Evaluating nitrogen isotopes as proxies for depositional environmental conditions in shales: Comparing Caney and Woodford Shales in the Arkoma Basin, Oklahoma

Abstract

In this study, we assessed the use of stable nitrogen isotopes (δ15N) to evaluate differences in water column conditions during the deposition of the Woodford Shale and Caney Shale in the Arkoma Basin in Oklahoma by comparing bulk δ15N values. The Woodford Shale was mainly deposited under anoxic bottom water column conditions, whereas the Caney Shale shows evidence of deposition under a suboxic/dysoxic deep water regime. Therefore, these two units can be used to test the utility of δ15N as a proxy for the differentiation of these intervals based on depositional environment and post-depositional processes, and provide insights into differences that may impact their assessments as reservoir and source rocks. We measured δ15Nbulk, organic carbon isotopes (δ13Corg), total organic carbon (TOC) and total nitrogen (TN) values from a core from the Arkoma Basin that contains sections of both Caney Shale and Woodford Shale. Bulk δ15N values are higher in the Caney Shale (average δ15Nbulk=9.5±0.8‰) than in the Woodford Shale (average δ15Nbulk=3.1±0.7‰); TOC values in the Woodford Shale are slightly higher than those in the Caney Shale (6±2% and 4±2%, respectively). No significant differences were identified in the δ13Corg and TN values between the two units. Given the depositional history of these units, we suggest that the difference in δ15N between the Caney Shale and the Woodford Shale primarily reflects differences in depositional environment, and that changes in δ15Nbulk values due to diagenesis or catagensis are subordinate.