Hydrocarbon Potential of Mississippian Chainman Shale, Railroad Valley, Nevada

Don E. French

The Chainman Shale has been identified as the source rock for the accumulations now being produced at Trap Spring, Grant Canyon, and Bacon Flat fields in Railroad Valley, Nevada. Because outcrops of the Chainman in the adjacent ranges area are at a low state of maturity, it is logical to assume that the oil has been generated as a consequence of burial of the source rock in the Railroad Valley basin, which developed beginning in the Miocene. An estimate of the amount of oil generated from the Chainman in this basin can be made using a method modified from Mackenzie and Quigley in 1988. The volume of the source rock under generating conditions can be estimated by establishing the distribution of the Chainman prior to basin formation with a paleogeologic map and determining the amount of that distribution present within an adequate thermal environment The geometry of the thermal environment is strongly influenced by a permeability barrier at the base of the valley-fill sediments that creates a thermocline. Pyrolysis data are used to establish the petroleum-generation potential of the source rocks in the generation site. This value is reduced by the percentage actually generated in the given thermal environment and the efficiency of expulsion from the formation. The quantity of petroleum available for accumulation must then be reduced by the percentage lost in migration. A reasonable estimate is that 300-800 million bbl of oil from the Chainman have been entrapped in the basin. This compares favorably with the estimates of the quantity found to date of about 50 million bbl of oil in place.

AAPG Search and Discovery Article #91022©1989 AAPG Annual Convention, April 23-26, 1989, San Antonio, Texas.