hanging wall folds occur in the deepwater fold–thrust belt: folds characterized by large interlimb angles that lack a sea-floor expression; folds with medium interlimb angles that show a clear sea-floor expression and normal faulting in the crest; and folds with small to medium interlimb angles, a clear sea-floor expression, and intensive crestal faulting associated with partial crestal failure. Each fold type seems to represent a distinct phase in the fold evolution between fold initiation and maturity. Measurements of the geometry of deepwater anticlines indicate that values of the fold-interlimb angle serve as a robust estimate for the maturity of a fold, with the youngest folds exhibiting the widest, the oldest folds the narrowest interlimb angles. Along the NW Borneo fold–thrust belt, slope geometries and structural styles vary significantly. The northern study area exhibits a multitude of steep, thrust-related fold-anticlines that are closely spaced (average distance 2 to 5 km) and commonly asymmetric. Associated thrusts range in dip from 308 near the present-day thrust front to 608 a few tens of kilometres landward. In contrast, the folds in the southern part of the study area are generally fewer, wider spaced (distance 4 to 15 km) and associated with thrust faults dipping between 158 near the deformation front and 408 below the present-day shelf edge. Total shortening measured along six regional shelf-to-basin transects across the NW Borneo

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deepwater fold–thrust belt ranges between 8 and 13 km. These values represent the sum of shortening calculated geometrically for three incremental time steps: late Pliocene to Recent (5–7 km per section), early Pliocene to late Pliocene (2–4 km per section), and Miocene to early Pliocene (ca. 3 km per section). The restoration results indicate that total shortening generally increases through time. The sea-floor expression of active folds indicates significant recent tectonic activity of deepwater faults and folds. A synoptic comparison of the shortening data derived from geometric restoration with data on shelf extension, fold and thrust frequency and the rate of change of fold-interlimb angles indicates that factors such as the presence of rigid subsurface obstacles (major rock inhomogeneities within the deforming wedge; non-uniform basement topography) and lateral variations in the sedimentary fill potentially contributed to differential deepwater folding and thrusting. These parameters can vary significantly depending on the individual fold– thrust location studied. Schlumberger is acknowledged for providing ‘Petrel’ under an Academic User License Agreement. Midland Valley is acknowledged for providing ‘2DMove’ for structural restoration. SMT is thanked for providing ‘The Kingdom Suite’ for seismic interpretation. Robert Hall, Eric Blanc and GSL Volume Editor Robert Scott provided thorough comments and valuable criticisms on the earlier version of the manuscript; their contribution to the final paper is gratefully acknowledged. This publication is a contribution to projects BA2136/2-3 and FR2119/1-1 supported by the Deutsche Forschungsgemeinschaft (DFG).

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Index Note: Figures are shown in italic font, tables in bold accretion 138, 163 shortening 90 accretionary wedge 93, 171 Ligurian unit/nappe 116 acquisition of data 3 Agogo structure/oilfield 34, 50– 51, 52 Ahlbrandt data 8, 9 Alabama thrust belt 65, 68 Albanian foreland 102, 108, 110 Alpine (north) ventures 10– 17 Alpine (south) venture 27, 28, 29 analogue modelling 2 Papua New Guinea Fold Belt 33, 40–52 anticline angles 175, 180, 181, 183 anticline geometry 176–177 apatite fission track 4, 39, 97, 101, 102 Apennines 121 Taiwan fold and thrust belt 141–143, 155, 159, 165 Apennines, heat flow 97, 98 Apennines, structure 27, 28 and thickness 83 Apennines see also Northern Apennines Appalachian thrust belt 58 gas 57, 69 seismic interpretation 63– 65 structure 57– 63 volume balance 65–68, 69 asthenosphere flow 97 back-arc extension 97 bacteria 125, 126, 130 balanced cross sections 2DMovew 39, 141 Keping Shan 74 Taiwan fold and thrust belt 144–156 Baluchistan, pressure-temperature modelling 106 basalt, Tarim Basin 77 bathymetry (palaeo-) and overpressure 93 Bavarian section 13, 14 belt-oblique (cross) fault 73, 77– 79, 81, 83 biogenic gas, Apennines 116, 126, 128, 130, 132 Birmingham anticlinorium 58, 64, 65, 68 blankening zone 176 Borneo, fold thrust belt 5 Borneo, NW offshore tectonics 169– 183 anticline geometry 176–177 geology 169– 171 restoration 169, 177– 178, 178– 180, 181 seismic data 171–173 structural styles 173 –175 Bosavi Lineament 34, 52 break-thrust 42, 51 restored 39 brines, Apennines 115– 133 burial (palaeo-) reconstruction 4, 95– 97, 101 calcite twins 105, 108– 110 burial, Taiwan fold and thrust belt 141, 143, 165

