Kinematics and growth of supra-salt fault systems: a field and subsurface analysis, Salt Valley salt wall, Paradox Basin, Utah

Abstract

Salt can provide the structure and seal necessary for hydrocarbon entrapment. However, it may lead to structural complexities, such as compartmentalizing a hydrocarbon reservoir through supra-salt faulting. Outcrop analog studies provide exceptional opportunities to observe how salt-influenced fault geometries evolved spatially and temporally. The Salt Valley salt wall in the northern Paradox Basin is an exceptional location to study supra-salt faulting due to the proximity of world-class outcrops and a 3D seismic reflection dataset. This proximity enables fault slip kinematic information from outcropping fault surfaces to be assigned to 3D fault geometries mapped in the subsurface. The Salt Valley supra-salt fault array is approximately 40 km long, trends parallel to and detaches downward onto the NW-plunging salt wall. Integration of 3D seismic reflection data, wells, published maps, satellite imagery, and structural field measurements enabled the interpretation of the spatial and temporal evolution of the fault array. Several kinematic analyses coupled with detailed geometric fault descriptions were used to determine the growth history of the studied subsurface fault array, which consists of a series of overlapping fault segments up to 12.5 km long, with throws of hundreds of meters, defining a series of crestal grabens and half-grabens. Outcropping faults are of similar length, though offset from the subsurface study by approximately 500 meters. This proximity facilitates field to subsurface correlation. Along the strike of the fault array, there are notable changes in the dip direction of the half-graben master faults and regions of varying fault strikes. These changes reflect heterogeneities of the top-salt geometry. Fault linkage analyses such as: fault throw-length (T-L); throw-distance (T-x); throw-depth (T-z), as well as qualitative distribution of fault throws from map and strike views indicate that the subsurface fault segments are over-displaced and have complex fault segment linkage histories. This over-displacement may be evidence for a hybrid fault growth model, where faults initially grew per the isolated fault model, but spent much of their growth history through coherent fault growth. Additionally, faults analyzed in this study display non-fractal throw distribution, indicating that faulting is controlled by localized zones of strain that coincide with asymmetries in Top-Paradox Salt.