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.

Evolution of diagenetic conditions and burial history in Buntsandstein Gp. fractured sandstones (Upper Rhine Graben) from in-situ δ¹⁸O of quartz and ⁴⁰Ar/³⁹Ar geochronology of K-feldspar overgrowths

In-situ δ¹⁸O measured in the quartz overgrowths help identify temperature and fluid origin variations responsible for cementation of the pore network (matrix and fracture) in the Buntsandstein Gp. sandstone reservoirs within the Upper Rhine Graben. The overgrowths record two types of the evolution of δ¹⁸O: 1) a monotonous decrease of the δ¹⁸Oovergrowth interpreted as linked to an increasing burial temperature and 2) random fluctuations, interpreted as pointing out the injection of allochthonous fluids in faulted areas, on the cementation processes of the pore network (both intergranular and fracture planes). Fluids causing the quartz cementation are either autochthonous buffered in ¹⁸O from clay illitisation; or allochthonous fluids of meteoric origin with δ¹⁸O below − 5%. These allochthonous fluids are in thermal disequilibrium with the host sandstone. The measured signal of δ¹⁸Oovergrowth measured from samples and calculated curves testing hypothetic δ¹⁸Ofluid are compared to T–t evolution during burial. This modelling proposes the initiation of quartz cementation during the Jurassic and is validated by the in-situ ⁴⁰Ar/³⁹Ar dating results obtained on the feldspar overgrowths predating quartz overgrowths. A similar diagenetic history is recorded on the graben shoulders and in the buried parts of the basin. Here, the beginning of the pore network cementation predates the structuration in blocks of the basin before the Cenozoic graben opening

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"

Geochemical, petrophysical and petrographical dataset of fractured Variscan granites of the Cornubian Batholith, SW United Kingdom

Outcrop analogue studies are of high priority as they allow the interpretation of geophysical surveys, minimizing the exploration risk, reliable predictions of reservoir properties and drawing conclusions on expected discharge, before drilling in a cost-wise way. Detailed knowledge of the petrophysical properties and petrographical characteristics of potential reservoir rocks improve the interpretation of forthcoming borehole logging and of fluid flow in different zones. Built datasets yet comprise the development of the necessary input for static and dynamic reservoir model parametrization. In the framework of EU H2020 funded ‘MEET’ project (‘Multidisciplinary and multi-context demonstration of EGS exploration and Exploitation Techniques and potentials’, grant agreement no:792037) outcrop analogue samples from the Cornubian Batholith, located in SW United Kingdom, were taken from the onshore plutons namely Land’s End, Tregonning-Godolphin, Carnmenellis, Carn Brea, Carn Marth and St. Austell. 47 granitic samples were collected from 23 outcrops representative for fresh, weathered, hydrothermally altered, fractured and faulted granites. From the outcrop samples, 338 cores with different diameters were obtained and the measurements of grain density, bulk density, permeability, porosity, acoustic wave velocity, thermal conductivity, thermal diffusivity, magnetic susceptibility, uniaxial compressive strength, Poisson’s ratio, Young’s modulus, tensile strength, shear strength, cohesion, coefficient of friction and shear modulus were performed on oven-dried cores under ambient temperature and atmospheric pressure in the HydroThermikum laboratory of the Technical University of Darmstadt. Petrographical and geochemical characterization were done for 47 outcrop analogue samples and for the drill cuttings, taken between 3,980 and 5,275 m (measured depth) in UD1; the production well of United Downs Deep Geothermal Power Project, first commercial deep geothermal project of UK, through thin section analysis, X-ray diffraction (XRD) and X-ray fluorescence (XRF). This dataset presents the geochemical, petrophysical and petrographical properties of Early Permian aged fractured Variscan granite specifically not overprinted by any younger tectonic regime. It forms a basis to other publications, where the results of the aforementioned multiparameter rock characterization are discussed in detail and are used for stochastic regional resource assessment, respectively. It follows the same database structure as PetroPhysical Property Database (P³) of Bär et al. (2020). Each analyzed core is provided with a sample ID according to the naming convention within MEET project as ‘MEETCW001001001’ (Project name - region name - location number - sample number - core number)

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