Characterization of geothermally relevant structures at the top of crystalline basement in Switzerland by filters and gravity forward modelling

Some of the major geothermal anomalies in central Europe are linked to tectonic structures within the top of crystalline basement, which modify strongly the top of this basement. Their assessment is a major challenge in exploration geophysics. Gravity has been proven to be suitable for the detection of mainly large scale lithological and structural inhomogeneities. Indeed, it is well known and proven by different wells that, for example, in northern Switzerland extended negative anomalies are linked to such structures. Due to depth limitation of wells, there vertical extension is often unknown. In this study, we have investigated the potential of gravity for the geometrical characterization of such basement structures. Our approach consists in the combination of the series of Butterworth filters, geological modelling and best-fitting between observed and computed residual anomalies. In this respect, filters of variable wavelength are applied to observed and computed gravity data. The geological model is discretized into a finite element mesh. Near-surface anomalies and the effect of the sedimentary cover were eliminated using cut-off wavelength of 10km and geological and seismic information. We analysed the potential of preferential Butterworth filtering in a sensitivity study and applied the above mentioned approach to part of the Swiss molasses basin. Sensitivity analyses reveal that such sets of residual anomalies represents a pseudo-tomography revealing the distribution of different structures with depth. This finding allows for interpreting negative anomalies in terms of 3-D volumes. Best-fitting then permits determination of the most likely 3-D geometries of such basement structures. Our model fits both, geological observations and gravity: among 10 deep boreholes in the studied area, six reach the respective units and confirm our distribution of the negative (and positive) anomalie

Interdisciplinary fracture network characterization in the crystalline basement: a case study from the Southern Odenwald, SW Germany

The crystalline basement is considered a ubiquitous and almost inexhaustible source of geothermal energy in the Upper Rhine Graben (URG) and other regions worldwide. The hydraulic properties of the basement, which are one of the key factors in the productivity of geothermal power plants, are primarily controlled by hydraulically active faults and fractures. While the most accurate in situ information about the general fracture network is obtained from image logs of deep boreholes, such data are generally sparse and costly and thus often not openly accessible. To circumvent this problem, an outcrop analogue study was conducted with interdisciplinary geoscientific methods in the Tromm Granite, located in the southern Odenwald at the northeastern margin of the URG. Using light detection and ranging (lidar) scanning, the key characteristics of the fracture network were extracted in a total of five outcrops; these were additionally complemented by lineament analysis of two different digital elevation models (DEMs). Based on this, discrete fracture network (DFN) models were developed to calculate equivalent permeability tensors under assumed reservoir conditions. The influences of different parameters, such as fracture orientation, density, aperture and mineralization, were investigated. In addition, extensive gravity and radon measurements were carried out in the study area, allowing fault zones with naturally increased porosity and permeability to be mapped. Gravity anomalies served as input data for a stochastic density inversion, through which areas of potentially increased open porosity were identified. A laterally heterogeneous fracture network characterizes the Tromm Granite, with the highest natural permeabilities expected at the pluton margin, due to the influence of large shear and fault zones

GeoLaB – Das geowissenschaftliche Zukunftsprojekt für Deutschland

Geothermische Energie kann bei der Dekarbonisierung des deutschen Energiesystems eine wichtige Rolle einnehmen. Um das große Potenzial der Geothermie im kristallinen Grundgebirge wirtschaftlich nutzbar zu machen, werden Ertüchtigungsmaßnahmen im Reservoir eingesetzt. Eine Voraussetzung für die öffentliche Akzeptanz solcher EGS („Enhanced Geothermal Systems“) ist jedoch die Minimierung der möglichen induzierten Seismizität. Ihre Kontrolle kann nur auf Basis des Verständnisses für die Prozesse und Wechselwirkungen des Fluids mit dem Reservoir erfolgen. Mit dem generischen Untertagelabor GeoLaB („Geothermal Laboratory in the Crystalline Basement“) sollen grundlegende Fragen der Reservoirtechnologie und Bohrlochsicherheit von EGS erforscht werden. Die geplanten Experimente werden wesentlich unser Verständnis der maßgeblichen Prozesse im geklüfteten Kristallingestein unter erhöhten Fließraten verbessern. Der Einsatz und die Entwicklung modernster Beobachtungs- und Auswertemethoden führen zu Erkenntnissen, die für eine sichere und ökologisch nachhaltige Nutzung der Geothermie und des unterirdischen Raumes von großer Bedeutung sind. Als interdisziplinäre und internationale Forschungsplattform wird GeoLaB in Kooperation mit der Deutschen Forschungsgemeinschaft, Universitäten sowie industriellen Partnern und Fachbehörden Synergien erzeugen und technisch-wissenschaftliche Innovationen hervorbringen