Composition and origin of coalbed gases in the Upper Silesian and Lublin basins, Poland

Coalbed gases in the Upper Silesian Coal Basin (USCB) are highly variable in molecular and stable isotope composition. Geochemical indices and stable isotope ratios for coalbed gases from USCB vary within the following ranges: CH4/(C2H6+C3H8) hydrocarbon index from 122 to more than 10,000; CDMI carbon dioxide-methane index {CDMI=[CO2/(CO2+CH4)] 100 (%)} from 0.0 to 21.0%; δ13C(CH4) from −79.9 to −44.5‰; δD(CH4) from −202 to −153‰; δ13C(C2H6) from-24.6 to −22.3‰; δ13C(C3H8) −24.7 ‰ (one sample); and δ13C(CO2) from −27.2 to −2.8‰. Only two coalbed gases were collected from the Lublin Coal Basin (LCB). Geochemical indices and stable isotope ratios for these samples show the following values: hydrocarbon index more than 10,000; CDMI=4.3 and 34.6%; δ13C(CH4)= −67.3 and −52.5‰; δD(CH4)= −201‰ (one sample); and δ13C(CO2)= −13.7 and −11.9‰. Methane, higher gaseous hydrocarbons (C2 to C5) and carbon dioxide occurring in the coalbed gases in both the USCB and the LCB were generated during the bituminous stage of the coalification process and, probably, during microbial reduction of carbon dioxide. However, depth-related isotopic fractionation which has resulted from physical (e.g. diffusion and adsorption/desorption) processes during gas migration cannot be neglected. In both basins the coalification process lasted no longer than several Ma, and was completed at the end of the Variscan orogeny (at the turn of Carboniferous and Permian).