The Geological Survey of Belgium (GSB) is involved in geothermal resources assessment at European scale with GeoElec and Thermomap (very shallow) projects and at regional scale with the geothermal plate-form of Wallonia. The GSB has recently completed a first evaluation of geothermal potential of the Walloon region for medium to high enthalpy (300-6000 m). In 2008, the U.S Geological Survey (USGS) has conducted an updated assessment of geothermal resources in the United States. The volume method was the primary scheme applied to identified geothermal systems in which the recoverable heat is estimated from the thermal energy available in a reservoir. In some European countries, the temperature data available generally allow to produce a heat flow map at great depth. The subsurface temperature measurements of Belgium were first compiled by Legrand in 1975 and updated by Vandenberghe & Fock in 1989. The temperature values from the coal and hydrocarbon exploration wells are significantly spread over the reservoirs. The geothermal gradients are strongly influenced by groundwater circulation. The fold and thrust belt context of the subsoil in Wallonia makes geothermal gradient interpretation, reservoir temperatures and reservoir volume difficult to assess. The first geothermal reservoirs identified at 1 kilometer depth were mapped by Berckmans & Vandenberghe (1998). The northern Campine and Anvers regions, the Hainaut basin, and the corridor between Liège and Visé were considered as potential areas. The waloon geothermal plate-form project consisted mainly in preparing and collecting deep geological structure and geothermal resource of the underground data. Geophysical, geological, temperatures and hydrogeological data required some up to date re-interpretation to match the current model knownledge of the deep geological underground of Wallonia. More details were given by a focused study on Liege area with a 3D model realized by Liege University and a chemical geothermometer analyse conducted by GSB. Two maps of geothermal energy interests were produced: one for low to medium depth (300-3000 m), and another one for great depth (3000-6000 m). They mainly represent cartography of the Devono-Carboniferous limestones and Lower Devonian quartzites for two geothermal horizons. Simplified versions of the two maps destinated to the public and policy makers were constructed according to the USGS geothermal resource and reserve terminology, illustrated in the Mc Kelvey diagram (1980). Berckmans A., Vandenberghe N., 1998. Use and potential of geothermal energy in Belgium. Geothermics 27: 235 - 242. Legrand, R. (1975). Jalons Géothermiques. Mémoire Explicatif Cartes Géologique, Mines Belgique, 16 :46 pp. Mc Kelvey (1980). US Geologcal Survey. Principles of a Resource/Reserve classification for Minerals, Circular 831. Vandenberghe N., Fock W. Temperature data in the subsurface of Belgium, 1989. Temperature data in the susbsurface of Belgium. Tectonophysics 164, 237-250.

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