calcite twins, palaeo-burial 105, 108– 110 calcite twins, shear stress 108, 109 Canadian Rocky Mountains failed venture 9–10, 11, 19 fluid flow 92, 93, 97, 105 vitrinite reflectance 99 carbonates 19, 98, 102 carbonate reservoir 2, 92, 104, 105, 129 Castel San Pietro, brine 126– 128, 133 chemistry 120, 121 Castrocaro, fluid emissions 128–130, 133 chemistry 120, 121 centrifuge 41 centrifuge analogue modelling 33, 40– 52, 53–54 plane layer 42–44, 46 pre-existing faults 44– 45 centripetal acceleration 40 Ceres basin model 88, 90, 93 Changhua thrust 144, 145, 146 Chaochou thrust 143, 160, 162 Chelungpu thrust 143, 158, 159 Chi-Chi earthquake 144– 146, 158 Chi-Chi fault 158 Chishan thrust 162 chromatography 116 Chukou thrust 143, 160, 161, 162, 165 clumped isotopes 110, 111 collision 72, 137, 139, 163, 165 collision, continent– continent 169 Cordilleran Fold Belt, structure and thickness 83 Cordilleran ventures 17–29 cost of exploration 9, 47 crustal doming 77 crustal shortening 72, 169 Cusiana–Cupiaga, Colombia 3, 19 Darai Plateau 34, 35, 36, 37, 39, 42, 53 de´collement 42, 43, 44, 45 Georgia thrust belt 57, 68 deep water reservoirs 101 deepwater fold and thrusts 169–183 deformation, Keping Shan 75 delta tectonics 169, 181 depth of hydrocarbon source, Apennines 122, 123, 125, 132 depth, palaeo- 97, 110 detachment Borneo 174, 175 Papua New Guinea 48, 50, 51, 53 Taiwan 141, 143, 144, 158 diagenesis 93, 95, 96, 103, 105 ductile duplex, gas plays in 57– 69 duplex thrust 24, 27, 40, 42 East Venezuela basin 17, 18, 22–25, 27, 29 El Furrial field 26, 27, 90, 91, 93, 95, 96 El Furrial thrust 88, 89

188 exhumation 137–139, 141, 163 extension and shortening 181 extensional fault 45, 51–52, 53 extensional structures 137, 139– 141, 144– 156, 161, 162, 163 failed ventures 9 –10, 11, 17, 29 fault reactivation 40, 41, 44–45, 51– 52 fault restoration 164, 177–178, 179 faulting mechanism 77, 78 faults, marginal basins 140 faults, normal 171, 176, 177 Ligurian unit 116, 123, 126, 130, 133 Taiwan fold and thrust belt 139–141, 144–156, 161, 162, 163 faults, strike slip 71 fission track 141 –143, 161 see also under apatite Floyd synclinorium 59, 62–63 fluid flow 88–95, 108, 111 fluid inclusions 4, 90, 93, 95, 96 fluid migration, Apennines 115– 133 pathway 118, 124, 127, 129, 131 Fly Platform 34, 37, 39, 42 fold maturity 183 fold-thrust belt, exporation problems 1– 6 frictional equilibrium, faulting 108 Garzon Massif, thermicity 97, 100, 101 gas 116 Apennines 126, 128, 130, 132 Georgia thrust belt 57, 69 Papua New Guinea 33, 34, 50, 52 gas, biogenic 116, 126, 128, 130, 132 gas, thermogenic 116, 121, 122, 125, 132– 133 gas vents 116 geochemistry Castel San Pietro brine 126–128 Castrocaro fluid emissions 128–130 Regnano mud volcano 121, 123, 125 Salsomaggiore brine 119, 120, 121– 123 Georgia, thrust belt 59– 63 geothermal gradient 87, 101, 103, 143 global petroleum reserves 7 gravitational delta tectonics 169, 181 gravitational flow 93, 111 gravity-driven shortening 5, 181 gravity, in modelling 40 Gulf of Mexico, deep passive margin 101 heat flow 95– 97, 98 Hedinia reservoir 35, 37, 39– 40 analogue modelling 44, 45, 51, 52, 53 Horn Mountain fold train 62, 63, 64, 65 Horseleg Mountain fold train 62–63, 64, 65 hydrocarbon exploration 1– 6, 33, 88 Keping Shan Thrust Belt 83– 84 Papua New Guinea Fold Belt 52– 53 re-evaluation in fold thrust belts 5 –6, 9 –29 hydrocarbon leakage 115 hydrocarbon modelling heat flow 87–88, 95– 97 palaeo-burial 103 –105, 108 –110 pore fluid pressure 88– 95

