Quantitative characterisation of the Triassic Sherwood Sandstone Group, suitability for long-term carbon capture and underground storage, Cheshire, United Kingdom

The Sherwood Sandstone Group (SSG) is a Triassic succession with prospects for carbon capture and storage (CCS). It is currently one of the main candidates in the UK for permanent, large-scale subsurface storage of CO2. Quantitative sedimentological analysis allows the lithological heterogeneity of the succession to be characterised. A sedimentological dataset acquired from exposures in the Cheshire Basin comprises fifty 1D sedimentary logs (total length ~650 m), five hundred palaeocurrent readings, and twenty architectural panels and photogrammetry panels, all acquired from a series of type-section outcrops. Additional subsurface datasets include core and well-log descriptions from well penetrations of the SSG in the East Irish Sea Basin (EISB) and analogous Triassic redbeds exposed in SE Spain. The datasets were examined using the Fluvial Architectural Knowledge Transfer Store (FAKTS) and the Database of Aeolian Sedimentary Architecture (DASA), two relational databases holding information on sedimentary architecture. Lithofacies types and associations thereof, architectural-elements and larger-scale depositional- element types are identified in fluvial deposits of the Chester Formation and mixed fluvial and aeolian deposits of the Helsby Sandstone Formation of the SSG in the Cheshire Basin. Fluvial elements include pebbly bedload-dominated channel belts, ribbon-like isolated channel fills and non-confined fines of overbank origin. Aeolian successions record the evolution of dune fields (ergs) with a range of dry, damp and flooded interdunes. Lithological heterogeneities occur at five scales: lamina-scale textural variability, internal bed variability arising from combinations of sedimentary structures, alternating facies units, architectural elements with associated bounding surfaces, and larger-scale depositional elements. The nature of these multi-scale heterogeneities is evaluated to predict resultant 3D lithological architecture and porosity-permeability
distributions, both critical for CCS reservoir appraisal. Data are used to constrain a three-
dimensional stratigraphic forward model, the Dune Architecture and Sediment Heterogeneity (DASH) model. Model outputs quantify 3D sedimentary heterogeneity at multiple scales: (i) facies units (e.g., trough cross bedded sandstone units), (ii) architectural elements (e.g. aeolian dune and interdune elements, fluvial floodplain and channel-fill elements), and (iii) depositional elements (e.g. aeolian dune-field successions, fluvial braid-belt successions). The migration and evolution of sedimentary systems over both time and space fundamentally controlled the resultant compound sedimentary architecture and thereby the 3D porosity-permeability distribution of the studied successions. The quantitative sedimentological models generated as an outcome of this study are important in the planning and implementation of CCS projects in the EISB.