From surface analogue to reservoir depth: A multiscale analysis of fracture permeability in the Fichtelgebirge granites of northern Bavaria - Integrating structural, hydraulic, and geophysical data for an improved understanding of fracture-controlled fluid flow in granitic geothermal reservoirs

This dissertation investigates the hydraulic and structural properties of fractured crystalline reservoirs using borehole data from three sites in the granites of the Fichtelgebirge, northern Bavaria, Germany: a deep geothermal borehole (~1830 m), two medium-depth drinking water wells (~150 m), and a shallow geoscientific test site (15–25 m) in a granite quarry. The study integrates borehole geophysics, hydraulic testing, photogrammetry, mineralogical analyses, and surface geophysics to evaluate how fracture orientation, dip, frequency, aperture, and spatial arrangement control hydraulic behaviour across depths and scales. At the quarry test site, four main fracture sets were identified within a faulted granite. Despite high fracture density, permeability is low (10⁻⁶–10⁻⁸ m/s) and spatially variable, with limited hydraulic connectivity between fractures. Permeability is independent of fracture frequency and depth, and high-pressure tests did not induce fracture reopening, likely due to mineralisation and in situ stress conditions. Results demonstrate strong scale dependency and show that fault zones with high fracture frequency do not necessarily correspond to increased permeability. The two shallow drinking water wells, although only metres apart, exhibit markedly different fracture distributions and productivity. Flow occurs primarily along individual fractures connected to the surface, and inflow cannot be directly correlated with fracture density or aperture. In contrast, the deep borehole reveals several major fracture zones that dominate groundwater inflow. Here, fracture frequency within zones and fracture orientation relative to the present stress field exert primary control on permeability. Hydraulically active fractures show high slip and dilation tendency, indicating potential for mechanical reactivation and stimulation. Alteration zones characterised by feldspar dissolution and illite precipitation frequently coincide with increased inflow. A critical assessment of borehole geophysical methods highlights image and resistivity logs as most effective for fracture detection, while fracture aperture proves unreliable for estimating permeability. Surface geophysical methods show limited applicability in this setting. Overall, the results emphasise the depth- and scale-dependence of fracture-controlled permeability in crystalline reservoirs. In shallow systems (~100 m), subhorizontal unloading fractures significantly enhance flow, whereas at greater depths tectonic fractures, fault zones, and their large-scale connectivity dominate. Direct extrapolation of surface observations to reservoir depth is therefore limited. These findings provide important implications for drinking water supply, geothermal energy development, and radioactive waste repository assessment in fractured granitic rocks.