The Triassic Mercia Mudstone Group as a host rock for radioactive waste: insights from a continuous core succession in North Yorkshire

The safe disposal of radioactive waste is an unresolved issue in the UK. As in many other countries, the search for an appropriate Geological Disposal Facility (GDF) includes considering Mesozoic mudstones as potential low-strength sedimentary host rocks. The Middle to Upper Triassic Mercia Mudstone Group and its offshore continuation, the Haisborough Group, combine two lithologies thought to have favourable host rock properties – mudstones and evaporites – and are a focus of current Community Partnership assessments in west Cumbria. However, these units are understudied, due to generally poor outcrop conditions and the dearth of core materials.

CASP had the chance to sample an onshore core from the Cleveland Basin north of Scarborough, North Yorkshire, UK, which offers a unique opportunity to study a continuous, near 300-m-thick succession of Middle to Upper Triassic strata encompassing the entire Mercia Mudstone Group. We are taking a multidisciplinary approach to the cored succession to assess its stratigraphy, composition and geomechanical behaviour. While primarily collected in the context of Geological Carbon Storage (GCS), the same data and properties, including porosity, permeability, clay content, rock strength and stratigraphic variability, are relevant to GDF assessment.

A total of 64 more or less evenly spaced core samples were taken from the entire succession, also accounting for facies heterogeneity. To best describe the composition of these rocks, we combine sedimentary logging and facies analysis, hand-held X-ray fluorescence (XRF) analysis, quantitative X-ray diffraction (QXRD) analysis, optical petrography and scanning electron microscope energy dispersive spectroscopy (SEM-EDS). Porosity-permeability and mercury injection capillary pressure (MICP) analyses are being carried out to assess the suitability of these strata as a seal. In addition, geomechanical analytical results from fresh core are available. These data will be integrated with palynostratigraphy from more than 130 samples and a comprehensive suite of wireline log data, to facilitate robust chronostratigraphic correlation. Select core and cuttings samples from offshore wells in the UK and Dutch sectors of the Southern North Sea are also being analysed using similar methods to address lateral variability. A sister project of CASP extends this dataset into the overlying marine Lower Jurassic Redcar Mudstone Formation of the Cleveland Basin, and may provide further insights relevant to GDF assessment, particularly in Lincolnshire.

Logging and facies analysis identified a conformal but distinct boundary between the Bunter Sandstone Formation and the overlying Mercia Mudstone Group. Given the near-coastal location of the well and the lack of a formal subdivision of the Mercia Mudstone Group in the Cleveland Basin, a preliminary lithostratigraphic framework is being constructed based on the Haisborough Group, focusing on sediment textures and the abundances of halite, anhydrite and dolomite cement for subdivision. Throughout the succession, optical petrographic, SEM-EDS and QXRD analyses identify anhydrite, clay minerals, dolomite, quartz and feldspar as common mineral components at varying proportions. Halite is largely confined to the Röt Halite Member of the Dowsing Formation, where it is pervasive. Elsewhere, compositional variation is mostly driven by the abundance of anhydrite, which occurs either finely dispersed, or concentrated in streaks, veins or nodules, often also healing fractures. QXRD analysis further records widely varying clay contents (10 to 65%), mainly illite + smectite, but also significant proportions of muscovite, chlorite and corrensite. Porosity-permeability and MICP analyses yield encouraging results with regard to validating the suitability of the Mercia Mudstone Group as a seal.