USING NITROGEN ISOTOPES AS A PROXY FOR DEPOSITIONAL WATER COLUMN REDOX STATE IN HYDROCARBON RESERVOIRS: WOODFORD SHALE AS A CASE STUDY

Assessment of unconventional hydrocarbon resources relies in part on an understanding of the original depositional conditions of the shale unit. Of particular interest is the characterization of the initial water column redox conditions, as these can often impact the development of the hydrocarbon resource play. Sedimentary nitrogen isotope measurements have been found to be reliable proxies for water column redox state in sediments that have been unaltered by catagenesis. By interpreting the measured δ¹⁵N_bulk values in the framework of a conceptual model of the relationship between bulk sedimentary δ¹⁵N values and deep-water oxygen levels, the water column redox conditions during deposition can be characterized as oxic, suboxic, or anoxic.

To determine if nitrogen isotopes can also be used as a proxy in shales that have undergone catagenic processes, we measured δ¹⁵N_bulk values in several cores from the Woodford Shale (Late Devonian-Early Mississippian). The δ¹⁵N_bulk 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 δ¹⁵N_bulk 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 δ¹⁵N_bulk values appear to have the strongest correlation with depositional redox state as determined by core characteristics and trace metal concentrations. Associations between δ¹⁵N_bulk and thermal maturity as evaluated by vitrinite reflectance, and δ¹⁵N_bulk 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.