Abstract

Coalbed methane is a natural by-product of the thermal maturation (coalification and devolatilization) of humic materials that contain type III vitrinitic kerogen. Although a large percentage of the methane generated by the coalification process escapes, some of it migrates into other reservoirs and the remainder is trapped within the coal itself., primarily absorbed within micropores of the coal.

Coalbed methane probably exists in all coal regions of Wyoming although many of the State's coal deposits are shallow and too thermally immature to generate substantial gas. Cretaceous and some Tertiary coal beds buried in the structurally deep portions of many Wyoming basins are thermally mature and have generated large volumes of methane. Biogenic coalbed methane exists in the Powder River Basin where relatively shallow, low rank Tertiary coal beds occur. Methane content per ton of coal is relatively low, but the great thickness of coal beds in the Powder River Basin have allowed large quantities of coalbed methane to accumulate.

In the Powder River Basin, there were 20 wells completed in the Tongue River Member of the Fort Union Formation as of March, 1989. These wells were producing from 3 to 504 MCF (thousand cubic feet) of gas daily; cumulative production was 1.425 billion cubic feet. Production from these wells, in the Recluse area of the basin, is not from the coal but from sandstone units adjacent to the coal beds. BETOP, Inc. is the first company to commercially produce methane from wells completed in Wyoming coal beds. They are now producing coalbed methane from four wells just north of Gillette, Wyoming. Three additional wells will bring production from the field to 2.5 million cubic feet of gas per day.

Using current estimates of total coal-in-place for the major coal-bearing areas in Wyoming and assuming that each ton of coal contains between five and 100 cubic feet of methane, Wyoming's estimated coalbed methane resource ranges from 7.25 to 145.0 trillion cubic feet. If only 50 percent of the lower number is produced, this would still be a very significant amount of gas.