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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.

Porosity and permeability alteration due to the thermo-poro-elastic stress field disturbance from the cold fluid injection is a deciding factor for longer, more economic, and safer heat extraction from an enhanced geothermal system (EGS). In the Soultz-sous-Forêts geothermal system, faulted zones are the main flow paths, and the resulting porosity–permeability development over time due to stress reorientation is more sensitive in comparison with the regions without faulted zones. Available operational and field data are combined through a validated numerical simulation model to examine the mechanical impact on the pressure and temperature evolution. Results shows that near the injection wellbore zones, permeability and porosity values are strongly affected by stress field changes, and that permeability changes will affect the overall temperature and pressure of the system, demonstrating a fully coupled phenomenon. In some regions inside the faulted zones and close to injection wellbores, porosity doubles, whereas permeability may be enhanced up to 30 times. A sensitivity analysis is performed using two parameters which are not well discussed in the literature the for mechanical aspect, but the results in this study show that one of them impacts significantly on the porosity–permeability changes. Further experimental and field works on this parameter will help to model the heat extraction more precisely than before.

Petrophysical characterization of the Los Humeros geothermal field (Mexico): from outcrop to parametrization of a 3D geological model Leandra M. Weydt1* , Kristian Bär1 and Ingo Sass1,2 Abstract The Los Humeros Volcanic Complex has been characterized as a suitable target for developing a super-hot geothermal system (> 350 °C). For the interpretation of geo- physical data, the development and parametrization of numerical geological models, an extensive outcrop analogue study was performed to characterize all relevant key units from the basement to the cap rock regarding their petrophysical properties, mineralogy, and geochemistry. In total, 226 samples were collected and analyzed for petrophysical and thermophysical properties as well as sonic wave velocities and mag- netic susceptibility. An extensive rock property database was created and more than 20 lithostratigraphic units and subunits with distinct properties were defined. Thereby, the basement rocks feature low matrix porosities (< 5%) and permeabilities (< 10–17 m 2 ), but high thermal conductivities (2–5 W m−1 K−1 ) and diffusivities (≤ 4·10–6 m2 s−1 ) as well as high sonic wave velocities (≥ 5800 m s−1 ). Basaltic to dacitic lavas feature matrix porosities and permeabilities in the range of < 2–30% and 10–18 –10–14 m2 , respectively, as well as intermediate to low thermal properties and sonic wave velocities. The pyro- clastic rocks show the highest variability with respect to bulk density, matrix porosity (~ 4– > 60%) and permeability (10 –18 –10–13 m2 ), but feature overall very low thermal conductivities (< 0.5 W m−1 K−1 ) and sonic wave velocities (~ 1500–2400 m s−1 ). Specific heat capacity shows comparatively small variations throughout the dataset (~ 700–880 J kg−1 K−1 ), while magnetic susceptibility varies over more than four orders of magnitude showing formation-related trends (10–6 –10–1 SI). By applying empirical correction functions, this study provides a full physiochemical characterization of the Los Humeros geothermal field and improves the understanding of the hydraulic and thermomechanical behavior of target formations in super-hot geothermal systems related to volcanic settings, the relationships between different rock properties, and their probability, whose understanding is crucial for the parametrization of 3D geologcal models.

Deep geothermal energy represents a key element of future renewable energy production due to its base load capability and the almost inexhaustible resource base. Especially with regard to heat supply, this technology offers a huge potential for carbon saving. One of the main targets of geothermal projects in Central Europe is the Upper Rhine Graben, which exhibits elevated subsurface temperatures and reservoirs with favorable hydraulic properties. Several decades of intensive research in the region resulted in a comprehensive understanding of the geological situation. This review study summarizes the findings relevant to deep geothermal projects and thus provides a useful working and decision-making basis for stakeholders. A total of nine geological units have been identified that are suitable for deep geothermal exploitation, comprising the crystalline basement, various sandstone formations and Mesozoic carbonates. An extensive lithostratigraphic, structural, geochemical, hydraulic and petrophysical characterization is given for each of these potential reservoirs. This paper furthermore provides an overview of the available data and geological as well as temperature models.

