Origin of hydrocarbon and non-hydrocarbon (H2S, CO2 and N2) components of natural gas accumulated in the Zechstein Main Dolomite (Ca2) strata in SW part of the Polish Permian Basin: stable isotope and hydrous pyrolysis studies

Abstract

The genetic identification of natural gas accumulated in the Zechstein Main Dolomite (Ca2) strata in the SW part of the Polish Permian Basin (PPB) was determined by means of analytical and experimental methods of petroleum geochemistry including molecular and isotopic compositions, i.e. δ13C of C1–C5 hydrocarbon gases and CO2, δ2H of C1–C3 hydrocarbon gases and δ34S of H2S of natural gas and gases generated during hydrous pyrolysis (HP) experiments and δ15N of N2 of natural gas as well as results of geochemical analyses of organic matter of the Ca2 source rock samples including Rock-Eval (RE) pyrolysis and elemental and stable C isotope compositions of kerogen. Four HP experiments conducted on two composite samples from Ca2 strata with different (II and III) prevailing genetic type of kerogen each, led to production of thermogenic gas which was used for geochemical comparison with natural gases accumulated within Ca2 strata. Hydrocarbon gases of analysed accumulations belong to two different genetic groups: (i) including gases sourced from sapropelic organic matter (type II kerogen) dispersed in the Ca2 strata produced during either both microbial and thermogenic processes or solely thermogenic processes and (ii) comprising gases not genetically related to the Ca2 source rock, originated from humic organic matter (type III kerogen) of high maturity from deeper buried strata. CO2 related to Ca2 strata primarily was produced during thermal decarboxylation of kerogen and carbonate dissolution by acid fluids, while CO2 unrelated to Ca2 strata was generated from thermal decomposition of deeper buried carbonate formations. H2S has been generated during thermal decomposition of sulphur-containing organic matter and thermochemical sulphate reduction (TSR) processes. The coexistence of CO2 and H2S indicates that these components might have been produced together by TSR and less microbial sulphate reduction (MSR) processes. Molecular nitrogen in analysed natural gases was generated mainly during thermal decomposition of organic matter, but atmosphere-derived molecular nitrogen is presumably also present.