Evaluation of redox versus thermal maturity controls on δ15N in organic rich shales: A case study of the Woodford Shale, Anadarko Basin, Oklahoma, USA

Abstract

Bulk sedimentary nitrogen isotopes (δ15Nbulk) have been primarily used to identify bottom water redox states during deposition in sedimentary environments that have not undergone significant diagenetic or catagenetic alteration. Recently, the utility of sedimentary δ15Nbulk as a paleoredox proxy in hydrocarbon-bearing shale intervals was demonstrated by qualitatively correlating the δ15Nbulk profiles from shale units to their depositional redox conditions. However, the effect of thermal maturity on the δ15Nbulk signal remains unknown. We analyzed samples from three cores from the Devonian–Mississippian age Woodford Shale in the Anadarko Basin with vitrinite reflectance values ranging from the oil window to the gas window in order to investigate how depositional redox conditions and thermal maturation affect the bulk sedimentary δ15N signals. Our results indicate that the δ15Nbulk values for the Woodford do not correlate with thermal maturity. Instead, the δ15Nbulk values primarily reflect the depositional redox conditions, which are supported by trace metal concentrations and depositional features such as burrow abundance and lamination. The expected relationship between δ15Nbulk and paleoredox conditions was observed both within each core and between cores on a basin wide scale, with samples deposited under suboxic bottom water conditions having higher δ15Nbulk values than those deposited under anoxic bottom water conditions. Our data also indicate that redox bottom water column conditions in the Anadarko Basin varied spatially and temporally during the deposition of the Woodford Shale.