Investigating the impact of heterogeneity on mudrock seals to CO₂ storage reservoirs, via the multiproxy characterisation of the well-exposed Lower Jurassic Redcar Mudstone Formation, Cleveland Basin, UK

Geological carbon storage depends on the assurance of long-term containment for injected carbon dioxide within the storage complex at depth. Critical to this is the assessment of seal potential in cap rock units, as well as their potential for secondary storage. However, mudrock seals are frequently evaluated as homogeneous media of variable thickness with assessments commonly based on sub-cm³ analysis of cuttings supplemented by modelling. The possible impact of heterogeneity in mudrocks on sealing capacity and the reactivity/adsorption of free phase CO₂ at and along reservoir-to-seal margins, therefore, requires improved constraint.

This research targets the Early Jurassic Redcar Mudstone Formation (>180 m thick) and overlying/underlying contacts, best exposed at Robin Hood’s Bay. These shallow marine mudstones are analogous units to seal complexes at incipient GCS sites in the North Sea. The study interval is well characterised by detailed biostratigraphic-lithostratigraphic frameworks, by legacy data from conventional/unconventional O&G exploration and by emerging data from burgeoning potash extraction.

Our initial study phase is focussed on characterising the vertical variability in mudrock heterogeneity over the 170 m thick study interval via sampling (~2 m spacing) and gamma spectrometry. Further, targeted high-resolution (1:25 scale) graphic logs cover 60% of the study interval, with parallel sampling and handheld XRF (~30 cm spacing). Similar targeted analysis examined fracture networks along relatively undeformed and deformed mudrock transects to constrain the impact of faulting and compositional heterogeneities on mechanical failure. SEM analysis of outcrop samples documents compositional properties. These efforts alongside upcoming analytical work will inform a mudrock microfacies/facies schema supported by high resolution by compositional data.

This study will enable us to constrain mudrock heterogeneity at outcrop scale and explore the relationships between composition, facies, deformation, and seal capacity/potenital. The wider ambition is to map such insights at progressively widening scales of investigation in the Cleveland Basin.