Characteristics of oils and their potential source rocks from the Cherokee Platform, Oklahoma

Abstract

The Woodford Shale is the most prolific source rock in the Anadarko Basin and Cherokee Platform and, over the past decade, it has become renowned for its high unconventional hydrocarbon potential. Recent Woodford Shale studies have emphasized facies heterogeneity based on lithology, sequence stratigraphy, and geochemical analyses. Data presented in this paper focus on the organic geochemical characterization and genetic relationship between the Woodford Shale and oils in the Cherokee Platform area, Pottawatomie County, Oklahoma. The shale samples were screened by total organic carbon (TOC) and Rock-Eval (RE) that indicate a high-quality Type I/II kerogen. Vitrinite reflectance measurements (VR%) of 0.59% for the Woodford Shale indicate the samples are at an early maturity stage in this area. The Woodford Shale generally has been subdivided into three members (Upper, Middle, and Lower), but in this study only the Upper and the top part of the Middle Woodford were characterized. Both intervals alternate between suboxic and anoxic depositional conditions, along with water column stratification and episodic euxinia or photic zone anoxia (PZA). A PZA episode has been described for Upper Woodford for the first time where C40 carotenoids and aryl isoprenoids show maximum abundance.The second part of the investigation involved oils, collected from wells in the area, and a detailed correlation with the Woodford Shale. The oils showed a close correlation with the Woodford, but maturity based on biomarkers may suggest a deeper source rock with equivalent organofacies. Many of the oils examined showed varying levels of biodegradation and possible mixing with a lighter more mature oil. The oils were compared with each other to determine possible relationships and assess the petroleum system on the southern Cherokee Platform.