Source rock potential of the Oligocene Menilite Formation in the Czech sector of the Subsilesian Unit (Flysch Carpathians)

Abstract

The Oligocene Menilite Formation represents the most important hydrocarbon source rock in the Flysch Carpathians. The formation is laterally uniform across long distances but shows strong vertical heterogeneity reflecting changes in depositional environments, which control the source rock potential. In the Czech Republic, the Menilite Formation is subdivided into the Subchert Member (nannofossil zone NP22), Chert Member (upper NP22 to lower NP23), Dynów Marlstone (NP23) and Šitbořice Member (upper NP23 to lower NP25). The present study describes in detail the Bystřice nad Olší section in the Subsilesian Unit where all four members are exposed including the uppermost part of the underlying Frýdlant Formation (Sheshory Marls). The present Frýdlant Formation has a negligible source rock potential compared to the Menilite Formation. The Menilite Formation in the Bystřice nad Olší section has mostly “good” source rock potential, although the TOC indicates “very good” potential. The Subchert Member (average TOC: 3.6 wt. %; HI up to 505 mg HC/g TOC) and the Chert Member (average TOC: 2.2 wt. %; HI up to 790 mg HC/g TOC) are the most prolific units. Based on organic petrography and HI, the kerogen is classified as type II and I. Indications of admixtures of type III kerogen are limited to samples with low TOC contents (<1 wt. %). The Source Potential Index (SPI) of the Menilite Formation in the Subsilesian Unit of the Czech Republic is estimated as ~1.15 t HC/m2 which is comparable to SPI calculated for sections in the Silesian and Ždánice units in the Czech Republic and for the Waschberg Zone in Austria. Significantly higher SPI values estimated for the Polish and especially the Ukrainian Carpathians (up to 74.5 t HC/m2) are caused by a wider stratigraphic range, several times greater thickness and higher total organic matter content.