SUBBITUMINOUS COAL LITHOTYPES AND GAS CONTENT IN WYODAK-ANDERSON COAL IN THE POWDER RIVER BASIN, WYOMING

Subbituminous coal beds in the Wyodak-Anderson coal zone of the Fort Union Formation generate about 70 percent of the gas produced from coal in the Powder River Basin, Wyoming. The producing coals are mainly the Anderson, Canyon, Big George, and Smith coal beds, which range from 20 to 200 ft thick. About 800 2-ft canisters of coal core, collected from the entire thickness of various coal beds were studied for their lithotypes, apparent rank, gas content, and cleat spacing.

The coals range from subbituminous C (subC) in the shallow margin of the basin to higher apparent rank of subituminous B (subB) and A (subA) toward the deeper part of the basin. Gas content increases in the higher rank coals and with depth. Cleat spacing ranges from 1.06 in. (27mm) for subC to 0.9 in. (23mm) for subA, with the latter having greater permeability. Coal lithotypes and cleat spacing vary laterally and vertically across and within a coal bed. Coal lithotypes include attritus; fine, medium, and coarsely laminated vitrain; woody vitrain; and fusain. The Wyodak-Anderson coal beds, on average, contain 58 percent attritus, 40 percent vitrain, and 2 percent fusain. The amount of gas in the attritus and vitrain lithotypes is uniform within coal ranks with subA containing more gas than subC and with gas production higher from coals with an apparent rank of subA and smaller cleat spacing. Desorbed average gas content for coals with apparent rank of subC and subA are 8.5 standard cubic feet per ton (scf/t) and 85 scf/t for attritus, 7.6 and 75 scf/t for finely laminated vitrain, 8.8 and 79 scf/t for medium to coarsely laminated vitrain, 4.2 and 79 scf/t for coarsely laminated vitrain, 7.4 and 79 scf/t for woody vitrain, and 2.1 and 96 scf/t for fusian. The good porosity and permeability of subA fusain is attributed to its ‘charcoal-like’ properties, which contribute to the high gas content and potentially to high gas flow through the reservoir.

The biogenic gas content of subbituminous coals in the Powder River Basin is controlled by the amount of gas generated and adsorbed in the coal lithotypes, which varies vertically within a coal bed. To accurately assess gas potential and saturation requires study across the entire thickness of the coal bed.