--> --> ABSTRACT: Coal-Bed Methane Potential of Tertiary Coal Beds and Adjacent Sandstone Deposits, Powder River Basin, Wyoming and Montana, by Dudley D. Rice, Romeo M. Flores; #91002 (1990).

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ABSTRACT: Coal-Bed Methane Potential of Tertiary Coal Beds and Adjacent Sandstone Deposits, Powder River Basin, Wyoming and Montana

Dudley D. Rice, Romeo M. Flores

Coal beds, as much as 250 ft thick, and adjacent sandstones in the Paleocene Tongue River Member of the Fort Union Formation are reservoirs for coal-derived natural gas in the Powder River basin. The discontinuous coal beds were deposited in raised, ombrotrophic peat bogs adjoining networks of fluvial channels infilled by sand. Coalbed thickness was controlled by basin subsidence and depositional environments. The coal resources in the basin are estimated to be as much as 1.3 trillion tons.

The average maceral composition of the coals is 88% huminite (vitrinite), 5% liptinite, and 7% inertinite. The coals vary in rank from subbituminous C to A (Ro values of 0.4 to 0.5%). Although the coals are relatively low rank, they display fracture systems. These systems include well-developed face and butt cleats, and secondary curvilinear joints dipping 20° to 40° from the face cleats.

Natural gas desorbed and produced from the coal beds and adjacent sandstones is composed mainly of methane with lesser amounts of CO2 (<10%). The methane is isotopically light values (^dgr13C1 values -56.7 to -60.9 ppt) and enriched in deuterium (^dgrD values -307 to -315 ppt). The gases are interpreted to have a bacterial origin--they were generated from the coals by fermentation prior to the main phase of thermogenic methane generation by devolatilization. Large amounts of water generated during early stages of coalification will have to be removed from the coal beds before commercial gas production can begin.

Desorbed amounts of methane-rich, bacterial gas in the Powder River basin are relatively low (tens of ft3/ton) compared to amounts of thermogenic coal-bed gases (hundreds of ft3/ton) from other basins. However, the total coal-bed gas resource in both the coal beds and the adjacent sandstones is considered to be large (as much as 40 tcf) because of the vast coal resources.

AAPG Search and Discovery Article #91002©1990 AAPG Rocky Mountain Section Meeting, Denver, Colorado, September 16-19, 1990