Mudrock Lithologies and Mechanical/Lithostratigraphy of the Woodford Shale Play, Anadarko Basin, West-Central Oklahoma.

Craig D. Caldwell
Cimarex Energy, Tulsa, OK

Seven mudrock lithologies, defined mainly on the basis of percent TOC and mineral content (primarily quartz, clay, and dolomite), compose the Lower, Middle, and Upper Woodford in the central part of the Anadarko Woodford Play where the Woodford is 175 to 330 ft. thick. The basal Woodford in this area is a TOC-poor (<2% TOC) clay-rich mudrock (41 to 52% clay), recording first transgression of the Woodford seas and deposition under more normal marine conditions than the overlying mudrock lithologies. The Middle Woodford and middle and upper parts of the Lower Woodford in the core area are composed of 10 to 30 ft intervals dominated by one of three lithologies: clayey mudrock (CM) (38% clay and 41% quartz), clayey siliceous mudrock (CSM) (27% clay and 55% quartz) and less common dolomitic clayey mudrock (DCM) (33% clay, 32% quartz, and 15% dolomite). These mudrock lithologies are organic rich with TOC values averaging 5 to 6.5% and core analysis gas-filled porosities averaging 5.6 to 6.8%. The Upper Woodford in the core area is predominantly CSM and siliceous mudrock (SM) (14.5% clay and 75% quartz). Quartz is largely biogenic but also includes scattered, detrital, silt-size grains. CSM present in the Lower, Middle, and Upper Woodford in the core area is generally restricted to the Upper Woodford in surrounding areas of the play where the Woodford thins to less than 150 ft. and the Lower and Middle Woodford are dominated by CM. CSM and SM are characterized by higher brittleness indices, lower stress, and are typically “easier to frac” than the more ductile clay-rich mudrocks, affecting lateral placement, completion procedures, and production.

AAPG Search and Discovery Article #90152©2012 AAPG Southwest Section Meeting, Fort Worth, Texas, 19-22 May 2012