Source-rock reservoirs geochemistry of Devonian–Mississippian mudrocks in central Oklahoma

Abstract

The Woodford Shale and the overlying Mississippian limestone constitute one of the prolific unconventional hydrocarbon targets in central and northern Oklahoma. With the advantage of horizontal drilling technology, Devonian and Mississippian mudrocks have become important topics for research to understand the petroleum system fundamentals, including sources of hydrocarbons for these unconventional reservoirs. In this study, organic geochemistry and petrography of Mississippian mudrocks and the Woodford Shale were examined. The key study core is from Lincoln County, Oklahoma, representing the Mississippian mudstone and Woodford Shale. Three additional cores in Payne County and one core from Logan County were studied. Core samples were assessed for hydrocarbon generation potential using Rock-Eval pyrolysis together with biomarker analysis using gas chromatography–mass spectrometry. Organic and thin-section petrography was performed to identify maceral composition and develop a sedimentary facies classification. Mudrocks within the Mississippian section exhibited adequate organic richness for petroleum generation, thereby augmenting the established generation potential of the Woodford Shale. Several unique Mississippian biomarkers were identified, including diterpenoids and extended tricyclic terpanes. Macerals of the Mississippian rocks exhibit dominance of lamalginite and bituminite, whereas Woodford macerals are high in telalginite. Based on the organic matter composition and sedimentology, six facies were identified within the Mississippian mudrocks. Furthermore, based on organic richness and key biomarker ratios, the Mississippian succession is divided into three units. Organic and sedimentological signatures suggest a shoaling-upward succession from the Woodford Shale to the Mississippian carbonate.