2014 Rocky Mountain Section AAPG Annual Meeting

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Undiscovered Petroleum Resources for the Woodford Shale and Thirteen Finger Limestone-Atoka Shale, Anadarko Basin


In 2010 the U.S. Geological Survey assessed undiscovered oil and gas resources for the Anadarko Basin province of Colorado, Kansas, Oklahoma, and Texas. The assessment included two continuous (unconventional) assessment units (AU); the Devonian Woodford Shale Gas AU mean undiscovered resources are 16 TCFG and 192 MMBNGL, and the Pennsylvanian Thirteen Finger Limestone-Atoka Shale Gas AU of the lower Atoka Group mean undiscovered resources are 6.8 TCFG and 82 MMBNGL. Oil resources of these AUs were assessed as conventional. Both AUs are mature for gas generation within the deep basin. Gas generation from the Woodford Shale started about 335 Ma, and from the Thirteen Finger Limestone-Atoka Shale about 300 Ma. Maturation results are based on vitrinite reflectance data, and on 1D and 4D petroleum system models that calculated vitrinite reflectance (Ro), and Rock-Eval and hydrous pyrolysis transformation ratios through time. The Woodford Shale Gas AU sweet spot and boundary were defined primarily on (1) isopach thickness from well-log analysis and published sources; (2) estimated ultimate recoverable production from existing, mainly horizontal wells; and (3) levels of thermal maturation. Measured and modeled Ro ranges from about 1.2% to 5% in the AU, which represents marginally mature to overmature for gas generation. The sweet spot included most of the Woodford Shale that was deposited within eroded channels in the unconformably underlying Hunton Group. The Thirteen Finger Limestone-Atoka Shale Gas AU has no known production in the deep basin, the boundary is based primarily on the gas generation window, and on thickness and distribution of organic-rich facies from these mainly thin shale and limestone beds.