The Lower Cretaceous “Vigla” Shales Potentiality to Be Source Rocks in the Ionian Basin, Greece

Abstract

As Lower Cretaceous “Vigla” shales have been suggested as one of the main source rocks for the Ionian Basin in Greece, a geochemical analysis was performed for “Vigla” shales in Kastos Island and the Araxos peninsula, far from the already studied areas. Results, based on Rock-Eval VI analysis, sample fractionation, and biomarkers analysis, showed that the studied rocks could be of low production capacity, are type II/III of kerogen, and can produce liquid and gas hydrocarbons for Kastos Island. Organic matter (total organic carbon-TOC 0.02–3.45%) of the studied samples is thermally immature, in the early stages of diagenesis, and was accumulated in an anoxic environment. Additionally, the geochemical analyses confirmed the combination of marine and terrestrial origin of the organic matter. On the other hand, TOC (0.01–0.72%) from the Araxos peninsula shows fair oil potential and type IV kerogen. The results based on the Odd–Even Predominance, OEP (27–31), OEP (2), and OEP (1), valued for samples AG1, AG2, AG5, and AG6, indicated an anoxic deposition environment. As the Ionian Basin was sub-divided into three sub-basins (internal, middle, and external) during its syn-rift evolution, different depositional conditions were developed from one sub-basin to the other, with different sedimentary thicknesses within the same sub-basin or among different sub-basins and with different amounts of TOC. The fact that there is a great difference in geochemical indices between the two studied areas during the same period suggests that probable different depositional conditions could exist. It seems that the richness in Kastos Island could be related to the neighboring Apulian Platform, whereas the poorness in the Araxos peninsula could be related to the Gavrovo platform, or the differences could be related to restrictions produced regions. The comparison with previous studies indicates that different quality and quantity of organic matter could be accumulated either within the same sub-basin or from one sub-basin to the other.