Down-Fan Grain-Scale Characterisation of Deep-Marine Reservoir Facies for CO2 Storage

When selecting reservoirs for CO2 storage, grain-scale heterogeneity is a key criterion. Lithological heterogeneity dictates the flow of a CO2 plume, where flow barriers allow for physical and residual trapping and may enlarge the CO2-reservoir-brine reaction front for mineral trapping reactions. For such reactions, mineralogical heterogeneity due to hydraulic fractionation creates a variable distribution of reactants, particularly clay minerals. Deep-marine reservoirs are voluminous and of excellent quality but also feature abundant heterogeneity laterally and vertically, due to a spatial variation of facies deposition, governed by flow processes, and stacking patterns of depositional elements (channels/lobes). Lithological analysis and representative facies sample collection was conducted in the Early to Middle Eocene Banastón System of the Aínsa and Jaca deep-water foreland Basins, in the Spanish Pyrenees. As an extensively well-characterised system, key deep-water sub-environments could be identified and can serve as subsurface reservoir analogues. Channel-lobe transition, and proximal, medial, distal and basin floor lobe sub-environments were investigated. Down-fan, sandstones transition from dominantly arenaceous high-density turbidites with scour-geometries to finer-grained, tabular high-density and interbedded, increasingly dilute, low-density turbidites. Mud preservation increases, as does calcilutite and hybrid-event bed deposition. Reactive mineralogy is therefore more likely concentrated in the more distal environment reservoirs, at the expense of reservoir quality. We use petrographic analysis to quantify and compare the mineralogical heterogeneity of facies within these environments, producing a relative reactive mineral index which can be compared to traditional reservoir quality to determine CO2 suitability of deep-marine reservoir facies and environments for effective and safe long-term storage.