ASSESSING 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 relative levels of anoxia within the geologic record and compare these signals to modern analogs (Algeo and Rowe, 2012). By applying this concept to a cored interval of the Woodford Shale in the Arkoma Basin, it is possible to develop an understanding of changing levels of circulation within the basin over geologic time. In this area, the Woodford Shale unconformably overlies the Hunton Limestone at an interpreted sequence boundary. The Woodford Shale is interpreted as a transgressive systems tract grading into a highstand systems tract. The top of the Woodford Shale in this area grades into the Sycamore Limestone.

The lower-most portion of the Woodford preserves a Mo-TOC signal that is consistent with a restricted basin that periodically receives influxes of oxygenated water. This is consistent with the onset of rising sea levels flowing into restricted basins and subsequently becoming isolated by localized conditions. The middle portion of the Woodford preserves a signal that indicates a high degree of circulation at the sediment-water interface. This level of circulation persists until just below the interpreted position of the maximum flooding surface. The upper-most portion of the Woodford preserves the most anoxic signal within this basin. However, this interpretation is ambiguous as this is a result of very low levels of Mo found within sediment. The presence of phosphate nodules in this interval suggest that there may have been sufficient oxygen present to send the Mo into solution, and any Mo found may have been adsorbed onto the surface of these minerals.

The ability to interpret the changing levels of bottom water anoxia will enable a greater degree of precision in targeting potential resources in hydrocarbon exploration. Furthermore, this information can be used to help improve our understanding of the changing environmental conditions of the Woodford Shale.