Paleoenvironment, organic matter maturity and the hydrocarbon potential of Menilite shales (Silesian Unit, Polish Outer Carpathians) - Organic and inorganic geochemical proxies

The Oligocene Menilite shales (Silesian Unit, Polish Outer Carpathians), an example of the organic-rich black shale facies of the Paratethys, were sampled in the two sections of Znamirowice and Kobielnik, representing proximal to the land and open-marine environments, respectively. Terrestrial organic matter (OM) imprinted in maceral and biomarker composition was probably supplied by the fresh-water runoff, as suggested by the total organic carbon (TOC)-total sulfur (TS) values and low Sr/Ba ratio in the Znamirowice section. In contrast, normal marine-salinity conditions and the scarcity of higher plant-derived biomarkers or macerals in the Kobielnik section suggest the deposition in a more distal environment. Abundant alginite and bituminite, high TOC content and EF P2O5 values, together with biogenic silica and the occurrence of triaromatic dinosteroids indicate a medium-to-high primary productivity, represented by algal-bacterial communities including silica secreting organisms. The inorganic proxies show that the analysed rocks were mainly deposited in unrestricted basin probably affected by temporary upwelling. Only a small number of samples approach the restricted basin model. Concentrations of redox-sensitive elements (e.g. Mo, U, V, Ni, Co) corroborating with biomarker proxies (homohopane index, pristane/phytane) allow us to suppose oxic conditions within the water column with periods of oxygen reduction in the bottom waters and mainly dysoxia/anoxia in the sediment during the Menilite shale formation. The studied Menilite shales are source rocks with good-to-excellent hydrocarbon potential, characterised by mostly immature OM. However, the hopane and sterane isomerisation ratios show a more advanced stage of thermal maturation in the Kobielnik section, reaching the threshold of the oil window. In the Znamirowice section the OM is still immature.

Automated summary

The Oligocene Menilite shales in the Polish Outer Carpathians represent different depositional environments, with the Znamirowice section showing terrestrial organic matter input and the Kobielnik section representing a more distal marine environment. The presence of abundant alginite indicates medium-to-high primary productivity, and the inorganic proxies suggest deposition in an unrestricted basin. The studied Menilite shales are considered to be source rocks with good-to-excellent hydrocarbon potential.