Assessing resources of enhanced geothermal (EGS) or medium deep geothermal systems (MDGS) for direct heat use and underground thermal energy storage (UTES) is a challenging task where usually diverse data sets of multiple origin and scale have to be compiled to obtain a comprehensive conceptual model of the subsurface, its structure and its properties. Within the research project “Hessen 3D 2.0” (BMWI-FKZ: 0325944), which aims to enhance the assessment of the prospective risk (‚Fündigkeitsrisiko’) for these kinds of geothermal projects, we established a workflow to implement and analyse such broad data sets. In a first step, comprehensive datasets of physical rock-, fluid- and reservoir properties are compiled which are based on investigations on relevant reservoir analogues, hydraulic test data from boreholes and borehole geophysical logs. The second step comprises the development of 3D geological models from a combination of borehole data, geological cross sections, seismic profiles, gravity and geomagnetic anomalies and geological maps to achieve the required detail on subsurface structure. This is prerequisite to distinguish the potentially usable reservoir units both within the crystalline or metamorphic basement and the sedimentary cover. Geostatistical analysis of the acquired comprehensive geothermal database is performed in a third step of the workflow; this allows for a parametrization of the geological model, for thermohydraulic subsurface modelling, and finally for the geothermal resource assessment. Such models, which consider the variability of rock and reservoir and fluid properties provide a thorough understanding of the subsurface temperature distribution, the dominant heat transport processes and hydraulic conditions. Finally, under consideration of both technical and economic boundary conditions and the statistics for the different relevant reservoir properties of the different geological units, assessment of hydrothermal, petrothermal and UTES potentials is performed directly with the 3D model. Therefore, a multiple-criteria approach, which assesses the quality of various rock and reservoir properties and their relevance for the different geothermal utilizations is implemented. This 3D-grid based method can be used for an identification and visualization of different geopotentials using various parameters to determine each potential. Thereby, to specify the grade of each potential under technical and economic requirements, threshold values for each parameter are defined. The approach described here allows for a stochastic assessment of the geothermal resources of a particular site of interest, including the determination of the probability of success and it provides the necessary numbers to attract investors to geothermal projects.

We provide a set of grid files that collectively allow recreating a 3D geological model which covers the Upper Rhine Graben and its adjacent tectonic domains, such as portions of the Swiss Alps, the Molasse Basin, the Black Forest and Vosges Mountains, the Rhenish Massif and the Lower Rhine Graben. The data publication is a complement to the publication of Freymark et al. (2017). Accordingly, the provided structural model consists of (i) 14 sedimentary and volcanic units; (ii) a crystalline crust composed of seven upper crustal units and a lower crustal unit; and (iii) two lithospheric mantle units. The files provided here include information on the regional variation of these geological units in terms of their depth and thickness, both attributes being allocated to regularly spaced grid nodes with horizontal spacing of 1 km. The model has originally been developed to obtain a basis for numerical simulations of heat transport, to calculate the lithospheric-scale conductive thermal field and assess the related geothermal potentials, in particular for the Upper Rhine Graben (a region especially well-suited for geothermal energy exploitation). Since such simulations require the subsurface variation of physical rock properties to be defined, the 3D model differentiates units of contrasting materials, i.e. rock types. On that account, a large number of geological and geophysical data have been analysed (see Related Work) and we shortly describe here how they have been integrated into a consistent 3D model (Methods). For further information on the data usage and the characteristics of the units (e.g., lithology, density, thermal properties), the reader is referred to the original article (Freymark et al., 2017). The contents and structure of the grid files provided herewith are described in the Technical Info section.

There is consensus that conduction is the main heat transfer mechanism in the lithosphere and generally controlled by (1) the heat input from larger mantle depths, (2) the heat internally produced in the lithosphere by the decay of radioactive elements, and (3) the variations in thermal conductivity within different lithospheric layers. To quantitatively assess the interaction of these controlling factors we use validated 3D conductive thermal models that integrate observations from wells, surface geology, seismic surveys, gravity and seismology and resolve various sedimentary, crustal, and mantle units. We analyze variations of the geothermal gradient laterally and with depth for the Upper Rhine Graben rift and two magma-rich passive continental margins of the Atlantic that are characterized by a similar crustal structure but a different age, the SW African margin and the Vøring margin. In general we find that the geothermal gradient decreases with increasing depth and shows varying lateral trends and values for the young rift system and the two different margins. In the young rift the distribution of heat is mainly controlled by variations of heat input from crustal domains of different radiogenic heat production and the insulating effect of the young unconsolidated rift sediments, that partly may also be affected by advective heat transport. For the passive margins the age-controlled depth of the oceanic thermal LAB causes the main differences between the two settings and we find an ongoing process of oceanic mantle cooling at the young Norwegian margin compared with the old thermally equilibrated SW African margin.

