Characterizing the physical properties of rocks from the Paleozoic to Permo-Triassic transition in the Upper Rhine Graben

Geothermal energy exploitation in the Upper Rhine Graben currently targets high-temperature anomalies in the crystalline Paleozoic basement at depths up to 5 km. However, at certain locations (e.g. Rittershoffen, France), geothermal installations are actively targeting resources at shallower depths where the Paleozoic granite transitions into the overlying Permo-Triassic sandstones. We here investigate the variation in physical properties—including porosity, P-wave velocity, permeability, uniaxial compressive strength, and thermal properties—in rocks that locally extend across the Paleozoic–Permo-Triassic boundary in four locations to the west of the Upper Rhine Graben. The lithologies investigated include sandstones, breccia, granites, metagranites, dolomite, and altered and unaltered volcanic rocks and represent the variety of lithologies at this transition. We note that while the porosity, permeability, thermal conductivity, and P-wave velocity of the Permo-Triassic sedimentary cover and Paleozoic crystalline basement rocks are consistent with values determined for rocks from exploratory and production boreholes at Soultz-sous-Forêts (France), the other lithologies (belonging to neither the sedimentary nor basement sequences) are conspicuously lower in porosity and permeability. Further, the attendant strength of these other lithologies could make them relatively unamenable to fracturing, reducing the possibility of fracture-controlled permeability in these units. Indeed, we conclude that in areas where these low-permeability and high-strength rocks act to cap the crystalline basement, hydrothermal convection may be curtailed and geothermal exploitation may be rendered untenable.