Occurrence and Geochemistry of Natural Gases, Piceance Basin, Northwest Colorado

The Piceance basin is a hydrocarbon-rich province that has natural gas production from reservoirs ranging in age from Late Jurassic to Eocene and large undeveloped resources of natural gas in coal beds and tight sandstone reservoirs of Cretaceous age. Gases from all producing intervals are of predominantly thermal origin and become isotopically heavier (δ13C1: −51.3 to −29.1 ‰) and chemically drier (C1/C1-5: 0.26 to 1.00) with increasing thermal maturity of reservoirs (Ro: 0.45 to 2.40%) over a depth range of 1100 to 11,702 ft (335–3567 m). Scatter in trend is attributed to source rock differences and considerable vertical and lateral migration. Based on chemical and isotopic composition, three major types of gases can be distinguished: those generated from mixed type II and III kerogens, those from dispersed type III kerogen, and those from coal. Gases generated from mixed type II and III kerogens are produced from the Upper Jurassic Morrison Formation, the Lower Cretaceous Cedar Mountain Formation, the Upper Cretaceous Dakota Sandstone, the Upper Cretaceous Mancos “B” producing interval, and marginal marine sandstones of the Upper Cretaceous lies producing interval. These gases are associated with minor amounts of oil and probably were generated from kerogen in the marine Mancos Shale. Gases generated from dispersed type III kerogen are produced from nonmarine sandstones of the Upper Cretaceous Williams Fork producing interval and from thermally immature reservoirs in the overlying Paleocene and Eocene Fort Union and Wasatch Formations. These nonassociated gases contain large amounts of CO2 and probably were generated from carbonaceous shales in the Williams Fork producing interval. Their presence in immature Fort Union and Wasatch reservoirs implies considerable vertical migration. The third type of gas is methane rich, is produced by devolatilization of humic coal, and is generally in coal beds of the Cameo-Fairfield zone of the Williams Fork producing interval. These gases are not the major source for adjacent sandstone reservoirs. A fourth, distinct type of isotopically light thermogenic gas occurs in immature reservoirs of the Eocene Green River Formation. This gas is inferred to have migrated from unidentified deeper, more mature source rocks.