Petrophysical properties are key to populate numerical models of subsurface process simulations and for the interpretation of many geophysical exploration methods. They are characteristic for specific rock types and may vary considerably as a response to subsurface conditions (e.g. temperature and pressure). Hence, the quality of process simulations and geophysical data interpretation critically depend on the knowledge of in-situ physical properties that have been measured for a specific rock unit. Inquiries for rock property values for a specific site might become a very time-consuming challenge given that such data are (1) spread across diverse publications and compilations, (2) heterogeneous in quality and (3) continuously being acquired in different laboratories worldwide. One important quality factor for the usability of measured petrophysical properties is the availability of corresponding metadata such as the sample location, petrography, stratigraphy, or the measuring method, conditions and authorship. The open-access database presented here aims at providing easily accessible, peer-reviewed information on physical rock properties in one single compilation. As it has been developed within the scope of the EC funded project IMAGE (Integrated Methods for Advanced Geothermal Exploration, EU grant agreement No. 608553), the database mainly contains information relevant for geothermal exploration and reservoir characterization, namely hydraulic, thermophysical and mechanical properties and, in addition, electrical resistivity and magnetic susceptibility. The uniqueness of this database emerges from its coverage and metadata structure. Each measured value is complemented by the corresponding sample location, petrographic description, chronostratigraphic age and original citation. The original stratigraphic and petrographic descriptions are transferred to standardized catalogues following a hierarchical structure ensuring intercomparability for statistical analysis. In addition, information on the experimental set-up (methods) and the measurement conditions are given for quality control. Thus, rock properties can directly be related to in-situ conditions to derive specific parameters relevant for modelling the subsurface or interpreting geophysical data.

A successful utilization of deep geothermal resources requires accurate predictions about the reservoir tem-perature distribution as well as an in depth knowledge of the hydraulic processes. The latter exert a direct in-fluence on the subsurface temperature and therefore on the potential productivity of geothermal reservoirs.The aim of this study is to investigate and quantify the influence that regional thermo-hydraulic processes exert on the geothermal configuration of potential reservoirs in the northern Upper Rhine Graben and ad-jacent areas. Specifically, we address the questions of (i) how the regional thermal and hydraulic configura-tions influence local reservoir conditions, and (ii) whether it is possible to improve subsurface predictions iteratively by relying on 3D numerical modeling techniques. Therefore, a highly detailed 3D structural model of the northern part of the Upper Rhine Graben and the adjacent areas was built. The model was then used for coupled 3D thermo-hydraulic simulations of the deep fluid flow and heat transport. By running different simulations, we systematically tested hypotheses for the regional fluid dynamics based on hydro-geochemi-cal analysis results. Springs, mineral and thermal, have been used for the validation process of the flow field and available temperature measurements for the validation of the thermal field. With this knowledge of the thermo-hydraulic mechanisms of heat transport, the next step will be to downscale this process understand-ing and implement it in predictions of geothermal potentials on a local scale for the urban area of Frankfurt am Main

A successful utilization of deep geothermal resources requires to make accurate predictions about the reservoirtemperature distribution as well as an in depth knowledge of the hydraulic processes exerting a direct influence onthe subsurface temperature distribution and therefore on the productivity of geothermal reservoirs.The aim of this study is to investigate and quantify the influence that regional thermo-hydraulic processes exert onthe geothermal configuration of potential reservoirs in the German federal state Hesse. Specifically, we addressthe question of how the regional thermal and hydraulic configuration influences the local reservoir conditionsand whether it is possible to improve subsurface predictions iteratively by relying on 3D numerical modelingtechniques. Therefore, a 3D structural model of Hesse is used as a basis for coupled 3D thermo hydraulicsimulations of the deep fluid and heat transport. To uncover the effects of process coupling, a stepwise workflow isfollowed. We first simulate the thermal and hydraulic field under steady-state conditions by means of two differentuncoupled simulations and then analyze the results of the coupled thermo-hydraulic steady-state simulations. Ina last effort, we investigate the influence of fluid viscosity and density varying with temperature and pressure intransient coupled simulations.As a result of our numerical simulations, Hesse can be differentiated into sub-areas differing in terms of the domi-nating heat transport processes. In a final attempt to quantify the robustness and reliability of the modeling results,we carry out an analysis of the modelling outcomes by comparing them to available subsurface temperature data.Modelled temperatures show different levels of fit with locally measured well temperatures. These differences inmodel fit indicate the need for either structurally refined models and/or iterative adaptions within realistic rangesof the hydraulic and thermal properties. Structural refinements can often only be handled with smaller-scalemodels, which will, in turn, benefit from the boundary conditions and improved process understanding as derivedfrom the regional modelling approach presented here.

