In a perspective of geothermal exploration in graben systems, we analyse the recently built consistent structural model of Soultz area (Dezayes et al., this issue) in order to understand the links between the regional graben fault system, the tectonic history of the Upper Rhine Graben (URG) and the local reservoir features (a low permeable naturally fractured granite).

The previous geological models of the area lack constraints concerning the geometry of the structures at the regional scale. To overcome this situation, the new 3D regional geological model of the Soultz area is based on a set of 2D seismic profiles (including recently reprocessed seismic lines) as well as former structural interpretations for oil exploration in the 70-80’s and numerous deep wells (Dezayes et al., this issue).

A network of 26 faults could be constructed in a 30x20x6 km model of the sedimentary cover and the top basement representing the faults as surfaces. The orientation of the major and generally synthetic faults (i.e. dipping eastward) is N22.5±7.5°E, whereas a second set strikes N0±10°E (mainly dipping westward), and a third one N45±10°E. The average dip of the faults is 60°.The fault density is approximately 3.7.10-4 m2 of faults per cubic meter.

The resulting 3D geological model confirms the tectonic situation of the Soultz site as a horst structure and indicates a maximum vertical offset of approximately 500m of the basement in the area of the Soultz geothermal anomaly.

A comparison between the smaller scale 3D fractured reservoir model based on borehole imaging, microseismicity and vertical seismic profiling (Sausse et al., 2010) and our 3D regional structural model suggests a strong correlation between the orientation of the partly stimulated fracture zones observed at reservoir scale and the secondary set of faults observed in our regional model.

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