Origin of Gaseous Hydrocarbons of the Outer Carpathians and Palaeozoic-Mesozoic Basement in Southeastern Poland

Natural gases accumulated both in the Polish Outer (Flysch) Carpathians and the Palaeozoic-Mesozoic basement are genetically related to thermogenic and microbial processes. Thermogenic gaseous hydrocarbons accumulated in the Dukla and Silesian units were generated from Type-II and -II/III kerogen of the Menilite Shales. Thermogenic gaseous hydrocarbons accumulated in the Sub-Silesian Unit migrated from the Silesian Unit. Natural gases accumulated in traps within the Middle Devonian, Lower Carboniferous and Upper Jurassic strata of the Palaeozoic-Mesozoic basement were mainly generated during thermogenic processes and only sporadically during microbial processes. The themogenic gases accumulated within Palaeozoic complex were generated from Type-II kerogen of the Ordovician-Silurian strata and gases accumulated within Mesozoic complex from Type-III (II/III) kerogen of the Middle Jurassic strata. In some traps within the Dukla and Silesian units of the Outer Carpathians and Palaeozoic-Mesozoic basement had already been formed and sealed as early as the initial, microbial transformation stage of the source rocks. These traps were successively supplied with thermogenic gaseous and liquid hydrocarbons generated from the same source organic matter at successively higher maturation stage.