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

Multiscale Characterisation of Fracture Patterns of a Crystalline Reservoir Analogue

For an accurate multiscale property modelling of fractured crystalline geothermal reservoirs, an enhanced characterisation of the geometrical features and variability of the fracture network properties is an essential prerequisite. Combining regional digital elevation model analysis and local outcrop investigation, the study comprises the characterisation of the fracture pattern of a crystalline reservoir analogue in the Northern Odenwald, with LiDAR and GIS structural interpretation. This approach provides insights into the 3D architecture of the fault and fracture network, its clustering, and its connectivity. Mapped discontinuities show a homogeneous length distribution, which follows a power law with a −2.03 scaling factor. The connectivity of the fracture network is heterogenous, due to a fault control at the hectometric scale. Clustering is marked by long sub-vertical fractures at the outcrop scale, and strongly enhance heterogeneity around weathered fracture and fault corridors. The multi-variable dataset created within this study can be used as input data for accurate discrete fracture networks and fluid-flow modelling of reservoirs of similar type

Multi-scale structural dataset of a crystalline reservoir analogue (Northern Odenwald)

For an accurate multi-scale property modelling of fractured crystalline geothermal reservoirs, an enhanced characterisation of the geometrical features and variability of the fracture network properties is an essential prerequisite. By the combination of regional digital elevation model analysis and local outcrop investigation, detailed insight into the 3D architecture of faults and fracture networks allows the quantification of structural parameters (fracture dimension, orientation, clustering and spacing). The structural dataset presented here contains the regional DEM interpretation at two resolutions (25 m and 1 m) of the Northern Odenwald and the LiDAR and GIS structural interpretation of 5 profiles acquired in the Mainzer Berg quarry between Darmstadt and Dieburg. This quarry exhibits the fracture network affecting a granodioritic pluton. Fracture length, orientation, dip, and fracture density and intensity are calculated for each profile. On GIS 2D datasets extracted from top and side views of the profiles, a clustering and spacing analysis between digitised items is performed and compared to the orientation of artificial scanlines. Power-law parametrisation is extracted from the length distribution of 2D and 3D datasets, with a, b coefficients. This multi-scale parametrisation of the fracture network can be used to construct near-surface discrete fracture network models. The dataset is a supplement to another publication (Bossennec al. 2021) that presents the structural organisation of crystalline rocks from the analogue of Mainzer Berg in the Northern Odenwald.V1.

Évolution des propriétés de transfert des grès par diagénèse et déformation : application aux formations du Buntsandstein Gp., Graben du Rhin

The Upper Rhine Graben (URG) is the target of a renewed development of the exploitation of oil and gas resources, since the discovery of the Roemerberg field in 2008. Strong improvements were made on the understanding of the petroleum system, and on the characterization of the oil and source rocks families, and migration pathways. The interactions between faults systems and fracture network, fluid flow and fracture infill, and fracture/matrix transfers, during the burial and rift opening, are the purpose of this PhD research, focused on Buntsandstein Gp. sandstone reservoir. Through the study of several outcrops on the URG shoulders, core and well logs data (Roemerberg field), the following aspects are investigated. Structural features of the fracture network in the reservoir and in the vicinity of major fault systems are characterized, with the identification of fracture families (infill and orientation), not always parallel to the major faults. Chemical and isotopic composition of matrix and fracture diagenetic phases are analysed, to determine conditions of cementation : Temperature, type of fluid, absolute dating when possible, or relative timing. The impact on petrophysical properties is assessed by investigating the influence of diagenetic processes on pore network properties, and on "bulk" petrophysical properties. These analytical results are integrated in a renewed conceptual model of fluid flow in the basin, and give new insights on the interactions between diagenesis, fault activity and fluid-flow, and their impact on petrophysical properties at the fault scale through geological times.Le Graben du Rhin est la cible d’un regain d’intérêt pour le développement de l’exploitation des ressources hydrocarbonées et géothermiques depuis une quinzaine d’années. De nombreuses améliorations ont été apportées à la compréhension de la géométrie des réservoirs et sur le système pétrolier, avec une caractérisation des huiles et des roches-mères, ainsi que sur les chemins de migration de l’huile. Cependant, il subsiste des interrogations sur les interactions entre systèmes de faille, réseau de fractures, circulations de fluides et cimentations, ainsi que sur les transferts entre matrice et fractures au sein des réservoirs, et l’évolution de ces propriétés au cours de l’histoire d’enfouissement. Cette thèse s’attache à la compréhension de ces interactions, en prenant pour objet d’étude les réservoirs gréseux du Buntsandstein Gp. Sur la base matérielle de plusieurs affleurements sur les épaules du fossé et des données de subsurface disponibles (comme celle du champs de Roemerberg), les caractéristiques du réseau de fractures au sein du réservoir et à proximité des zones de failles majeures ont été déterminées, avec l’identification de familles (remplissages, orientation) avec une variabilité de leur agencement par rapport aux failles majeures. La détermination de la composition chimique et isotopique des remplissages de fractures et des phases diagénétiques affectant la matrice a permis d’améliorer les modèles de conditions de cimentations : température, type de fluide, datation relative, ou absolue lorsque les méthodes analytiques s’y prêtent. L’impact de ces processus sur les propriétés pétrophysiques a été estimé, par le biais de la caractérisation du réseau poreux et la quantification des propriétés pétrophysiques globales ("bulk"). Ces résultats analytiques sont ensuite intégrés à un modèle conceptuel de circulation de fluides à l’échelle du bassin, répertoriant la mise en place des différentes minéralisations, apportant un nouveau regard sur le résultat à l’échelle des zones de failles des interactions entre diagénèse, tectonique, et circulations de fluides, au cours de l’histoire géologique du bassin

