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Geological Controls of the Organic-Inorganic Distributions Within the Woodford Shale in Northern Oklahoma U.S.A: Integrating for Finding the Unconventional Sweet Spots


The Woodford Shale (Upper Devonian-Lower Mississippian) is a world-class source rock and is one of the main unconventional reservoir in the Mid-Continent region of the U.S.A. This formation is an outstanding oil and gas producer, and is also a good analog for other siliceous unconventional resource shales, particularly if they are underlain by carbonates. The northern part of the state of Oklahoma has a recent high amount of interest due to its exceptional oil and gas reserves and production. The primary goal for this study is to resolve key integrations of geological, geochemical and geophysical characteristics that define the controls on the organic-inorganic facies distribution. The Woodford Shale in the study area has been divided into three members (upper, middle and lower) and has been described principally as an organic-rich black shale with intercalations of chert, siltstone, sandstone, dolostone layers. In addition, laminations of reddish-brown clay, greenish clay and organic matter with scattered siliceous concretions are also present. By integrating the organic parameters (Rock Eval Pyrolysis), inorganic parameters (X-ray Fluorescence and X-Ray Diffraction), well logs correlation, 3D Seismic multi-attribute analysis, 3D seismic inversion, and the sequence stratigraphy of the Woodford Shale, a geological characterization reveals that the Woodford Shale sequence is characterized by variable thickness that are related to subaerial erosion and karstification of the unconformity surface on top of the underlying pre-Woodford Carbonate Strata (Hunton Group). This typical paleo-topography, represented by karsted terrains and incised valleys, shows the transition of a well-developed thicker section of Woodford Shale from a deeper highly restricted anoxic and organic-rich environment, towards a shallower, thinner, and less organic-rich interval. The Woodford Shale is both thicker and more TOC-rich where the underlying Hunton Group is heavily eroded, acting as the depositional fairway for thicker Woodford Shale sections. Seismic analysis at and above the unconformity surface produced by the erosion of the Hunton Group, reveals pod shaped intervals of high TOC that may have been deposited as very restricted intervals. We interpret the structural lows to be isolated pods represented by locally increased accommodation space and highly restricted Woodford shale organic rich deposits, therefore identifying more innovative unconventional reservoir sweet spots.