Relation Between Extensional Geometry of the Northern Grant Range and Oil Occurrences in Railroad Valley, East-Central Nevada

Abstract

In the northern Grant Range, heterogeneous Neogene extension was dominated by synchronous arching and attenuation. Attenuation was accomplished along a stacked set of attenuation faults that formed at low angles to bedding as the Paleozoic carbonate and Paleogene rocks arched about a north-northwest axis. The style and amount of attenuation was controlled by lithologic character and structural depth of rock units and by geometry of the arch. On the steeper west side of the Grant Range arch, the curviplanar low-angle attenuation faults converge into a single shallowly west-dipping fault zone along which the stratigraphic juxtaposition of Mississippian units over Middle Cambrian units and Late Cretaceous granite marks the zone of maximum attenuation. The distinct geometry o the arched, westward-converging, low-angle fault array is seen in windows into the deeper structure low on the west side of the range. We conclude that arching and heterogeneous extension resulted from uplift of the Grant Range relative to the structural basin of Railroad Valley to the west. This structural differentiation is expressed as a complex zone of subparallel-to-bedding, shallow-dipping attenuation faults rather than as a simple high-angle range-front fault. Seismic and drill-hole data indicate that low-angle attenuation faults in the range extend into Railroad Valley and control the structure buried in the valley. Mississippian and Paleocene to Eocene petroleum source rocks and Devonian to Oligocene reservoir rocks in Railroad Valley oil fields are in extensively fractured rocks of the upper plate to the major extensional fault system. Thus, relatively cold upper-plate rocks, immature with respect to hydrocarbon generation, were brought relatively down into contact with hotter lower-plate rocks by Neogene attenuation faulting. Oil in Railroad Valley, which is sourced from rocks as young as Eocene, was probably generated by this juxtaposition during Neogene crustal attenuation, and subsequently migrated into upper-plate fractured reservoi s.