Origin and migration of oil and natural gas in the central part of the Ukrainian outer Carpathians: Geochemical and geological approach

The generation and migration processes, as well as the influence of secondary processes, on oil and natural gas accumulated in Lower Cretaceous–lower Miocene strata of the central part of the Ukrainian outer Carpathians are discussed. Type II and II/III kerogens from the Oligocene–lower Menilite beds of the Boryslav-Pokuttya nappe generated oil and natural gas mainly during the middle stage of the oil window. These hydrocarbons accumulated in the fields of the Boryslav-Pokuttya and Skiba nappes. Water washing is the dominant secondary process occurring in many accumulations, regardless of reservoir rock, whereas biodegradation is insignificant and is clearly evident only in natural gas from one seep in the abandoned Starunia ozocerite mine. All hydrocarbon gases are thermogenic and were generated from mixed organic matter containing various proportions of type II and III kerogens. Only one natural gas accumulation in the Pivnichna Dolyna field contains some microbial hydrocarbon contribution. Carbon dioxide mainly originated from thermogenic kerogen decarboxylation and sporadically from microbial processes. Molecular nitrogen was mainly generated during low-temperature thermal transformation of organic matter and could also partly originate from NH4-rich illites of the clayey facies of the Oligocene–lower Miocene Menilite beds. The integration of the geochemical data with geological setting and maturation history of source rocks enabled us to reconstruct hydrocarbon migration paths, revealing the complicated trap-filling process.