Abstract Basin evolution of the U.S. Chukchi shelf involved multiple phases, including Late Devonian–Permian rifting, Permian–Early Jurassic sagging, Late Jurassic–Neocomian inversion, and Cretaceous–Cenozoic foreland-basin development. The focus of ongoing exploration is a petroleum system that includes sag-phase source rocks; inversion-phase reservoir rocks; structure spanning the rift, sag, and inversion phases; and hydrocarbon generation during the foreland-basin phase. Interpretation of 2-D seismic and sparse well data documents the presence, in the south-central part of the shelf, of a series of en-echelon, north-south trending monoclonal fold limbs that display up to 1+ km (3,300 ft) of structural relief. These folds, which are located above the tips of rift-phase normal faults, are interpreted as inversion structures formed by maximum compressive stress oriented obliquely to the strike of rift-phase normal faults. Erosional relief on a Jurassic unconformity, growth strata in the overlying Upper Jurassic to Neocomian strata, and east-dipping clino-forms in a high accommodation depocenter east of the inversion structures indicate profound structural influence on sedimentation. Oil-prone source rocks, reservoir-quality sandstone, migration pathways, and structural closure are linked intimately across the Jurassic unconformity, which reflects inversion. Thus, all these key petroleum systems elements were in place when Triassic source rocks entered the oil generation window during Cretaceous–Cenozoic stratigraphic burial.

Abstract The Dinkum graben system beneath the central to eastern Beaufort shelf of Arctic Alaska comprises a complex of grabens and horsts that records multiple phases of extension and contraction spanning the Mississippian through Early Cretaceous (Neocomian). The graben system extends from about 150°W eastward for more than 200 km, approximately parallel to the Alaska Beaufort Sea coast. The eastern extent of the graben system (east of about 145.5°W) is masked by deep burial beneath Cenozoic strata and by complex Cenozoic structures. The graben system developed above regional basement that includes the pre-Mississippian Franklinian sequence, interpreted as Late Proterozoic-Devonian strata deformed and metamorphosed during the Ellesmerian orogeny. Franklinian rocks display in seismic data a range of variably dipping structural and metamorphic fabrics described in a companion abstract (Connors and Houseknecht). Previously published interpretations of the Din-kum graben suggest two phases of extension related to rift opening of the Amerasia basin, a Jurassic phase characterized by generally south-dipping normal faults and an Early Cretaceous phase characterized by generally north-dipping normal faults. However, our interpretation of 2D seismic data, tied to well control near the coast and potential fields data across the Beaufort shelf, documents a geologic history commencing with Mississippian extension accommodated by both north- and south-dipping normal faults detached along the variably dipping basement fabrics. Growth strata indicate that pulses of extension in the graben system occurred during the Mississippian, Late Triassic–Early Jurassic, and Neocomian. Further, certain faults accommodate Mid-Late Jurassic growth strata that grade from positive to negative growth along strike, suggesting inversion of older structures, perhaps by stresses oblique to older fault planes. This polyphase deformational history is reflected in a complex graben system that accommodates Mississippian-Neocomian strata at least 5 km thick in places. The newly recognized presence of pre-Jurassic strata in the Dinkum graben system has significant implications regarding petroleum systems. Upper Triassic growth strata likely include oil-prone source rocks in the Shublik Formation, corroborated along the southern margin of the graben by oil accumulations ( e.g. , Northstar) with implausible migration pathways from sources to the south, and by chemistry that suggests a Triassic source rock containing more detrital components and less carbonate than typical Shublik of the North Slope. Considering the timing of extensional pulses discussed above, the presence of Lower Jurassic and Neocomian source rocks also is likely. Although all these source rocks likely are thermally overmature in deeper parts of the graben, shallower parts of the graben, horsts within the graben, and southern and northern margins of the graben may be in the oil window, and may have been charged from the graben.

