Trends in Gas Content in Eastern Kansas Coals and Implications for Future Coalbed Natural Gas Exploration

K. David Newell and Timothy R. Carr
Kansas Geological Survey, University of Kansas, Lawrence, KS

Adsorbed gas content in eastern Kansas Middle Pennsylvanian coals decreases from the Cherokee basin northward into the Forest City basin. Maximum gas contents in the Cherokee basin register approximately 350 scf/ton (dry weight, as received). Less data are available for the Forest City basin, but coals there have at most about half the gas content of coals in the Cherokee basin. The decrease in gas content is likely due to a northward decrease in maturation away from the Arkoma basin, and lesser biogenic contributions to coalbed natural gas in the deeper parts of the Cherokee and Forest City basins. This latter effect is influenced by a west-northwestward increase in formation-water salinity away from outcrops on the basin flanks in western Missouri. NNE-SSW production fairways are anticipated to parallel the present-day or paleo-contacts between saline basinal water to the west, and relatively fresh water to the east.

Cherokee basin coalbed gas methane δC13 is commonly less than -50 ? (PDB), indicating microbial and mixed microbial-thermogenic sourcing. The coalbed natural gases with the more negative δC13 methane are usually dry, which is also characteristic of microbial derivation. Methane δD is around -220 ? (SMOW), indicating CO2 reduction is the dominant biogenic process.

Isotopic data indicate that methane δC13 for conventional gas accumulations, regardless of reservoir age, are more negative on the eastern flank of the Forest City and Cherokee basins. This suggests that some conventional and coalbed gases could be "secondary biogenic gases" in which methanogenic bacteria modify preexisting hydrocarbons.