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Integrated isotopic and organic geochemical constraints on the depositional controls and source rock quality of the Neogene Kalamaki sedimentary successions (Zakynthos Island, Ionian Sea)

Kontakiotis G., Karakitsios V., Maravelis A.G., Zarkogiannis S.D., Agiadi K., Antonarakou A., Pasadakis N., Zelilidis A.

This study investigates the oxygen and carbon (δ18O, δ13C) stable isotope ratios, as well as the organic geochemical features of the 146-m-thick composite Kalamaki section located in the southwest margin of the Hellenic Fold and Thrust Belt (Zakynthos Island). The section includes Late Miocene–Pliocene pre-evaporitic, evaporitic, and post-evaporitic deposits. The obtained bulk geochemical data (Rock–Eval II, bitumen extraction, and fractionation) and biomarker compositions indicate poor–fair source-rock potential predominantly promising for gas production. TOC values indicate the presence of samples that might be of slight interest (0.50–0.95%) in the evaporitic interval and samples that are worthy of further investigation (up to 1.31%) in the pre-evaporitic sequence. The organic matter type (type III kerogen), along with the presence of plant remains (leaves) in the studied deposits reflect the mainly terrestrial origin of the organic material. Tmax and Production Index values further indicate that most of the sediments (138 out of 195 samples) are immature for oil generation and were not under high temperature conditions during burial, whereas 29 of them correspond to the mature oil stage. Oil generation requires deeper burial, which may have occurred offshore in the Ionian Sea. A marine paleoenvironment is indicated that received organic material from both marine and terrigenous sources. Isotopic values imply influence of continental fresh-water inputs in a high salinity, evaporative depositional setting. Furthermore, our findings provide evidence that the climate has favored the development of these source rocks by triggering relative sea-level fall, which in turn enhanced the preservation of organic matter by increasing sea-water salinity and suppressing carbonate deposition.

Mediterranean Geoscience Reviews. 2021, 3