Die tatsächliche Nutzung petrothermaler Reservoire zur Strom- und Wärmegewinnung ist in Europa nach wie vor im Wesentlichen auf Soultz-sous-Forêts beschränkt, während einige weitere Forschungsprojekte in der Planungsphase sind. Bisherigen Potenzialermittlungen zufolge liegen jedoch mehr als 90 % des geothermischen Potenzials Deutschlands in genau diesen Reservoirsystemen. Im Verbundprojekt „Hessen 3D 2.0“ (BMWi-FKZ: 0325944) werden im Teilprojekt I petrothermale Potenziale von Hessen untersucht, quantifiziert und in einem geologischen -geothermischen 3D-Modell visualisiert. Das hierfür notwendige 3D-Strukturmodell ist mit Schichtenverzeichnissen der hessischen Bohrdatenbank des Hessischen Landesamtes für Naturschutz, Umwelt und Geologie (HLNUG) sowie geologischen Schnitten und Karten erstellt worden. Zur Definition der geothermischen Modelleinheiten sind die im hessischen Grundgebirge auftretenden Gesteinstypen mittels petrophysikalischer Analysen charakterisiert und durch eine statistische Datenanalyse klassifiziert worden. Im kristallinen Grundgebirgsteil konnten so die Modelleinheiten (1) Granit, (2) Granodiorit, (3) Gneis sowie (4) Diorit und Gabbro unterschieden werden. Diese Modelleinheiten sind im 3D-Modell als eigenstände Volumina abgebildet und mit den ermittelten petrophysikalischen Gesteinseigenschaften druck- und temperaturabhängig parametrisiert. Wo Bohrlochdaten, geologische Karten und Schnitte räumlich begrenzte Einblicke in den Untergrund bieten, stellt die Analyse des gemessenen Schwerefelds insbesondere für tiefere Krustenbereiche eine ergänzende oder auch validierende Methode dar. Mit den neu erhobenen Schwere-messungen der Hessischen Verwaltung für Bodenmanagement und Geoinformation (HVBG) liegt uns ein Datensatz vor, mit dem wir Dichteheterogenitäten mittels 3D gravimetrischer Vorwärtsmodellierung (Software IGMAS+) abbilden und hinsichtlich vorhandener Lithologie-unterschiede und Störungen interpretieren können. Mit diesem Beitrag werden methodische Abläufe und Ergebnisse der geologischen 3D-Struktur - sowie Gravimetrie-modellierung, die die Basis der Potenzialabschätzung darstellen, vorgestellt.

