The Eastgate borehole was drilled in December 2004 at the site of the former Lafarge cement works at Eastgate village in Weardale, County Durham. It was funded by the DECC Geothermal Challenge (Phase 1), was the first deep geothermal exploration to be drilled in the UK for over 20 years and was drilled to a depth of 995 m. It penetrated 4 m of superficial deposits, 267.5 m of Lower Carboniferous strata (which included the Whin Sill) followed by 723.5 m of Devonian Weardale Granite. The borehole was designed to intercept deep fracture hosted brines associated with hydrothermal vein systems.
The borehole encountered abundant brines at temperatures of around 46°C within natural fracture networks that exhibited very high permeability within the Weardale Granite. These results suggest a mean geothermal gradient of 38°C km-1 which exceeds the UK average geothermal gradient of 21°C km-1 and imply that a borehole sunk to a typical production depth of 1800 m could provide temperatures in the range 75 – 80°C.
The borehole demonstrated that the fracture system within the granite could supply appreciable quantities of water, changing the common perception that granite based geothermal resources are purely hot dry rock prospects that require artifical hydraulic stimulation. The Eastgate well is a research well and as such investigations into its performance will continue.