Abstract Indian basement faults, which bound three orogen-perpendicular palaeotopographic ridges of Precambrian Indian basement south of the Himalaya, extend to the base of the Indian lithosphere and to the northern extent of the Indian lithosphere underneath Tibet. In the eastern Himalaya, the active orogen-perpendicular Yadong–Gulu graben is aligned with an earthquake-generating strike-slip fault in the high Himalaya. We argue that the graben results from crustal necking during reactivation of the underplated basement fault. In the central Himalaya, along-strike diachronous deformation and metamorphism within the Himalayan metamorphic core, as well as lateral ramps in the foreland thrust belt, spatially correspond to the Lucknow and Pokhara lineaments that bound the subsurface Faizabad Ridge in the Indian basement. Analogue centrifuge modelling confirms that offset along such deep-seated basement faults can affect the location, orientation and type of structures developed at various stages of orogenesis and suggests that it is mechanically feasible for strain to propagate through a melt-weakened mid-crust. We suggest that inherited Indian basement faults affect the ramp-flat geometry of the basal Main Himalayan Thrust, partition the Himalayan range into distinct zones, localize east–west extension resulting in the Tibetan graben and, ultimately, contribute to lateral variability in tectonic evolution along the orogen's strike.

ABSTRACT Previous work shows that the Burnt Bluff Group was deposited as a series of shallow- to moderate-water-depth facies in a tropical marine setting in the Michigan Basin during the Llandoverian. New interest in the unit for both hydrocarbon resources (subsurface) and aggregate resources (outcrop) is driving research in this poorly understood unit. New cores, as well as investigation of the outcrop belt in the Upper Peninsula of Michigan, allow further elaboration of the depositional model. The traditional interpretation of the Burnt Bluff Group consists, in stratigraphic order, of the open-marine deposits of the Lime Island Formation, the restricted lagoonal–tidal flat Byron Formation, and the open-marine deposits of the Hendricks Formation. The contacts with the underlying Cataract Group and overlying Manistique Group are gradational. Identification and description of facies in both cores and outcrop sections provide new constraints on the stratigraphic nomenclature of the Burnt Bluff Group. New outcrop and core data require the revision of the depositional model for portions of the group because typical Byron-like facies are found interbedded with Hendricks-like facies in both the Byron and Hendricks Formations. Limited age data from published research combined with the new facies model for the Burnt Bluff Group suggest that the unit was deposited as a time-transgressive package on a carbonate ramp during the Llandoverian.

Abstract: The boundaries between pairs of adjacent fault segments within normal fault arrays define a spectrum of structures, from relay ramps where the length of overlap between the fault segments is much larger than the separation, through low aspect ratio (overlap/separation) relay ramps and ultimately to underlapping fault segments. Where fault segments underlap, transfer of displacement between them is accommodated by a connecting monocline. When displacement increases and a through-going fault forms, relay ramps are preserved as fault-bounded zones of elevated bed dip and monoclines are preserved as areas of normal drag. Therefore, the orientation and magnitude of bed dips within and adjacent to a fault zone, and the numbers of segments seen on a cross-section through it, depend largely on the aspect ratios of relay ramps in the initial fault array. The aspect ratio of relay ramps varies between different fault systems. An analysis of the geometry of 512 relay ramps from 13 different fault systems suggests that the main controls on aspect ratio are the strength of the sequence at the time of faulting and the underlying structure.

Abstract As hydrocarbon-prone basins mature through time, stratigraphic traps become increasingly important as hosts for yet-to-find reserves. Explorationists strive to reduce the uncertainty in reservoir distribution and quality, but considerable complications exist in the evaluation of stratigraphic traps owing to the inextricable links between stratigraphy, trap definition and their subsequent risking. This study quantifies the relationships that exist between reservoir geometries and the rates of reservoir property degradation in a turbidite sandstone pinch-out zone. The investigation focuses on the Paleocene Forties Sandstone Member of the Everest and Arran fields of the East Central Graben of the UK North Sea. We utilized standard seismic interpretation techniques and integrated stratigraphic and petrophysical analysis of wireline log data to map deep-water turbidite sandstone terminations and develop a predictive model for reservoir property changes close to the feather edge. The Forties Sandstone Member thins systematically up on to a palaeoramp on the eastern basin margin of the Central Graben. Results reveal that the net reservoir sandstones pinch out after the turbidite flows had traversed 5 km across the palaeoramp. The gross interval is predicted to completely terminate 6.4 km up the palaeoramp. The reservoir properties decrease in concert with the thinning trend in the wedge zone as a function of the interaction of palaeotopography and the hydraulics of the decelerating flows. The inclination of the counter-regional slope is considered to be a key controlling factor that determines the rate of thinning and thus the termination position of the sandstones and their concomitant reservoir property decline. The results of this study demonstrate that characterization of pinch-outs into distinct zones based on a palaeotopographic template can be of utility in stratigraphic and combination trap definition. This work also has wider implications for prospect risking, volumetric analysis, the population of properties and geological modelling of stratigraphic traps.

Abstract Because of the importance of understanding the association of folds and fractures in the hydrocarbon and mining industry, a considerable amount of work has been undertaken to establish the geometric relationship between these structures. The structures are linked in a variety of ways. Sometimes, as for example in the formation of fractures in the inner and outer arcs of the hinge zones of single-layer folds or of accommodation thrusts in the hinge regions of multilayer folds, it is the process of folding that generates the fractures. At other times, as in the formation of a fault-bend fold and other types of forced folds, the reverse is true. In this paper an attempt is made to look briefly at the various types of fold-fracture associations found in nature and, by considering the evolution of a typical fold–thrust belt, obtain an insight into the controls on the temporal and spatial organization of the different types of folds and their associated fractures that form in this tectonic regime. The role of fluids in the initiation of both folds and thrusts is considered, as is the subsequent impact of these structures on fluid migration. It is shown that understanding the links between stress, fluid pressure, fracturing and folding provides a clear insight into the fluid mechanics operating in an active fold–thrust belt.

Abstract The impact of geometric uncertainty on across-fault flow behaviour at the scale of individual intra-reservoir faults is investigated in this study. A high resolution digital elevation model (DEM) of a faulted outcrop is used to construct an outcrop-scale geocellular grid capturing high-resolution fault geometries (5 m scale). Seismic forward modelling of this grid allows generation of a 3D synthetic seismic cube, which reveals the corresponding seismically resolvable fault geometries (12.5 m scale). Construction of a second geocellular model, based upon the seismically resolvable fault geometries, allows comparison with the original outcrop geometries. Running fluid flow simulations across both models enables us to assess quantitatively the impact of outcrop resolution v. seismic resolution fault geometries upon across-fault flow. The results suggest that seismically resolvable fault geometries significantly underestimate the area of across-fault juxtaposition relative to realistic fault geometries. In turn this leads to overestimates in the sealing ability of faults, and inaccurate calculation of fault plane properties such as transmissibility multipliers (TMs).