Karstified and fractured Lower Carboniferous (Mississippian) limestones of the UK ‒ A cryptic geothermal reservoir

Karstified and fractured Lower Carboniferous (Mississippian) limestones of the UK have been evaluated for their geothermal potential. This wholly undeveloped resource is calculated to be in excess of 26 million GWh (95 EJ) for the P90 case and as much as 35 million GWh (129 EJ) for the P50 case for six regions in England and Wales, ranging in scale from Kent at 0.04 million GWh (P50) to Northern England at 15 million GWh (P50). The evaluation used three sources of evidence to assess the distribution, current burial depth, likely temperatures and transmissivity of these limestones. Seismic and outcrop data, well logs and core from drilled wells and water chemistry data from thermal springs were used to demonstrate the presence of deep, water bearing, permeable limestones. Geothermometry calculations using spring-water composition data provided clues about temperatures at which the issuing water equilibrated with its original host rock indicating that water now reaching the surface had acquired its solutes at temperatures significantly in excess of 30 °C and depths much greater than 1km. The UK uses about 2.5 EJ per annum yielding a resource to use ratio of 40-50 years assuming no natural inflow or reinjection of cooled water for reheating.