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

Abstract

Abstract We established the generation and migration processes as well as the influence of secondary processes of oil and natural gas accumulated in Lower Cretaceous-Oligocene strata of the western part of the Ukrainian Outer Carpathians based on the results of organic geochemical analyses of oil and natural gas related to the geological setting and source organic matter. The Oligocene Menilite beds from the Boryslav-Pokuttya Nappe are the main source rock for oil and natural gas accumulated in the fields of the Boryslav-Pokuttya and Skiba nappes. Co-sourcing of oils and natural gas accumulated in the Skiba Nappe in the study area by both Menilite and Spas beds of this nappe cannot be excluded. In some cases (e.g. Novoshidnytsya field) oil and natural gas originated within the Oligocene Menilite beds of the Boryslav-Pokuttya Nappe, migrated through thrusts and fissure zones and accumulated in the Lower Cretaceous reservoirs of the Skiba Nappe. Oil from small Vovche field and Pohar seep of the Krosno Nappe was generated by Menilite source rocks of this tectonic unit and migrated to the traps. Sporadically oils underwent secondary processes, mainly biodegradation and occasionally water washing. Hydrocarbon gases and carbon dioxide were generated mainly during low-temperature thermogenic processes from Type II kerogen, and partly during the microbial CO2 reduction process. Only natural gas from the Mala Volosyanka oil and gas seep of the Krosno Nappe was documented to have undergone biodegradation.