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Utilizing HHXRF to Assess Variable Bottom Water Anoxia Within the Late Devonian Woodford Shale in the Arkoma Basin, Southern Oklahoma


By comparing the concentration of Mo (ppm) and TOC (wt%) within sediments it is possible to interpret the changing relative degrees of bottom water circulation within the geologic record. The Woodford Shale is interpreted as TST grading into HST. The ability to interpret the changing levels of bottom water anoxia enables a greater degree of precision in targeting potential hydrocarbon resources. However, direct measurement of TOC can be time-consuming and is a destructive process. It is also possible to document changing circulation patterns, non-destructively using Hand-Held X-Ray Fluorescence (HHXRF), within a basin by utilizing changing trends in redox sensitive trace metals (such as Mo, Ni, and Cu) and a semi-quantitative approximation of the Degree of Pyritization (herein referred to as aDOP) based on an idealized formula for pyrite (FeS2). This chemofacies approach produces a qualitatively similar interpretation to the changing Mo-TOC signal in the Woodford Shale and can evaluate lateral trends in changing bottom water circulation. In the Woodford Shale, distal regions preserve the greatest degree of circulation. Proximal regions preserve highly variable conditions with more restricted conditions being more common in the lowermost Woodford Shale and circulation generally improving upsection. This information helps improve our understanding of the changing environmental conditions of the Woodford Shale. The lowermost Woodford preserves a Mo-TOC signal that is consistent with a restricted basin that periodically receives influxes of oxygenated water consistent with the onset of rising sea levels flowing into restricted basins that subsequently become isolated by localized conditions. The middle Woodford preserves a signal that indicates an increased degree of circulation at the sediment-water interface. This level of circulation persists until the maximum flooding surface. The presence of phosphate nodules in the uppermost Woodford suggest sufficient oxygen present to send the Mo into solution and indicates active upwelling and circulation with the Paleotethys. The Framvaren Fjord is a reasonable modern analog for the lowermost portion of the Woodford Shale. The Cariaco Basin is a good modern analog for the middle and uppermost Woodford Shale.