Chalk is a highly porous carbonate rock that hosts major hydrocarbon reservoirs worldwide and may also serve as a geological reservoir for unwanted pollutants such as CO₂. We evaluate a ~200-m-thick Eocene chalk unit found ~2 km beneath the southern Israeli continental shelf as a potential storage reservoir, using data from three boreholes (Ashkelon-1, Yam-2, and Yam-West-1), compared with available Eocene outcrops in Israel. The aim of this study is to determine how diagenetic processes affected porosity and permeability in exposed versus buried marine chalk.

Cathodoluminescence shows that most primary sedimentary components (e.g., Nummulites, other fossils and micrite) retain non-luminescent signals, whereas cements display zoned luminescence reflecting multiple diagenetic phases. U–Pb ages indicate that primary components are Eocene in age (55-33.9 Ma), with some Oligocene overprints, whereas most cements formed during the Late Eocene (42-33.9 Ma), Oligocene, and Miocene.

Stable isotopes reveal that marine chalks (δ¹⁸O = 1.5 to −1.9‰; δ¹³C = −3.3 to 1.3‰, VPDB) are markedly different from exposed Eocene rocks and Oligocene–Miocene vein calcites, which show more depleted values (δ¹⁸O = -7.12‰ to -12.45‰; δ¹³C = -1.17‰ to -10.84‰, VPDB). The studied outcrops plot along mixing trends between these end members, indicating substantial diagenetic overprinting. Porosity and permeability data in the Eocene outcrops confirms that cementation was the main mechanism reducing reservoir quality. The carbonate fossil-rich mud rocks affected by submarine landslides/turbidities, were turned by diagenesis into cemented limestones, while fine-grained chalks not affected by turbidities were least altered.

Late Eocene cementation likely reflects submarine mass-movement events, whereas Oligocene–Miocene cementation was driven by meteoric water infiltration following marine regression. The buried offshore chalks largely preserve their primary porosity and are overlain by ~1800 m low-permeability marls/shales caprock, indicating that the Eocene carbonate sequence beneath the shelf is a promising target reservoir for CO₂ storage.