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. Geochemical indices and stable isotope ratios vary within the following ranges: hydrocarbon (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%, δ¹³C(CH₄) from ?66.1 to ?24.6‰, δD(CH₄) from ?266 to ?117‰, δ¹³C(C₂H₆) from ?27.8 to ?22.8‰, and δ¹³C(CO₂) from ?26.6 to 16.8‰. Isotopic studies reveal the presence of 3 genetic types of natural gases: thermogenic (CH₄, higher gaseous hydrocarbons, and CO₂), endogenic CO₂, and microbial CH₄ and CO₂. Thermogenic gases resulted from coalification processes, which were probably completed by Late Carboniferous and Early Permian time. Endogenic CO₂ migrated along the deep-seated faults from upper mantle and/or magma chambers. Minor volumes of microbial CH₄ and CO₂ occur at shallow depths close to the abandoned mine workings. “Late-stage” microbial processes have commenced in the Upper Cretaceous and are probably active at present. However, depth-related isotopic fractionation which has resulted from physical and physicochemical (e.g. diffusion and adsorption/desorption) processes during gas migration cannot be neglected. 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.