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.
The Upper Rhine Graben (URG) represents a region with a high potential for deep geothermal projects in Central Europe due to a significantly increased geothermal gradient of more than 100
Fluid flow in fractured reservoirs depends on a multitude of parameters and processes, such as the density, orientation, length, opening and roughness of fractures, stress conditions or the influence of mineralization
The characterization of fracture networks in the Tromm Granite was performed using a combination of established techniques at multiple scales. Outcrops distributed over the entire pluton were analyzed using the light detection and ranging (lidar) technique
Overview of the study area:
The crystalline Odenwald at the northeastern margin of the URG is the largest outcrop of the Mid-German Crystalline High (MGCH), extending over 50 km from Heidelberg to Darmstadt (Fig.
In comparison, the Bergsträßer Odenwald is dominated by Variscan plutonic rocks intruded into a metamorphic volcanic–sedimentary series
The Tromm Granite forms a ca. 60 km
While an interlocking of the two plutons characterizes the contact between the Tromm Granite with the Weschnitz Granodiorite, the eastern boundary constitutes a 1–2 km wide heterogeneous westward-dipping mylonitization and cataclasic zone along the Otzberg Fault
Most of the mapped and interpreted faults strike NNE–SSW to NNW–SS, which approximately corresponds to the measured orientation of
The interdisciplinary and multi-scale fracture network characterization was carried out using the following methods. The first part focuses on structural geological investigations and DFN modelling. The second part presents the applied geophysical acquisition techniques in detail. A summary of all investigations into the Tromm Granite is given in Fig.
Overview map of the surveys conducted in the Tromm Granite:
Two DEMs of the Tromm Granite were examined with respect to the density, length and orientation of lineaments. The high-resolution DEM with a cell size of 1 m allowed detailed structural investigations. In addition, the satellite-based Shuttle Radar Topography Mission (SRTM) model with a resolution of 1 arcsec was used to identify regional structural features.
Lineaments are natural, rectilinear surface features that are uniquely identifiable and likely reflect subsurface structures, i.e., faults, discontinuities, or weakness zones. It should be noted that shallow dipping faults may not appear as linear structures and thus may be underrepresented, especially in areas of strong relief. However, most faults in the Tromm Granite are assumed to be rather steeply dipping.
The methodology of lineament analysis is described in previous studies, e.g., in
Five abandoned quarries located across the Tromm Granite were selected for detailed structural analysis of the fracture network (Fig.
Interpretation of a scanned outcrop wall from the quarry in Ober-Mengelbach (Profile A in Fig.
The raw lidar data were first imported into RiSCAN PRO to merge individual scans. Further analysis of the point clouds was performed using the open-source software CloudCompare and QGis. The point cloud was resampled to less than 2 million points to reduce the computational effort of the following steps. Afterwards, the orientations of the surface normals were calculated by triangulating between the points, and these normals were converted to the dip and dip directions. Based on this, the Ransac shape detection plugin was applied to automatically extract the orientations of continuous fracture planes
Besides automatic plane recognition, the lidar data were also manually interpreted in QGis to investigate the fracture length, density and connectivity (Fig.
The results of the lineament and outcrop analyses were finally summarized in a normalized trace length cumulative-frequency plot with a power law fitted to the data, which describes the relationship between the frequency and the cumulative distribution of fracture lengths
DFN models were generated with the software FracMan to quantitatively model the hydraulic properties of the fractured crystalline basement based on the structural parameters acquired in the field. Fracture orientations were implemented by performing a cluster analysis of the dip directions and dip angles extracted from the lidar data. The fracture density was defined along a virtual horizontal borehole using the calculated P10 values. The fracture length distribution was set according to the computed power law. A lower cutoff of 70 cm was applied, as significant censoring, i.e., under-representation of short fractures, occurs below this length. The effective fracture aperture largely governs the hydraulic conductivity of fractures. Due to exhumation and weathering processes, measured aperture values at near-surface outcrops are usually not reliable
For a sufficient number of discontinuities, the fractured basement behaves like an anisotropic porous medium. The equivalent porous medium (EPM) permeability tensor can thus be calculated for a DFN model by, e.g., the approach of
During two surveys in summer 2020 and spring 2021, gravity measurements at 431 stations along 11 profiles were conducted in the Tromm Granite (Fig.
