Minor and Trace Element and Re-Os Chemistry of the Upper Devonian Woodford Shale, Permian Basin, West Texas: Insights into Metal Abundance and Basin Processes

Abstract

The trace and minor element and Re–Os geochemistry of the Upper Devonian Woodford Shale are analyzed in order to characterize elemental abundances, to identify associations among trace elements and to constrain paleoceanographic conditions and depositional processes. This organic-carbon-rich mudstone in the Permian Basin, west Texas, is a major source of hydrocarbons in the basin and is coeval with many other Upper Devonian shales in North America. The Woodford lacks enrichment in many trace metals. Only Mo, U, S and Se are significantly enriched. Other redox sensitive elements are depleted or similar to average shale composition, including Pb, Bi, Cr, Ti, Cu, Zn, Co, and V. Elements associated with granitic sources such as rare earths, Th, Ce, and TiO2 are also depleted relative to average shale; this appears to be related to be a source control. A strong basin reservoir effect is noted among several redox sensitive elements, including Mo, Cu and Ni, which largely accounts for the depletion. Dilution by biogenic silica had an additional effect on metal concentrations. Multivariate factors analysis identified associations between elements, including groupings of: rare earth elements; elements enriched in granitic crust; silica, varying antithetically with elements in carbonate minerals; organic carbon, Mo and U; V; phosphate; Fe and S. Noteworthy among the results are the different behavior of redox-sensitive elements, suggesting different precipitation mechanisms or varying dependence on reservoir effects. A strong redox effect is noted in the TOC/Ptot ratio at approximately the Frasnian–Famennian boundary, indicating an abrupt transition to an anoxic column boundary that coincides with a short-term significant fall in sea level. This suggests that anoxia was induced by isolation of the basin from the global ocean. However with the exception of the uppermost Famennian, initial 187Os/188Os values determined from Re–Os geochronology for the Permian Basin are similar to correlative sections of the Appalachian and Peace River Basins of North American and the Rhenohercynian basin of Europe. This indicates that although the Permian Basin became restricted during the upper Devonian and early Mississippian, ocean connectivity remained between regional and global basins.