Abstract

Compositional (i.e., hydrocarbon gas wetness) and isotopic analyses (C¹³, D for methane) for gases desorbed from Pennsylvanian (Desmoinesian) Cherokee Group coals from Montgomery County, KS and Cass County, MO suggests that these gases are of mixed microbial and thermogenic origin. Analyses of conventionally-produced gases from Pennsylvanian and Mississippian reservoirs in eastern Kansas indicate that these gases also range from microbial to thermogenic in origin, but thermogenic gases are more common west of a line extending from Wyandotte County (near Kansas City) to Chautauqua County, KS (along the KS-OK state line). Thermogenic processes increase deeper into the Cherokee and Forest City basins and southward into Oklahoma, whereas biogenic processes affect gas compositions farther east along the shallow flanks of these basins. Initial data indicate Kansas coalbed gases have heating values comparable to gases produced from conventional reservoirs. Helium can be an important but variable constituent in gas from coals and organic-rich shales. Helium values approaching 1% have been reported. The mixed origin of the conventional and coalbed gases in eastern Kansas has implications for predicting the quantity and quality of coalbed-gas production trends. Different production fairways of separate biogenic and thermogenic origin are possible.