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

Abstract

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.