Publisher © Czech Geological Survey, ISSN: 2336-5757 (online), 0514-8057 (print)

Source rock potential of the Menilite Formation in the Czech Republic


Petr Jirman, Eva Geršlová, Miroslav Bubík

Geoscience Research Reports 53, 2020, pages 127–136

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Published online: 12 October 2020

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The Oligocene Menilite Formation represents the most important source rock in the Outer Flysch Carpathians. However, only a few studies describe its source rock potential on the territory of the Czech Republic. This overview study deals with 163 samples from the Oligocene Menilite Formation from the Silesian, Subsilesian and Ždánice units in the Czech Republic (Fig. 1) taken from authors´ previous research. All samples were measured for the total organic carbon content (TOC). Based on it, and with respect to the sampling interval, a subset of 122 samples was used for Rock-Eval pyrolysis. The data are in this study newly completed by additional 35 archival samples from 8 boreholes showing both the TOC and Rock-Eval parameters. Based on the dataset, the genetic potential (GP) and hydrogen index (HI) were calculated. Additionally to 3 archival samples with known oxygen index (OI), 10 samples were newly measured and the OI calculated. The source rock potential was evaluated for each Menilite Formation member separately excluding the 9 older archival samples that cannot be assigned to a particular member. Those samples are interpreted as “undivided” Menilite Formation.
The thermal maturity was evaluated by Rock-Eval temperature Tmax. Majority of the studied samples have Tmax < 430 °C (average 418 °C) indicating a presence of immature organic matter (Table 1). In such case, the TOC and HI are considered original. The Menilite Formation has generally a “good” source rock potential (Fig. 2) according to the Rock-Eval GP (average 12.6 mg HC/g rock, range 0.07 - 122.8 mg HC/g rock), whereas the TOC (average 3.19 wt.%, range 0.20 - 16.5 wt.%) indicates “very good” potential. This is presumably caused by inert organic carbon content within the overall TOC. The Chert Member is the most productive among the members according to both the TOC (average 5.19 wt.%), and GP (average 37.0 mg HC/g rock). Based on the refine insight of Rock-Eval HItrue (Table 2), and according to Dahl et al (2004), the organic matter in the Menilite Formation is classified as kerogen type II (mixed) and I (sapropelic). The kerogen type classification based on the raw (original, not recalculated) Rock-Eval data only (Fig. 3) is misleading due to inert organic carbon content within the overall TOC (Table 2), and several low TOC samples, which significantly reduce the true HI (HItrue, Table 2).
Summarizing the data, the Menilite Formation may represent very promising source rock in case of higher thermal maturity which is a limiting factor here. Thermal maturity conditions favourable for hydrocarbons generation are expected especially beneath the Magura Group of Nappes due to higher burial depths.