Triassic geothermal clastic reservoirs in the Upper Rhine Graben

1. Tertiary Jurassic Paleozoic crystalline basement Buntsandstein Thickness of Buntsandstein Temperature at the top of Buntsandstein Top of Buntsandstein Exploitable heat in the Buntsandstein Heat in place in the Buntsandstein Triassic geothermal clastic reservoirs in the Upper Rhine Graben Temperature at 1500m Temperature at 5km The goal of this study is to make a first assessment of the Buntsandstein reservoirs (lower Trias) at the Rhine Graben regional scale in order to delimit the most favourable areas for exploration and exploitation of the geothermal resource. >In France, the main exploited low enthalpy geothermal reservoirs lie within the Jurassic fractured limestone formation of Dogger located in the central part of Paris Basin, where 34 doublets have been heating 600 000 people for 30 years. The development of renewable energy combined with the energy demand implies to search other geothermal reservoirs either below the Paris area or either in other promising areas in the country. >In this framework, Ademe and BRGM co-funded the CLASTIQ project (CLAyed SandSTones In Question), in order to study the deep geothermal potential of the Rhine Graben for heat production, or even electricity if temperature conditions are high enough. Dogger Trias Structural map of the Upper Rhine Graben and temperature distribution extrapolated at 1500m depth (GGA Hannover database in Genter at al., 2004). Cross-section though the Paris basin Promising geothermal area in France. Green: area with geothermal installations, orange: promising areas; red: high potential geothermal area. >The Rhine Graben is a Cenozoic graben belonging to the west European rift system (Ziegler, 1990). It is well-known because of numerous studies for petroleum and mining exploration (boreholes, geophysical surveys…). Several major subsidence phases related to the Rhine graben tectonics generated variable sediment thicknesses. >The geothermal resource is located in argillaceous and clastic formation of the lower Triassic unit, namely the Buntsandstein formation. >This study is based on old maps from Geothermal Synthesis of the Upper Rhine Graben (Munck et al., 1979) published by the Commission of European Communities. We have digitalized and georeferenced the characteristic maps of Buntsandstein formation (top depth of the layer, bottom depth of the layer and temperature at the top). The digitalized data have been interpolated by kriging in taking into account the presence of faults. Soultz Rhine river Baden-Baden Core of Buntsandstein sandstones. Depth of the top of the Buntsandstein from Munck et al., 1979. Schematic geological map. Cross-section through the Rhine Graben. • >We obtain 500m resollution grid map of: • The depth of the top of the Buntsandstein; • The thickness of the Buntsandstein; • The temperature at the top of the Buntsandstein. • >To quantify the geothermal resource, we calculate the quantity of heat in the reservoir volume. • >However, only a part of this resource could be extracted. The recovery factor R quantifies the exploitable reserve. Heat in place Q = r . Cv . V . (Ti - Tf) (Muffler & Cataldi, 1978) Heat exploitable Qexpl = Q . R Recovery factor R = RT . RG Temperature factor RT = (Ti – Tinj) / (Ti – Tf) Geometric factor RG = 0.33 for doublet in aquifer (Lavigne, 1978) • Results • It appears that the north part of the Upper Rhine Graben constitutes a more favorable area than the south part. In the north part, the top of the Buntsandstein reservoir is located around 2000-3000m depth with a temperature of about 150°C and its sandstone thickness is around 500-600m. The exploitable geothermal potential is between 15 and 30 GJ/m2 (7 GJ/m2 for the exploited Dogger in the Paris Basin). • This study confirms that the Buntsandstein formation in the Upper Rhine Graben has a good geothermal potential for heat or even electricity production. • Some promising areas have been identified and some new geophysical acquisition such as 2D or 3D seismic profiles should be very helpful in providing relevant information about the reservoir structure. r : rock density (2200 kg/m3) Cv : heat capacity (710 J/Kg.K) V : reservoir volume Ti : initial temperature of rock Tf : final temperature of rock (10°C) Tinj : injection temperature (25°C) Authors References: Genter A., Guillou-Frottier L., Breton J.P., Denis L., Dezayes Ch., Egal E., Feybesse J.L., Goyeneche O., Nicol N., Quesnel F., Quinquis J.P., Roig J.Y., Schwartz S. (2004) - Typologie des systèmes géothermiques HDR/HFR en Europe. Rapport final. BRGM/RP-53452-FR, 165 p., 75 fig., 10 tabl. Lavigne J. (1978) – Les resources géothermaiques françaises. Possibilités de mise en valeur. Ann. Des Mines, April, p.1-16. Muffler P. & Cataldi R. (1978) – Methods for regional assessment of geothermal resources. Geothermics, 7, p.53-89. Munck F., Walgenwitz F., Maget P., Sauer K, Tietze R. (1979) – Synthèse géothermique du Fossé rhénan Supérieur. Commission of the European Communities. BRGM Service Géologique Régional d’Alsace – Geologisches Landesamt Baden-Württemberg. Ziegler P. (1992) – European Cenozoic rift system. Tectonophysics, 2008, p.91-111. Dezayes Chrystel1, Genter Albert2, Tourlière Bruno3 1BRGM Dpt Geothermal Energy, 2EEIG Heat Mining, 3BRGM GEO ENGINE Final Conference – Vilnius, Lithuania – 12-15 February 2008