The successful development of shallow coalbed methane resources from low rank coal in the Powder River Basin in Wyoming and Montana has helped to stimulate (or renew) interest in the potential for coalbed methane development in the Denver Basin. The Denver Basin contains an estimated 30-35 billion tons of subbituminous coal and lignite in the Laramie and Denver Formations at depths of less than 3,000 ft, and although there is currently no coalbed methane production in the basin, the Gas Research Institute (GRI) estimates as much as 2 trillion cubic feet (Tcf) of coalbed methane (in-place) within these two formations. Of this total, GRI (1999) suggests that some 0.3 Tcf of methane may be recoverable. In order to better evaluate the coalbed methane potential in the Denver Basin, the USGS Front Range Infrastructure Resources Project initiated a study in the greater Wattenberg area (GWA) in order to gain some perspective on the coal-bed methane potential in the Upper Cretaceous Laramie Formation. The GWA incorporates about 2,900 mi2 in parts of Adams, Boulder, Denver, Jefferson, Larimer, Morgan, and Weld counties, and extends from T. 2 S. to T. 7 N., and from R. 61 W. to R. 69 W. The area includes most of the Boulder-Weld coal field, and additional areas where Laramie Formation coal was mined in the past. In the GWA, commingled gas production from all Cretaceous units is allowed, and recently relaxed drill-spacing requirements (spacing < 40 acres) might encourage re-completion efforts in existing wells to tap into additional pay zones. Potential coal-bed methane resources in the Laramie Formation overlie targets of current gas production in deeper, older Cretaceous strata, and may constitute a shallow, "behind-pipe" resource in existing gas wells.
The main coal-bearing zone in the Laramie Formation is present within the lower 300 ft of the formation. The maximum thickness of the coal zone in the GWA is about 290 ft and the minimum thickness is on the order of 75-80 ft. Total (cumulative) coal thickness within the coal zone ranges from a few feet or less (traces of coal) to as much as 35 ft. Maximum depth to the top of the zone exceeds 1,300 ft, although in most of the GWA the depth to the top of the coal zone is less than 1,000 ft. The thickest total coal accumulations are at depths of less than 500 ft, in and near the Boulder-Weld coal field. Individual coal-bed thickness can vary from less than 1 ft to as much as 9 ft, and the number of coal beds within the coal zone varies from 2 to 12. Average (arithmetic mean) as-received heat-of-combustion (Btu/lb) values for Laramie coal beds, based on analyses of coal mine and coal core samples, range from 7,200 to more than 9,900 Btu/lb. Heat-of-combustion values for coal in the Boulder-Weld coal field are typically 1,000 to 1,500 Btu/lb higher than for Laramie coal in other areas in the Denver Basin. Total coal gas contents, determined from the desorption of Laramie Formation coal core samples in three drill holes in or adjacent to the GWA, ranged from 0 to as much as 24 cubic ft/ton. At least eight mines in the Boulder-Weld coal field experienced mine fires or explosions during their production history, and an additional eight coal mines reported the presence of gas. Perhaps some of the most compelling evidence for the gassy nature of Laramie Formation coal was recorded in the Eagle Mine in the northeastern part of the Boulder-Weld coal field, where more than 7,000 cubic ft of gas per day (28 cubic ft of gas per ton of mined coal) was emitted during the first quarter of 1976.
Favorable coal geologic factors for a potential coalbed methane resource in the Laramie Formation in the GWA include the documented presence of coalbed gas, the relatively continuous distribution of Laramie coal beds in subsurface throughout the area, cumulative (total) coal-bed thickness exceeding 30 ft, and individual coal-bed thickness of as much as 12 ft locally. Certain other coal geologic factors, however, could restrict or even negate the prospective development of this resource. For example, in places where total coal accumulations exceed 20 ft, the lower Laramie coal zone is generally shallow (less than 500 ft), and coal beds are near faulted and undermined areas in the Boulder-Weld coal field. The shallow depth and proximity to faults and abandoned underground mines could result in gas leakage into mined-out cavities, or leakage to the surface via faults or up-dip migration to nearby outcrops. In certain areas of the GWA, where greater coal zone depths might enhance methane retention, reported total coal accumulations are typically less than 20 ft, and commonly less than 10 ft. The limited volume of coal in these areas could severely diminish the coalbed methane resource potential. Another factor that could constrain Laramie Formation coalbed methane development is the close association of coal beds with the Laramie-Fox Hills aquifer. This aquifer, which is one of the primary sources of fresh water for residential, agricultural, and commercial use, includes sandstone beds in the lower, coal-bearing part of the Laramie Formation, as well as sandstone in the underlying Fox Hills Sandstone and uppermost Pierre Shale. Dewatering of the coal, which may be required to develop the methane resource, could result in the lowering of subsurface water levels through time. Thus, careful consideration in regard to the development of Laramie Formation coalbed methane resources will be required to ensure that associated water production will not compromise the integrity of this important Front Range water supply.