Evolution of transfer properties of sandstones by diagenesis and deformation : Case study on Buntsandstein Gp. sandstones, Upper Rhine Graben

Le Graben du Rhin est la cible d’un regain d’intérêt pour le développement de l’exploitation des ressources hydrocarbonées et géothermiques depuis une quinzaine d’années. De nombreuses améliorations ont été apportées à la compréhension de la géométrie des réservoirs et sur le système pétrolier, avec une caractérisation des huiles et des roches-mères, ainsi que sur les chemins de migration de l’huile. Cependant, il subsiste des interrogations sur les interactions entre systèmes de faille, réseau de fractures, circulations de fluides et cimentations, ainsi que sur les transferts entre matrice et fractures au sein des réservoirs, et l’évolution de ces propriétés au cours de l’histoire d’enfouissement. Cette thèse s’attache à la compréhension de ces interactions, en prenant pour objet d’étude les réservoirs gréseux du Buntsandstein Gp. Sur la base matérielle de plusieurs affleurements sur les épaules du fossé et des données de subsurface disponibles (comme celle du champs de Roemerberg), les caractéristiques du réseau de fractures au sein du réservoir et à proximité des zones de failles majeures ont été déterminées, avec l’identification de familles (remplissages, orientation) avec une variabilité de leur agencement par rapport aux failles majeures. La détermination de la composition chimique et isotopique des remplissages de fractures et des phases diagénétiques affectant la matrice a permis d’améliorer les modèles de conditions de cimentations : température, type de fluide, datation relative, ou absolue lorsque les méthodes analytiques s’y prêtent. L’impact de ces processus sur les propriétés pétrophysiques a été estimé, par le biais de la caractérisation du réseau poreux et la quantification des propriétés pétrophysiques globales ("bulk"). Ces résultats analytiques sont ensuite intégrés à un modèle conceptuel de circulation de fluides à l’échelle du bassin, répertoriant la mise en place des différentes minéralisations, apportant un nouveau regard sur le résultat à l’échelle des zones de failles des interactions entre diagénèse, tectonique, et circulations de fluides, au cours de l’histoire géologique du bassin.The Upper Rhine Graben (URG) is the target of a renewed development of the exploitation of oil and gas resources, since the discovery of the Roemerberg field in 2008. Strong improvements were made on the understanding of the petroleum system, and on the characterization of the oil and source rocks families, and migration pathways. The interactions between faults systems and fracture network, fluid flow and fracture infill, and fracture/matrix transfers, during the burial and rift opening, are the purpose of this PhD research, focused on Buntsandstein Gp. sandstone reservoir. Through the study of several outcrops on the URG shoulders, core and well logs data (Roemerberg field), the following aspects are investigated. Structural features of the fracture network in the reservoir and in the vicinity of major fault systems are characterized, with the identification of fracture families (infill and orientation), not always parallel to the major faults. Chemical and isotopic composition of matrix and fracture diagenetic phases are analysed, to determine conditions of cementation : Temperature, type of fluid, absolute dating when possible, or relative timing. The impact on petrophysical properties is assessed by investigating the influence of diagenetic processes on pore network properties, and on "bulk" petrophysical properties. These analytical results are integrated in a renewed conceptual model of fluid flow in the basin, and give new insights on the interactions between diagenesis, fault activity and fluid-flow, and their impact on petrophysical properties at the fault scale through geological times

