Developmental geology of coalbed methane from shallow to deep in Rocky Mountain basins and in Cook Inlet–Matanuska basin, Alaska, U.S.A. and Canada

Abstract

The Rocky Mountain basins of western North America contain vast deposits of coal of Cretaceous through early Tertiary age. Coalbed methane is produced in Rocky Mountain basins at depths ranging from 45 m (150 ft) to 1,981 m (6,500 ft) from coal of lignite to low-volatile bituminous rank. Although some production has been established in almost all Rocky Mountain basins, commercial production occurs in only a few. Despite more than two decades of exploration for coalbed methane in the Rocky Mountain region, it is still difficult to predict production characteristics 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 stages of coalification, thermogenic, formed during the main stages of coalification, or late stage biogenic, formed as a result of the reintroduction of methane-generating bacteria by groundwater after uplift and erosion. Examples of all three types of coalbed gases, and combinations of more than one type, can be found in the Rocky Mountain region. Coals in the Rocky Mountain region achieved their present ranks largely as a result of burial beneath sediments that accumulated during the Laramide orogeny (Late Cretaceous through the end of the Eocene) or shortly after. Thermal events since the end of the orogeny have also locally elevated coal ranks. Coal beds in the upper part of high-volatile A bituminous rank or greater commonly occur within much more extensive basin-centered gas deposits which cover large areas of the deeper parts of most Rocky Mountain basins. Within these basin-centered deposits all lithologies, including coals, sandstones, and shales, are gas saturated, and very little water is produced. The interbedded coals and carbonaceous shales are probably the source of much of this gas. Basin-centered gas deposits become overpressured from hydrocarbon generation as they form, and this overpressuring is probably responsible for driving out most of the water. Sandstone permeabilities are low, in part because of diagenesis caused by highly reactive water given off during the early stages of coalification. Coals within these basin-centered deposits commonly have high gas contents and produce little water, but they generally occur at depths greater than 5,000 ft and have low permeabilities. Significant uplift and removal of overburden has occurred throughout the Rocky Mountain region since the end of the Eocene, and much of this erosion occurred after regional uplift began about 10 Ma. The removal of overburden generally causes methane saturation levels in coals to decrease, and thus a significant drop in pressure is required to initiate methane production. The most successful coalbed methane production in the Rocky Mountain region occurs in areas where gas contents were increased by post-Eocene thermal events and/or the generation of late-stage biogenic gas. Methane-generating bacteria were apparently reintroduced into the coals in some areas after uplift and erosion, 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 by the cost of water disposal.