Die mitteltiefen und petrothermalen Potenziale für geothermische Stromerzeugung, Direktwärmenutzung und saisonale Wärmespeicherung von Hessen werden derzeit im Rahmen des Projektes “Hessen 3D 2.0” untersucht. Ziel ist die Ausweisung von Gebieten in denen eine geothermische Nutzung wirtschaftlich erfolgsversprechend ist und in denen geringe Fündigkeitsrisiken zu erwarten sind. Als Grundlage für die Potenzialbewertung werden zahlreiche bestehende Datensätze zusammengetragen, miteinander verschnitten und durch neue eigene Untersuchungen ergänzt. In einem ersten Schritt werden die petrophysikalischen, thermischen und mechanischen Eigenschaften der geothermisch relevanten Formationen sowie der Reservoirfluide ermittelt und in Datenbankanwendungen dokumentiert, vereinheitlicht und für die weitere Auswertung vorgehalten. Dies beinhaltet Labormessungen an Aufschlussanalog- und Bohrkernproben, hydraulische Bohrlochtestdaten sowie bohrlochgeophysikalische Untersuchungen. Als zweiter Schritt werden auf Basis digitalisierter Bohrdaten, geologischer Profilschnitte, interpretierter seismischer Profile sowie geologischer Kartenwerke die bestehenden 3D-Modelle verbessert und weiter untergliedert, um nutzungsbezogen geothermische Modelleinheiten, die für petrothermale Systeme, Wärmespeicherung oder Direktwärmenutzung geeignet sind, zu unterscheiden. Diese Modelleinheiten werden anhand der umfänglichen Datenbank geothermischer Gesteins- und Reservoireigenschaften attributiert, um als Basis für die Potenzialausweisung oder thermohydraulischen Modellierungen zu dienen. Die thermohydraulischen Modellierungen unter Berücksichtigung der Variabilität der Gesteins- und Gebirgseigenschaften sowie realistischer Randbedingungen sollen zur verbesserten Prognose des Untergrundtemperaturfeldes beitragen sowie ein besseres Verständnis der regional dominierenden Wärmetransportprozesse (konduktiv oder konvektiv) ermöglichen. Insbesondere in Regionen für die nur wenige Temperaturmessungen aus tiefen Bohrungen vorliegen, ist eine deutlich bessere Temperaturprognose sowie ein deutlich verbessertes Verständnis der Hydraulik im tiefen Untergrund, das entscheidend für den Betrieb offener geothermischer Systeme ist, zu erwarten. Für die 3D Potenzialausweisung werden technische und wirtschaftliche Rahmenbedingungen sowie die Kenntnis der statistischen Verteilung der geothermischen Kennwerte berücksichtigt, um stochastisch abgesicherte Prognosen als Basis für die Angabe der Fündigkeitsrisiken zu ermöglichen. Abschließend sollen die identifizierten geothermischen Potenziale mit den Wärmeüberschüssen und dem Wärmebedarf der Stadt Frankfurt a. M. verschnitten werden, um wirtschaftlich interessante Standorte für zukünftige Geothermieprojekte zu identifizieren

The Upper Rhine Graben (URG) has a large socioeconomic relevance as it provides a great potential for geothermal energy production in Germany and France. For the utilisation of this energy resource it is crucial to understand the structure and the observed temperature anomalies in the rift basin. In the framework of the EU-funded “IMAGE” project (Integrated Methods for Advanced Geothermal Exploration, grant agreement no. 608553), we apply a data-driven numerical modelling approach to quantify the processes and properties controlling the spatial distribution of subsurface temperatures. Typically, reservoir-scale numerical models are developed for predictions on the subsurface hydrothermal conditions and for reducing the risk of drilling non-productive geothermal wells. One major problem related to such models is setting appropriate boundary conditions that define, for instance, how much heat enters the reservoir from greater depths. Therefore, we first build a regional lithospheric-scale 3D structural model, which covers not only the entire URG but also adjacent geological features like the Black Forest and the Vosges Mountains. In particular, we use a multidisciplinary dataset (e.g. well data, seismic reflection data, existing structural models, gravity) to construct the geometries of the sediments, the crust and the lithospheric mantle that control the spatial distribution of thermal conductivity, radiogenic heat production, permeability, and hence temperatures in the URG. By applying a data-based and lithology-dependent para-meterisation of this lithospheric-scale 3D structural model and a 3D finite element method, we calculated in a first step the steady-state conductive thermal field for the entire region. Available measured temperatures (down to depths of 5 km) were used to validate this physics-based 3D thermal model, but also 137 revealed heat trans-port by hydrothermal convection. In a second step we performed numerical simulations of coupled fluid and heat transport on a smaller-scale and higher resolved model, for which the lithospheric-scale model provides the thermal boundary conditions. We show that the Variscan upper crustal domains with their different radiogenic heat production controls the regional thermal field. Highest temperatures are predicted for the URG, where a thermal blanketing effect due to thick thermally low-conductive sediments is locally modified by additional fluid flow.

