Eva Schill

Localization of temperature anomalies in the Upper Rhine Graben: insights from geophysics and neotectonic activity

The European Cenozoic Rift System hosts major temperature anomalies in Central Europe. In its central segment, the Upper Rhine Graben (URG), temperatures range from 75°C to nearly 150°C at a depth of 2000 m. Different hypotheses have been suggested to explain the localization of these anomalies. Our review and comprehensive interpretation of gravimetric and magnetic data, as well as neotectonic activity patterns, suggests that low-density, mostly magnetic and fractured granitic basement is systematically associated with major temperature anomalies. Further analyses provide insight into different heat transport processes contributing to the localization of these anomalies. Magnetic and gravity anomalies are known to represent lithological variations associated with the pre-Permian. We show their spatial relationship with positive temperature anomalies in the URG. Correlation between magnetics and temperature reveal a mean contribution of heat production to the temperature anomaly of about 10–15°C. A slightly higher mean value is obtained from correlation between gravity and temperature, which may be attributed to effects resulting from fracture porosity. The spatial relationship between temperature anomalies and neotectonic patterns indicates compressional shear and uplift regime for the major anomalies of the central segment of the URG. This is in agreement with different numerical models indicating free convection on fracture zones linked to faults. Our findings show that about 15–25% of the temperature anomaly can be attributed to variation in heat production. Hydrothermal circulation convection along faults, activated by the tectonic context, may explain the remaining 75–85% of the temperature anomalies.

Inferring the in situ stress regime in deep sediments: an example from the Bruchsal geothermal site

BackgroundKnowledge of the ambient state of stress is of crucial importance for understanding tectonic processes and an important parameter in reservoir engineering. In the framework of the 2,500-m deep geothermal project of Bruchsal in the central part of the Upper Rhine Graben, new evidence is presented for the stress field in deep-seated sedimentary rocks.MethodsWith a sophisticated data analysis based on the concept of critical stress ratios, we evaluate the quality and uncertainty range of earlier stress field models in the Bruchsal area. New data from borehole logging and leak- off tests in deep sediments are used to propose an alternative stress profile for this part of the Upper Rhine Graben.ResultsThe revised stress field model for the Bruchsal area predicts a normal with transition to strike-slip faulting regime. Stress field perturbations and potential decoupling process within specific clay-, salt-, and anhydrite-bearing units of the Keuper can be observed.ConclusionBy comparison with other models, we can show a regional consistency of our stress field model that is reliable throughout the central Upper Rhine Graben extending from Bruchsal in the East to the Soultz-sous-Forêts EGS site in the West.

Impact of gravity, collector surface roughness and fracture orientation on colloid retention kinetics in an artificial fracture

The interaction of monodisperse fluorescent carboxylated polystyrene colloids (25nm and 1000nm diameter) with a cut granodiorite surface (Grimsel granodiorite; Switzerland) and with acrylic glass is investigated both experimentally and numerically. Colloid transport experiments are conducted in a parallel plate type fracture flow cell with an aperture of 0.75mm at pH5 under low ionic strength (1mM NaCl) and under laminar flow (7mL/h) conditions. The study focuses on the effect of residence time, colloid size, collector material and fracture orientation on colloid retention. Long colloid residence times are achieved by stop-flow experiments. Using atomic force microscopy and, more specifically, the colloid probe technique surface roughness and force distance information of the collector material (granodiorite or acrylic glass) as a function of probe size (cantilever) are obtained. The experiments are modeled using COMSOL Multiphysics® (2-D numerical simulations). The experimental and the modeled results lead to the conclusion that large colloids (1000nm diameter) undergo sedimentation and deposition on the surface during stop-flow. Collector interaction is not affected by the surface roughness variation. Contrariwise, for the investigated 25nm colloids sedimentation does not play a role under the experimental conditions and collector interaction is triggered by surface inhomogeneities such as surface roughness.

Delineation of geothermally relevant Paleozoic graben structures in the crystalline basement of Switzerland using gravity

Summary Although many are suspected from seismic observations, few and far between Permo-Carboniferous (PC) troughs at the top of the Variscian basement in Switzerland are confirmed by wells. The well-known and geothermally relevant Northern Swiss trough with its top in a depth of about 1.5 km has been first identified in its spatial extension by gravity measurements revealing its smaller density with respect to the crystalline basement. Further south, towards the Alps, paleo-graben structures are expected to occur with a top depth down to about 5 km following the deepening of the top crystalline basement. A sensitivity study using Butterworth filter of different wavelengths applied to realistic 2D and 3D geological models was conducted in order to understand the gravity variation with increasing depth of the top of the troughs. It reveals high potential in the delimitation of the PC troughs at significant depth as well as in tracing their vertical geometry using the different wavelengths. Application on the real data confirms the interpretation of negative anomalies in terms of PC troughs.