Composition and origin of coalbed gases in the Lower Silesian basin, southwest Poland

Coalbed gases in the Lower Silesian Coal Basin (LSCB) of Poland are highly variable in both their molecular and stable isotope compositions. Geochemic al indices and stable isotope ratios vary within the following ranges: hydr ocarbon (C-HC) index C-HC = CH4/(C2H6+C3H8) from 1.1 to 5825, wet gas (C2 ) index C2+ = (C2H6+ C3H8+ C4H10+ C5H12) / (CH4+ C2H6+ C3H8+ C4H10+ C5H12) 100 (%) from 0.0 to 48.3%, CO2-CH4 (CDMI) index CDMI = CO2/ (CO2+ CH4) 100 (%) from 0.1 to 99.9%, delta C-13(CH4) from -66.1 to -24.6%o, deltaD(CH4) f rom -266 to -117%o, delta C-13(C2H6) from -27.8 to -22.8%o, and delta C-13( CO2) from -26.6 to 16.8%o. Isotopic studies reveal the presence of 3 geneti c types of natural gases: thermogenic (CH4, higher gaseous hydrocarbons, an d CO2), endogenic CO2, and microbial CH4 and CO2. Thermogenic gases resulte d from coalification processes, which were probably completed by Late Carbo niferous and Early Permian time. Endogenic CO2 migrated along the deep-seat ed faults from upper mantle and/or magma chambers, Minor volumes of microbi al CH4 and CO2 occur at shallow depths close to the abandoned mine workings . "Late-stage" microbial processes have commenced in the Upper Cretaceous a nd are probably active at present. However, depth-related isotopic fraction ation which has resulted from physical and physicochemical (e.g. diffusion and adsorption/desorption) processes during gas migration cannot be neglect ed. The strongest rock and gas outbursts occur only in those parts of coal deposits of the LSCB which are dominated by large amounts of endogenic CO2. (C) 2001 Elsevier Science Ltd. All rights reserved.