Abstract

The Woodford Shale in the Arkoma Basin of Oklahoma is brittle and highly fractured. In some areas these fractures are cemented with calcite, which was studied in core samples using thin-section petrography, fluid-inclusion analysis, and carbon and oxygen stable-isotope geochemistry to characterize the precipitating fluid and to develop a paragenetic sequence for the diagenetic events.

Based on the relative timing of the formation of the Woodford Shale and Pennsylvanian deformation of the Arkoma Basin, initial movement and fracturing must have occurred between 326–345 ma., relatively early after the compaction of the Woodford Shale. Where the Woodford Shale overlies the Hunton Group carbonates, the fractures are likely to contain calcite. The precipitating fluid had minimum entrapment temperatures that range from 81.8° to 110° C and are consistent with the indicated geothermal gradient of the Arkoma Basin. One-phase-petroleum inclusions in the fracture-filling calcite indicate that hydrocarbons were migrating as the fractures were being cemented. The wide range of salinity values calculated from the microthermometry indicates a multiple-fluid history and is supported by the zonation of calcites observed using cathodoluminescence petrography. The wide range of depleted δ¹³C over a smaller range of depleted δ¹⁸O_calcite indicates the incorporation of variable amounts of isotopically light carbon derived from the oxidation of organic carbon, indicating diagenesis in the zone of thermochemical sulfate reduction.