COAL COMPOSITION OF THE BIG GEORGE COALBED (FORT UNION FORMATION) JOHNSON COUNTY, WYOMING

Measured gas content from the core extracted from the Big George coalbed (Fort Union Formation, Paleocene) Powder River Basin, Wyoming, can be related to coal chemistry, macroscopic petrology, and palynology. A previous study by the USGS in 1984 indicated that seven samples from a 202-ft-thick section of the Big George coalbed (well B23-BG1C) in eastern Johnson County, Wyoming, desorbed an average of 64 scf/t of coalbed gas. In the present study, a 138 ft section of coal was cored from the Michiwest 32-16 well, two miles east of the 1984 study site. Fifty samples were collected and the gas was desorbed. Later the coal lithotypes of the core samples were macroscopically described. Palynologic and standard chemical analyses were also performed. Gas content of the coal core ranged from 35.01 to 90.52 scf/t and averaged 70.43 scf/t. Calorific values of the samples ranged from 9975 to 10492 Btu/lb (as-received basis) and the samples are sub-bituminous B in rank (moist ash-free basis). The data revealed that thick, woody laminated coal directly correlates with high gas content. Lower gas content correlates with the presence of high ash and carbonaceous shale units. Palynologic data indicate that coal intervals dominated by taxodiaceous-cupressaceous-taxaceous trees probably produced the woody laminated coal. Our study of the Michiwest 32-16 core provides a new method of determining controls of the gas content of the Big George coalbed. Megascopic descriptions combined with palynologic and gas desorption data may enable a prediction of high and low gas content of Powder River Basin coalbeds.