Abstract

Nineteen gas wells that produce from Cretaceous-age reservoirs from nine different fields in the Bighorn Basin were sampled and analyzed for chemical and isotopic compositions. The analyses were compared with those from a much more extensive suite of gases from the Wind River Basin to the south. Although both basins have similar Cretaceous through Eocene stratigraphic sequences and similar geologic histories, only the Wind River Basin produces large quantities of gas from Cretaceous and lower Tertiary rocks. For the first time, variations in carbon isotopic ratios of propane and ethane are discussed along with variations in carbon isotopic ratios in methane in the two basins in order to gain a more complete understanding of the origins of these gases.

There is considerable evidence supporting vertical migration of gases from deep mature zones (Rm 1.4 to >2.5) into shallow, marginally mature to immature reservoir rocks in both basins. Thermogenic gases are somewhat more mature in the Wind River Basin than in the Bighorn Basin, but this difference may largely be due to sample bias since there is such little deep gas production in the latter. Many of the gases from relatively shallow depths in both basins contain a mixture of gas from deep, thermally mature source rocks and biogenic methane. The biogenic component was detected by studying the disequilibrium between carbon isotopic ratios of methane versus ethane and propane. One gas sample from a depth of 13,150 ft in the Bighorn Basin also appears to include a biogenic component. The biogenic methane was probably generated during the last 10 Ma as a result of regional uplift and erosional downcutting which allowed methane-generating bacteria to reinvade rich source rocks.

Some mature, deep-derived gases in the Wind river Basin contain methane that is isotopically too heavy to be in equilibrium with the ethane and propane fractions. This heavy methane may be the result of biodegradation, but these samples come from depths as much as 18,050 ft, which is probably too deep for microbial activity. It is suggested that these deepsourced gases contain a component of isotopically heavy methane derived from Type III organic matter in coal.

Although there are some differences in the stratigraphy, deformational histories, and thermal maturities between the Wind River and Bighorn Basins, it is unlikely that these differences can explain the relatively small amount of gas production from Cretaceous and lower Tertiary rocks in the Bighorn Basin when compared to the Wind River Basin. More likely, this difference is due to the lack of exploration for gas in the Bighorn Basin.