A New Saturated and Aromatic Hydrocarbon‐Based Scheme to Decipher the Origin of Extracted Organic Matter in the Outer Carpathians: From the Jurassic to the Miocene

In this review, published and new geochemical data characterising 17 potential source rocks for hydrocarbons in the Polish part of the Outer Carpathians have been used to establish a new scheme that can be useful for oil–source rock correlations. Rock and drill core samples, ranging in age from Jurassic to Miocene, were analysed using Rock‐Eval pyrolysis. The results confirmed that the best source rock is shale from the Oligocene Menilite Beds, particularly the so‐called deep shale, occurring at depths greater than 4000 m. The Palaeocene/Eocene mudstone interbeds in the Istebna Beds can serve as a source rock for natural gas, although only locally, due to their highly irregular geochemical parameters. Lower Cretaceous strata, specifically the fine‐grained sediments of the Spas Shales, Lgota Beds and Veřovice Beds, appear to be significant for microbial generation of methane. Molecular analysis of extractable organic matter, combined with pyrolytic data, revealed a wide range of organic matter maturation stages, from immature to overmature. These analyses also identified rare zones of high maturity at shallow depths in source rocks, confirming the upwards displacement of once deeply buried strata in the Outer Carpathians. Additionally, the saturated and aromatic hydrocarbon‐based compounds and their ratios revealed characteristic biomarkers and polycyclic aromatic hydrocarbons (PAHs) that differentiate various shale‐ (mudstone, claystone) or marl‐rich formations within specific lithostratigraphic units. These chemical compounds serve as strong tools for deciphering the origin of oils in the Outer Carpathians, regardless of the organic matter's maturity level. This age‐based molecular characterisation highlights the particular importance of higher plant‐derived compounds, such as oleanane isomers, which are most abundant in the Oligocene samples but also present in Kimmeridgian shale. Aromatised terpenoid precursors, in turn, are abundant in shale from the Menilite Beds as well as in the Krosno and Istebna beds. Terrigenous organic matter is also accompanied by algal‐derived components (e.g., dinosteroids) and bacterial contributions (e.g., hopanes). Finally, the analysed organic matter was deposited in sedimentary basins under varied redox conditions. Anoxic environments were confirmed by high bisnorhopane/C 30 hopane ratios, whereas oxic to suboxic conditions were indicated by high pristane/phytane ratios, suggesting a mixed oxic–anoxic water column.