ABSTRACT: Chemical and Isotopic Compositions of Natural Gases in the Wind River Basin, Wyoming

JOHNSON, RONALD C., and DUDLEY D. RICE, U.S. Geological Survey, Denver, CO

Variations in the chemical and isotopic compositions (isotope{13}C[1], isotope{13}C[2], isotopeD[1]) of gases from the Wind River basin, central Wyoming, were studied to better understand the origins of the gases. Gases from all producing intervals are predominantly thermal rather than biogenic in origin (C[1]/C[1-5] of 0.92 to 1.0, isotope{13}C[1] of -31.12 to -47.40 o/oo). In general, gases become isotopically heavier and chemically drier with increasing depth (2321 to 18,050 ft) and increasing thermal maturity of the reservoir rock from a vitrinite reflectance (Rm) of 0.50 to 2.6%, but considerable vertical migration has altered this trend in some areas. For example, gases from shallow, marginally mature reservoirs (Rm 0.5%) of the Eocene Wind River Formation from the Pavillion fie d in the western part of the basin are isotopically heavy (isotope{13}C[1] of -39.24 to -40.20 o/oo) and chemically dry (C[1]/C[1-5] of 0.95 to 0.97), and must have migrated from a deeper more mature source. Gases from the Madden field along the deep trough of the basin become only slightly heavier isotopically (isotope{13}C[1] of -34.81 to -31.82 o/oo) and chemically drier (C[1]/C[1-5] of 0.95 to 1.0) through over 12,000 ft of section (5556 to 18,050 ft) again suggesting considerable vertical migration.

Gases from the Paleocene Fort Union Formation vary considerably in chemical and isotopic composition. At Fuller Reservoir field in the central part of the basin, shallow (2,00-3500 ft) Fort Union gas associated with a waxy oil is much wetter chemically (C[1]/C[1-5] of 0.84) and isotopically lighter (isotope{13}C[1] of -46.99 o/oo) than Fort Union gases from depths of 4500-7000 ft (C[1]/C[1-5] of 0.94 to 0.95, isotope{13}C[1] of -34.79 to -36.19 o/oo). Fort Union gases from Waltman field in the eastern part of the basin are isotopically light (isotope{13}C[1] of -44.24 to -47.40 o/oo) and chemically wet (C[1]/C[1-5] of 0.91 to 0.96).

A single coalbed methane well has been completed in the western part of the basin, the Arco 1-15 Tribal well at Riverton dome in the southeast corner of the Wind River Reservation. Gas from this well, which is completed in Upper Cretaceous Mesaverde coals at depths of 3270 to 3839 ft, appears to be of thermogenic origin (isotope{13}C[1] -46.15 o/oo, isotope{13}C[2] -26.06 o/oo, C[1]/C[1-5] 0.98). Shallow coalbed gases (307 to 818 ft) desorbed from core of the thermally immature (Rm 0.4 to 0.6%) Upper Cretaceous Mesaverde Formation in the Wind River Reservation have highly varied chemical and isotopic compositions and appear to have complex and varied origins. Coalbed gases from the Hudson area in the southeastern corner of the Reservation have the isotopic compositions of a thermally enerated gas (isotope{13}C[1] -47.0 to -55.91 o/oo, isotope D[1] -248 to -253 o/oo). Coalbed gases from the Pilot Butte area about 25 mi to the northwest appear to be a mixture of biogenic and thermogenic gas. The methane fraction is isotopically light (isotope{13}C[1] -61.85 to -66.21 o/oo, isotope D[1] -280 to -286 o/oo) and is probably largely biogenic, but the gases contain as much as 5.6% C[2-5] and this fraction is probably of thermogenic origin.

AAPG Search and Discovery Article #91017©1992 AAPG Rocky Mountain Section Meeting, Casper, Wyoming, September 13-16, 1992 (2009)