Timing and Paragenesis of the Calcite Fracture Fill in the Woodford Shale

Abstract

The Woodford Shale in the Arkoma Basin of Oklahoma is brittle and highly fractured. In some areas of the basin these fractures are cemented with calcite cement. This study uses petrography, fluid inclusion analysis, and carbon and oxygen stable isotope geochemistry to analyze the calcite fracture fill obtained from core samples to characterize the precipitating fluid and to develop a paragenetic sequence for the diagenetic events occurring within the shale that led to the precipitation of the calcite fracture fill. Because of the timing of the formation of the Woodford shale and the deformation of the Arkoma basin in the Pennsylvanian time, initial movement and fracturing of the Woodford shale must have occurred between 326-345 ma., relatively early after the formation and compaction of the Woodford shale. Where the Woodford shale overlies the Hunton Group rocks, the fractures are likely to contain calcite fracture fill because the Hunton serves as a source of carbonate, facilitating the precipitation of calcite. The precipitating fluid had minimum entrapment temperatures that range from 81.8-110 C and are consistent with the indicated geothermal gradient of the Arkoma Basin. One phase petroleum inclusions found in the calcite fracture fill 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 13C over a smaller range of depleted 18Ocalcite 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.