Geochemical and geophysical evidence on the geothermal potential of Caledonian granites in Britain

Abstract

TECHNIQUES for the extraction of geothermal energy from hot dry granite are being developed with considerable success, principally at Los Alamos, New Mexico^1–3. The method involves drilling to depths where the mean temperature exceeds about 160 °C and then using hydraulic fracturing techniques to create an artificial fracture system through which water is circulated and recovered for energy extraction by heat exchangers. At Los Alamos, the required bottom hole temperatures are reached within 3 km of the surface (>50 °C km⁻¹). Although gradients in Britain do not reach this value, a recent economic assessment (J. Garnish, personal communication, based on data in refs 2 and 4) of UK hot rock potential indicates that the Los Alamos technique may become cost effective where the required temperatures occur at <5 km depth; an average minimum thermal gradient of 30–35 °C km⁻¹. The Cornubian granite batholith of Hercynian age and, to a lesser extent the Tertiary granites of northwestern Scotland, have been considered to have potential as hot rock geothermal sources^4,5, but less emphasis has been placed on the Caledonian granites of Britain. Recent studies of heat flow in Britain^6,7, together with an increasing volume of interpreted geochemical and geophysical data, suggest that certain Caledonian granites of Lower Devonian age may have potential as hot rock sources comparable with that of the Cornubian batholith. Few direct measurements of temperature in Caledonian terrain exist although northern England is known to have high heat flow^6,7, and here we review the available data which have a bearing on the geothermal potential of these granites.