GRANITE AS A GEOTHERMAL RESOURCE IN THE NORTHEAST

In the absence of volcano-derived hydrothermal activity and high heat flow, granitic plutons provide an alternative geothermal resource from which heat may be usefully extracted. Compared with other crustal rocks, granites contain higher concentrations of the heat producing elements (K, U, Th). Additionally, they are more homogeneous and have simpler fracture systems than surrounding country rock, allowing for stimulation through hydro-fracking of large (>1 km3) geothermal reservoirs. However, not all granites are created equal! Those with heat production > 5 μW/m³, or with deep batholithic roots, are the most promising. Estimated temperatures at a given depth are related to the heat production, thickness and thermal conductivity of the granite. For example, the Carnmenellis pluton in Cornwall, England (which will be drilled in 2012) is estimated to have temperatures in excess of 170 ⁰C at a depth of 5 km, which is sufficient for co-production of electricity and hot water for heating. More importantly, granite bodies that are buried beneath thick sequences of thermally insulating sediments are also potential geothermal targets. Most successful examples to date include the Soultz sur Forêt project in France, with temperatures of 200 ⁰C at a depth of 5 km. (and which is currently producing electricity), Innamincka, Australia, with temperatures of 250 ⁰C at a depth of 4 km. (which will be producing in 2012), and the seismically ill-fated project in Basel, Switzerland. Surely, if such projects involving the geothermal potential of granites, can succeed elsewhere, they can succeed here in the granite-rich Northeast? The geothermal potential of the Conway Granite, NH has long been recognized. Other possibilities include the Fitchburg pluton, MA, and granites buried beneath the Carboniferous sediments of the Narragansett Basin and the Triassic sediments of the Connecticut River Valley.