ABSTRACT: Thermal Maturity and Coalbed Methane Potential of the Greater Green River, Piceance, Powder River, and Raton Basins

SCOTT, ANDREW R., and WILLIAM A. AMBROSE, Bureau of Economic Geology, University of Texas at Austin, Austin, TX

Coal rank, gas content, and present-day burial depth are important factors that control the coalbed methane potential of a basin. Methane in coalbeds can be thermogenic and/or biogenic in origin. The migration of methane within coalbeds has important implications for coalbed methane development and should also be considered in evaluating the coalbed methane potential of a basin. Higher-rank coals generate more methane and less carbon dioxide than lower-rank coals. Therefore, thermally mature coals at relatively shallow depths are preferred coalbed methane targets. On this basis, the coalbed methane potential of four major western basins was evaluated.

Coal rank in the Greater Green River basin ranges from subbituminous to semianthracite. Vitrinite reflectance (Ro) values at 6000 ft range from 0.55 to 0.60%, and typical gas contents range from 100 to 375 scf/ton; gas contents of coals less than 2000 ft are generally less than 100 scf/ton. Coal rank in the Piceance basin ranges from semianthracite in the deeper parts of the basin to high-volatile C bituminous along the basin margins. Coals in the Piceance basin are high-rank (Ro of 0.7-1.5% at 6000 ft) and have reached the thermogenic threshold of methane generation; typical gas contents range from 100 to 450 scf/ton, indicating good coalbed methane potential for this basin. Thermally immature coals (Ro < 0.50%) in the Powder River basin have gas contents of less than 75 scf/ton. oal rank in the Raton basin ranges from high-volatile C bituminous to low-volatile bituminous along the Purgatoire River. High coal rank at shallow depths (Ro > 1.5% at 3,000 feet) suggests that the Raton basin has a good coalbed methane potential based on coal rank and burial depth. Typical gas contents range from 150 to 400 scf/ton, except along the eastern margin of the basin where gas contents are less than 100 scf/ton.

Coal rank, burial depth, and gas content data indicate that the Raton and Piceance basins have the highest coalbed methane potential. However, abnormally high gas contents of low-rank coals (>2000 ft) in the Greater Green River basin indicate the presence of biogenic and/or migrated thermogenic gases. Therefore, parts of this basin may have a greater coalbed methane potential than indicated by previous studies. The Powder River basin and shallow coal beds (<2000 ft) in the Greater Green River basin have a lower coalbed methane potential due to low coal rank and gas contents.

AAPG Search and Discovery Article #91012©1992 AAPG Annual Meeting, Calgary, Alberta, Canada, June 22-25, 1992 (2009)