TRANSITION METALS AND REGIONAL PALEOENVIRONMENTAL CHARACTERISTICS OF DEVONIAN BLACK SHALE: IMPLICATIONS FOR GAS SOURCE BED PROSPECTIVITY

The Woodford Shale of the US southern midcontinent is an Upper Devonian/Lower Mississippian black shale sequence that may span up to 32-million years of time, including the Frasnian-Famennian boundary. Black shales are commonly enriched in transition metals, and the Woodford Shale is 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 both U and total organic carbon (TOC), our previous work demonstrates that the Woodford Shale is characterized by two sample populations. One population exhibits a strong linear relationship between U as determined at outcrop via gamma-ray spectrometry (or GR) and TOC from hand samples. The second population exhibits anomalously high GR values, for which a linear relationship with TOC was not observed. We hypothesize that reducing conditions produce black shales which can be described by linear equations of GR versus TOC, but when chemical conditions become euxinic, relationships fail to emerge.

To test this hypothesis, and better constrain the depositional conditions that produce gas shales, we have determined trace metal concentrations (Fe, Mn, U, Zn, Mo) in Woodford Shale outcrops from southern Oklahoma. Trace metal distributions are assessed within a strict stratigraphic context to better constrain the role of redox in gas shale development. Enrichment of trace metals peak in the middle member, which is also associated with some of the highest TOC concentrations. However, high TOC concentrations are also observed in portions of both the lower and upper members, where trace metal enrichments are absent. By comparing our results with those from equivalent stratigraphic intervals in the Chattanooga Shale, we have developed a model for the regional distribution of anoxic/euxinic conditions during the Upper Devonian. This model provides new insights to the geochemical response of the Devonian seas to climatic changes, and provides insight into gas-shale development, which can be used in further exploration efforts.