Coal Beds--Source Rock and Reservoir: ABSTRACT

Abstract

Coal beds are known to exist in parts of almost all major sedimentary basins in the conterminous United States from outcrop to depths in excess of 15,000 ft (4,570 m). Coal-bearing strata often exist interbedded with tight gas reservoirs, making it very difficult to differentiate the tight gas resource from the methane-from-coal beds resource. Studies of the methane-from-coal beds resource have determined that it may exceed 400 tcf. Measurements of methane in coal beds have shown that much gas generated during coalification is not currently present in the coal beds and may have escaped into stratigraphically contiguous formation which are now tight gas reservoirs. Bituminous coals have been analyzed that contain more than 600 ft3 of methane per ton of coal, or approximately 1 mmcf of gas per acre-foot of coal. More than 5,000 ft3 is generated during the thermochemical alteration of peat to the low volatile bituminous-semi-anthracite boundary. Only a fifth to a third of that gas appears to be retained in the coal. The excess gas may be a source for other reservoirs. Analysis of coal samples collected throughout the United States shows gas contents ranging from less than 25 ft3/ton for subbituminous coals in the Powder River and San Juan basins to more than 500 ft3/ton for coals in parts of the Green River, Raton, San Juan, and Appalachian basins. In the Powder River basin, where the coal resource is very large, even the low gas content coals have potential as producing gas reservoirs. End_of_Article - Last_Page 521------------