Abstract

Variations in the chemical and isotopic compositions (δ¹³C₁ or methane) of gases from the Wind River Basin, central Wyoming, were studied in order to better understand the origins of the gases. Gases from all producing intervals in conventional reservoirs at depths ranging from 2,321 to 18,050 ft are predominantly thermal in origin (C₁/C_1-5 of 0.82 to 1.0, δ¹³C₁ of -31.12 to -47.40‰). Most gases sampled from conventional reservoirs appear to have migrated from deeper, more mature source rocks. Gases were collected from three fields where reservoirs from several stratigraphic levels are productive, the Madden field along the deep basin trough, and the Pavillion and East Riverton Dome fields in the western part of the basin. Considerable vertical migration has occurred at all of these fields. At Madden, for example, gases become only slightly heavier isotopically (δ¹³C₁ of -34.81 to -31.82‰) and chemically drier (C₁/C_1-5 of 0.95 to 1.0) through more than 12,000 ft of section (5,556 to 18,050 ft). At Pavillion, gases from the shallow (3,437 to 3,564 ft), immature reservoirs (R_m 0.5 percent) in the lower Eocene Wind River Formation are isotopically heavy (δ¹³C₁ of -39.24 to -40.20‰) and were generated by mature to post-mature source rocks.

The lacustrine Waltman Shale Member of the Paleocene Fort Union Formation, which is present throughout much of the eastern two-thirds of the basin, appears to inhibit the vertical migration of gas from deeper sources. Few gas fields produce from the marginally mature reservoirs above the Waltman Shale Member, and the gas that is produced from this interval appears to have been generated in the Waltman. At Fuller Reservoir field, in the central part of the basin, gas from shallow (2,500-3500 ft), marginally mature (R_m 0.60 to 0.65 percent) reservoirs in the Fort Union Formation above the Waltman Shale Member is associated with waxy oil, is wet chemically (C₁/C_1-5 of 0.84) and is isotopically light (δ¹³C₁ of -46.99‰). This gas and oil appears to have been generated in the Waltman Shale Member deeper in the basin. Gases from below the Waltman Shale Member are chemically dry (C₁/C_1-5 of 0.94 to 0.95) and isotopically heavy (δ¹³C₁ of -34.79 to -36.19‰) and probably migrated from underlying Upper Cretaceous source rocks. In contrast, at Pavillion field, west of the pinchout of the Waltman Shale Member, mature gases from probable Upper Cretaceous source rocks were able to migrate into the shallow marginally mature reservoirs of the lower Eocene Wind River Formation.

A single coalbed methane well is producing in the western part of the basin, at Riverton Dome in the southeast corner of the Wind River Reservation. Gas from this well, which is completed in Upper Cretaceous Mesaverde Formation coals at depths of depths of 3,270 to 3,839 ft., appears to be of thermogenic origin (δ¹³C₁ -46.15‰, C₁/C_1-5 0.98). Shallow coalbed gases (307 to 818 ft) desorbed from cores of the thermally immature (R_m 0.40 to 0.54 percent) 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 generated gas (δ¹³C₁ -47.0 to -55.91‰). Coalbed gases from the Pilot Butte area about 25 miles to the northwest appear to be a mixture of biogenic and thermogenic gas. The methane fraction is isotopically light (δ¹³C₁ -61.85 to -66.21‰) and is probably largely biogenic, but the gases contain as much as 5.6 percent C₂₊ and this fraction is probably of thermogenic origin. These coals appear to be too thermally immature to have generated significant quantities of thermogenic gas, and it is suggested that the thermogenic component of these gases migrated into the coals from a deeper, more thermally mature source.