The Origin and Habitat of Hydrocarbons of the Polish and Ukrainian Parts of the Carpathian Province

Abstract

Multiple petroleum systems occurring in different tectonic elements of the Polish and Ukrainian parts of the Flysch Carpathians, their foredeep, and the Paleozoic–Mesozoic basement have been identified and characterized using various geochemical methods. The Carpathian flysch sequence, containing the main oil fields in the area, contains two main potential source rock intervals: the Early Cretaceous and Oligocene. The Oligocene Menilite shales, which are widespread in the Polish and Ukrainian parts of the Carpathian flysch belt, contain high-quality source rocks, with mainly type II kerogen and high petroleum potential. A suitable tectonic position in several units of the flysch belt, especially in its frontal nappes of the Ukrainian part, provided the maturation level of this series, corresponding to different parts of the oil window. Modeling of hydrocarbon generation and expulsion in Menilite rocks shows that these processes are related mainly to Miocene overthrusting. Geochemical studies of oils from different fields occurring in the flysch sequence show that they belong to the same family. The presence of oleanane probably indicates that they have been generated from Menilite rocks. Lower Cretaceous organic-rich rocks, which contain mainly type II and III kerogen with good petroleum potential and a maturation level corresponding to the oil window, should be considered as another important potential source rock sequence in both the Polish and Ukrainian Carpathian flysch belt. The autochthonous Miocene nonfolded molasse sequence contains most of the gas fields in the Polish and Ukrainian Carpathian Foredeep. Geochemical studies of Badenian and Sarmatian strata show that they contain sufficient amounts of immature terrestrial organic matter, which generated methane-rich microbial gases. Molecular and isotopic compositions of natural gases from Miocene reservoirs confirm that they were produced during microbial carbon dioxide reduction. Molecular and stable isotope compositions of gases from Paleozoic–Mesozoic reservoirs of the Polish part of the basement show that they were generated by both microbial and thermogenic processes. Gas from the Lachowice field in Devonian rocks is a typical nonassociated thermogenic gas, generated from type III kerogen with a high degree of maturation. This gas probably migrated from Carboniferous source rocks. The oils accumulated in Upper Cretaceous and Upper Jurassic reservoirs in the Polish part and, southeast of the Ukrainian part of the basement, contain oleanane and other specific biomarkers and belong to the same geochemical family. They were probably generated from the Oligocene flysch rocks and Middle Jurassic beds.