Daniel G. Carpenter
Extension characterized by full grabens and half-grabens and tilted, folded, and faulted range blocks is geometrically and kinematically constrained by geologic and geophysical data. Rootless gravity-slide blocks are common secondary features to high-angle normal faults, which exert primary control over crustal extension.
A synthetic seismogram from the only test well (Mobil Virgin River 1-A; TD = 5,962.5 m), seismic reflection, and Bouguer gravity data (up to 70 mgal of relief) indicate over 7,600 m of low-density Tertiary sediments in the Virgin Valley basin. Several thousand meters of sediment are within the oil-generative window. Neogene basalt flows suggest geothermal gradients higher than today.
Lacustrine limestone in the Oligocene-Miocene Horse Spring Formation contains cryptalgalaminate, a potential hydrocarbon source. The Horse Spring Formation is overlain unconformably by the Miocene-Pliocene Muddy Creek Formation. These formations were deposited in association with movement on the Virgin-Beaver Dam Mountains fault, as indicated by fanning-upward reflector geometry. The formations are incorporated into a major rollover anticline. Mississippian Chainman Shale, penetrated to the west by Chevron's Colorock Quarry well, indicates a possibility for Sevier-age hydrocarbon generation by thrust loading.
Porosity to helium ranges from 16.6 to 17.9%, and permeability to air ranges from 19 to 86 md for the Cenomanian Baseline Sandstone, a potential reservoir that crops out in the North Muddy Mountains and is downdropped into the Virgin Valley basin. SEM and petrographic studies indicate slight compaction, low-clay content, and good reservoir potential. Traps exist in the Virgin Valley basin because of flexure of Sevier orogenic structures and the presence of Paleozoic through Cenozoic strata on the western and eastern margins of the basin during Cenozoic extension.
AAPG Search and Discovery Article #91030©1988 AAPG Annual Convention, Houston, Texas, 20-23 March 1988.