Abstract: Coal and In-Place Gas Resources of the Greater Green River Basin, Wyoming and Colorado

Andrew R. Scott, Roger Tyler, Douglas S. Hamilton, Naijiang Zhou

Previous estimates of coal and in-place gas resources from the Greater Green River basin have ranged from 49 to 302 billion tons of coal and 0.2 to 31 Tcf of gas. Coal resource estimates were based primarily on coal mining data supplemented by geophysical logs, and in-place gas resources were subsequently estimated by simply multiplying the coal tonnage by fixed gas content values.

Detailed net coal-maps for the Upper Cretaceous Mesaverde Group and lower Tertiary Fort Union Formation constructed in subsurface studies indicate that previous coal resources for the basin have been greatly underestimated. Moreover, previous resource estimates have not consistently factored in gas content, ash content, and density data. Preliminary total coal resource estimates using no depth restrictions are approximately 1,277 billion tons,

whereas coal resources at drilling depths less than 7,500 feet are 688 billion tons. Coalbed methane resources, based on curves of ash-free gas content versus depth established from measured gas-content data, range from approximately 314 Tcf to 84 Tcf for the same depth restrictions.

Although the Mesaverde and Fort Union contain nearly equal amounts of coal (628 and 649 billion tons, respectively), 84 percent of the gas resources are found in the Mesaverde, reflecting greater maximum burial depth and higher coal rank. At depths less than 7,500 feet, the Fort Union represents 75 percent of the coal resource (445 billion tons) but only 33 percent of the in-place gas, reflecting lower coal rank (subbituminous to high-volatile B bituminous). The largest Mesaverde coal and gas resources are found in the Sand Wash basin, whereas the most significant Fort Union resources are located off the northwest flank of the Rock Springs Uplift.

AAPG Search and Discovery Article #90986©1994 AAPG Annual Convention, Denver, Colorado, June 12-15, 1994