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

Abstract

This study investigates the oxygen and carbon (δ 18 O, δ 13 C) 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. T max 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 freshwater 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 seawater salinity and suppressing carbonate deposition.

Multiple crises preceded the Mediterranean Salinity Crisis: Aridification and vegetation changes revealed by biomarkers and stable isotopes

“…Between 7.16 to 6.88 Ma and 6.80 to 6.54 Ma, mean BIT values of 0.73 indicate a more terrigenous/soil source of organic matter that contrast BIT indices of 0.46, indicating an dominantly aquatic production for the interval between 6.87 and 6.80 Ma. This observation is consistent with results from Crete (Ploutis section) and Zakynthos (Kalamaki section) indicating a mixed origin of the organic matter during the Late Miocene (Kontakiotis et al, 2020(Kontakiotis et al, , 2021. The Nile and the Saharan rivers may have served as the main east Mediterranean suppliers of clastic sediments during Tortonian-Messinian (Gladstone et al 2007), with additional contributions from mainland Greece (Karakitsios et al, 2017), western Anatolia, and the proto-Crete islands due to their proximity and position within the Aegean-Mediterranean domain.…”

Integrated Assessment of Marine-Continental Transitional Facies Shale Gas of the Carboniferous Benxi Formation in the Eastern Ordos Basin

“…The organic matter abundance is an important geochemical index to measure the quality of black organic-rich shale to generate gas [15][16][17][18]. As an important part of the organic-rich shale, organic matter abundance provides a necessary material basis for shale gas resources [19,20].…”

“…Previous studies have shown that in the Benxi Formation in the eastern Ordos Basin, a barrier coastal sedimentary system is mainly developed, and shelf sedimentary system is developed locally [14,23]. The sedimentary system can be subdivided into several microfacies, including sand flat, mixed flat, mud flat, marl flat, barrier sand bars, lagoon mud, and mud continental shelf microfacies [16]. In the Hutian Member at the bottom of the Benxi Formation, iron-aluminum-bearing mudstone and shale is mainly developed, which are the initial deposits of the Late Paleozoic epeiric sea, and superimposed on the underlying Majiagou Formation, which has experienced 150 million years of exposure and denudation after the deposition of Majiagou Formation [23].…”

Quartz Origins and Paleoenvironmental Controls on Organic Matter Accumulation of Marine Shale in the Ordovician Wulalike Formation, Northwestern Ordos Basin, China: Significance for Shale Gas Exploration

“…The OM is the prerequisite for shale gas formation and its sedimentary environment is directly related to hydrocarbon generation. The factors controlling OM enrichment can be summarized into two categories: OM input factors (e.g., paleoproductivity, paleoclimate, and terrigenous detrital input) and preservation condition factors (e.g., redox conditions, paleowater depth, and sedimentation rates) [17][18][19][20]. Three modes of OM enrichment have been proposed: (1) the high productivity model [9]; (2) the anoxic preservation model [21,22]; and (3) superposition of the two models above [23,24].…”