Initial Assessment of the Sealing Capacity of Caprocks in the Ionian Zone Reservoirs, Greece

Summary This study evaluates the sealing capacity of various lithologies of the Ionian zone in Greece, through the analysis of 11 outcrop and 6 well samples using X-ray Diffraction (XRD) and Mercury Injection Capillary Pressure (MICP) techniques. Samples were collected from Epirus and Lefkas (Greece) and wells in Crete. XRD analysis identified mineralogical compositions, particularly focusing on evaporitic lithotypes. MICP testing, conducted with a Poremaster60GT porosimeter, assessed pore size distribution and capillary entry pressures, providing estimates of the maximum hydrocarbon (HC) and CO2 column heights each lithology could retain. Results showed that shale samples exhibited homogeneous microporosity and high sealing efficiency, capable of retaining significant oil and CO2 columns, constrained primarily by geomechanical properties. Mudstones demonstrated slightly larger pore sizes but retained excellent sealing potential for oil (up to 1315 m) and CO2 (up to 269 m). In contrast, evaporitic samples, impacted by weathering and reduced plasticity, exhibited wider macropore distributions and significantly lower sealing capacities (oil: 5–66.61 m; CO2: ∼1–13.26 m). These findings highlight the strong capillary sealing potential of shales and mudstones, while suggesting that evaporitic rock performance in outcrop samples may underestimate true reservoir conditions due to surface degradation and MICP limitations in microporosity detection.