Hydrocarbon potential of the Lower Cretaceous (Barremian–Albian) Shypot Formation in the Chornohora nappe, Ukraine

Abstract

Organic matter-rich rocks occur in the Carpathians, both in the Lower Cretaceous and Oligocene. Whereas, the Oligocene Menilite Formation has been intensely studied, the hydrocarbon potential of Lower Cretaceous rocks is less well understood. In the present paper a 405 m thick succession of the lower part of the Shypot Formation in the Chornohora nappe (Ukraine) is studied using 94 outcrop samples. Maturity parameters for the Lower Cretaceous rocks indicate peak oil maturity (~0.85 % Rr) and organic carbon content averages 2.8 wt. % for all samples. As a result of the enhanced maturity, the hydrogen index (88 mg HC/g TOC) and the remaining petroleum potential (2 tHC/m2) are low. Comparisons with coeval rocks from the same tectonic unit, but with lower maturity suggest that the original petroleum potential was significantly higher (4 tHC/m2). Probably about 2 tHC/m2 were generated during deep burial (6 km?), but were lost during uplift and erosion. Macerals analysis reveals a mixed type III-II kerogen, with domination of terrigenous components, which is also supported by HI values of nearby marginal mature samples (~200 mg HC/g TOC). Lower Cretaceous organic matter-rich rocks are found along the entire Carpathian arc. A compilation of published data for age-equivalent rocks across the Carpathian Fold-Thrust Belt shows that HI values are mainly controlled by maturity as well as the moderately high original HI values. Most of these rocks contain predominantly type III-II kerogen, whereas Lower Cretaceous rocks in the Skole-Skyba nappe near the Polish–Ukrainian border contain type (III-) IV kerogen.