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Geochemical Signatures of Stratigraphic Sequences and Sea-Level Change in the Woodford Shale, Permian Basin, West Texas

Mnich, Cheryl 1; Hemmesch, Nikki 1; Harris, Nicholas B.1
1 Geology and Geological Engineering, Colorado School of Mines, Golden, CO.

The Woodford Shale is considered one of the main source rocks in the Permian Basin, West Texas and has recently become a target for shale gas exploration. We have carried out high-resolution geochemical studies in a continuous Woodford core from Pecos County, TX, both to characterize variations in composition within the shale, important for well completions, and to assess how sea-level changes influence redox conditions, biologic productivity, and sedimentation rate. We integrate the high-resolution geochemistry with a detailed sequence stratigraphic interpretation of the same interval and demonstrate that the organic and inorganic chemistry of the shale changes systematically with sea-level fluctuations. Longer wavelength geochemical variation may be related to climatic changes or tectonic influences.

Quartz content is highest in the transgressive and highstand systems tracts, and increases over the full length of the core. Quartz is positively correlated with TOC, suggesting it is biogenic in origin. The TOC/P ratio increases in the lowstand and transgressive systems tracts reflecting increased reducing conditions and high biologic productivity.

The Woodford was influenced by both carbonate and siliciclastic deposition, based on antithetic variation in silicate- and carbonate-associated elements. Lowstand, transgressive, and highstand systems tracts (LST, TST and HST, respectively) each have unique geochemical signatures that record changes in redox conditions, production and sedimentation rate. The LST is characterized by: increasing reducing conditions, decreasing productivity and increasing sedimentation rate. The TST is characterized by: increasing reducing conditions, increasing productivity, and decreasing sedimentation rate. The HST is characterized by: decreasing reducing conditions, decreasing or stable productivity, and increasing sedimentation rate. Additionally, the Woodford spans the Frasnian-Famennian boundary (F-F) of the Devonian, which has been geochemically characterized around the world. In this study, the F-F coincides with the boundary between the traditional Upper and Middle Woodford units, as signaled by: an upward decrease in TOC; abundant phosphate nodules at and above the boundary; and the presence of increased terrigenous organic matter.

Based on these results the TST’s are the best targets for shale gas, as they have the highest TOC, high quartz content, and the lowest clay and carbonate content.


AAPG Search and Discovery Article #90090©2009 AAPG Annual Convention and Exhibition, Denver, Colorado, June 7-10, 2009