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

摘要

In this study, we assessed the use of stable nitrogen isotopes (δ~(15)N) 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 δ~(15)N 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 δ~(15)N 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 δ~(15)N_(bulk), 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 δ~(15)N values are higher in the Caney Shale (average δ~(15)N_(bulk)=9.5 ± 0.8‰) than in the Woodford Shale (average δ~(15)N_(bulk)=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 differenceswere identified in the δ~(13)C_(org) and TN values between the two units. Given the depositional history of these units, we suggest that the difference in δ~(15)N between the Caney Shale and theWoodford Shale primarily reflects differences in depositional environment, and that changes in δ~(15)N_(bulk) values due to diagenesis or catagensis are subordinate.