The Triassic Mercia Mudstone Group: Sealing the UK's net zero future

The Middle to Upper Triassic Mercia Mudstone Group (MMG) is a heterogeneous succession of fine-grained siliciclastic and evaporite rocks that is widespread across the UK and northwest Europe. The MMG's proven efficacy as a caprock in many Triassic hydrocarbon systems has led to its evaluation for several subsurface storage and disposal applications including as: 1) a host rock for a Geological Disposal Facility (GDF) for radioactive waste, 2) a caprock for carbon capture and storage (CCS) and, 3) both a host and seal for geological hydrogen storage. Utilising published core and outcrop data, this paper provides a detailed review and synthesis of the lithostratigraphy, lithological variability, depositional environments, mineralogy and burial diagenetic evolution of the MMG. We demonstrate that, despite generally favourable sealing properties such as low bulk permeability, the MMG exhibits significant heterogeneities arising from variations in grain size, mineral assemblages, evaporite content, depositional facies and burial history. Clay- and evaporite-rich intervals promote sealing, ductility and fracture self-sealing, whereas localised silt- and sand-rich horizons, pre-existing fracture networks, and brittle cemented intervals may act as fluid conduits. Key gaps remain in mineralogical, petrophysical and geomechanical data, particularly from offshore settings most relevant to future storage and disposal projects. We conclude that the MMG has strong potential to support multiple energy transition technologies but emphasise that safe, long-term containment requires site-specific characterisation, an improved understanding of heterogeneity across multiple scales and integration of geological, geochemical and geomechanical data to robustly predict seal performance under coupled thermal, hydraulic, mechanical and chemical perturbations.