Aeolian sedimentary heterogeneity in the Triassic Sherwood Sandstone Group, UK: implications for CO ₂ storage

Understanding lithologic and stratigraphic heterogeneity is useful for planning carbon capture and storage (CCS) projects. In the UK East Irish Sea Basin, depleted hydrocarbon reservoirs of the Triassic Sherwood Sandstone Group (SSG) are targets for large-scale geological storage of CO ₂ . In the adjoining Cheshire basin, large saline aquifers of the SSG also have potential to act as CCS reservoirs. This study characterises the sedimentary architecture and facies organization of the part of the SSG represented by deposits of aeolian origin in the Cheshire Basin in a quantitative manner, to predict heterogeneity at multiple scales, and to enable detailed geological modelling of this succession. Outcrop datasets acquired from the Cheshire Basin comprise twenty sedimentary logs (total length ∼116 m) and one hundred and fifty-two palaeocurrent readings. Additional subsurface data include core and outcrop descriptions from well penetrations of the SSG in the Cheshire Basin. Eleven aeolian facies types and five facies associations are identified in the Helsby Sandstone Formation of the SSG in the Cheshire Basin. This succession records the evolution of sinuous-crested aeolian dunes whose migration was oblique to the dominant palaeowind. Dune-scale bedforms were separated by a range of dry, damp and flooded interdunes. Sedimentological data from outcrops and the subsurface were incorporated into the Database of Aeolian Sedimentary Architecture (DASA) – a sedimentological database – and synthesised to provide inputs with which to constrain three-dimensional forward stratigraphic models generated by the Dune Architecture and Sediment Heterogeneity (DASH) model. The model outputs demonstrate how decimetre- to metre-scale mudstone units representative of damp to wet aeolian interdunes form low-permeability baffles between relatively more porous and permeable sandstone units of aeolian dune origin. The lateral extent and continuity of interdune deposits acting as low-permeability baffles varies both horizontally and vertically in the succession, from isolated lenses to laterally continuous sheet-like bodies, ranging from 0.1-3.5 m in vertical thickness and 10s-100s m in lateral extent. These variations arose in response to climatic changes during the evolution and accumulation of the aeolian system. Model results demonstrate a three-dimensional complexity in preserved aeolian sedimentary architecture that is not apparent in two-dimensional outcrop exposures or one-dimensional well data but are below resolution typically afforded by seismic data. The quantitative modelling results provide insights into relationships between the morpho-dynamic evolution of aeolian bedforms, preserved sedimentary architecture and petrophysical heterogeneity, the predictions of which are useful for informing the development of CCS projects.