Abstract

Oil and wet gas-condensate production has been established from the Niobrara Formation and underlying Codell Sandstone member of the Carlile Shale in the deeper part of the Denver basin. Dry gas production has been estabblished from chalk reservoirs in the Niobrara on the shallow east flank of the basin. The shallow east-flank gases are chemically dry, isotopically light with respect to δ13_C values, and are probably of local biogenic origin. The Niobrara contains organic-rich laminations characterized by type II kerogen. This type of organic matter is capable of generating oil and wet gas-condensate when subjected to thermal degradation and cracking processes. The thermal maturity pattern present in the Niobrara Formation appears to explain the occurrence of thermogenic oil and wet gas-condensate in the deeper part of the basin and biogenic dry gases on the shallow east flank.