Reservoir and Diagenetic Characterization of the Lower Cochrane Member, Hunton Group - Lincoln and Logan Counties, Oklahoma

Abstract

The Ordovician - Devonian Hunton Group is an important carbonate reservoir in central Oklahoma. The lower Cochrane member of the Hunton Group was affected by sea level changes during the Llandoverian (Early Silurian), which favored early karsting throughout the reservoir. The dolomitization in the lower Cochrane member is the result of seawater migration through the reservoir at near surface temperatures. The dolomite carbon and oxygen isotopic signature (averaging δ18O = -4.01 / 0.69 and δ13C = 0.13 / 0.76) likely were partially reset during circulation of meteoric fluids that precipitated later calcite cements. Dolomite, together with calcite cements, was precipitated during the Silurian prior to the deposition of the Devonian Misener Sandstone and Woodford Shale. Sand, silt, and clay from Devonian siliciclastics occlude porosity, filling fractures and vugs after calcite cementation, and possibly after sparse silica cement. Stylolites, resulting from pressure-dissolution, represent the last diagenetic event recognized in the reservoir. Hydrocarbon production in the study area is likely related to fracturing and reservoir thickness rather than lithofacies. In general, the dolomite in the lower Cochrane member is more porous and permeable than limestone but its lack of uniformity affects overall reservoir quality.