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Controls on Compartmentalization Within Supra-Salt Crestal Fault Systems: A Case Study From the Salt Valley Salt Wall, Paradox Basin, Utah

Abstract

Salt-cored anticlines provide excellent structural trapping geometries in host strata along their crests and flanks. They are frequently associated with complex crestal fault arrays that develop in response to both localisation of regional tectonic strain within the overburden, and dissolution of salt by erosion and groundwater circulation during periods of shallow burial or sub-aerial exposure. The resulting crestal fault arrays often exhibit complex fault block configurations characterised by two or more opposing half-graben systems separated by transverse accommodation zones. Such fault block configurations have the potential to compartmentalise hydrocarbon traps, but are very poorly imaged on even high-quality 3D seismic datasets.

Examples of opposing crestal fault block configurations are present along the exposed crests of salt walls from the Salt Anticline Region of the Paradox Basin, Utah. This work presents an analysis of the evolution of the crestal fault system in Cache Valley, which is located at the termination of the underlying Salt Valley salt structure. Detailed structural mapping provides input to a 3D geometrical model of the fault system, which is restored and analysed to derive fault heave and displacement-length relationships.

Two distinct fault populations are identified within Cache Valley. A dominant E-W-trending set comprises the main crestal fault system, which we attribute to an early phase of regional tectonic extension; this fault system is composed of several kinematically-linked, but oppositely-dipping half-grabens, separated by transverse accommodation zones. A second set of faults trending NE-SW are present only in the east of Cache Valley, and are postulated to be related to the transfer of regional tectonic strain between the Salt Valley salt wall and the Fisher Valley salt wall, which is offset by 5km to the north-east. We show that interaction between the two fault populations influenced the position of transfer zones within the crestal fault system. Subsequent late-stage gravitational collapse of the salt wall crest, induced by salt dissolution, was facilitated by reactivation of these pre-existing fault trends, resulting in a highly compartmentalised crestal fault block configuration. This work demonstrates the potential for significant compartmentalisation associated with the evolution of crestal fault systems, and has wider implications for hydrocarbon migration modelling and prospect evaluations in salt basins.