Anatomy of reservoir-scale normal faults in central Utah: stratigraphic controls and implications for fault zone evolution and fluid flow

Analysis of fault zone structure and composition of two intermediate-displacement faults in the Colorado Plateau reveal how fault structure varies as a function of lithology, and how faults impact fluid flow. The Little Grand Wash fault cuts Jurassic Summerville through Cretaceous Mancos Shale rocks, and consists of a complex set of interweaving fault strands. Fault relays are developed where sandstone and shale are cut by the fault in roughly equal amounts. Ancient and modern fluid flow is documented by the presence of travertine and tufa deposits, an oil seep, and CO ₂ gas seeps. Analysis of the water, travertine, and gas composition indicate that the CO ₂ emanates from a reservoir 1.5-2 km deep, and charges a shallow aquifer. Cross-fault flow is inhibited, and the gas and water flows in the footwall damage zone of the fault. Analysis of the Bighole fault in the San Rafael Swell shows how fault structure varies with displacement. The fault is exposed entirely in the aeolian Jurassic Navajo Sandstone, and consists of a dense fault core interpreted to be a densely packed set of deformation bands bounded by a narrow slip surface. The fault zone consists of conjugate deformation band sets in the hanging wall and footwall of the fault, and the fault core thickness does not vary significantly with net slip.