Molecular Organic Geochemistry of the Oil and Source Rock in Railroad Valley, Eastern Great Basin, Nevada, United States

Abstract

A comprehensive geochemical study of oils from Railroad Valley and two candidate source rock intervals from the nearby Egan Range, was conducted in order to establish oil-oil and oil-source rock correlations. Total organic carbon analyses showed high organic content in the Mississippian Chainman Shale. However, outcrop samples of the Paleogene Sheep Pass Formation Member B are organically lean. Strata in both of these units are mature, and tend to be oil-gas prone. Biomarker analysis of oil samples revealed that two different oil families exist. Group 1 oils (Trap Spring and Grant Canyon oils) appear to originate from marine shale source rocks that were deposited under normal marine salinity and dysoxic conditions, as shown by high Pr/Ph ratios, low homohopane index, and high diasterane/steranes ratios. Group 1 oil correlates with Chainman Shale source rock extracts. Group 2 oils (Eagle Spring, Kate Spring, and Ghost Ranch oils) are lacustrine-derived and have low Pr/Ph, high gammacerane, good preservation of homohopane, and low diasterane/sterane ratios. The abundance of oleonane and dinosterane provides good evidence that oils belonging to this group are derived from source rocks younger than the Cretaceous, which points to the Sheep Pass Formation Member B. Our comprehensive geochemical study of oil also suggests that the oils from Kate Spring and Ghost Ranch are slightly different from oils from Eagle Spring but they are still closely related. Detail geochemical analysis were able to show that a difference in source rock facies and source rock depositional conditions in the lacustrine system serves as a key control that resulted in those differences. Stable carbon isotope data clearly showed two different groups. Group 1 oils have low δ13CSAT and high δ13CAROM, which is indicative of a marine source rock. On the other hand, Group 2 oils appear to have high δ13CSAT and high δ13CAROM, which suggests a lacustrine-derived oil. Additionally, diamondoid analyses showed most of my oil have low abundances of diamondoids, which suggest that intense oil cracking has not yet occurred. The result of this research shows that two different intervals (the Chainman Shale and the Sheep Pass Formation Member B) serve as effective source rocks in this basin. This new understanding of effective source rock(s) in this basin will significantly improve the hydrocarbon play concept as well as open the new perspective of hydrocarbon exploration within the Basin and Range area.

AAPG Datapages/Search and Discovery Article #90216 ©2015 AAPG Annual Convention and Exhibition, Denver, CO., May 31 - June 3, 2015