Evaluating Nitrogen Isotope Measurements in Unconventional Hydrocarbon Reservoirs

Abstract:

Nitrogen isotope (δ¹⁵N) measurements from core samples taken from unconventional hydrocarbon reservoirs may provide important information on depositional environment, reservoir characterization, and post-depositional processes. In order to evaluate the potential of nitrogen isotopes as geochemical proxies for resource evaluation, we measured δ¹⁵N_bulk values for six Woodford Shale (Late Devonian-Early Mississippian) cores and three Caney Shale (Early Mississippian) cores and compared the profiles with other geochemical, lithological, maturation, and well-log data. The strongest correlation is between δ¹⁵N_bulk and redox-sensitive trace metals and other redox proxies, as predicted by previous research into δ¹⁵N_bulk values. This indicates that δ¹⁵N_bulk can be used in unconventional reservoirs as a proxy for depositional redox conditions. Unlike other redox proxies, δ¹⁵N_bulk reflects the redox state of the deep-water column, rather than that of the deposited sediment, providing a representation of water column processes during deposition. The δ¹⁵N_bulk proxy also appears not to be overprinted by catagenic processes. Associations of δ¹⁵N_bulk with thermal maturity, gamma ray response, and catagenesis and diagenesis proxies were found to be minimal. The ^δ15N_bulk profiles do not appear to be overprinted during catagenesis and therefore are not a reliable record of post-depositional processes. Including nitrogen isotope analyses in a geochemical assessment can provide valuable information about the original redox state of the reservoir unit, and assist in characterizing depositional environment.