Origin of gaseous hydrocarbons, noble gases, carbon dioxide and nitrogen in Carboniferous and Permian strata of the distal part of the Polish Basin: Geological and isotopic approach

Abstract

Sixteen natural gas samples collected from Pennsylvanian and Permian (Rotliegend and Zechstein Limestone) strata of the distal part of the Polish Basin in Wielkopolska and Lower Silesia were analysed for molecular composition and stable carbon isotope composition of hydrocarbons (CH4, C2H6, C3H8, n-C4H10 and i-C4H10) and CO2, stable hydrogen isotope composition of methane, stable nitrogen isotope composition of N2, and stable isotope composition of noble gases (He, Ne, Ar, Kr, and Xe). Thirteen of analysed hydrocarbon gases reveal complete [δ13C(CH4)>δ13C(C2H6)>δ13C(C3H8)] and partial inversed isotopic trends from methane to propane thus they have a very complicated generation, migration and accumulation history and range of their source rock horizons. Two source rock horizons occur NE from Wolsztyn Ridge. They contain mixed types III/II and II/III kerogens of Pennsylvanian age and Mississippian and/or Devonian age, respectively. One source rock horizon occurs SW from Wolsztyn Ridge. It contains type III and mixed type III/II kerogens of Pennsylvanian age. The kerogens in all source rock horizons generated gaseous hydrocarbons at two separate maturity stages: about 0.5 to 0.8% in vitrinite reflectance scale at the first maturity stage, and over 1.3% in vitrinite reflectance scale at the second maturity stage. High concentrations of He in analysed gases are mostly a product of α-decay of U and Th enriched in crustal materials. A small contribution of He and Ne of mantle origin to the gas reservoirs was inferred. Radiogenic 4He/40Ar ratios are higher than the average production rate ratio of about 5 for crustal materials, which might have been caused by a selective supply of lighter He through crustal rocks surrounding the gas reservoirs, or higher (U+Th)/K ratio than the crustal average. CO2 from analysed gases was mainly generated during thermogenic processes of transformation of organic matter, although some gases can contain components from endogenic processes and from thermal destruction of Zechstein Limestone and probably Precambrian carbonates. N2 was mainly generated during thermal transformation of organic matter and had a large component released from NH4-rich illites. Very high N2/40Ar for the gases might be caused by accelerated thermogenic production of N2 under the condition of high heat flux from volcanic activity at late Pennsylvanian and early Permian age. Deep-seated faults, mainly framing the Wolsztyn Ridge, perform important role in migration and mixed in different proportions of noble gases originated from upper mantle and from mineralisation by radioactive minerals their fault surfaces and occurrence of Pennsylvanian–Lower Permian volcanites in crust as well as hydrocarbon gases, CO2 and N2, also from thermal destruction of NH4-rich illites, from “gas generation kitchens” of source rock horizons to the traps.