Aeromagnetic dataset from the Tromm Granite in the southern Odenwald

The crystalline basement is due to its vast heat content a major target for geothermal utilization, with the largest resources tied to permeable fault zones. However, as in the Upper Rhine Graben, the basement has often been explored by only a few wells or seismic lines penetrating the sedimentary cover, which poses significant risks for the development of geothermal power plants. Outcrop analog studies help to overcome this lack of structural data. The Tromm Granite was identified as suitable analog for the basement in the northern Upper Rhine Graben, located at the northeastern rift margin. A drone-based aeromagnetic survey was conducted in March 2022, covering an area of about 13.3 km² in the center of the pluton. This dataset allows a refined mapping and characterization of potentially permeable fault zones. With this publication, the acquired aeromagnetic dataset is made freely available. Attached is the text file "Magnetic_dataset_full.txt" which contains all measurements of the magnetic field. In addition, georeferenced grids of the magnetic field intensity (TMI) as well as the reduction to the magnetic pole (RtP) are given. Details about the data acquisition and the basic processing steps are summarized in the text file "Survey and processing details.txt"

Finite element simulation of permeable fault influence on a medium deep borehole thermal energy storage system

Solutions for seasonal energy storage systems are essential for the reliable use of fluctuating renewable energy sources. As part of the research project SKEWS, a medium deep borehole thermal energy storage system with a depth of 750 m is under construction at Campus Lichtwiese in Darmstadt, Germany, to demonstrate this innovative technology. Prior to the design of SKEWS, the geological context in the surroundings of the project location was investigated using archive drilling data and groundwater measurements. The geologic survey suggests the assumption that the uppermost part of the intended storage domain is crosscut by a normal fault, which displaces the Permian rocks east of Darmstadt against granodioritic rocks of the Odenwald crystalline complex. A 3D finite-element numerical model was implemented to estimate the effect of the potentially higher hydraulic conductivity of the fault zone on the planned storage system. For this purpose, a storage operation over a time span of 30 years was simulated for different parametrizations of the fault zone. The simulations reveal a limited but visible heat removal from the storage region with increasing groundwater flow in the fault zone. However, the section of the borehole thermal energy storage system affected by the fault is minor compared to the total depth of the system. This only constitutes a minor impairment of the storage efficiency of approximately 3%. In total, the amount of heat extracted varies between 320.2 GWh and 326.2 GWh for the different models. These findings can be helpful for the planning and assessment of future medium deep borehole thermal energy storage systems in fractured and faulted crystalline settings by providing data about the potential impact of faults or large fractures crosscutting the storage system

Multi-scale folds/faults/fractures network characterization in the Saar-Lorraine coal basin

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Interdisciplinary Dataset on the Fracture Network of the Tromm Granite, Southern Odenwald, SW Germany

This data publication is a supplement to the paper by Frey et al. (2021, in prep.), which describes the interdisciplinary and multi-scale characterization of the fracture network in the Tromm Granite. The crystalline basement in the Upper Rhine Graben (URG) represents an attractive reservoir for deep geothermal projects due to the comparatively high temperatures and the abundance of large-scale faults and fractures. A spatially resolved description of the fracture network, which strongly influences the hydraulic properties in the subsurface, is crucial for reducing exploration risks. However, only a few wells were drilled into the basement of the URG and image logs are sparse and often not openly accessible, causing considerable uncertainties. To circumvent this problem, a multidisciplinary outcrop analogue study was performed in the Tromm Granite, located in the southern Odenwald. Using a LiDAR scanner, comprehensive structural geological datasets were collected in five abandoned quarries and evaluated both automatically and manually. This dataset was extended by a regional lineament analysis based on two digital elevation models with 1 m and 1 arcsecond resolution. This allows a comprehensive description of the fracture network in the Tromm Granite regarding the density, length, orientation and connectivity of fractures. Discrete fracture network (DFN) models were developed for two outcrops and the equivalent permeability tensors under reservoir conditions were calculated. Thereby, the influence of different parameters, such as fracture orientation, density, aperture and mineralization was investigated. In addition to the classical structural geological data, geophysical measurements were carried out to allow a refined identification of the naturally permeable zones. Along 11 profiles, 431 gravity measurements were performed and a complete Bouguer correction with a reference density of 2670 kg/m³ was applied. Together with existing gravity field data from LIAG, HVBG, LVermGeo and LGL, an extended Bouguer anomaly map was interpolated. By applying high-pass filters, the regional part of the gravity field was subtracted. Stochastic inversion of the anomaly yields a 3D density model of the subsurface down to 2 km depth, from which fracture porosity can be inferred. In addition, concentration measurements of radon activity were made along one profile. Areas of elevated concentration indicate the existence of permeable faults or fracture corridors. In particular, combining radon measurements with other methods, such as gravity, provides an more accurate estimate of subsurface porosity and permeability.v1.