For the regional gravity signal analysis, ca. 5300 additional data points provided by the Leibniz Institute for Applied Geophysics (LIAG) and the Hessian Administration for Land Management and Geoinformation (HVGB) were used within a radius of 50 km around the survey area. Together with the newly acquired data, a Bouguer anomaly map with a nominal resolution of 20 m was calculated using the minimum curvature interpolation method. A series of high-pass filters with cutoff wavelengths of 10, 5, and 2 km were then applied to subtract the regional gravity field.
A stochastic 3D inversion of the high-pass filter Bouguer anomaly (10 km cutoff wavelength) was performed to infer the density distribution and the porosity in the subsurface. The commercial platform GeoModeller (Intrepid Geophysics), which employs a Monte Carlo Markov chain algorithm to invert geophysical data, was used for this purpose. A detailed discussion of the methodology is available in Guillen et al. (2008). The model domain has extensions of 7 km in the E–W direction and 6 km in the N–S direction and a depth of 2 km. The upper boundary is defined by the 10 m DEM. Given the relative homogeneity of the pluton with respect to the matrix density and the lack of structural input data, an unconstrained inversion was performed. The continuous model was converted into a discrete cuboid voxel model with a cell size of
The algorithm first calculates the geophysical effect of the starting model, in this case a homogeneous half-space, and then uses a Bayesian approach to determine the likelihood of the model. In subsequent iterations, random variations of the model are generated according to the probability distribution of the rock density. Models that lead to a reduction in the deviation between the calculated and measured gravity anomalies have a higher likelihood and are stored. After 250 million iterations, a large collection of possible models have been generated, allowing statistic evaluation.
Finally, the porosity is estimated, assuming the abovementioned homogeneity of the Tromm granite, using
Radon is a naturally occurring radioactive gas that is concentrated in the soil air. The most abundant Rn isotope, with a proportion of ca. 90 %, is Rn-222, with a half-life of 3.82 d, which is formed in the decay series of U-238. Permeable fault zones may provide migration pathways where Rn-222 transport to the surface is enhanced. Consequently, elevated radon concentrations are expected in the close vicinity of hydraulically active faults
Measurements of the activity concentration [Beq m
Figure
The main strike of the mapped faults ranges from 160 to 170
Summary of the lineament analysis in the Tromm Granite area:
Summary of fracture network properties for all outcrops analyzed in the Tromm Granite area.
OMB
In total, five outcrops of varying size, distributed over the entire Tromm Granite area and hence representing the heterogeneity of the pluton, were investigated using lidar (Figs.
Summary of the outcrop analysis in the Tromm Granite area (faults and outline of the Tromm Granite from
The most extended outcrop examined is an abandoned quarry with dimensions of ca. 150
The two smaller outcrops in Hammelbach and Weschnitz Valley are located at the northeastern border of the Tromm Granite. Here, a fine- to medium-grained cataclastic granite is predominant, which was considerably affected by the adjacent Otzberg Shear Zone. Consequently, the P21 is highest here: 10.82 and 9.07 m m
Figure
The IXY topology was examined to quantify the connectivity of the encountered fracture networks; this is expressed as the average number of connections per line,
DFN models were created for the two outcrops in Borstein and Weschnitz Valley, which represent the end members of the Tromm Granite in terms of fracture density (Fig.
To test the transferability of the results to crystalline reservoirs in the URG, a comparison with hydrogeological data, e.g., from Soultz-sous-Forêts, is useful. Here, the mean permeabilities of the fractured granitic basement range from
It should be noted that the hydraulic properties of fractured reservoirs are subject to strong spatial variations. For example, permeability can be increased by several orders of magnitudes close to active faults. In contrast, at larger distances from these faults or large-scale fractures, the mean permeability of the basement is of the order of
Illustration of DFN models with 1 % open fractures for
Summary of the DFN modeling. The Oda permeabilities in the
Figure
The strongest positive anomaly of 1 to 1.5 mGal is located north of Wald-Michelbach and coincides with a major lineament. Similarly, a positive anomaly of 0.5 to 1 mGal can be observed along the presumed fault zone between Zotzenbach and Wald-Michelbach. The strongest negative anomaly, with an amplitude of ca.
