Mechanisms of generation of hydrogen sulphide, carbon dioxide and hydrocarbon gases from selected petroleum fields of the Zechstein Main Dolomite carbonates of the western part of Polish Southern Permian Basin: Isotopic and geological approach

In this paper we present the results of analyses of molecular and isotopic (S-34/S-32 in H2S, C-13/C-12 in CO2, CH4, C2H6 and C3H8, and H-2/H-1 in CH4) compositions of natural gas and stable sulphur isotope composition of S-organic compounds of oils from the Zechstein Main Dolomite carbonate reservoir of PZ2 cycle and sulphates of Basal Anhydrite of PZ2 and Upper Anhydrite of PZ1 cycles of the western part of Polish Upper Permian Basin. These results related to geological and palaeogeological conditions reveal that: (i) H2S could be generated mainly during thermochemical sulphate reduction (TSR) at the turn of Jurassic and Cretaceous and, in negligible volumes, during microbial sulphate reduction (MSR) and also during thermal decomposition of S-containing compounds of dispersed organic matter and oil; (ii) CO2 was generated during microbial and thermal transformations of organic matter, TSR and MSR processes, and during carbonate dissolution by fluid migration; (iii) the hydrocarbon components were generated by both microbial and thermogenic processes. The microbial methane was generated by fermentation and also during carbon dioxide reduction. Thermogenic hydrocarbons were generated during a single generation process of early low-temperature stage of "oil window" from type II kerogen.