EUSTATIC SEA-LEVEL CONTROLS ON ORGANIC CARBON PRESERVATION IN THE WOODFORD AND BARNETT SHALES, SOUTHERN OKLAHOMA AND NORTHERN TEXAS

The Woodford Shale is an Upper Devonian/Lower Mississippian black shale sequence that may span up to 32-million years of time, including the Frasnian-Famennian boundary. The Barnett Shale is a Mississippian-aged black shale found in the Fort Worth Basin. Black shales are commonly enriched in transition metals. The Woodford and Barnett Shales are no exception, with U concentrations exceeding 100 ppm in the middle member of the formation. While the details of the chemical reactions and feedbacks responsible for trace metal enrichment are debated, these elements continue to be valuable for constraining paleoenvironmental conditions and potential source rock characteristics. While anoxic conditions are thought to favor the preservation of high concentrations of trace metals and total organic carbon (TOC), debate continues concerning the specific role of anoxia in the preservation of TOC, as well as the importance of eustatic sea-level shifts in the development of anoxia during Woodford and Barnett deposition—and by extension in other shale gas units.

To test this hypothesis, and better constrain the depositional conditions that produce gas shales, we have defined meter- to decimeter-scale depositional sequences in these deposits in core and outcrops from southern Oklahoma and northern Texas. Detailed cm-scale descriptions augmented by SEM and thin-section petrography and interpretations of modern suites of wireline logs establish a sequence stratigraphic framework within which to examine variations in TOC concentrations and associated trace metal enrichments. Specifically, gamma-ray curves and variations in a suite of redox-sensitive metals are used to establish the role of glacial eustatic sea-level changes in the development of anoxia and TOC preservation. We are also utilizing our previously developed two-box model of U diffusion into the Woodford to provide constraints on sedimentation rates and associated fluxes of organic carbon to the sediments. These results provide new insights to the geochemical response of the Devonian seas to climate changes, and the development of gas shales, which can be used in further exploration efforts.