Turonian-Senonian organic-rich sedimentary strata and coal facies in Parnassos-Ghiona Unit, Central Greece: An assessment of palaeoenvironmental setting and hydrocarbon generation potential

Abstract

In the past few years, scientific research related to the hydrocarbon potential in Greece has been focused on the formations of the Ionian Unit in the western areas of the country, where oil-seepages are well known. In contrast, Parnassos-Ghiona geotectonic Unit (PGU), Central Mainland Greece, has mainly been studied for the presence of bauxite ores. However, Upper-Cretaceous organic-rich strata, including local coal lenses, are occurring on top of the b3 bauxite horizon across PGU. In the present study, two underground mine sites, at Vagoneto and Gouves, where coal lenses occur, as well as an outcrop of the b3 bauxite horizon and the overlying gray limestone (PBF section) have been studied by means of organic petrology, mineralogy and inorganic geochemistry, as well as Rock-Eval analysis. The aim of the study is to evaluate the palaeoenvironmental conditions that favoured the organic matter accumulation subsequently to the bauxite formation, as well as the assessment of the hydrocarbon potential of the Upper Cretaceous strata within PGU. The study of the sections of interest revealed the establishment of three different modes of organic matter depositional settings, including paralic mires on the top of the lateritic mud deposition, paralic mires developed upon the Lower Cretaceous limestone bedrock, as well as shallow-marine basins with limited water circulation, followed by a deepening trend upwards. The low/very low grade humic coal lenses at the Vagoneto and Gouves sites formed by accumulation of mostly herbaceous plants, being characterized by elevated amounts of detrovitrinite and inertinite, and variable amounts of liptinites usually in the form of sporinite and alginite. Accordingly, the Rock-Eval data indicate the occurrence of all kerogen types, with the Types III and IV generally prevailing. The overall coal-petrographic data indicate intense oxidation in the palaeo-mires, possibly related to the dry warm Turonian climate and/or the influence of brackish waters. Additionally, the thermal maturity derived from both vitrinite reflectance and Rock-Eval analysis, point to Medium Rank Bituminous coals or, in terms of hydrocarbon generation, to the early oil window stage. The data suggest a good-to-excellent gas potential for Parnassos-Ghiona Unit, whereas the oil-potential seems to be limited.