Structural Styles, Their Plate-Tectonic Habitats, and Hydrocarbon Traps in Petroleum Provinces

Abstract

Broadly interrelated assemblages of geologic structures constitute the fundamental structural styles of petroleum provinces. These assemblages generally are repeated in regions of similar deformation, and their associated hydrocarbon traps can be anticipated prior to exploration. Styles are differentiated on the basis of basement involvement or detachment of sedimentary cover. Basement-involved styles include wrench-fault structural assemblages, compressive fault blocks and basement thrusts, extensional fault blocks, and warps. Detached styles are decollement thrust-fold assemblages, detached normal (growth faults and others), salt structures, and shale structures. These basic styles are related to the larger kinematics of plate tectonics and, in some situations, to particular depositional histories. Most styles have preferred plate-tectonic habitats: (1) wrench at transform and convergent plate boundaries; (2) compressive fault blocks and basement thrusts at convergent boundaries, particularly in forelands and orogenic belts; (3) extensional fault blocks at divergent boundaries in all stages of completion and certain parts of convergent boundaries; (4) basement warps in a variety of plate-interior and boundary settings; (5) decollement thrust-fold belts in trench inner walls and foreland zones of convergent boundaries; (6) detached normal faults, usually in unstable, thick clastic wedges (mostly deltas); (7) salt structures primarily in nterior grabens that may evolve to completed divergent boundaries; and (8) shale structures in regions with thick overpressured shale sequences. Important differences in trend arrangements and structural morphologies provide criteria for differentiation of styles. These differences also result in different kinds of hydrocarbon traps. Wrench-related structural assemblages are concentrated along throughgoing zones and many have en echelon arrangements. The basic hydrocarbon trap is the en echelon anticline, in places assisted by closure directly against the wrench fault itself. Compressive and extensional fault styles typically have multiple, repeated trends, which combine to form zigzag, dogleg, or other grid patterns. Their main trap types are fault closures and drape folds above the block boundaries. Basement warps (domes, arches, etc) are mostly solitary features and commonly provide long-lived positive areas for hydrocarbon concentration in broadly flexed closures. Most decollement thrust-fold structures are arranged in long, sinuous belts and are repeated in closely spaced, wavelike bands. Effective closures include slightly to moderately disrupted compressive anticlines and lead edges of thrust sheets. Most detached normal are listric that occur in coalescing, cuspate bands parallel with the strike of contemporaneous sedimentation. Their basic hydrocarbon traps are associated rollover anticlines which are uniquely concentrated along the downthrown sides of major faults. Salt and shale structures are present both as buoyantly rising pillows, domes, ridges, etc, and as highly complex injected features caused by tectonic forces. Stratigraphic factors, such as truncation, wedging, onlap, and unconformity, add to the variety of traps n all styles. In many places the structures of a petroleum province are either, or both, a gradation between the described fundamental styles and a mix of several styles. These structures can be further complicated by superimposition of fundamentally different tectonic environments. Additional modification of structures can result from still other factors inherent in the deformed region or in the particular tectonic event.