FRENCH, DON E., Independent Geologist, Billings, MT, and JEFFREY B. HULEN, University of Utah Research Institute, Salt Lake City, UT

ABSTRACT: Origin and Nature of the Hydrocarbon Seal in Railroad Valley, Nye County, Nevada

Over 33 million bbl of oil have been produced from the Basin-Range oil fields of Railroad Valley in eastern Nevada. Oil-reservoir rocks are Devonian to Tertiary age, forming structural or structural/stratigraphic traps beneath a widespread seal at the base of an alluvial and lacustrine, Miocene-Pliocene valley-fill sequence. This seal is mandated by Railroad Valley drilling results to date: sub-valley-fill strata are oil rich, but of 850,000 ft of valley fill penetrated by 214 wells, fewer than 150 ft have yielded oil shows. Temperatures from drill-stem tests also indicate that the seal commonly coincides with a thermocline. The nature and origin of the seal have been investigated at Bacon Flat and Grant Canyon fields in east-central Railroad Valley, and in nearby Horse Camp basin, wh re a contemporaneous valley-fill sequence is exposed. At all three sites, the basal valley fill consists of tuffaceous clastic rocks in which volcanic ash has been altered to montmorillonite. At the two oil fields, these clay-rich rocks have also been hydrothermally kaolinized and silicified, enhancing the permeability barrier at the unconformity. The sealed zone and associated thermocline could affect the thermal regime of the basin, and thereby the size and distribution of potential hydrocarbon generation sites; they could mask the presence of commercial geothermal systems. Improved understanding of the sealing processes also could help constrain models of hydrocarbon-associated precious-metal deposits.

AAPG Search and Discovery Article #90993©1993 AAPG Rocky Mountain Section Meeting, Salt Lake City, Utah, September 12-15, 1993.