Besides these larger anomalies, short-wavelength variations of the gravity signal in the range of
Results of the gravity survey:
Results of the stochastic gravity inversion are shown as differences from the homogeneous density of 2670
Results of the gravity inversion. Difference between the inverted density and the initial density of 2670
At greater depths, the inverted density model becomes more diffuse and the density variations are generally smaller. At 0 m a.s.l., the negative anomaly in the west and the positive anomaly in Gadern can still be clearly recognized. In contrast, the density reduction at the eastern edge is very weak. At 1000 m b.s.l., the variations have a very long wavelength and range between only
A comparison of the radon activity concentration in soil air with the corresponding Bouguer anomalies is shown in Fig.
Based on the extensive structural geological investigations at the five outcrops and the lineament analysis, a more comprehensive description of the fracture network in the Tromm Granite has been obtained. Scale independence of the fracture length distribution was demonstrated with a power-law exponent of ca.
Like the fracture length, the connectivity of the fracture network seems to be independent of scale or location. All outcrops and lineament maps indicate a dominance of Y-nodes, which is in clear contrast to the northern Odenwald, where I- and X-nodes represent the largest share
Compared to fracture length and connectivity, the orientation of the fracture sets shows some scale-dependent and spatial variations. In the outcrops Ober-Mengelbach, Borstein and Streitsdölle, the fracture orientations are controlled by the main fault direction of
Similar to the fracture orientation, the fracture density is subject to considerable lateral changes, which can be attributed to the influence of large-scale tectonic structures, especially at the pluton margins. In the eastern part of the Tromm Granite, the basement is deformed by the nearby Otzerg Shear Zone. As a result, there is an evident accumulation of lineaments, and the outcrops show by far the highest fracture density. Medium fracture densities were found in Ober-Mengelbach, in the southern part of the pluton, i.e., at the border with the Schollenagglomerat. Although this area lacks pronounced long fault zones, the lithological heterogeneities led to more intense granite deformation than in the central Tromm Granite. Accordingly, the lowest fracture density was found in the Borstein and Streitsdölle outcrops.
In summary, the Tromm Granite is likely not characterized by a complete fractal fracture network, which is consistent with, e.g., studies from the western rift shoulder
The measured gravity anomalies provide insights into the subsurface density distribution of the Tromm Granite (Figs.
In general, individual faults can rarely be accurately traced with the gravity data acquired for the Tromm Granite, as the influence of fracture porosity on bulk density is too small. Instead, areas can be identified where a high density of faults and fractures leads to increased porosity and thus to a significant density reduction. Accordingly, the Tromm Granite is potentially structurally weakened at the contact with the Weschnitz Pluton in the western part of the study area. Unfortunately, there are neither larger outcrops nor available well data, leaving this assumption speculative. The slightly smaller negative anomaly at the eastern boundary with the Buntsandstein can be explained by the proximity to the Otzberg Shear Zone. Here, the pluton is presumably characterized by similar structural properties to those in the Hammelbach and Weschnitztal outcrops, which means that the fracture density and thus the porosity are increased. Interestingly, the anomaly does not extend over the entire damage zone at the eastern margin of the Tromm Granite, but is concentrated in a limited area with a high density of intersecting lineaments. A possible explanation is that the fractures are partially mineralized with, e.g., barite
Positive gravity anomalies of up to 1.5 mGal can be observed at the southern Tromm Granite along two fault zones. In Gadern, several lamphropyric intrusions were mapped and, as in the quarry of Ober-Mengelbach, localized amphibolitic zones are present. These mafic rocks have a considerably higher density (2700–3100 kg m
Radon measurements were carried out along just one profile due to the high time consumption of this method. Accordingly, a regional interpretation of the results is only possible to a limited extent. Nevertheless, the determined radon anomalies give helpful indications about the architecture of the analyzed fault zones. Two distinct radon peaks indicate localized permeable fracture zones in the granite. The highest activity correlates with a negative Bouguer anomaly, which further supports this assumption. Interestingly, the peaks are not located directly above the assumed positions of the faults, but in the damage zone a few meters to tens of meters to the sides, suggesting low permeability in the fault core
The Tromm Granite represents a suitable site for the planned geothermal underground research laboratory (GeoLaB), as the main criteria proposed by
Furthermore, the Tromm Granite is a well-suited outcrop analogue for the crystalline basement in the URG. The granitic body has a similar mineralogical composition to the reservoir rocks, e.g., in Soultz-sous-Forêts or Rittershoffen
A DFN parameter study was carried out to estimate the hydraulic properties of the Tromm Granite under assumed reservoir conditions (Fig.
