A review of multi-scale sedimentological heterogeneities in the Ormskirk Sandstone Formation of the East Irish Sea Basin and their impact on the injection and migration of CO ₂

Heterogeneities in siliciclastic sedimentary rocks manifest as variations in physical properties, composition and fabric. Depositional environments control primary heterogeneity, with transport mechanisms and sequence stratigraphic context also playing key roles. Secondary (diagenetic) heterogeneity develops post-deposition via compaction, dissolution and cementation. Understanding how these heterogeneities influence CO 2 injection and migration within siliciclastic reservoirs is essential for carbon capture and storage (CCS) planning. The Ormskirk Sandstone Formation (OSF), the target reservoir for the HyNet North West CCS project, contains depositional, diagenetic and structural heterogeneities. This paper presents a comprehensive review of these features, focusing on heterogeneities of depositional and diagenetic origin; structural heterogeneities are acknowledged but lie beyond the scope of this review. Heterogeneities in the OSF occur at multiple scales. At the smallest scale, diagenetic processes such as cementation, compaction and grain coatings alter reservoir quality and contribute to heterogeneity at the pore and facies scale. At the facies scale, depositional fabrics such as laminae and cross-stratification, along with the spatial arrangement of depositional elements, reflect shifting palaeoenvironmental conditions and sedimentological processes. These controls give rise to complex architectures, including porous, highly permeable aeolian sandsheets interbedded with heterolithic and often cemented interdune facies associations, and multilateral, stacked fluvial channel deposits. At the reservoir-zone and reservoir scales, the stacking and lateral interfingering of architectural elements, such as bars, channels, dune sets and overbank deposits, generates stratigraphic heterogeneity that governs CO 2 plume geometry, flow pathways and storage efficiency.