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Atlantic Rim Coalbed Methane Play: The Newest Successful CBM Play in the Rockies

Lamarre, Robert A.1, Stephen K. Ruhl1
1 Lamarre Geological, Denver, CO
2 Anadarko Petroleum Corporation, The Woodlands, TX

The Atlantic Rim Coalbed Methane Play is rapidly becoming one of the most significant new CBM discoveries in the Rockies. The play is on the shallow eastern margin of the Washakie Basin, in Carbon County, Wyoming. The play area is 55 miles long by 5 miles wide, targeting prospective coals in the Almond and Allen Ridge Formations of the Mesaverde Group.

Data from wells within three different productive areas indicate that the relatively low rank coals (high volatile C bituminous) contain 175 to 350 scf/t gas on a dry, ash-free basis. The total coal thickness ranges from 40 to 80 feet, at depths of 200 feet to 3000 feet. Adsorption isotherms indicate that the coals are fully saturated. The coals are over-pressured with pressure gradients of 0.48–0.67 psi/ft. Water quality within the coals ranges from 1000–1450 ppm total dissolved solids. Potential reserves in coals of both formations may be as high as 3.0 Bcf per well, based on 160-acre spacing. Sixteen wells are producing (Sept. 2003) 5148 Mcfd and 26,171 Bwpd, for an average of 322 Mcfd and 1261 Bwpd per well.

The over-pressuring and unusually high gas contents are probably related. We suspect that the play is a large stratigraphic trap, with the gas being held in place by downdip flow of meteoric water from the high Sierra Madres to the east. This influx of fresh water has probably contributed biogenic gas to the coal reservoir. Coals and carbonaceous shales that are deeply buried near the center of the Washakie Basin are generating thermogenic gas today. Some of this gas has probably migrated updip to the east to help form the large coalbed methane occurrence that is currently being exploited.