Minimizing induced seismicity during stimulation and operation represents a major challenge for deep geothermal exploitation of the crystalline basement
Apart from hydrogeological properties, the temperature of the reservoir is an important parameter for any geothermal prospection. The thermal field in the URG has been extensively studied in the past
In addition to deep EGS projects, underground heat storage will significantly contribute to reducing emissions in the future energy supply. Thereby, the seasonal fluctuations of other renewable energy sources such as solar and wind energy can be compensated for. It is expected that medium-depth borehole heat exchangers (MD-BHE) in the crystalline basement have the highest efficiency among comparable technologies and can be applied almost anywhere that the basement is situated near the surface
The fracture network characterization of the Tromm Granite has led us to the following conclusions:
Combining outcrop and lineament analysis allows for a more comprehensive description of the main fracture network characteristics. While fracture length distribution and connectivity are mostly scale independent, fracture orientation and density vary significantly across the Tromm Granite. The latter two parameters are heavily affected by the crustal-scale Otzberg Shear Zone. Hydraulic properties of the fractured basement under reservoir conditions can be estimated with DFN models and validated by hydraulic test data from deep boreholes. However, the calculated permeabilities are associated with large uncertainties, as the stress conditions and therefore the fracture aperture are highly unconstrained. Gravity and radon measurements enable more advanced mapping of potentially permeable zones. The fracture porosity can be inferred from the inverted density model, where homogeneous subsurface conditions are present. Lithological variations and mineralization prevent exact porosity quantification. Structural investigations and gravity anomalies show that the most suitable hydraulic properties are expected at the margin of the granitic pluton, where regional-scale fault zones influence the fracture network. Moreover, during the cooling of a granitic pluton, the margins were more affected by the circulation of residual fluids and are therefore more altered than the center, allowing the creation of preferential hydraulic pathways. The Tromm granite is a suitable site for GeoLaB, as the composition is generally homogeneous and representative of the reservoirs of the URG, high fracture density and connectivity were observed, the stress conditions and fracture orientation are favorable for reactivation, the hydraulic boundary conditions are controllable, and a sufficient overburden is ensured.
The presented research data can be found at
The conceptualization of the study and the choice of methodology were made by MF, CB, and ES. The field investigations were conducted by MF, CB, and LS. Data preparation, formal analysis, and validation visualization were performed by MF. Acquisition of funding and project management were in the hands of KB and IS. The work was supervised by KB and IS. Resources were provided by IS and ES. MF wrote the original draft. All co-authors reviewed and edited the manuscript and approved the final version.
The contact author has declared that neither they nor their co-authors have any competing interests.
Publisher’s note: Copernicus Publications remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.
First of all, we would like to thank Cäcilia Boller for conducting one part of the gravity measurements. We are grateful that the HLNUG, LIAG and HVGB provided the borehole, gravity and digital elevation data. We thank Sebastian Schröder and the municipalities of Wald-Michelbach and Rimbach for giving us access to the quarries in the Tromm Granite.
The research was funded by the Interreg NWE program (grant no. NWE892) through the Roll-out of Deep Geothermal Energy in North-West Europe (DGE-ROLLOUT) project
This paper was edited by Virginia Toy and reviewed by two anonymous referees.