Characterization of the Basement/Cover Unconformity Based on Saint Pierre Bois Quarry Observations (Rhine Graben, France)

According to the literature, deep geothermal energy reserves are associated with naturally fractured/faulted reservoirs in various geological contexts. In the Upper Rhine graben, several deep geothermal plant exploit the transition zone between crystalline basement and sedimentary cover as a reservoir for heat and power. However, the complexity of this zone makes characterization of its heterogeneities a great challenge to the development of geothermal resources.

At the Saint Pierre Bois quarry, on the western graben border, the granitic crystalline basement is overlain by arkoses. Fracture orientation measurements and rock sampling were conducted on all accessible quarry benches, providing rich datasets for both the granite and the arkose.

The fracture dataset in granite can be interpreted as a large evolved Riedel N80°E fault zone. The core zone is the most highly weathered-fractured zone in the quarry. The two major fracture sets (E-W and N30°E) identified in the damage zone represent shear fracture in the R-direction and P’ fracture, respectively. The E-W structures were the pathways of deep hot fluid circulation at the cover/basement transition. Most of the smallest cataclastic structures associated with this stage are sealed, whereas the highest porosities are present at the margins of large breccia corridors.