The Upper Rhine Graben (URG) is a promising target for deep geothermal energy production. As part of the EU-funded project “IMAGE” (Integrated Methods for Advanced Geothermal Exploration, grant agreement no. 608553), we aim to understand the processes that control the temperature distribution in the subsurface of the URG by using numerical simulations. Reservoir-scale numerical models are often used to predict the hydrothermal conditions and to reduce the risk of drilling non-productive geothermal wells. One major problem related to such reservoir-scale models is setting appropriate boundary conditions that define, for instance, how much heat enters the reservoir from greater depths. To overcome this problem, we combine lithospheric-scale 3D structural and thermal modelling with higher resolved hydrothermal models of the sedimentary basin fill in a workflow of successive modelling steps. After model pa- rameterization with thermal rock properties we solve the steady-state conductive heat equation first to calculate the 3D conductive thermal field and to understand the first-order controlling factors of the regional tem-perature distribution. In a second step we took into account the fluid flow due to permeability variations in the porous media and the influence of major faults, which locally change the temperature distribution. These numerical simulations of coupled heat and fluid transport were performed on smaller and higher resolved models of the basin fill for which the conductive thermal model provide the thermal boundary conditions. We present the methodological workflow, the developed 3D structural and thermal models, and assess how heat transport mechanisms in response to lithological and structural features on different scales control subsurface temperatures.

© 2015, Springer-Verlag Berlin Heidelberg.The Permocarboniferous siliciclastic formations represent the largest hydrothermal reservoir in the northern Upper Rhine Graben in SW Germany and have so far been investigated in large-scale studies only. The Cenozoic Upper Rhine Graben crosses the Permocarboniferous Saar–Nahe Basin, a Variscan intramontane molasse basin. Due to the subsidence in this graben structure, the top of the up to 2-km-thick Permocarboniferous is located at a depth of 600–2900 m and is overlain by Tertiary and Quaternary sediments. At this depth, the reservoir temperatures exceed 150 °C, which are sufficient for geothermal electricity generation with binary power plants. To further assess the potential of this geothermal reservoir, detailed information on thermophysical and hydraulic properties of the different lithostratigraphical units and their depositional environment is essential. Here, we present an integrated study of outcrop analogues and drill core material. In total, 850 outcrop samples were analyzed, measuring porosity, permeability, thermal conductivity and thermal diffusivity. Furthermore, 62 plugs were taken from drillings that encountered or intersected the Permocarboniferous at depths between 1800 and 2900 m. Petrographic analysis of 155 thin sections of outcrop samples and samples taken from reservoir depth was conducted to quantify the mineral composition, sorting and rounding of grains and the kind of cementation. Its influence on porosity, permeability, the degree of compaction and illitization was quantified. Three parameters influencing the reservoir properties of the Permocarboniferous were detected. The strongest and most destructive influence on reservoir quality is related to late diagenetic processes. An illitic and kaolinitic cementation and impregnation of bitumina document CO2- and CH4-rich acidic pore water conditions, which are interpreted as fluids that migrated along a hydraulic contact from an underlying Carboniferous hydrocarbon source rock. Migrating oil and acidic waters led to the dissolution of haematite cements in the lower Permocarboniferous formations. During the Eocene, subsidence of the Upper Rhine Graben porosities and permeabilities of the sandstones of these formations were strongly reduced to 2.5 % and 3.2 × 10−18 m2. The second important influence on reservoir quality is the distinct depositional environment and its influence on early diagenetic processes. In early stage diagenesis, the best influence on reservoir properties exhibits a haematite cementation. It typically occurs in eolian sandstones of the Kreuznach Formation (Upper Permocarboniferous) and is characterized by grain covering haematite coatings, which are interpreted to inhibit cementation, compaction and illitization of pore space during burial. Eolian sandstones taken from outcrops and reservoir depths exhibit the highest porosities (16.4; 12.3 %) and permeabilities (2.0 × 10−15; 8.4 × 10−16 m2). A third important influence on reservoir quality is the general mineral composition and the quartz content which is the highest in the Kreuznach Formation with 73.8 %. Based on the integrated study of depositional environments and diagenetic processes, reservoir properties of the different Permocarboniferous formations within the northern Upper Rhine Graben and their changes with burial depth can be predicted with satisfactory accuracy. This leads to a better understanding of the reservoir quality and enables an appropriate well design for exploration and exploitation of these geothermal resources.

