Coal-bed Methane Potential in Montana

CAMPEN, ELIZABETH, Consultant, Billings, MT

Montana's coal resources are the second largest of the United States, with coal underlying approximately 35% of the state. These resources are estimated at 478 billion tons. Associated coal-bed methane resources are estimated to be 14 tcf. The coals of Montana range from Jurassic to early Tertiary in age and from lignite to low-volatile bituminous in rank.

Thickness, rank, maceral composition, and proximate and ultimate analyses all vary vertically and laterally. The state contains eight major coal resource areas. A large percentage of Montana's coal consists of the Paleocene Fort Union lignites of eastern Montana, generally considered of too low a rank to contain significant methane resources. Most of the state's other coal deposits are higher in rank and contain many recorded methane shows.

During Cretaceous and Tertiary times, regressive-transgressive cycles resulted in numerous coal-bearing sequences. Major marine regressions allowed the formation of large peat swamps followed by transgressions which covered the swamps with impervious marine shales, preventing the already forming methane from escaping.

About 75% of Montana's coal is less than 1000 ft below the ground's surface, making it ideal for methane production. Associated water appears to be fresh, eliminating environmental problems. Pipelines are near to most of the major coal deposits. Exploration for coal-bed methane in Montana is still in its infancy but at this time shows commercial promise.

AAPG Search and Discovery Article #91010©1991 AAPG Rocky Mountain Section Meeting, Billings, Montana, July 28-31, 1991 (2009)