On the fringe of reservoir potential: distal lobe architecture and CO2 storage—learnings from the Skoorsteenberg Formation, South Africa

CO2 transport and fate in the subsurface is strongly influenced by relative permeability, which governs interactions between injected CO2 and formation water, as well as trapping efficiency and seal effectiveness. Permeability variability and heterogeneity have a strong influence on CCUS performance, making lateral facies changes and stratigraphic architecture critical controls on fluid flow. Unit 5 of the Skoorsteenberg Formation, South Africa, provides an exceptional outcrop analogue for assessing heterogeneity across distal lobe fringes. The lower stratigraphy comprises 10–40 m of thin-bedded siltstones deposited by low-density turbidity currents, overlain by medium-bedded, very fine-grained sandstone packages. Proximal and lateral distal fringe packages contain mud-rich current ripples and low-amplitude bed-waves, whereas the frontal fringe is dominated by hybrid beds with abundant mudclasts and grooves. These are overlain by unusually thick (≤8 m), laterally discontinuous massive sandstones with elevated mud content, forming finger-like bodies that transition laterally into thin-bedded turbidites, creating a web-like network of interconnected elements. Lateral and downdip transitions from mixed grain-size bedform-dominated deposits to argillaceous hybrid beds indicate longitudinal flow transformations from turbulent to increasingly laminar conditions across the distal fringe. The enigmatic massive sandstones, which thicken dramatically downdip, are interpreted as a type of transitional flow deposit, comparable to those observed in fine-grained systems such as the Mississippi Fan. The distal lobe fringe documented here, with heterogeneous elements and reactive mineralogy, can act as geochemical hotspots. Their pinch-out into sealing lithologies makes them potential stratigraphic traps and preferential CO2 storage targets, which ongoing work is evaluating through characterization of mineralogy and reservoir quality.