@phdthesis{Schintgen2016,
  author    = {Schintgen, Tom Vincent},
  title     = {The geothermal potential of Luxembourg},
  url       = {http://nbn-resolving.de/urn:nbn:de:kobv:517-opus4-87110},
  school      = {Universit{\"a}t Potsdam},
  pages     = {XXII, 313},
  year      = {2016},
  abstract  = {The aim of this work is the evaluation of the geothermal potential of Luxembourg. The approach consists in a joint interpretation of different types of information necessary for a first rather qualitative assessment of deep geothermal reservoirs in Luxembourg and the adjoining regions in the surrounding countries of Belgium, France and Germany. For the identification of geothermal reservoirs by exploration, geological, thermal, hydrogeological and structural data are necessary. Until recently, however, reliable information about the thermal field and the regional geology, and thus about potential geothermal reservoirs, was lacking. Before a proper evaluation of the geothermal potential can be performed, a comprehensive survey of the geology and an assessment of the thermal field are required. As a first step, the geology and basin structure of the Mesozoic Trier-Luxembourg Basin (TLB) is reviewed and updated using recently published information on the geology and structures as well as borehole data available in Luxembourg and the adjoining regions. A Bouguer map is used to get insight in the depth, morphology and structures in the Variscan basement buried beneath the Trier-Luxembourg Basin. The geological section of the old Cessange borehole is reinterpreted and provides, in combination with the available borehole data, consistent information for the production of isopach maps. The latter visualize the synsedimentary evolution of the Trier-Luxembourg Basin. Complementary, basin-wide cross sections illustrate the evolution and structure of the Trier-Luxembourg Basin. The knowledge gained does not support the old concept of the Weilerbach Mulde. The basin-wide cross sections, as well as the structural and sedimentological observations in the Trier-Luxembourg Basin suggest that the latter probably formed above a zone of weakness related to a buried Rotliegend graben. The inferred graben structure designated by SE-Luxembourg Graben (SELG) is located in direct southwestern continuation of the Wittlicher Rotliegend-Senke. The lack of deep boreholes and subsurface temperature prognosis at depth is circumnavigated by using thermal modelling for inferring the geothermal resource at depth. For this approach, profound structural, geological and petrophysical input data are required. Conceptual geological cross sections encompassing the entire crust are constructed and further simplified and extended to lithospheric scale for their utilization as thermal models. The 2-D steady state and conductive models are parameterized by means of measured petrophysical properties including thermal conductivity, radiogenic heat production and density. A surface heat flow of 75 ∓ 7 (2δ) mW m-2 for verification of the thermal models could be determined in the area. The models are further constrained by the geophysically-estimated depth of the lithosphere-asthenosphere boundary (LAB) defined by the 1300 °C isotherm. A LAB depth of 100 km, as seismically derived for the Ardennes, provides the best fit with the measured surface heat flow. The resulting mantle heat flow amounts to ∼40 mW m-2. Modelled temperatures are in the range of 120-125 °C at 5 km depth and of 600-650 °C at the crust/mantle discontinuity (Moho). Possible thermal consequences of the 10-20 Ma old Eifel plume, which apparently caused upwelling of the asthenospheric mantle to 50-60 km depth, were modelled in a steady-state thermal scenario resulting in a surface heat flow of at least 91 mW m-2 (for the plume top at 60 km) in the Eifel region. Available surface heat-flow values are significantly lower (65-80 mW m-2) and indicate that the plume-related heating has not yet entirely reached the surface. Once conceptual geological models are established and the thermal regime is assessed, the geothermal potential of Luxembourg and the surrounding areas is evaluated by additional consideration of the hydrogeology, the stress field and tectonically active regions. On the one hand, low-enthalpy hydrothermal reservoirs in Mesozoic reservoirs in the Trier-Luxembourg Embayment (TLE) are considered. On the other hand, petrothermal reservoirs in the Lower Devonian basement of the Ardennes and Eifel regions are considered for exploitation by Enhanced/Engineered Geothermal Systems (EGS). Among the Mesozoic aquifers, the Buntsandstein aquifer characterized by temperatures of up to 50 °C is a suitable hydrothermal reservoir that may be exploited by means of heat pumps or provide direct heat for various applications. The most promising area is the zone of the SE-Luxembourg Graben. The aquifer is warmest underneath the upper Alzette River valley and the limestone plateau in Lorraine, where the Buntsandstein aquifer lies below a thick Mesozoic cover. At the base of an inferred Rotliegend graben in the same area, temperatures of up to 75 °C are expected. However, geological and hydraulic conditions are uncertain. In the Lower Devonian basement, thick sandstone-/quartzite-rich formations with temperatures >90 °C are expected at depths >3.5 km and likely offer the possibility of direct heat use. The setting of the S{\"u}deifel (South Eifel) region, including the M{\"u}llerthal region near Echternach, as a tectonically active zone may offer the possibility of deep hydrothermal reservoirs in the fractured Lower Devonian basement. Based on the recent findings about the structure of the Trier-Luxembourg Basin, the new concept presents the M{\"u}llerthal-S{\"u}deifel Depression (MSD) as a Cenozoic structure that remains tectonically active and subsiding, and therefore is relevant for geothermal exploration. Beyond direct use of geothermal heat, the expected modest temperatures at 5 km depth (about 120 °C) and increased permeability by EGS in the quartzite-rich Lochkovian could prospectively enable combined geothermal heat production and power generation in Luxembourg and the western realm of the Eifel region.},
