Structural inheritance and hydrothermal alteration impact on fluid circulation in a clay-rich shear zone

Abstract

Deep geothermal power plants in the Upper Rhine Graben (URG) harness natural hot brines circulating within the fracture network of basement rocks. The nature and geometry of the fault network must be documented in detail to reduce the risk of targeting low-permeability structures during drilling. Fault zones and associated fracture networks exhibit variable hydraulic properties depending on the nature of their deformation, hydrothermal alteration, and mineral infills. The Schauinsland mine, located on the eastern shoulder of the URG, is considered an analogue for exploited geothermal basement reservoirs. It provides a 3D exposure of a clay-rich shear zone and a perpendicular ore vein, analogous to present-day brine circulation pathway encountering a heterogeneously permeable structure. Petrophysical, petrographical, mineralogical and geochemical investigations of this shear zone offer the opportunity to reconstruct its formation and the associated fluid pathways. A statistical analysis of the dataset was carried out to highlight correlations between deformation and hydrothermal alteration processes. Through repeated seismic cycles, the core zone shifted from a conduit to a barrier for fluid circulation, due to the precipitation of secondary minerals within the remaining open spaces, resulting in a multi-core structure. Observations show that the damage zones within the transition zone of the shear zone likely constitute the optimal zone for present-day fluid circulation.