Multiscale-multiproxy seal assessments of Mesozoic mudrock units in North Yorkshire, a potential aid to screening and modelling radioactive waste disposal facilities

Lower strength sedimentary rocks, namely low-permeability, fine grained sedimentary successions or mudrocks, have been identified as prime lithologies for geological disposal facilities (GDFs) in the UK (Turner et al., 2023). Triassic to Early Jurassic mudrock and mudrock-halite units comprise approximately 10% of the UK’s onshore outcrop and subcrop by area (~26,000 km²). A considerable volume of these rocks are sited within the 200–1000 m subsurface realm appropriate for GDFs.

CASP’s research programme in the Cleveland Basin, North Yorkshire has sampled and logged more than 600 m of continuous stratigraphy. The studies have targeted both outcrop and shallow borehole core to examine the impacts of heterogeneity in mudrock units on seal potential for geological carbon sequestration (GCS). The studied successions include the lower Lias Group in outcrop, primarily at Robin Hood’s Bay, as well as the entire Mercia Mudstone and lower Lias groups in a nearby onshore core. Here, the results of the initial phase of this investigation, addressing ~180 m of the lower Lias Group in outcrop, are presented.

The aim of the outcrop investigation is to link a comprehensive catalogue of primary (compositional and sedimentological) and secondary (deformation-related) heterogeneities to variations in seal integrity and potential. To capture compositional variability, we combined sedimentary logging (1:200; 1:25) and facies analysis, hand-held gamma/X-ray fluorescence (XRF) analysis, quantitative X-ray diffraction (QXRD) analysis, optical petrography (microfacies analysis) and scanning electron microscope energy dispersive spectroscopy (SEM-EDS). Mechanical stratigraphy is examined via field measurements and lab analyses. Targeted porosity-permeability and mercury injection capillary pressure (MICP) analyses underpin assessments of seal potential.

Following compositional analyses, the sediments are identified as dominantly siliceous shales, and classified into groupings/subgroupings based on mineral content. Broadly speaking, compositional heterogeneity is secondary to textural heterogeneity; the latter is strongly modulated by the shallow marine depositional mode and setting. The initial poro-perm assessments largely track this primary trend, modified by pervasive bioturbation. As suggested by previous studies (e.g. Hobbs et al., 2012), the compositional and clay-type signatures contrast with those of the Lias Group of southern England, but provide a closer analogue to deeper units prospective for GDF development in Lincolnshire. Mechanical stratigraphy confirms increased fracture density towards fault damage zones and suggests that only a small (mineralised) subset of surficial fracture networks are likely to be an issue at GCS depths (>1800 m); however, their relationship with shallower GDF target depths is less clear. Initial bed-by-bed correlation to local subsurface data ~6 km from the outcrop investigations reveals a largely layer-cake stratigraphy and close alignment in gamma/XRF signatures. The results are encouraging with respect to the seal potential of the succession.

The results of this study, as well as the subsurface datasets produced by the wider research programme, can be employed to inform desk-based GDF site screening. Where appropriate, our data may also feed into multiscale site geomodels evaluating geomechanical or reactions-based behaviour, which will be key to demonstrating site safety. Additionally, this dataset can help to understand data gaps associated with borehole-only appraisals.