The Geology of Basle in View of Deep Heat Mining

Abstract

There is a strong possibility that the frst Enhanced Geothermal System (EGS) pilot-plant in Switzerland will be situated in Basle. This may be the case on account of a number of practical, economic, political and geothermal reasons. This paper concentrates on the geological aspects which led to the conclusion, that Bade may be one of the most suited regions for a EGS pilot-plant in Switzerland. The City of Bade lies in the southeastern corner of the Rhinegraben, a failed rift system of Oligocene age. The discussed area lies close to the triple point of the trough, the tabular Jura mountains to the east and the folded Jura mountains to the south. The Cenozoic graben fill sequence is well documented as well as the underlying Mesozoic sedimentary sequence. A major unknown however is the thickness of Permocarboniferous sediments, which are known to occur in narrow troughs in Northern Switzerland. The top of the cristalline basement is expected at a depth of 2 to 2.5 km. A temperature gradient of up to 40"C/km in the sedimentary sequence is believed to be the expression of the insulation effect of the sediments. However a thinned crust as result of the rifting may contribute to an increased temperature gradient in the basement as well. Tectonic activity is monitored by a dense network of modem digital seismic stations. They document one of the highest seismically active areas of the Rhinegraben system. These data offer good indications of the state of stress of the deeper subsurface in the area.