Abstract Gravity collapse structures are common features on passive margins and typically have a tripartite configuration including an updip extensional domain, a transitional domain and a downdip compressional domain with a common detachment underlying the system. A number of studies have classified these systems, yet few document the wide variations in geometry. This study documents the gravity collapse structures of the Namibian and South African Orange Basin; these structures represent some of the best imaged examples of this important process. We first demonstrate the geometry and kinematic evolution of these systems, focusing on examples of the tripartite configuration from a typical collapse. We then highlight the significant variability in the structures of the system and describe features such as cross-cutting in margin-parallel sections, portions of the system with multiple detachments, systems with stacked synchronous detachments and the temporal evolution of faults within the system. By integrating our observations from a number of sections, we present a model explaining the spatial and temporal evolution of the system. This enables us to discuss likely causes of collapse structures and also, by placing the system into a well-constrained stratigraphic context, how the presence of both maximum flooding surfaces and early margin deltaic sequences have a fundamental control on the resulting collapse geometry.

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 palaeo-thermal 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 thin-skinned 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.

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.

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 décollement 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.

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. 50°. 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.

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 pre-existing 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.

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 palaeo-burial 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 °C 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.

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.

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 sub-Yuching 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.

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.

Abstract 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.