© 2015 Elsevier B.V. Rotliegend siliciclastic sediments in southern Hessen (Germany) are a good example of dissolution of detrital feldspars, which is a common feature in many sandstones. Dissolution occurred after mechanical compaction of the lithic-rich sandstone, which experienced framework collapse with pores and pore connections filled and obstructed by deformed ductile lithic grains (pseudomatrix) thereby reducing pore space to microporosity., The advanced degree of compaction and reduced porosity caused low permeability and low hydraulic conductivity of the rock mass. This is further reduced by the presence of wackes and shales that occur intercalated with the sandstones. Feldspar dissolution thus took place in low permeable sediments when large-scale flow of meteoric or acidic fluids is ruled out as a cause of feldspar dissolution. Mineral precipitation (illite, kaolinite, and albite) took place within pseudomatrix and detrital matrix as well as in secondary pores created by feldspar dissolution. Feldspar was the source for the authigenesis. The system was thus closed during burial after framework collapse, and diagenetic reactants in the form of detrital components were already present within the system. The original mass was preserved, but redistributed and diagenetic minerals were the local sinks for the dissolved reactants, precipitating within the system. This also suggests that burial diagenesis in general might be more mass conservative than usually assumed.Rotliegend sandstones thus form a case where, despite of the lack of external exchange of mass by fluid flow, major diagenetic processes did take place and significantly modified the original mineralogy and texture. Feldspar diagenesis can take place from other processes than mere large-scale flushing of open systems as often supposed. It implies that the volumes of rock affected by feldspar diagenesis may be much larger than anticipated based upon the common hold believe that feldspar diagenesis is linked to unconformities and surface weathering or dissolution in near-surface aquifers.

The aim of this work is the establishment of measurement technologies for temperature, displacement and strain measurement for the use in thermo-mechanical qualification tests at hot space structures. For the temperature measurement, the use of thermocouples for surface temperature measurement of ceramic matrix composites, based on C and SiC was investigated and optimized concerning chemical compatibility, fixation and ageing behaviour at temperatures up to 1600°C inair and N$_{2}$-atmosphere at 10$^{-2}$ mbar and 10$^{-3}$ mbar. For the non-contacting temperature measurement, a new pyrometric system including on-line emissivity measurement and multireflexion compensation was optimized and qualified for the temperature range between 700°C and 1600°C. In combination with this pyrometric system, a new thermocouple calibration method was developed and patented which allows a calibration even above 1700°C. Additionally, the use of high temperature strain gauges applied at ceramic matrix composites was optimized and qualified for application temperatures up to 1000°C. Detailed investigations were performed concerning chemical compatibility, service life and measurement accuracy. For the temperature range up to 1600°C, a high temperature extensometer was developed and patented, which allows a relative movement compensation of the measurement object. An additionally developed and patented high temperature displacement measurement sensor allows a displacement measurement up to the surface temperatures of 1400°C. With the investigated and qualified HT-measurement technologies, the necessary requirements for the component qualification of the hot structures of the space plane HERMES for limit reentry conditions (T$_{max}$: 1600°C) can be fulfilled.

The aim of this work is the clearification of R-curve behaviour of non-transforming ceramics, in particular aluminum- oxide exhibiting incrystalline fracture. Investigations of crack growth in controlled bending experiments were performed using -3-Ptf and 4-Pt-bending samples of differing sizes under inert conditions. The fracture experiments were realized using several loading techniques, for example constant and varying displacement rates, load rupture (P = 0) and relaxation tests (v = 0). In addition unloading and reloading experiments were performed to investigate hysteresis curves and residual displacements in accordance with R-curve behaviour.During the crack-growth experiments, the crack extension was measured in situ using a high resolution immersion microscop. With this technique, the fracture processes near the crack tip (crack activity zone) was observed as well. The crack resistance as a function of crack extension (R-curve) was determined using differing calculation methods. All of the methods used resulted in approximately identical R-curves, within the statistical error band. The crack resistance at initiation R$_{o}$was 20 N/m. The crack resistance increased during approximately 3 mm of growth to a maximum of 90 N/m. A decrease in the crack resistance was determined for large a/W (crack length normalized with sample height) values, independant of the calculation methods. The R-curve behaviour was interpreted as due to a functionalresistance behind the observed crack tip, which arises from a volume dilatation in the crack activity zone while the crack proceeds.