Organic geochemistry of the Woodford Shale, southeastern Oklahoma: How variable can shales be?

Abstract

Woodford Shale samples, obtained from a cored outcrop in southeastern Oklahoma, were geochemically analyzed to determine vertical variations of organic facies, thermal maturity, and an evaluation of their depositional environments. Total organic carbon values ranged from 5.01 to 14.81%, indicating a good source rock potential. In this area, the Woodford Shale is marginally mature, as indicated by vitrinite reflectance values. Rock-Eval data revealed that the samples are dominated by type II kerogen. Biomarker ratios, based on pristane, phytane, steranes, and hopanes, show a mix of marine and terrigenous organic matter. High-salinity conditions and water density stratification also prevailed during deposition of this formation, as indicated by the presence of gammacerane. The Woodford Shale was subdivided into lower, middle, and upper members based on the integration of geochemical and geologic data. Moreover, the presence and extent of photic zone anoxia (PZA) were determined by the presence of aryl isoprenoids. The lower and upper Woodford Shale members were deposited under dysoxic to suboxic conditions and episodic periods of PZA. The middle member was deposited under anoxic conditions and persistent PZA. In addition, aryl isoprenoids helped infer the position of the chemocline during deposition of the different members. The relative hydrocarbon potential parameter was used in determining transgressive and regressive cycles within the Woodford Shale. This study undoubtedly demonstrates the significant lithologic and chemical variability that occurs within shales. The application of this workflow to regional studies can have a direct influence on exploration and production activities in shale-gas systems.