Potential Gas Shales in Oklahoma, U.S.A.

Brian J. Cardott
Oklahoma Geological Survey, Norman, OK

Black shales, considered important hydrocarbon source rocks and cap rocks, are now being evaluated as gas reservoirs. The current gas-shale activity in Oklahoma is in the Woodford Shale (Upper Devonian-Lower Mississippian; equivalent to the Ohio Shale and other Devonian gas shales in the eastern United States), Caney Shale (Mississippian; equivalent to the Barnett Shale in Texas), and Excello Shale Member (Pennsylvanian). Gas produced from the Excello Shale Member is included with gas reported from the Mulky coal (an impure coal below the Excello Shale Member).

Data gathered on these potential gas shales include depth, thickness, thermal maturity, kerogen type, and kerogen quantity. Thermal maturity is determined by vitrinite reflectance. Kerogen type and quantity are determined by Rock-Eval pyrolysis.

An Oklahoma gas-shales database, available on the Oklahoma Geological Survey Web site, contains information on gas wells completed in the Woodford Shale or Caney Shale. To date, initial potential gas rates range from 8 to 2,100 thousand cubic feet of gas per day from 62 wells at vertical depths from 763 to 9,983 ft. Three horizontal wells with lateral lengths from 834 to 3,037 ft have been drilled in the Woodford Shale in Coal and Pittsburg Counties in eastern Oklahoma.

Shale-gas production in Oklahoma is a frontier play. Unresolved factors for successful gas-shale wells include the affect of natural versus induced fractures on gas production, optimum thermal maturity for oil generative organic matter, and best completion practices.