--> Petroleum Geochemistry and Mudstone Diagenesis of the Woodford Shale, Anadarko Basin, USA — An Integrated Approach

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Petroleum Geochemistry and Mudstone Diagenesis of the Woodford Shale, Anadarko Basin, USA — An Integrated Approach

Abstract

We undertook an integrated organic geochemical and petrologic study of the Upper Devonian-Lower Mississippian Woodford Shale on cores recovered from thermally mature and liquids-rich mudstone reservoirs in the Anadarko basin of south-central Oklahoma, USA. The purpose of the work was to identify the critical mechanisms that control oil and natural gas generation, expulsion, migration, and retention in an active petroleum source rock that is also a producing unconventional reservoir. We identified five microfacies in the Woodford Shale; 1) siliceous mudstone; 2) silicified mudstone; 3) chert and argillaceous chert; 4) argillaceous, siliceous dolostone; and 5) phosphatic mudstone. All of these microfacies exceed the minimum TOC and S2 threshold values for effective petroleum source rocks. The original HI values, calculated from visual kerogen data, indicate oil-prone organic matter in the rocks. Thermal maturity approximates the boundary between low-volatile and high-volatile liquid generation. Sixty to 75% of the petroleum generation process is complete with a 61 to 83% expulsion efficiency. Plots of oil crossover effect and oil saturation indices denote Woodford Shale intervals that retained adequate volumes of hydrocarbons for potential economic production. Reservoir quality in the productive intervals is controlled by the diagenetic fabric of the mudstones. Mineral matrix porosity was reduced mostly by compaction, quartz cementation, and bitumen expulsion from kerogen during burial. Organic-matter porosity, formed during petroleum generation and expulsion, now comprises greater than 99% of the observed effective pore volume. The organic-matter pores occur within an extensive post-oil solid bitumen network which also provides a primary migration path for hydrocarbons.