METHANOGENIC PATHWAYS IN THE POWDER RIVER BASIN: THE GEOLOGIC FACTOR

Previous studies of methanogenic pathways from coalbed natural gas (CBNG) in the Powder River Basin, based on methane d¹³C and dD (Deuterium), recognized both methyl fermentation and carbon dioxide (CO₂) reduction; however these data were not correlated to the coal geology of the CBNG reservoirs. Our investigations of these isotopes, as well as dD and d¹⁸O from CBNG produced water, d¹³C (CO₂), and gas composition variations (CH₄, CO₂, N₂, and C₂+), discovered significant differences between CBNG reservoirs. A total of 165 CBNG wells, between depths of 170-2,231 ft, were sampled for gases and co-produced water from 7 CBNG reservoirs in the Paleocene Fort Union Formation and 3 CBNG reservoirs in the Eocene Wasatch Formation. Fort Union samples are mainly from the Anderson and Canyon coal beds of the Wyodak-Anderson coal zone, which account for 70% of gas production in the basin.

Although the compositional field ratio plot of methane d¹³C and dD implies a fermentation pathway, 70% of the samples are derived from CO₂ reduction when the fractionation of D of water and methane is taken into account. Statistical analysis shows a significant difference in the percent of CO₂ reduction between the Anderson and Canyon coal reservoirs. Light methane d¹³C (-60 to -83‰) is distributed along the margins of the basin and heavier methane d¹³C (-51 to -60‰) is in the deeper part of the basin; methane dD is depleted along the basin margins. The basin-wide distribution of methane d¹³C implies groundwater recharge stimulated microbial “new” gas, probably by CO₂ reduction, along the basin margins; whereas, “old” or mixed microbial and migrated thermogenic gases exist in the basin center. Burial and thermal maturation history based on vitrinite reflectance (0.30 - 0.66%) from coal and organic-rich shale indicate temperatures of 20º - 80ºC, which is within the range of methanogenesis and early thermogenesis in coal with low C₁/C₂+C₃ ratio.

Methane d¹³C and gas composition (e.g., N₂ and CH₄) differ between Wasatch and Fort Union CBNG reservoirs and between different Fort Union CBNG reservoirs implying that geologic factors played a significant role in methanogenic pathways in the basin. Thus, microbiological, biogeochemical, and hydrological studies should correlate their data and results to the coal geology of the basin in order to be meaningful.