NITROGEN ISOTOPES IN THE WOODFORD SHALE-ARKOMA BASIN: δ15N AS A PROXY FOR UNCONVENTIONAL RESOURCES

Stable nitrogen isotopes have been used to trace the origin and fate of sedimentary organic matter in marine environments. Based on this premise, the efficacy of nitrogen isotopes as proxies in the discovery of hydrocarbons and unconventional resources in the Woodford Shale (Upper Devonian-Lower Mississippian) of the Arkoma Basin is explored here. We will present δ¹⁵N_org, δ¹³C_org, and TOC values from 3 different locations within the Woodford, and compare them to conventional petroleum drilling parameters such as resistivity and gamma ray logs. Our results indicate that δ¹⁵N_org values are highest in the upper section of the Woodford then decrease with depth. In contrast, δ¹³C_org values are lowest in the upper section of the Woodford, with a sudden rise in the middle of the measured section, followed by a decrease in the lower section. TOC has a positive correlation with δ¹⁵N_org and a negative correlation with δ¹³C_org. Resistivity is often used by conventional petroleum and gas shale analysis methods to determine the presence of high hydrocarbon concentrations and thus the likelihood of productivity. Samples from outside of the Woodford Shale deposits generally show more enriched nitrogen isotopic values coupled with low resistivity values. In the Woodford, we generally see a decrease in δ¹⁵N_org accompanied by an increase in resistivity, though not all samples follow this trend. Our data indicates that there may be a negative correlation between resistivity and δ¹⁵N_org.