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Inorganic Elemental Analysis of Woodford and Mississippian Mudrocks: Implication for Petroleum Systems Analysis


The Woodford Shale and the overlying Mississippian Limestone constitute one of the major oil and gas producing intervals across the Anadarko basin and adjacent shelves. Known for its organic-richness and generation potential, the Woodford Shale has long been recognized as a major source rock for produced oils from the Mississippian Limestone. However, variations in crude-oil composition, together with the presence of secondary organic-rich mudrocks within the Mississippian Limestone, provide another hydrocarbon charge source. Recent organic geochemical studies showed evidence for a contribution to the produced oils from Mississippian mudrocks, in addition to the Woodford Shale. Here, we use inorganic elemental analyses as an additional tool for unraveling source rock depositional settings and secondary processes associated with hydrocarbon generation and migration. In this study, a collection of oil samples, together with core samples from Logan County in north-central Oklahoma, was examined using inductively-coupled plasma mass spectrometry (ICP-MS). Additionally, rock samples were analyzed using Rock-Eval for hydrocarbon generation potential assessment, and oil samples were processed for overall n-alkane profiles using GC-FID. Based on TOC and elemental composition signatures, samples from the organic-rich beds within the Mississippian section were divided into three intervals, and compared with the Woodford Shale samples. Average TOC values for organic-rich Mississippian rocks increase down section with an average of 5.8%, while TOC values for the Woodford Shale average 7.1%. The depth profile trend of major and trace elements such as Mg, Al, Fe, V, Ni etc. were compared with Rock-Eval and GC-FID data to evaluate organic matter type, preservation and redox condition, and hydrocarbon generation potential. Additionally, an inorganic elemental fingerprint was developed for the rocks and compared with that of the crude-oil samples, with the aim to understand the use of elemental fingerprinting as a tool for oil-source correlation and/or secondary alteration processes as a function of hydrocarbon migration.