Isotopic geochemistry and habitat of the natural gases from the Upper Carboniferous Žacleř coal-bearing formation in the Nowa Ruda coal district (Lower Silesia, Poland)

Abstract

The Nowa Ruda coal district is located in the SE part of the Lower Silesian Coal Basin. The analysed coals have vitrinite reflectance from 1.12 to 1.56% and δ 13C values from −24.6 to −23.2%. Natural gases accumulated in coal seams reveal considerable variability of molecular and isotopic compositions. Geochemical indices and stable isotope ratios vary in the following ranges: C 2+ wet gas index from 0.6 to 40.7%; carbon dioxide-methane index (CDMI) from 1.9 to 99.9%; δ 13C(methane) from −66.1 to −28.6%; δD(methane) from −256 to −211%; δ 13C(carbon dioxide) from −24.4 to −7.0%; and δ 13C(ethane) from −27.8 to −25.5%. Stable isotope studies allow distinction between thermogenic, endogenic (magmatic and/or upper-mantle derived) and probably even bacterial gases. The intensity of migration, mixing and accumulation of the natural gases of various origins appear to be variable in time and space. The variability of both the molecular and the isotopic compositions of the analysed gases probably results from the specific primary reactions active during the generation of the thermogenic gases in coal seams (decomposition of coal-seam matrix and/or cracking of n-alkanes) and/or in humic dispersed organic matter of coaly mudstones. An isotopic fractionation may also develop during the migration of the natural gases through the ultra-fine pores of coals. The results of this paper suggest that in the case of gases connected with coal seams, the existing gas classifications cannot be directly applied to their genetic interpretation.