Coalbed methane potential of the Denver Basin has long been neglected because the coals were judged to be too thin, too shallow, and too immature to support commercial gas production. With the success of the Powder River Basin coalbed methane play, which has coals of similar rank and depth, this study was undertaken to quantify the gas held by Denver Basin coals by volumetric means. Powder River Basin coals were used as an analog because coals in both basins are generally shallow, Cretaceous to Paleocene in age, subbituminous in rank, and roughly the same temperature.
Coals of the Powder River Basin contain both sorbed and free gas. The sorbed gas contribution is calculated with a sorption isotherm and reservoir pressure, while the free gas component is calculated from conventional gas reservoir engineering principles. This study considered both sorbed and free gas contributions.
The average Bureau of Land Management (BLM) isotherm for Powder River coals published by Crockett and Meier (ref 2) was used to estimate gas content. Analogous to Powder River coals, coalbed density was assumed to be 1.35 grams/cubic centimeters (gm/cc). Sorbed gas was calculated as the product of area, coal thickness, coal density, and gas content. Coal porosity was assumed to be 10 percent and gas formation volume factors were calculated assuming the gas to be pure methane.
This study determined that coals of the Denver Formation hold 704 billion cubic feet of gas (bcf) and those of the Laramie Formation contain 1539 bcf, for a total of 2243 bcf. The total Denver Basin coalbed methane resource is therefore estimated to be 2.24 trillion cubic feet of gas (tcf) with approximately two-thirds of the gas held in the Laramie formation coals and one-third in the Denver formation coals.