Origin of hydrocarbon and non-hydrocarbon (H2S, CO2 and N2) components of natural gas accumulated in the Zechstein Main Dolomite carbonate reservoir of the western part of the Polish sector of the Southern Permian Basin

Abstract

The origin and migration of hydrocarbon and non-hydrocarbon (hydrogen sulphide, carbon dioxide and molecular nitrogen) components of natural gas accumulated within the Zechstein PZ2 (Stassfurt) Main Dolomite (Ca2) carbonate reservoir in the western part of the Polish sector of the Southern Permian Basin (SPB) were established based on molecular and isotopic compositions (δ13C[CH4, C2H6, C3H8, n-C4H10, i-C4H10, n-C5H12, i-C5H12 and CO2], δ2H[CH4], δ15N[N2] and δ34S[H2S]) related to geological settings and 1-D modelling of generation processes. Hydrocarbon gases were generated during both microbial and thermogenic processes. Primary microbial processes ended at the turn of the Late Permian and Early Triassic, when oil and thermogenic gas generation processes began. The reconstruction of the thermal evolution and transformation of organic matter in this part of the SPB shows that thermogenic processes ended in the Late Jurassic in the Grotów Peninsula (Zone C), and in the Late Cretaceous in the western part of the Wielkopolska Platform and Gorzów Platform (zones A and B). The presence of the microbial component in analysed accumulations indicates that traps within the Ca2 carbonate strata were formed and sealed by the overlying impermeable anhydrites and salts of PZ2 (Stassfurt), PZ3 (Leine) and PZ4 (Aller) cycles preventing its escape. Accumulation of natural gas in Ca2 traps took place in at least two stages and had a cumulative character. First, microbial gases accumulated, then thermogenic gases associated with oil were generated from type II kerogen in the “oil window”. An insignificant methane component was generated from type III kerogen and migrated to Ca2 reservoir from Carboniferous strata. H2S was most probably generated during both microbial (MSR) and thermochemical sulphate reduction (TSR) of anhydrites in the PZ1 (Werra) cycle, and additionally from thermal decomposition of organic sulphur components of oil and kerogen in the Ca2 carbonate rocks. MSR ended during the Late Permian. TSR was temporarily halted as a result of Early Cretaceous uplift and probably resumed on a minor scale after the Late Cretaceous. CO2 was predominantly generated during thermogenic processes of kerogen decarboxylation, carbonate dissolution processes, and TSR and MSR together with H2S. A lesser contribution of CO2 from microbial fermentation was also observed. To a large extent N2 was mainly generated during thermal transformation of type II kerogen dispersed in the Ca2 carbonate strata and probably partly migrated from Carboniferous high-maturity type III kerogen to Ca2 reservoirs through fault systems.