A comprehensive geochemical study of oils from Railroad Valley, Nevada and two candidate source rock intervals from the nearby Egan Range, was conducted in order to establish oil-oil and oil-source rock correlations. Analyses consisted of total organic carbon, Rock-Eval pyrolysis, and vitrinite reflectance for source rock samples, as well as biomarker, diamondoids, and stable carbon isotope analyses on source rock extracts and oil samples.
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 immature to mature, and tend to be oil-gas prone.
Biomarker analysis of oil samples revealed that two different oil groups 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. In addition, age related biomarker parameters showed this oil group to be derived from a source rock that is older than Cretaceous. 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. High gammacerane and high C24 tetracyclic suggest that the oil in this group was derived from a source rock deposited under hypersaline conditions. 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. My comprehensive geochemical study of oil also suggests that the oils from Kate Spring and Ghost Ranch are different from oils from Eagle Spring but they are still closely related. I hypothesize 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, which supports my biomarker data. Group 1 oils have low δ13C SAT and high δ13CAROM, which is indicative of a marine source rock. On the other hand, Group 2 oils appear to have high δ 13CSAT and high δ13CAROM, which suggests a lacustrine-derived oil. Additionally, diamondoid analyses showed most of my oil and source rock extracts have low abundances of diamondoids, which suggest that intense oil cracking has not yet occurred.
The results 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. Specifically, oil fields in the western and southern part of the basin (Trap Spring and Grant Canyon) were charged by the Chainman Shale source rock, whereas the Sheep Pass Formation member B was the main contributor of the reservoired oils in the eastern part of the basin (Eagle Spring, Kate Spring, and Ghost Ranch). 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.
