Fault zones in layered carbonate successions: from field data to stress field models

Abstract

We present a field-study based workflow to develop 3D-numerical models on local stress fields within fault zones hosted in layered rocks at reservoir depths. As an example we use the carbonate successions of the Upper Muschelkalk (Middle Triassic) in the Upper Rhine Graben, Southwest Germany, that form a reservoir for deep geothermal energy. Steps of the workflow include (A) characterization of fault-zone units and mechanical layering, (B) estimations of rock mechanical properties and (C) assumptions on the in situ stress regime. Results of 3D-numerical models of fault-zone local stress fields at reservoir depth of 2900 m show pronounced differences depending on (1) fault-zone orientation, (2) impact of maximum horizontal stress SH in the given stress regime (normal faulting, strike-slip faulting or transitional normal to strike-slip faulting), (3) fault-zone scale and (4) contrast in mechanical properties. Soft fault damage zones and fault cores trending at a minor angle to SH (0°–30°) concentrate less stress than comparable units at higher angles to SH (60°–90°), in particular in the strike-slip regime. The impact of mechanical layering increases with horizontal compression. This may result, for example, in formation of barriers to fracture propagation and thus lower probability of forming well-interconnected fracture networks necessary for fluid flow in reservoirs. Comparisons to estimated fault-zone stress states such as dilation and slip tendencies, show that their prediction in the study area is difficult because of the current transitional stress regime (normal to strike-slip faulting) and, in addition, varying orientations of SH.