Gas generation in Carboniferous source rocks of the Variscan foreland basin: implications for a charge history of Rotliegend deposits with natural gases

Abstract

Numerical modelling of the Carboniferous–Permian petroleum system in the Polish Basin was carried out using PetroMod software. The Carboniferous source rocks contain organic matter mostly of a humic nature (gas-prone Type III kerogen). Subordinately, only in the Lower Carboniferous deposits, kerogen of algal marine origin and mixed Type II/III kerogen occur. The quantity of dispersed organic matter is variable, but usually below 2% TOC. In the Carboniferous section, a progressive increase in the maturity of organic matter with depth was observed, from approximately 0.5% Rr at the top of the Westphalian in marginal parts of the Carboniferous basin to over 5.0% Rr at the bottom of the Lower Carboniferous in the eastern Fore-Sudetic Homocline. The thermo- genic generation of hydrocarbons continued from the late Westphalian (eastern Fore-Sudetic Homocline and partly Pomerania) throughout the Mesozoic, up to the Late Cretaceous. The advancement of this process is va- riable in different parts of the Carboniferous basin, reaching up to 100% of kerogen transformation in the zones of maximum maturity of organic matter. However, the most intensive periods of gas generation and migration were the Late Triassic and the Late Jurassic. The most prospective areas are located NE of Poznań–Kalisz line and SW of Poznań.