Microstructure of Mineralised Fracture Seals in Evaporitic Mudrocks

Summary The Mercia Mudrock Group is a sequence of siltstones, claystones, and evporites that is an extensive regional caprock and a prospective host rock for radioactive waste disposal in the UK. However, numerous natural fractures pose a leakage risk to infrastructure projects relying on MMG’s low permeability. Precipitation of evaporitic minerals into the fracture space lowers the permeability, restoring the seal. However, this specific form of sealing may create mechanical contrast between the fracture fill and the host mudrock, which can be exploited by stress perturbations, brought on by reservoir pressurisation, tectonic forces, and tunnelling. This study uses optical microscopy, SEM, and EDX, to analyse the interface between fracture fill and host rock at a small scale, as to assess possible reactivation. The main finding is that the interface is not always a discrete surface. Coarse grained mudrocks may share a cementing mineral with the fracture, which should limit the mechanical contrast, in contrast to clay rich mudrocks.