Organic geochemical study and genetic correlation of natural gas, oil and Menilite source rocks in the area between San and Stryi rivers (Polish and Ukrainian Carpathians)

We studied the source rocks and petroleum accumulations in the Carpathian Mountains of the Polish-Ukrainian border area, between the San and Stryi rivers. A total of 108 Oligocene Menilite Formation rock samples, 12 oil and 15 gas samples accumulated in the Oligocene reservoirs, and one oil and gas sample from an Upper Cretaceous reservoir were analysed. The aim of this study was to evaluate the petroleum potential of the Menilite Formation in this area and to evaluate potential genetic correlations between source rock and petroleum. The Menilite Formation represents a prolific source rock with TOC content ranging from 2 to 7 wt%, locally exceeding 20 wt%. Algal type II and mixed type II/III kerogens dominate but sometimes type III kerogen and, rarely, type I kerogen occur. Thermal maturity of the Oligocene Menilite Formation increases from outer to inner tectonic units, from the immature in the Boryslav-Pokuttya and Skole units to the overmature in the southern parts of Silesian and Dukla units. As a consequence, petroleum potential decreases in the same order. Geochemical analyses reveal that oil and gas generation and migration processes within the Carpathian flysch succession are complicated. All analysed oils were generated from the Menilite Formation during early and peak stages of the “oil window”. Associated gas is of low-temperature thermogenic origin, sometimes containing an admixture of microbial gas. High-temperature thermogenic gas occurs only in the Polish part of the Silesian Unit.