EVALUATING UNCONVENTIONAL RESOURCE PLAYS USING NITROGEN ISOTOPES

Evaluation of unconventional resource plays requires knowledge of the formation history of the reservoir, particularly characterization of the depositional environment for the unit and any subsequent changes to the organic matter present. While bulk sedimentary nitrogen isotope (δ¹⁵N) values have been determined to be an accurate proxy for water column redox state in well-preserved sedimentary systems, the use of the proxy in unconventional and hydrocarbon systems is relatively new. In addition, the effects of post-depositional changes such as thermal maturation, compartmentalization, and fluid migration on the δ¹⁵N proxy have not been assessed.

A series of case studies was designed to evaluate the use of bulk sedimentary δ¹⁵N values as a depositional water column redox proxy in unconventional shale plays, and to characterize any processes that might control alteration of the δ¹⁵N signal. The results of three sets of shale samples are presented: Devonian-aged Woodford Shale and Caney Shale samples from cores drilled in the Arkoma Basin in Oklahoma; samples from three Woodford Shale cores from the Anadarko Basin, Oklahoma; and Ohio Shale core samples from the central Appalachian Basin in Kentucky. In these studies, the δ¹⁵N data was combined with other isotopic, geochemical and lithological measurements, which include carbon isotopes of organic carbon, trace metal concentrations, vitrinite reflectance, and gamma ray logs.

The results from these case studies support the use of δ¹⁵N measurements as a depositional redox proxy for unconventional and hydrocarbon plays, as the δ¹⁵N values primarily reflect water column oxygen concentrations during deposition. Comparison between cores across a basin indicates that thermal maturation processes do not appear to overprint the δ¹⁵N signal. Additionally, comparison of the inorganic and organic sedimentary nitrogen isotope fractions may provide evidence regarding fluid migration pathways during catagenesis and/or compartmentalization within an unconventional resource unit. In conclusion, measurement and interpretation of δ¹⁵N values as part of a multi-proxy geochemical analysis can provide important details regarding the depositional and catagenic history of an unconventional or hydrocarbon resource play.