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

Dudley D. Rice, Ronald C. Johnson

The Piceance basin is a hydrocarbon-rich province that has major resources of natural gas in coal beds and tight sandstone reservoirs of Cretaceous age. In general, gases from all producing intervals are of thermal origin and become isotopically heavier (^dgr¹³C₁ values = -51.3 to -29.1 ppt) and chemically drier (C₁/C_1-5 values = 0.41 to 1.00) with increasing level of thermal maturity (R_o values = 0.45 to 2.4%) over a depth range of 1,100 to 11,702 ft. Maturity increases to the south because of higher paleotemperatures.

Based on chemical and isotopic composition, three types of gases can be distinguished: those generated from (1) mixed types II and III kerogens, (2) dispersed type III kerogen, and (3) coal, shown in series from chemically wettest to driest and isotopically lightest to heaviest at equivalent levels of thermal maturity. Type 1 gases are produced from marine sandstones, such as those of the Mancos B and Iles formations, and are associated with oil. Type 2 gases are in nonmarine sandstones of the Williams Fork Formation, are nonassociated at all levels of maturity, and contain large amounts of carbon dioxide (as much as 22%) derived from thermal decomposition of type III kerogen. Methane-rich type 3 gases are produced by devolatilization of humic coal and are contained in coal beds of th Cameo-Fairfield zone of the Williams Fork Formation. They have not migrated into adjacent sandstone reservoirs.

Gases in immature Tertiary reservoirs are similar in chemical and isotopic composition to those of underlying reservoirs. They are interpreted to have migrated from deeper, more mature Cretaceous source rocks.

AAPG Search and Discovery Article #91022©1989 AAPG Annual Convention, April 23-26, 1989, San Antonio, Texas.