Origin and secondary processes in petroleum in the eastern part of the Polish Outer Carpathians

Abstract

A geochemical study of crude oil and natural gas from producing wells and seeps within the Lower Cretaceous–Oligocene sandstone reservoirs of the most eastern part of the Polish Outer Carpathians was conducted for the evaluation of their origin, migration pathways and the effects of secondary processes (biodegradation, water washing and evaporative fractionation). The petroleum was generated from mixed marine/terrigenous organic matter (type-II/III kerogen) occurring mostly in Oligocene Menilite beds in the Silesian and Skole nappes. Shales within Upper Cretaceous–Palaeocene Istebna and Lower Cretaceous Veřovice, Lgota and Spas beds are considered as an additional source of hydrocarbons (mainly gaseous). Biodegradation processes were recorded in numerous oils, especially occurring in seeps, but the extent of these processes is not severe, because steranes and terpanes were not affected. All oils are more-or-less water washed, while the evaporative fractionation processes are developed only on a minimal scale. The natural gases are primarily of thermogenic origin, usually co-genetic with oil and often contain components of secondary microbial degradation of both oil and natural gas (13C-enriched CO2 and 13C-depleted CH4). Gases with the highest microbial methane contribution have been recorded in petroleum accumulated in Dwernik and Łodyna fields.