ALBERT

Description: The formation of mineral and energy resources involves the interaction of groundwater flow, mechanical deformation, mass and heat transport processes. Thereby, groundwater flow patterns, temperature field, and fluid-rock interactions are all interdependent. This calls for a unified description linking the coupling between the different scales and related physical phenomena involved. A mathematical formulation of the main driving processes affecting basin fluid and heat transport allows developing numerical models as tools to examine the interactions of simultaneously active processes and variable parameters within the constraints given by physical principles and taking into account proper temporal and three dimensional spatial scales. Therefore, the usage of mathematical models is justified by the help they bring in the understanding and verification of specific mechanisms acting in natural systems. In Section “Basin Analysis” at GFZ German Research Centre for Geosciences mathematical models of increasing degree of complexity are applied to the study of energy and mass transport processes in complex sedimentary basins.

Description: The European Molasse Basin is a Tertiary foreland basin at the northern front of the Alps, which is filled with mostly clastic sediments. These Molasse sediments are under‑ lain by Mesozoic sedimentary successions, including the Upper Jurassic aquifer (Malm) which has been used for geothermal energy production since decades. The thermal field of the Molasse Basin area is characterized by prominent thermal anomalies. Since the origin of these anomalies is still an object of debates, especially the negative ones represent a high risk for geothermal energy exploration. With our study, we want to contribute to the understanding of the thermal configuration of the basin area and with that help to reduce the exploration risk for future geothermal projects in the Molasse Basin. For this, we conducted 3D basin‑scale coupled fluid and heat transport simulations to reproduce the present‑day thermal field of the Molasse Basin by con‑ sidering conduction, advection, and convection as heat‑driving mechanisms. Within this paper, we show how the temperature distribution of the Molasse Basin, including the pronounced thermal anomalies, can be reproduced by coupled fluid flow and heat transport simulations following a multi‑scale 3D‑modelling approach. We find that the shallow thermal field is strongly affected by basin‑wide fluid flow. Further‑ more, we show that the temperature distribution at the depth of the Malm aquifer is strongly influenced by the hydraulic conductivity of the Foreland and Folded Molasse Sediments and that hydraulically conductive faults have only a minor influence on the regional temperature distribution. Moreover, we show that the positive and negative thermal anomalies are caused by the superposed effects of conductive and advective heat transport and correlated with the geological structure.

Description: The European North Alpine Foreland Basin is a Tertiary wedge-shaped foreland basin at the northern front of the European Alps. The Molasse Sediments are underlain by Mesozoic sedimentary successions, which include the Upper Jurassic aquifer (Malm), a major target for geothermal energy production. The Molasse Basin has been used for geothermal energy production since decades due to its specific thermal configuration. The thermal field of the basin shows increasing temperatures from north to south and pronounced positive and negative thermal anomalies at depths of exploration interest. Between these anomalies, temperature differences of more than 40 K may occur over a small horizontal distance of just a few kilometres, a phenomenon which could so far not be explained based on the present-day knowledge. Though, a high amount of data about the structure as well as the distribution of temperatures and thermal properties in the European Molasse Basin exists, knowing the temperature distribution is not enough to reduce the exploration risk in the European Molasse Basin. Rather, an understanding of the heat driving mechanisms and the origin of the pronounced temperature anomalies is of high importance to reduce the uncertainty in predicting the extraction temperature and discharge of geothermal power plants. To explain the origin of the pronounced thermal anomalies in the German Molasse Basin, first a lithospheric-scale 3D structural model was constructed based on freely available depth and thickness information which includes the Molasse Basin as well as the South German Scarpland and some parts of the Alps. Areas not covered with measured data were constrained with isostatic calculations and 3D gravity modelling. In a second step, the present-day 3D steady-state conductive thermal field of the German Molasse Basin was calculated based on the gravity constrained lithospheric-scale 3D structural model. The predicted temperature distribution indicates that the thermal field is controlled by conductive heat transport in the lithospheric mantle and the crystalline crust. Shallower parts of the thermal field are strongly controlled by a thermal interdependence between the Alpine area and the basin itself and by the underlying crystalline crust related to their contrasting thermal properties. Furthermore, the results indicate that the distinct thermal anomalies in the German Molasse Basin are partly triggered by the structural configuration of the crust and the presence of the Tauern Body. To assess the influence of fluid flow on the shallow thermal field of the German Molasse Basin, coupled fluid flow and heat transport simulation were conducted which succeeded to reproduce the observed thermal anomalies in the German Molasse Basin. In contrast to assumptions of previous studies no permeable faults were needed to reproduce these thermal anomalies. The resulting coupled thermal field indicates that the temperature distribution is primarily controlled by conductive heat transport, but also strongly affected by basin-wide as well as local fluid flow especially at shallower depths. In particular, the results show that the positive and negative thermal anomalies are caused by a combination of conductive and advective heat transport and may be correlated to the permeability of the Molasse Sediments, to the facies controlled permeability distribution in the Upper Jurassic aquifer (Malm) and to the spatial distribution of the Cretaceous Purbeck formation.