USING NITROGEN ISOTOPES AS A PROXY FOR DEPOSITIONAL WATER COLUMN REDOX STATE IN HYDROCARBON RESERVOIRS: WOODFORD SHALE AS A CASE STUDY
To determine if nitrogen isotopes can also be used as a proxy in shales that have undergone catagenic processes, we measured δ15Nbulk values in several cores from the Woodford Shale (Late Devonian-Early Mississippian). The δ15Nbulk values were then combined with other geochemical and lithological proxies, including trace metals, thermal maturity, and gamma ray logs, to evaluate and constrain the use of δ15Nbulk as a redox proxy. The Woodford Shale samples were also compared with similar δ15N and geochemical measurements from the Caney Shale (Early Mississippian), an interval deposited under different initial conditions that is generally less productive. The measured δ15Nbulk values appear to have the strongest correlation with depositional redox state as determined by core characteristics and trace metal concentrations. Associations between δ15Nbulk and thermal maturity as evaluated by vitrinite reflectance, and δ15Nbulk and gamma ray response were found to be minimal. While the original δ15N values of the initial, pre-catagenesis material cannot be determined, bulk sedimentary δ15N values appear to be reliable proxies for depositional water column redox conditions in the Woodford Shale. As a result, inclusion of nitrogen isotopes in a geochemical assessment can provide valuable information in the evaluation of unconventional resource plays.