Petroleum potential of Middle Jurassic rocks in the basement of the Carpathian Foredeep (Ukraine) and oil-to-source correlation with oil in Upper Jurassic reservoirs
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Organic matter-rich Middle Jurassic rocks occur in the Mesozoic basement of the Carpathian Foredeep in Ukraine. Eighty-nine core samples from the Mosty-2, Korolyn-6 and Korolyn-2 wells as well as three oil samples from the Kokhanivka and Orkhovychi oil fields in the Kokhanivka Zone, were investigated. Bulk geochemical data as well as maceral analysis have been determined, and a subset of these samples, including all oil samples, have been investigated for biomarker composition. Middle Jurassic strata are rich in organic matter (average: 4.19 wt. %) and reach TOC contents of up to 14.98 wt. %. However, HI values are low, typically around 100 mg HC/g TOC, and only reach a maximum of 242 mg HC/g TOC, indicating dominance of gas-prone, Type III kerogen. T max values as well as random vitrinite reflectance measurements for the Mosty-2 (434 °C; 0.69-0.71 % Rr), Korolyn-6 (443 °C; 0.78-0.82 % Rr) and Korolyn-2 (448 °C; 0.85-0.90 % Rr) wells indicate that thermal maturity increases with depth from marginally mature to mature, suggesting that hydrocarbon generation may have occurred. However, biomarker data suggests no genetic link between these rocks and Upper Jurassic oils, as C 27-C 28-C 29 sterane distributions, Pr/Ph ratios, DBT/Phenanthrene ratios and isotope data display significant differences between the two. New geochemical data, along with published biomarker data on Upper Jurassic rocks and crude oils belonging to the same oil family as the Upper Jurassic oils in the Ukrainian Carpathians, showed that Upper Jurassic rocks from the Korolyn-6 well present a better fit and are the source for the analysed oils.