Fault anatomy, porosity and pore connectivity: the La Sarraz Fault system

The understanding of the distance to failure (criticality) of the faults in the Alpine foreland is of strategic importance for the development the geothermal resources in Switzerland. Fault evolution involves highly coupled processes leading to a complex anatomy of fault zones. In turn, this complex anatomy will have a leading impact on current fault

properties, behavior and stability. It is thus required to develop geological-tectonic models of faults relevant for the Swiss foreland basin that go beyond a simplistic geometrical description of the geological features but include geo-mechanical attributes like stress, strength, criticality and permeability at a scale relevant for project development. This is an important but challenging task. Conceptual bases should be laid down in order to enable this type of model to be developed.

To tackle this problematic, our study will initially rely on an outstanding natural laboratory situated along the La Sarraz fault system because this NW-SE oriented right-lateral fault system shows spectacular outcrops in quarries in the Éclepens region. In an initial phase, our project focusses on a detailed description of the fault anatomy, porosity and pore connectivity and their impact on overall fault characteristics.

The objective of the current project is to generate a detailed understanding of the fault pattern at the quarry scale and to relate it to variations in fault anatomy and fault rocks characteristics down to the pore scale. These characteristics will be linked to hydromechanics properties of the fault segments in order lay down fault maturation and fault anatomy conceptual models relevant for the Swiss foreland basin. Initially, we will perform structural analyses of fault patterns in the quarry and the vicinity to constrain the paleo-stress field. Photogrammetric methods and lidar scan will be used for this purpose. Subsequently we will make a detailed study of the structural features on distinct fault damage zones to determine the fault anatomy and kinematics. The fault will be drilled through and image with borehole televiewers in order to decipher its detailed anatomy. Finally, we will sample the damage zone from the undeformed border to the highly

strained central section and analyze the samples in a multi-scaled X-ray micro-tomography scanner (CTscan). The CTscans

will provide quantitative assessments of the damage and associated pore space geometry. The high-resolution 3D CTscan images will serve as the numerical domain from which we overlay with a finite element or finite difference mesh in order to model fluid flow through the pore system and assess variation in permeability.