TEXTURAL CONTROLS ON COALBED METHANE CONTENT IN THE SUBBITUMINOUS COAL OF THE POWDER RIVER BASIN

The variability of methane gas content in Powder River Basin coals can, to a large extent, be accounted for through textural changes in the coal. Two coal cores of 82 ft and 85 ft from the Tertiary Fort Union Formation, Wyoming, were thoroughly studied for methane content and coal type. Both cores were sampled in continuous 2-ft intervals and methane content was measured. Gas content ranged from 5 to 13 standard cubic feet (scf)/ton in one core, and from 11 to 17 scf/ton in the other. After desorption, the cores were described macroscopically. Four major coal types, which had previously been observed in the Powder River Basin, were identified in the two cores. All intervals of each coal type were quantitatively point counted and the thickness of each vitrain band measured. Vitrain band abundance in both cores ranged from 9% to 63% of the total coal volume. Grain sizes of vitrain were between 1 mm (0 f) to more than 160 mm (-7.3 f). It was found that the highest gas contents were generally associated with the highest proportions of vitrain bands. There is also some correlation of higher methane content with increased grain-size of the vitrain macerals. Coal types with greater abundances of vitrain have a high permeability, thus allowing greater desorption of gas. However, this relationship breaks down in the presence of abundant fusain layers (fossil charcoal), which are highly permeable and would preferentially draw gas to these parts of the seam.