The significant increase in United States gas production from coal beds over the past 5 yr has encouraged exploration and development of coal gas resources worldwide. Accurate assessment of coal and coal gas resources and delineating areas within basins containing the largest resources are important aspects of resource development. Previous resource studies may have overestimated or underestimated coal gas resources because the ash and density terms of resource equations were inconsistently or inappropriately considered.
In basins where coal analysis data are sparse, coal gas resources are best calculated on an ash-free basis. The density contrast between ash-forming minerals and organic matter is large enough that the weight percent ash is much larger than the corresponding volume percent. Therefore, a correction factor relating weight percent ash-free coal and ash yield (determined from proximate analysis) to ash-free coal volume is required to accurately calculate coal gas resources. Rather than using one density value, as has been done in previous studies, our calculations require that bulk coal density (including mineral matter) be distinguished from ash-free coal density.
Coal and coal gas resources of the Williams Fork and Fort Union formations in the Sand Wash basin, determined from modified resource equations, are 291 billion tons (short tons) (264 billion t [metric ton]) and 79 Tcf* (2.2 Tm3). These resources are significantly higher than previous estimates of basin resources of 34.5 billion tons (31.3 billion t) of coal and 14 Tcf (0.4 Tm3) of coal gas. The Williams Fork Formation contains 79% of the coal and 95% of the coal gas resources, reflecting greater maximum burial depth and higher gas contents. The Fort Union Formation contains 21% of the coal, but only 5% of the coal gas resources. Coal gas resources are greatest in the central part of the basin where Williams Fork resources approach 70 Gcf*/mi2 (0.8 Gm3/km2). In comparison, the San Juan basin, the nation’s most prolific coal gas–producing basin, has maximum coal gas resources approaching 35 Gcf/mi2 (0.4 Gm3/km2). The high resource density in the Sand Wash basin is due to greater net coal thickness rather than high gas content.