Fault zones in potential geothermal reservoir rocks in the Upper Rhine Graben: Characteristics, permeability implications, and numerical stress field models

Fault zones in carbonate successions of the Middle Triassic Muschelkalk are potential target structures of hydrothermal projects in the Upper Rhine Graben (URG). A mixed-method approach was applied to make assumptions on fault-zone permeability structures, stress states and local stress fields. This thesis contributes to exploration of fault-related Muschelkalk reservoirs in the URG and draws comparison with the already successfully tested hydrothermal reservoir of the Lower Triassic Bunter sandstone. To define fault-zone characteristics and estimate related permeability structures in Muschelkalk rocks, fault zones of different types (e.g., normal, inverted, oblique-slip) and different scales (displacement: medium-scale: 1-10 m; large-scale >10 m) were analyzed in detail. The outcrop analogue study of one large-scale fault zone in thick-bedded sandstones of Bunter age provided the opportunity for comparison. Special emphasis in both sedimentary successions was given to damage-zone and fault-core characteristics as well as to characteristics of associated fracture systems (e.g., density, aperture distribution, connectivity, vertical extension). Fracture systems formed in rather homogenous units have a positive effect on reservoir permeability. They may provide, in particular in proximity to the fault core, potential fluid pathways across multiple layers and enhanced fracture connectivity of comparatively short fractures. In contrast, fracture systems in strongly mechanically layered units may have less impact on reservoir permeability. Fault cores show partially significant complexity, comprising mainly sealing, but also conductive structures. Large-scale fault zones, i.e., in reservoir-scale, in both potential reservoir units are best described as combined conduit-barrier systems. They show potentially conductive fracture systems in damage zones (and additionally in Bunter: transition zone) and a low-permeable to sealing fault core. To improve knowledge about fault-zone local stress fields within the layered Muschelkalk reservoir (reservoir depth: 2.900 m) 3D-numerical models were developed, using the finite element software COMSOL Multiphysics®. Pronounced differences in local stress fields occur, depending on (1) orientation, (2) impact of maximum horizontal stress SH, (3) fault-zone scale and (4) contrasts in rock-mechanical properties. Clear dependency of fault-zone orientation on stress magnitude and displacement follows for stress regimes with high horizontal compression. In particular, large-scale fault-zones at 30° to SH may be favorable for SH-induced horizontal displacement within soft fault-zone units since highest displacement-values occur. Decrease of stress magnitudes in soft fault rocks diminishes towards fault zones oriented perpendicular to SH. Impact of mechanical layering increases with increasing horizontal compression, resulting in vertically heterogeneous stress fields. To make assumptions on the hydraulic activities of fault zones, analytical estimations on slip and dilation tendencies for analyzed fault zones at reservoir conditions are presented. Results reveal stress state variations of each fault zone, attributed to the current transitional stress regime and varying orientations of the maximum horizontal stress SH. Results of presented outcrop analogue studies help to make profound assumptions on fault-zone permeability structures and thus to define promising drilling targets in the URG. In this context, sedimentary successions were found which can be excluded as potential geothermal reservoirs. Findings of 3D-numerical models could help to support the strategy of possibly needed stimulation treatments in the Muschelkalk reservoir. Moreover, results of this thesis gain insights on potential problems during the drilling operation in Muschelkalk reservoirs, e.g., the likelihood of a vertically heterogeneous stress field.