Abstract
New and old magnetic and gravimetric data acquired in the frame of a geothermal project have been processed in N. Vosges and the adjacent Rhinegraben in order to identify the nature and tectonic framework of the graben basement and to select areas with a geothermal potential. In maps, the ENE–WSW striking units of N. Vosges can be followed beneath the sedimentary cover as far as the N35°-striking Upper Rhenish Shear Zone, which shifts these units towards NE. The magnetic basement S and SW of Strasbourg is interpreted as akin to the southern Champ du Feu Massif, consisting of I-type granitoids. Combined with new and old reflection seismic sections, the data show that the petrologic and tectonic contexts in the Illkirch-Grafenstaden area, where a geothermal project is planned, which is similar to the deep geothermal wells of Soultz and Rittershoffen 70 km to the north. The target zone is a N–S fault, partly normal and partly transpressive, which offsets the magnetic, I-type granitoids with high contents of uranium and thorium by about 800 m. Further exploitation of oil along the N–S footwall block attests to the geothermal potential of the area. The NW–SE transpressional regime favours the formation of cracks and increases the porosity and consequently fluid convection. The close borehole ESC-7 and modelling of the temperature distribution lead to expectations of a temperature of at least 150 °C on top of the granite and show that the temperature anomaly results from the heat-productive granitoids and from the blanket effect of the low thermal conductivity tertiary sediments.
Highlights
Since the eighteenth century, the Rhinegraben has been known for its high-temperature anomalies, in particular in northern Alsace, where the shallow oil reservoirs of Merkwiller-Pechelbronn, west of Soultz-sous-Forêts, were dug, drilled and exploited in the 1960s (Fig. 1)
Geothermal and geophysical data for the northern Vosges and central Upper Rhinegraben Previous geophysical investigations have shown that the magnetic anomalies, which strike NNE–WSW to NE–SW in the northern Vosges, continue in the same direction across the Rhinegraben towards the area of geothermal interest
In geophysical maps, the ENE–WSW striking units of the northern Vosges can be followed beneath the Mesozoic to Quaternary cover as far as the N35°-striking Upper Rhenish Shear Zone, which shifts these units towards the NE in the Baden-Baden area
Summary
The Rhinegraben has been known for its high-temperature anomalies, in particular in northern Alsace, where the shallow oil reservoirs of Merkwiller-Pechelbronn, west of Soultz-sous-Forêts, were dug, drilled and exploited in the 1960s (Fig. 1). There, the temperature gradient reaches 10 °C/100 m, mainly within the first 1000-m depth (Genter et al 2010). Edel et al Geotherm Energy (2018) 6:13 ODENWALD URSZ Wasselonne AB.
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