Characterization and origin of natural gases of the Anadarko Basin

Abstract

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 (&delta; 13 C 1 values range from -49.8 to -33.2 ppt) and chemically drier (C 1 /C 1&ndash;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 <3,000 ft. Gases display little compositional variation &delta; 13 C 1 values range from -46.4 to -39.9 ppt, C 1 /C 1&ndash;5 values range from 0.69 to 0.96). Because organic-rich, mature source rocks are not present in the area, gases probably were generated in the central basin from Pennsylvanian or older source rocks during the mature stage of hydrocarbon generation. This implies migration over distances as much as several hundred miles. In the Sooner trend, associated gases are produced from Silurian, Devonian, and Mississippian carbonates at depths as great as 9,600 ft and were generated from type-II kerogen during the mature stage of hydrocarbon generation. Associated oil correlates with extracts of the Upper Devonian and Lower Mississippian Woodford Shale. Gases are isotopically lighter (&delta; 13 C 1 values of -47.3 to -40.6 ppt) and chemically wetter (C 1 /C 1&ndash;5 values of 0.67 to 0.99) than those derived from type-III kerogen at an equivalent level of thermal maturity.