DEVELOPMENTAL GEOLOGY OF COALBED METHANE FROM SHALLOW TO DEEP IN ROCKY-MOUNTAIN BASINS AND IN COOK INLET MATANUSKA BASIN, ALASKA, USA AND CANADA

The Rocky Mountain basins of western North America contain vast deposi ts of coal of Cretaceous through early Tertiary age. Coalbed methane i s produced in Rocky Mountain basins at depths ranging from 45 m (150 f t) to 1,981 m (6,500 ft) from coal of lignite to low-volatile bitumino us rank. Although some production has been established in almost all R ocky Mountain basins, commercial production occurs in only a few. Desp ite more than two decades of exploration for coalbed methane in the Ro cky Mountain region, it is still difficult to predict production chara cteristics of coalbed methane wells prior to drilling. Commonly cited problems include low permeabilities, high water production, and coals that are significantly undersaturated with respect to methane. Sources of coalbed gases can be early biogenic, formed during the early stage s of coalification, thermogenic, formed during the main stages of coal ification, or late stage biogenic, formed as a result of the reintrodu ction of methane-generating bacteria by groundwater after uplift and e rosion. Examples of all three types of coalbed gases, and combinations of more than one type, can be found in the Rocky Mountain region. Coa ls in the Rocky Mountain region achieved their present ranks largely a s a result of burial beneath sediments that accumulated during the Lar amide orogeny (Late Cretaceous through the end of the Eocene) or short ly after. Thermal events since the end of the orogeny have also locall y elevated coal ranks. Coal beds in the upper part of high-volatile A bituminous rank or greater commonly occur within much more extensive b asin-centered gas deposits which cover large areas of the deeper parts of most Rocky Mountain basins. Within these basin-centered deposits a ll lithologies, including coals, sandstones, and shales, are gas satur ated, and very little water is produced. The interbedded coals and car bonaceous shales are probably the source of much of this gas. Basin-ce ntered gas deposits become overpressured from hydrocarbon generation a s they form, and this overpressuring is probably responsible for drivi ng out most of the water. Sandstone permeabilities are low, in part be cause of diagenesis caused by highly reactive water given off during t he early stages of coalification. Coals within these basin-centered de posits commonly have high gas contents and produce Little water, but t hey generally occur at depths greater than 5,000 ft and have low perme abilities. Significant uplift and removal of overburden has occurred t hroughout the Rocky Mountain region since the end of the Eocene, and m uch of this erosion occurred after regional uplift began about 10 Ma. The removal of overburden generally causes methane saturation levels i n coals to decrease, and thus a significant drop in pressure is requir ed to initiate methane production. The most successful coalbed methane production in the Rocky Mountain region occurs in areas where gas con tents were increased by post-Eocene thermal events and/or the generati on of late-stage biogenic gas. Methane-generating bacteria were appare ntly reintroduced into the coals in some areas after uplift and erosio n, and subsequent changes in pressure and temperature, allowed surface waters to rewater the coals. Groundwater may also help open up cleat systems making coals more permeable to methane. If water production is excessive, however, the economics of producing methane are impacted b y the cost of water disposal. (C) 1998 Elsevier Science B.V.