--> Gas geochemistry of three tight-gas-sand fields, Rocky Mountains

2014 Rocky Mountain Section AAPG Annual Meeting

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Gas geochemistry of three tight-gas-sand fields, Rocky Mountains


Tight-gas-sand reservoirs in the Rocky Mountains are a major natural gas resource; yet gas compositions in these fields are largely undocumented, and controls on gas composition are poorly understood. I report a large data set of gases from: Jonah Field, Greater Green River Basin; the Mamm Creek – Rulison – Parachute – Grand Valley fields, Piceance Basin; and Greater Natural Buttes Field, Uinta Basin. The data are used to characterize gas sources, generation mechanisms, thermal maturities, and secondary alteration. The range of production gas compositions in the three fields is similar, from pure methane to 35% C2+, with a small number of wetter gas samples. The δ13C of methane ranged from −25 to −45%; wetter gases were isotopically heavier and less variable. The three fields are distinguishable by δ13C of CO2, δD of the C2+ gases, ratio of i-C4 to n-C4 and noble gas parameters. Jonah mud gases are generally wetter and isotopically lighter with increasing depth, with a wide range of compositions at any depth. Piceance gases become dryer and isotopically heavier with depth, with a shift to lighter values for the deepest samples. The stratigraphic variability in gas compositions suggests complex migration pathways, probably through fracture systems and more permeable beds. Gas compositions are influenced by (1) mixing of gases from primary cracking with gases from secondary cracking of oil; (2) primary cracking at varying thermal maturity; (3) bacterial oxidation of hydrocarbon gases, yielding gases that are dry, with isotopically heavy hydrocarbons and light CO2. This effect is pervasive in the upper part of the gas column at Jonah Field and locally significant in the Piceance wells.