1. Geographic distribution of geochemical properties of microbially generated Seelyville
and Springfield coalbed gases in the southeastern part of the Illinois Basin, Indiana
Panel a: Several coal members in the Illinois Basin (i.e., the extent of the grey area in the map insert) contain economically significant quantities of coalbed gas. We compiled chemical and isotopic gas characteristics along the southeastern margins of the basin in Indiana and scrutinized regional versus local gas variations in Seelyville and Springfield coal beds. Relatively high permeability and shallow depths of burial of these Indiana coals ( meters) allowed inoculation with methanogenic microbial consortia, thus leading to microbial methane generation along the eastern marginal part of the Illinois Basin.
Panel b: These relatively low-rank, high volatile bituminous coals with a vitrinite reflectance Ro ~ 0.6 % contain significant amounts of coal gas (~3 cm3/g) with
≥ 97 volume % microbial methane.
Panel c: Geographic lateral and vertical variations of coal gas parameters are evident in many geochemical parameters, for example in the ratio of
[methane/(ethane + propane)].
Panel d: The stable isotopic 13C/12C ratio of methane, and the carbon isotopic difference between carbon dioxide and methane in Springfield and Seelyville coals in Indiana are typically small.
The graphs were adapted from a manuscript by Strąpoć et al. that has been submitted to the International Journal of Coal Geology.