Coalbed Methane in the San Juan and Powder River Basins: Differences and Similarities

Andrew R. Scott, Altuda Energy Corporation, 401 Austin Highway, Suite 209, San Antonio, TX 78209, phone: 210 829 8080, [email protected]

The San Juan Basin of Colorado and New Mexico and the Powder River Basin of Wyoming and Montana are major coalbed methane plays in the United States. Cretaceous coal seams in the San Juan Basin contain an estimated 84 Tcf of gas, whereas Tertiary coals of the Powder River Basin contain approximately 39 Tcf. Ultimate recoveries for these two basins are more than 10 and 24 Tcf, respectively, with the higher recovery efficiency in the Powder River due to unusually high permeability.

San Juan and Powder River Basin coals differ significantly in terms of coal rank and gas content values, yet both basins are economic successes. Understanding the synergistic interplay among key factors affecting coalbed methane producibility explains why basins with markedly different hydrogeologic characteristics are economically viable. Most of the coal beds in the San Juan Basin have reached the thermal maturity level required to generate significant amounts of thermogenic gases. Secondary biogenic gas generation associated with meteoric recharge after basin uplift and cooling has locally increased gas contents (150 to 600+ scf/ton). All key hydrogeologic factors come together synergistically to make the San Juan Basin the most prolific coalbed methane basin in the world. The Powder River coal beds coal beds are low rank (subbituminous) and gas contents are generally less than 30 scf/ton; only secondary biogenic and, possibly, early thermogenic gases are present. However, the basin is economic for coalbed methane because of the presence of highly permeable, shallow, thick, coal beds and lower drilling costs.