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Hydrocarbons are being produced at depths ranging from 4,000 to 8,500 ft along the axis of the Denver basin. On the basis of chemical and isotopic composition, gases from the three main reservoirs of Cretaceous age are interpreted to be of thermogenic origin. Gases from the Terry and Hygiene Sandstone Members of the Pierre Shale, the youngest reservoir, are the isotopically lightest (^dgr13C1 values range from -55.7 to -49.2 ^pmil) and chemically wettest (C1/C1-5 values range from 0.67 to 0.83), and are associated. Gases from the Codell Sandstone Member of the Carlile Shale generally become isotopically heavier (^dgr13C1 values range from -47.8 to -43.9^pmil) as they become chemically drier (C1/C SUB>1-5 values range from 0.76 to 0.8). During the main part of mature stage, oil and associated gas (isotopically lightest and chemically wettest) were generated from type II kerogen. During the hotter, later part of the stage, wet gas (isotopically heaviest and chemically driest) and condensate were generated from residual kerogen and from heavier hydrocarbons previously generated. Variations in character of the gases from the "J" sandstone, the oldest reservoir, are similar to those of the Codell; they become isotopically heavier (^dgr13C1 values range from -47.9 to -43.1^pmil) as they become chemically drier (C1/C1-5 values range from 0.84 to 0.87). Gases from "J" are interpreted to have been generated at similar levels of maturity as those of the Codell, but from type III kerogen. These gases are nonassociated and are isotopically heavier and chemically drier at similar levels of maturity than are those generated from type II kerogen.
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