Controls on Bacterial Gas Accumulations in Thick Tertiary Coal Beds and Adjacent Channel Sandstones, Powder River Basin, Wyoming and Montana

RICE, DUDLEY D., and ROMEO M. FLORES, U.S. Geological Survey, Denver, CO

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 about 3 sq mi in size, adjoining networks of fluvial channels infilled by sand. Coal-bed thickness was controlled by basin subsidence and depositional environments.

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 [R(0) 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 degrees to 40 degrees from the face cleats.

Natural gas desorbed and produced from the coal beds and adjacent sandstones is composed mainly of methane with lesser amount of Co(2) (<10%). The methane is isotopically light [(13)C(1) values -56.7 to -60.9 ppt) and enriched in deuterium (Delta D values -307 to -315 ppt). The gases are interpreted to be generated by bacterial processes and the fermentation pathway, prior to the main phase of thermogenic methane generation by devolatilization. Large amounts of bicarbonate water generated during early stages of coalification will have to be removed from the fracture porosity in the coal beds before desorption and commercial gas production can take place.

Desorbed amounts of methane-rich, bacterial gas in the Powder River basin are relatively low (<60 Scf/ton) compared to amounts of thermogenic coal-bed gases (hundreds of Scf/ton) from other Rocky Mountain 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 (as much as 1.3 trillion tons).

AAPG Search and Discovery Article #91004 © 1991 AAPG Annual Convention Dallas, Texas, April 7-10, 1991 (2009)