INDEX prediction 101– 103 unroofing 97– 101 hydrocarbon see also under petroleum hydrothermal circulation 90, 105, 106 imbricate belt, Papua New Guinea 36, 37, 40 modelling 42, 51 stratigraphy 35 imbricate thrust sheet 13–15, 25, 159 Georgia 63 Keping Shan 71 inversion 3, 141, 182 Georgia 65, 68, 69 Papuan Fold Belt 37, 39, 45, 52, 53 inversion, thick skinned 4 inverted fault, Taiwan fold and thrust belt 161, 162, 165 isostatic correction 177, 179 isotope study 93, 110– 111 karst, burial 105, 106 Keping (Frontal) Fault 73, 74–76, 78, 79, 82 Keping Shan Thrust Belt, China 4 3D model 73 hydrocarbon exploration 83–84 structure 72–79, 80– 82 kinematic modelling 106, 110 Eastern Venezuela 88–90 Taiwan fold and thrust belt 141– 158 Kingston–Chattooga composite thrust 58–59, 62, 63, 65, 68, 69 Kutubu oilfield 3, 37, 39–40, 44, 45, 48, 51 shortening in 52, 53– 54 layer parallel shortening 90, 93, 95, 105, 108, 109, 111 stylolites 93, 110 Ligurian unit/nappe 116 –118, 122 –128, 130–133 Little Sand Mountain syncline 62, 63, 64, 65 Llanos, Colombia 3, 4, 20– 21 Lombardia, wells 27, 28 Londonderry High 36, 38, 39, 52 Luku thrust 143, 160 Lunhou thrust 143, 160, 161, 162 Macal project, failure 24– 25, 27 magnetic susceptibility anisotropy 93–95, 103 Mananda anticline 47 Manila Trench 138, 165 mantle plume 77 mass movement 175, 176 maturity rank 89, 95, 98, 103, 104, 110 Garzon Massif 97, 100–101 maximum efficiency rate 7 mechanical stratigraphy modelling 40, 41, 42–44, 46, 51– 52 Meishan balanced section 142–143, 158–159 meteoric fluids, origin 93 meteoric water 119, 123, 126, 129, 130, 133 chemistry 132 methane 116, 126, 128, 132, 133 generation 123, 125, 130 Mexico (Eastern), fluid inclusions 105, 107 micro-thermometry 95 Mirdita Ophiolite (Albania), thermicity 100–102, 108, 110 Moose Mountain Field 3

