Quantifying the heterogeneity of deep-water depositional environments for CO 2 storage: an outcrop-based example from the Hecho Group (Spanish Pyrenees)

Characterising heterogeneity at a range of scales for CO 2 storage is critical to balance reservoir quality (net storage and injection feasibility) against internal trapping efficiency. Here, the deep-water Banastón System (Middle Eocene Hecho Group) of the South Pyrenean Foreland Basin is utilised as a subsurface reservoir analogue for application to subsurface settings. An 80 km down-dip transect was analysed to quantify lithological, mineralogical and reservoir heterogeneity from bed to depositional element scale, including sedimentological description, permeability estimates and petrographic analysis. Reservoir geometries include amalgamated and scoured channel-fill and channel-lobe transition deposits, proximal lobes that show compensational stacking and which, down depositional-dip, transition into increasingly tabular and compartmentalised medial lobes and distal lobe fringes. Quartz and extrabasinal lithics dominate proximal turbidites, feldspar and mica are abundant in medial-lobe deposits, whilst distal fringe turbidites and hybrid beds are clay matrix-rich, fine-grained carbonate-rich, micaceous and overlain by carbonate-rich mud, thereby altering the reactive mineral composition from proximal to distal deposits. Proximal to medial lobes feature good reservoir characteristics for net storage of CO 2 , tabular geometries with inherent vertical permeability heterogeneity that would be conducive to lateral plume spread, and mineralogical heterogeneity that could promote CO 2 -reservoir-brine reactions, indicating their potential as ideal storage sites.