Petrophysical controls on fluid storage and wettability in Cenozoic diatomites from Norway and Greece

Diatomite is a siliceous sedimentary rock characterized by exceptionally high porosity and inherently low permeability, representing an extreme pore system in which fluid storage and transport are strongly governed by mineralogy and pore-scale connectivity. This study compares the petrophysical and mineralogical characteristics of Cenozoic diatomites from a subsurface reservoir on the Norwegian Continental Shelf (Lark Formation) with diatomite outcrop analogues from Crete and Gavdos (Greece). The results show that fluid storage and transport in diatomites are primarily controlled by mineralogical heterogeneity and pore-throat architecture rather than bulk porosity alone. The Norwegian reservoir diatomite contains a higher proportion of clay minerals and exhibits mixed-wet behavior with comparatively lower recovery during imbibition tests. In contrast, the Greek outcrop diatomites are more siliceous and display stronger water-wet behavior and more efficient capillary-driven recovery. However, the presence of carbonate phases in the Greek samples limits their suitability as direct analogues for clay-rich reservoir diatomites on the Norwegian Continental Shelf. All investigated diatomites exhibit high porosity and remain chemically stable under CO2-rich, low salinity water conditions at reservoir-representative pressure and temperature. These findings highlight the importance of mineralogical variability and pore connectivity in controlling wettability and fluid flow in diatomaceous formations. They also provide a framework for evaluating their behavior in hydrocarbon reservoirs and subsurface CO2 storage settings.