The origin of deep geothermal anomalies in the German Molasse Basin: results from 3D numerical models of coupled fluid flow and heat transport

Abstract

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 underlain 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 considering 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. Furthermore, 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.