  language  = {en}
}
@phdthesis{Fuchs2013,
  author    = {Fuchs, Sven},
  title     = {Well-log based determination of rock thermal conductivity in the North German Basin},
  url       = {http://nbn-resolving.de/urn:nbn:de:kobv:517-opus-67801},
  school      = {Universit{\"a}t Potsdam},
  year      = {2013},
  abstract  = {In sedimentary basins, rock thermal conductivity can vary both laterally and vertically, thus altering the basin's thermal structure locally and regionally. Knowledge of the thermal conductivity of geological formations and its spatial variations is essential, not only for quantifying basin evolution and hydrocarbon maturation processes, but also for understanding geothermal conditions in a geological setting. In conjunction with the temperature gradient, thermal conductivity represents the basic input parameter for the determination of the heat-flow density; which, in turn, is applied as a major input parameter in thermal modeling at different scales. Drill-core samples, which are necessary to determine thermal properties by laboratory measurements, are rarely available and often limited to previously explored reservoir formations. Thus, thermal conductivities of Mesozoic rocks in the North German Basin (NGB) are largely unknown. In contrast, geophysical borehole measurements are often available for the entire drilled sequence. Therefore, prediction equations to determine thermal conductivity based on well-log data are desirable. In this study rock thermal conductivity was investigated on different scales by (1) providing thermal-conductivity measurements on Mesozoic rocks, (2) evaluating and improving commonly applied mixing models which were used to estimate matrix and pore-filled rock thermal conductivities, and (3) developing new well-log based equations to predict thermal conductivity in boreholes without core control. Laboratory measurements are performed on sedimentary rock of major geothermal reservoirs in the Northeast German Basin (NEGB) (Aalenian, Rhaethian-Liassic, Stuttgart Fm., and Middle Buntsandstein). Samples are obtained from eight deep geothermal wells that approach depths of up to 2,500 m. Bulk thermal conductivities of Mesozoic sandstones range between 2.1 and 3.9 W/(m∙K), while matrix thermal conductivity ranges between 3.4 and 7.4 W/(m∙K). Local heat flow for the Stralsund location averages 76 mW/m², which is in good agreement to values reported previously for the NEGB. For the first time, in-situ bulk thermal conductivity is indirectly calculated for entire borehole profiles in the NEGB using the determined surface heat flow and measured temperature data. Average bulk thermal conductivity, derived for geological formations within the Mesozoic section, ranges between 1.5 and 3.1 W/(m∙K). The measurement of both dry- and water-saturated thermal conductivities allow further evaluation of different two-component mixing models which are often applied in geothermal calculations (e.g., arithmetic mean, geometric mean, harmonic mean, Hashin-Shtrikman mean, and effective-medium theory mean). It is found that the geometric-mean model shows the best correlation between calculated and measured bulk thermal conductivity. However, by applying new model-dependent correction, equations the quality of fit could be significantly improved and the error diffusion of each model reduced. The 'corrected' geometric mean provides the most satisfying results and constitutes a universally applicable model for sedimentary rocks. Furthermore, lithotype-specific and model-independent conversion equations are developed permitting a calculation of water-saturated thermal conductivity from dry-measured thermal conductivity and porosity within an error range of 5 to 10\%. The limited availability of core samples and the expensive core-based laboratory measurements make it worthwhile to use petrophysical well logs to determine thermal conductivity for sedimentary rocks. The approach followed in this study is based on the detailed analyses of the relationships between thermal conductivity of rock-forming minerals, which are most abundant in sedimentary rocks, and the properties measured by standard logging tools. By using multivariate statistics separately for clastic, carbonate and evaporite rocks, the findings from these analyses allow the development of prediction equations from large artificial data sets that predict matrix thermal conductivity within an error of 4 to 11\%. These equations are validated successfully on a comprehensive subsurface data set from the NGB. In comparison to the application of earlier published approaches formation-dependent developed for certain areas, the new developed equations show a significant error reduction of up to 50\%. These results are used to infer rock thermal conductivity for entire borehole profiles. By inversion of corrected in-situ thermal-conductivity profiles, temperature profiles are calculated and compared to measured high-precision temperature logs. The resulting uncertainty in temperature prediction averages < 5\%, which reveals the excellent temperature prediction capabilities using the presented approach. In conclusion, data and methods are provided to achieve a much more detailed parameterization of thermal models.},
  language  = {en}
}