Structural and diagenetic controls on flow pathways in fractured Triassic Muschelkalk and Keuper limestones, southern Germany – Implications for geoenergy exploration

Understanding structural and diagenetic interactions is key in analyzing flow pathways in tight lithologies in the focus of geo-energy production as e.g., geothermal energy. Fracture spacing and clustering is crucial for reservoir production success and reduces uncertainty in reservoir exploration and utilization. This outcrop analog study evaluates diagenesis, fractures, vein cement generations, kink bands, and faults in tight fractured limestones of the Upper Muschelkalk-Lower Keuper transition on the eastern Upper Rhine Graben shoulder in SW Germany. Early and burial diagenesis led to cementation of former pore spaces and therefore drastically reduced the matrix porosity and permeability (porosity: 0.13–10.87 %, permeability: <0.0001 mD to 9.7 mD). Highest permeabilities are recorded in samples containing partially sealed veins and stylolites (up to 9.7 mD) at 1.2 MPa confining stress. The impact of increasing confining stress on the permeability of undisturbed limestones, as well as limestones containing stylolites and partially sealed veins, indicate that besides an undisturbed host rock sample and a sample containing stylolites, partially sealed veins preserve higher permeability at 30 MPa confining stress (41 % of initial value, compared to 16 and 11 % of the initial value). Fracture cluster analyses using the normalized correlation count method indicates that clustering around a breached kink band and associated fault is not symmetrically arranged and contains fracture sets of different strike. Fracture clusters also exist away from the fault at the breached kink band. Slip and dilation tendencies indicate that clustered fracture sets striking NNE-SSW parallel to the Rhine graben rift, WNW-ESE, and NW-SE parallel to the in-situ maximum principal horizontal stress are more likely to contribute to fluid flow as they are suitably oriented in the present-day stress field. Breached, decameter-scale reverse kink bands are the first reported in the region, c. 180–200 km N-NNE of the Alpine deformation front. Kink bands are most likely related to compression by far field stresses induced by the Alpine orogeny during the Eocene, and show partially cemented fault planes indicating locally persevered pore space. The applied methods of structural and diagenetic reservoir quality assessment and obtained outcomes aid in the understanding of fluid migration pathways for geoenergy applications in the Upper Rhine Graben area. Further, the results are also transferrable to other fractured tight reservoirs worldwide, which can help to solve problems for energy or heat supply that are of societal importance. • Partially sealed veins & stylolites retain permeability at 30 MPa confining pressure in tight limestones. • Fracture cluster occur in undisturbed host rock and faulted host rock. • Conjugate kink bands indicate a compressional tectonic phase linked to the Alpine orogeny. • Fracture clusters striking NNE-SSW, WNW-ESE, and NW-SE are potential fluid conduits.