Multi-scale fault and fracture networks of the UK’s Mississippian carbonate platforms: implications for extracting geothermal energy

Unlocking the low enthalpy geothermal potential of the UK through heat extraction from Mississippian carbonate platforms (MCP) could support the transition to net-zero carbon emissions by providing a sustainable heat source. However, MCPs remain underexplored compared to analogous formations in Belgium and the Netherlands, largely due to limited understanding of fracture-controlled fluid flow in these low-porosity carbonate systems. This study evaluates the structural and hydraulic characteristics of MCPs through a multi-scale fault and fracture connectivity analysis across regional, local, and outcrop scales in North Wales, adjacent to the Menai Straits Fault System. Using regional fault map and seismic reflection data, drone photogrammetry, outcrop description, and bed scale fracture mapping, we assess fracture geometry, intensity, orientation, and connectivity. Fracture stability was evaluated through slip tendency (Ts) and dilation tendency (Td) analyses under varying stress scenarios, and permeability tensors were derived to characterize flow orientation. Results demonstrate a strong scale-dependency in fracture connectivity. Connectivity coefficients (CL) increase from 0.64 at regional scale to 3.03 at bed scale, while CB (Connection per branch) values rise from 0.71 to 1.58, indicating that smaller-scale datasets capture more complex and transmissive networks. Reactivation analysis shows that fractures subparallel to present-day stress, SHmax (170°) exhibit high dilation tendencies (Td > 0.8), favouring open fluid pathways. Cross-cutting relationships provide field evidence for stress rotation, with implications for multi-phase fracture development and enhanced connectivity near the Menai Straits Fault System. These findings suggest that stress field evolution and structural inheritance significantly influence fracture permeability architecture. The integration of outcrop and subsurface data improves characterization of sub-seismic fracture networks, offering valuable guidance for geothermal drilling strategies and reservoir development in underutilized carbonate platforms.