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. Maximum burial temperatures were between 124 to 134°C. Sixty to 75 % of the petroleum generation process is complete and expulsion efficiency was 61 to 83 %. Plots of oil crossover effect and oil saturation indices denote Woodford Shale intervals that retained adequate volumes of hydrocarbons for commercial petroleum production.

Reservoir quality in these productive intervals is controlled by the diagenetic fabric of the mudstones. Mineral matrix porosity was reduced by compaction, quartz cementation, and bitumen expulsion from kerogen during deeper burial. However, significant mineral matrix porosity was initially preserved in microfacies that underwent early quartz cementation, and relatively larger amounts of pre-oil bitumen filled these spaces prior to the onset of petroleum generation and organic-matter porosity formation. Organic-matter porosity now comprises greater than 99 % of the observed pore volume, and occurs within an extensive and well-connected post-oil solid bitumen network which serves as an effective petroleum migration path.