Characterization and origin of natural gases of the Anadarko Basin

Natural-gas production in the Anadarko basin is from three geographically separated areas that can be differentiated by age of reservoir and by inferred nature of thermal origin of the gases. In the central basin, nonassociated gases are produced mainly from Upper Mississippian and Pennsylvanian sandstones. Gases become isotopically heavier (δ¹³C₁ values range from -49.8 to -33.2 ppt) and chemically drier (C₁/C_1–5 values range from 0.74 to 0.99) with increasing level of thermal maturity. Gas samples are from depths as much as 21,600 ft. Gases were generated mainly from interbedded shales with type-III kerogen during the mature and postmature stages of hydrocarbon generation. Deviations from the trend are due to mixing and migration of gases generated at different levels of thermal maturity over the past 250 m.y. In the giant Panhandle-Hugoton field, nonassociated gases are generally produced from Permian carbonates at depths