INDEX Moran anticline 50, 53 seismic lines 46– 50 structural style 36, 45– 51 Mount Kinabalu, uplift 181, 182 mud volcanoes, Apennines 115, 116, 123– 126, 127, 128 mushwads 57, 64, 65, 68 see also ductile duplex Northern Apennines petroleum system 115– 133 fluid emission 116, 117, 118, 119–123 mud volcano 115, 116, 123–126, 127, 128 sampling and analysis 116 Northwest Borneo Trough 174, 175 Novaya Zemlya fold thrust belt 5 obduction 101 oil exploration, Papua New Guinea 33, 34 ophiolites, thermicity 100– 102, 108, 110 organic acid 132 Oriente province, Colombia 17, 18, 19– 21 palaeo-burial 87–111 calcite twins 105, 108– 110 fluid inclusions 103–105 heat flow 95–97 pore fluid pressure 88–95 reconstruction 103– 105, 107, 108–110 salt distribution 101– 103 Papua New Guinea Fold Belt 2 –3, 4, 33–54 analogue modelling 33, 40–52 hydrocarbon prospectivity 52– 53 seismic interpretation 36– 40, 44, 46 stratigraphy 35, 36 structural models 36 tectonics 34– 40 passive margin 19, 20, 21, 87, 110 Apulia 108 Gulf of Mexico 101 Paua anticline 34, 36, 47, 49, 50, 51, 53 Peikang High 144– 156, 162, 165 permeability 88, 90, 95 petroleum modelling see hydrocarbon petroleum reserves 7– 9 petroleum system synthesis, Northern Apennines 121– 123, 125– 126, 130, 133 Philippine Sea Plate 137, 139, 165 piezometry, palaeo- 108, 110 Pingchi thrust 161, 162, 165 Piqiang Fault 73, 76, 77, 78, 79, 81 Pirital front 26, 27 Pirital thrust 88, 89 plasticine modelling clay 35, 41 Po valley basin 27, 29, 117 pore fluid pressure 88–95, 108, 110 porosity 90, 95 Porretta, fluid emissions 116, 130–133 chemistry 120, 121, 132 Potwar Basin 101, 103, 104 pre-existing faults, modelling 44– 45 pressure/temperature modelling 106, 107 pressure/temperature value 103, 105 processing seismic data 171 –173 quartz cementation 95, 96, 105

189

ramp, lateral 71, 77, 78 reactivation Georgia thrust belt 65, 68 Papuan fold belt 51– 52 reconstruction, palaeo-burial 105, 108–110 Regnano mud volcano 123– 126 brine chemistry 120, 121 reserve estimates, fold–thrust belts 2, 7 –8 Papua New Guinea Fold Belt 33, 52 Timor Sea 38 reservoir prediction temperature, pressure and fluid flow 87–111 restoration 2 2DMovew 39, 141 Albanides 102 deepwater fold and thrust zone 169, 177–178, 178 –180, 181 ductile duplex 66–67 thrust belt structure 65–69 restored thrust 39 reverse fault 160, 162 Rock-Eval 88 Rocky Mountains, magnetic susceptibility anisotropy 93–95 roll-back subduction 97 Rome thrust (Appalachians) 59 Saergan Fault 77, 78 salient Keping Shan 71, 72 Tennessee 57, 58 saline water 115– 133 salinity values 93 Salsomaggiore, brine emission 116, 119 –123, 133 brine chemistry 120, 121 salt diapir 101 Salt Range 101, 103, 104 salt redistribution 101, 103, 104 Sanchakou Fault 76, 77– 79, 80, 83 Saranda anticline, burial 109, 110 Schuangtung thrust 143, 159, 161, 163, 165 sea floor expression of deepwater folds 171, 174, 176–177, 180, 183 sea-floor spreading 171 section restoration see restoration sedimentation 171, 176–177, 183 seepage, Apennines 115– 133 seismic data 169, 170 processing 171–173 seismic interpretation Georgia thrust belt 63– 65 Moran anticline 46–50 Papua New Guinea Fold Belt 36– 40, 44, 46 seismic prolife Appalachians 64 Borneo, NW offshore 172– 174 East Venezuela 24 Papua 44, 48, 50 Sequatchie anticline 57, 58, Serran˜a, burial reconstruction 95 Serrania province, Venezuela 17, 18, 22– 25, 27, 29 shear stress, calcite twins 108, 109 shortening 5, 46, 49, 177, 179, 183 values 180, 181