Abstract As Offshore Nigeria enters a third decade of deep-water exploration, unsuccessful wells in the structures of the deep-water Outer Fold and Thrust Belt have spurred a reevaluation of plays in the regional basin. Key lines from a newly-acquired seismic data set having 10 km long-offset, deep-tow acquisition parameters, and modern PSDM processing are examined here and show significant improvements in deep imaging. The interpretation of these lines advances the understanding the Paleogene Akata Shale and structural styles of mobile shale features and focuses new attention towards exploration leads of older sediments in intermediate water depths of the Inner Fold and Thrust Belt. The data show a clearer imaging of crustal structure, and the interpretation of the Tertiary supports the view of the offshore Nigeria as a linked extension to a contraction system driven primarily by gravity spreading. The Inner Fold and Thrust Belt of deep to intermediate water depths has been difficult to image in past data sets showing only thick sections of seismically opaque facies commonly interpreted as shale ‘diapirs’ and only thin sediments. Deep tow data here reveals several deep areas of stacked thrust sheets, within the Paleogene strata, as well as associated floor and roof detachments interpreted throughout the delta. The formation of these ‘duplexes’ uplifted existing Neogene thrusted sediments and folded these sediments often to very shallow depths where they are eroded near the present-day water bottom. Improved resolution of the lower Miocene and Oligocene sediments shows a robust deposition of these sequences that are involved in the inner belt structuring. The Inner Fold and Thrust Belt shows features that form a variety of hidden, deeper reservoir targets, structures of different timings, and areas where the deeper imbricates of the Akata could provide thickened source rock intervals.

Abstract The northwestern Gulf of Mexico basin has emerged as an archetype example of a robust, progradational passive margin system that induces substantial translation over underlying detachments due to gravitational loading. Despite this recognition, the difficulties in deep imaging of seismic data have continued to obscure key features of this deformation. Megaregional, 2D, long-offset PSDM data help advance the interpretation of the Paleogene and Miocene of the Gulf of Mexico. We present results from new seismic line composites made up of reprocessed PSDM legacy onshore data (sourced from SEI and GPI ), and newly acquired ocean bottom cable data and marine streamer data acquired and processed by GX Technology. Key lines from this dataset link robust, onshore shelf lowstand wedges to deep-water sediments and more clearly image deep structural styles and salt remobilization events. The lines span approximately 300 miles (500 km) from onshore Texas to the ultra-deep water and finally show the full size of geologic features, including a regional salt weld that starts onshore at the top Eocene and extends for over 100 km, ramping up to Oligocene. The interpretation highlights the effects of gravitational forces on the stratigraphic section and delineates prominent extensional faults that sole-out at major detachment levels and are linked to a newly recognized Paleogene thrust belt, as well as to previously documented Oligo-Miocene contractional belts. Significant lateral translation occurs along these detachments. The data image a key fault connection from Oligo-Miocene extensional faults down to the Louann detachment surface, and the interpretations provide viable scenarios for Oligo-Miocene expansion to drive the Perdido fold belt along the Louann detachment.

ABSTRACT Shear fault-bend folding produces ramp anticlines with very distinctive shapes. They are characterized by long, gentle backlimbs that dip less than the fault ramp, in contrast with classical fault-bend folding. Backlimb dips and limb lengths increase progressively with fault slip, by a combination of limb rotation and kink-band migration. We summarize and apply two simple end-member theories of shear fault-bend folding involving a weak décollement layer of finite thickness at the base of a ramp: (1) simple-shear fault-bend folding, in which the layer undergoes an externally imposed bedding-parallel simple shear with no basal fault, and (2) pure-shear fault-bend folding, in which this basal layer slides above a basal fault and shortens and thickens above the ramp, with no externally applied bed-parallel simple shear. In the limit of large displacement, the fold geometry in pregrowth strata approaches the geometry of classical fault-bend folding, with abacklimb dip that approaches the ramp dip. However, even in these cases, growth strata may record the history of limb rotation that is characteristic of a shear fault-bend fold heritage. We demonstrate that these theories are in agreement with well-imaged seismic examples from the Nankai Trough and Cascadia accretionary wedges, which show substantial shears (40–65°) over stratigraphic intervals of a few hundred meters.

ABSTRACT We describe local structural controls on deposition above contractional fault-related folds that yield patterns of stratigraphic onlaps, pinch-outs, and facies transitions that are diagnostic of folding mechanism. In folds that grow by kink-band migration, stratigraphic onlaps and pinch-outs that formed at emergent fold scarps are incorporated into fold limbs and aligned along growth axial surfaces. In contrast, the positions of these same features in structures that grow primarily by limb rotation are more variable and are controlled directly by sedimentation-to-uplift ratio. We present kinematic models and natural examples that integrate seismic reflection data and well control to describe these structural influences on growth stratigraphy. An understanding of this interplay between local deformation and deposition helps us infer the positions of subtle pinch-outs that may provide hydrocarbon traps and can yield a detailed history of structural development.