Investigation of Natural Permeability in Graben Systems: Soultz EGS Site (France)

Abstract

Two different approaches have been employed to investigate the localization of fault zones and natural permeability at regional and local levels in extensional tectonic settings with potential for EGS technology. Regional distribution of potential permeable fault zones has been analyzed dynamically using a new 2D geodynamic model. Numerical simulation of instability of a viscoelastic-plastic lithosphere require a lower crustal viscosity in the order of 5•10^21 to 10^22 Pa s to match the deformation pattern of the Upper Rhine Graben (URG). Models clearly show a localization of deformation at regional level, but do not explain the internal fault distribution on the horst level as it can be seen at the Soultz. This has been investigated further using geophysical means. The high-resolution 3D structural regional model of the Soultz area is used as a basis for new geophysical interpretation and inversion of gravimetric and magnetic data. Several gravimetric and magnetic anomalies can be linked to thermal anomalies, which in general the Upper Rhine Graben are known to be linked to free convection and thus, linked to porosity and permeability changes.