190 shortening, crustal 72, 90, 169 shortening, oilfield 52– 54 Siberia (West) province 8 silicone putty 35, 41 Simms Mountain anticline 62–63, 64, 65 slab detachment 88, 97 South China Sea 170, 172– 174, 182 opening of 171 South Tien Shan Fault 72–73 spectrometry, Raman 111 spectroscopy, atomic absorption 116 squeegee episode 90– 93, 105 stratigraphy Keping Shan thrust belt 79 Papua New Guinea Fold Belt 35, 36 seismic 175, 176–178 Taiwan fold and thrust belt 141, 157 stress and seal parameters 53 stress indicators 108, 109, 110 stress inversion technique 108 strike slip fault 71 structural styles 173–175 structural variability, controls on 71–84 structure, analogue modelling 33– 54 mechanical stratigraphy 40, 41, 42–44, 46, 51– 52 structure, Cordilleran province 17 structure, Georgia thrust belt 57– 63 structure, Papua New Guinea Fold Belt 36 evolution 50, 53–54 structure, Taiwan fold and thrust belt 138, 158– 162 stylolites 93, 110 sub-Yuching fault 150 –156, 161, 162, 163 sub-Yuching uplift 160– 161, 165 Tachienshan thrust 143, 159, 160, 161, 163, 165 Taichung balanced section 142–143, 159–160 Taiwan fold and thrust belt 137–165 3D model 161–162, 163 balanced cross sections 144–156 geology 139 plate tectonic setting 137–139 pre-existing normal faults 139–141, 144– 156, 161–164 stratigraphy 141, 157 structure 138, 158–162 thermal history 141 –143 tar belt, fluid flow 111 tar belt, Orinoco 18 Tarim Basin 72, 75, 77, 83 tectonics, Papua New Guinea Fold Belt 34–40

INDEX tectono-thermal evolution 123, 125, 130 temperature 97, 105, 107 Albanides 101 Salt Range, Pakistan 103, 104, 106 Taiwan fold and thrust belt 142– 143, 163 Venezuela 90, 95, 96 Tennessee salient 57, 58 thermal history Apennines 123, 125, 126, 130 Taiwan fold and thrust belt 139, 141–143, 165 thermal maturity 3 –4, 121 thermal modelling 97, 101, 103, 104 thermal spring 116 thermicity, Albanides 100– 102, 108, 110 thermogenic gas 116, 121, 122, 125, 132–133 thickness and structure 71, 76, 79, 81– 83 thick-skinned tectonics 24, 25, 27, 28 thin-skinned tectonics 4, 23, 40, 52, 53– 54, 171 thrust ramp 51 Tien Shan Foreland 71–84 orogenic belt 72 Timor Sea, oilfield analogue 36, 38, 52 topography-driven fluid flow 90–93 Toro anticline 39, 45, 51 triangle back thrust 13, 18, 21, 27 Tulungwan thrust 143, 159, 160 Tunisia, pressure-temperature modelling 106 Tyrrhenian Sea, opening 97 unroofing, Garzon Massif 97–101 uplift rate 137 –139 vein sets 110 Venezuela (East) basin 18, 22–25, 27, 29 petroleum modelling 88– 92, 95, 96 Vienna Basin 12, 15–16, 17 vitrinite reflectance 39, 88, 99, 110, 132 volume balance, ductile duplex 65– 68, 69 volume mass balance 3, 4 Western Foothills Fold and Thrust Belt see Taiwan fold and thrust belt Yijianfang Fault 79, 83 Yuching balanced section 142–143, 160–161 Yuching, sub-uplift 160– 161, 165 Zagros Fold Belt 2, 5 structure and thickness 83 Zagros– Mesopotamide province 8 zircon fission track 141– 143, 165 Zongwe anticline 37, 39, 45

Onshore fold–thrust belts are commonly perceived as ‘difficult’ places to explore for hydrocarbons and are therefore often avoided. However, these belts host large oil and gas fields and so these barriers to effective exploration mean that substantial unexploited resources may remain. Over time, evaluation techniques have improved. It is possible in certain circumstances to achieve good 3D seismic data. Structural restoration techniques have moved into the 3D domain and increasingly sophisticated palaeo-thermal indicators allow better modelling of burial and uplift evolution of source and reservoirs. Awareness of the influence of pre-thrust structure and stratigraphy and of hybrid thick and thin-skinned deformation styles is augmenting the simplistic geometric models employed in earlier exploration. But progress is a slow, expensive and iterative process. Industry and academia need to collaborate in order to develop and continually improve the necessary understanding of subsurface geometries, reservoir and charge evolution and timing; this publication offers papers on specific techniques, outcrop and field case studies.