Outcrop-based gamma-ray characterization of the Woodford Shale of south-cental Oklahoma

Scope and method of study. The late Devonian (Frasnian, Fammenian) and basal Mississippian (Tournasian) in age Woodford Shale was studied in Murray, Carter, and Pontotoc Counties, Oklahoma. The purpose of this study was to determine if a relationship exists between outcrop lithofacies and gamma-ray response and if ultimately this relationship can be extended into the subsurface. Over 500 gamma-ray readings were collected from five studied outcrops to complete this work. Thin-section analysis, total organic carbon, x-ray diffraction and vitrinite reflectance data were also integrated in this study.; Findings and conclusions. The Woodford Shale falls into the uraniferous shale category (>20 ppm) with uranium readings as high as 106 ppm. Uranium is the main constituent controlling gamma-ray character. The greatest occurrence of uranium occurs in the lower Woodford and in the top of the middle Woodford.; The bulk of the Woodford contains two main shale habits, fissile and non-fissile. Lithofacies found in or associated with the non-fissile category are phosphatic shale, siliceous shale, dolomite beds, phosphate nodules and rare occurrences of chert and limestone. The fissile shale habits are composed of laminations that are <1 cm thick and are the most organic-rich of the Woodford shales.; Thin-section analysis of the upper Woodford indicates that radiolarians are the main source for silica in the Woodford. Thin-sections cut from phosphate nodules and surrounding shales suggests that radiolarians provide the nuclei for phosphate development.; The Woodford outcrop-based gamma-ray profiles can be correlated into the subsurface. This correlation is based upon pattern recognition of the connected points that compose a gamma-ray profile in comparison with the gamma-ray logs which are continuous curves.