Mid-Continent Section

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Composition and mechanical properties of the Woodford Shale, northern Oklahoma


The Woodford Shale in southern Oklahoma is silica-rich as a result of radiolarian content and ranges from around 70 percent bedded chert for the Arkansas Novaculite to 10 to 50 percent for cherty Woodford in the Arkoma Basin and outcros on the Arbuckle and Criner uplifts. Bedded cherts have higher natural fracture density, with fractures terminating in adjacent clay-rich beds. In contrast, the Woodford Shale in northern Oklahoma lacks bedded chert, but contains silica-rich bands that appear to nucleate on detrital-silt-rich laminae. Thin section microscopy reveals that all detrital silt is not the same. Silt grains surrounded by clay result in relatively high silica content as determined by x-ray, but silica bands do not develop. Silica cement appears when silt grains are in contact, increases at the expense of clay content, and alters wireline log response and mechanical properties. Intervals with lower-clay content exhibit higher resistivity and lower neutron porosity than clay-rich intervals. Silica-cement, augmented by carbonate cement and sulfides, imparts competence and brittleness to the Woodford Shale. Competence is evident in the smooth outer core surface in cemented intervals that resisted erosion during coring. In contrast, clay-rich intervals are eroded, especially parallel to bedding, generating a rougher outer core surface. Cemented intervals are brittle, generating a propensity to naturally fracture or propagate fractures during hydraulic stimulation.