Abstract

Based on chemical and isotopic composition, two groups of thermogenic natural gas can be distinguished in the Uinta Basin. Gases in group A are characterized by wetness (C₂₊) values between about 5 and 9 percent, methane ¹³C values greater than −40‰, ethane ∂¹³C values greater than −28.5‰, and methane ∂D values greater than −185‰, and were generated from oxygen-rich type III (nonmarine) kerogen during late stages of catagenesis and metagenesis. This nonassociated gas occurs in reservoirs of the Upper Cretaceous Mesaverde Group and Tertiary Wasatch and Green River Formations in the greater Natural Buttes field. The Wasatch and Green River gases occur in reservoirs that are marginally mature in reference to hydrocarbon generation, yet are almost identical in composition to gases in the deeper Mesaverde Group. The gases in the Wasatch and Green River are interpreted to have migrated vertically from deeper, more mature source rocks with type III kerogen in the Mesaverde. This interpretation indicates that gas production in tight reservoirs of the Wasatch and GreenRiver will be restricted to areas where pathways for vertical migration, such as faults and fractures, are developed.

In contrast, gases in group B have C₂₊ values ranging from about 2 to 23 percent, methane ∂¹³C values less than −46.9‰, ethane ∂¹³C values less than −34.5‰, and methane ∂D values less than −228‰, and were generated at intermediate stages of catagenesis from mainly hydrogen-rich type I (lacustrine) kerogen associated with sediments deposited in ancient Lake Uinta (Tertiary Green River Formation). These wet gases are associated with oil and are in reservoirs of the Green River Formation in the greater Altamont-Bluebell and greater Red Wash fields. The oils and associated gases were probably generated in the deep, overpressured Tertiary part of the basin in the Altamont-Bluebell area. Methane-rich gas and some oil subsequently migrated laterally (eastward) by hydrodynamic flow into shallow, immature reservoirs at the greater Red Wash field.