Geological and geophysical characterisation of the exploration boreholes EB1 and EB2, Inde Syncline, Rhenohercynian Fold-and-Thrust Belt and Weisweiler Horst, Lower Rhine Embayment, Germany

Within the framework of the industrial transformation (“Strukturwandel”) in the Rhenish Lignite Mining Area, alternative heat sources are becoming increasingly important to meet municipal heat demand. This need becomes even more pressing with the phase-out of the lignite-fired coal power plant in Weisweiler in 2029. It currently provides approx. 165 MWth of heat to district heating networks in the Rhenish Lignite Mining Area. The development of medium-deep (>400 m) and deep geothermal reservoirs (> 1,500 m) could be a partial solution to provide renewable heat to single buildings, residential or commercial neighbourhoods, or districts of the city of Aachen via the existing district heating network. Two exploratory wells (EB1 and EB2) were drilled next to an existing drill site by RWE Power AG within the exploration framework for geothermal energy and the construction of new research facilities on the premises of the power plant in Weisweiler. The first well, EB1, was drilled in October 2023 with a measured total depth of 100 m while the second one, EB2, was drilled in February 2024 with a measured total depth of 506 m. EB1 was equipped with a 3-component seismometer while a double-U heat exchanger was installed in EB2. Both boreholes are equipped with fibre optic cables along their entire length. An enhanced geothermal response test to estimate the effective thermal conductivities was performed in the second borehole. The cuttings, the core material, and geophysical logs provide information about the distribution of Cenozoic and Palaeozoic deposits of the Weisweiler Horst of the Lower Rhine Embayment. The Palaeozoic deposits of the Inde Syncline of the Rhenohercynian Fold-and-Thrust Belt are encountered at a depth of approximately 70 m. The observations enable the characterisation of depositional sequences (cyclothems) of the Upper Carboniferous and support the evaluation of the structural and petrophysical properties of the encountered alternating layers of mudstone, siltstone, sandstone, and hard coal. Current structural geological models of the area can now be revised to include the more certain stratigraphic boundaries of the Breitgang and Aussenwerke formations. Similarly, it will be possible to recalculate the depth distribution of the Lower Carbon-iferous Kohlenkalk carbonates by performing an uncertainty analysis. These boreholes mark the initial phase of geothermal exploration of the potential reservoirs around Weisweiler. Seismic campaigns and deep exploration wells are planned to continue exploring the subsurface of the region in the coming years.