Continued documentation of the lithology and radioisotope concentrations of the GE1 borehole, Högsbo

Abstract

During 2021, Geogruppen got a mission by Gothenburg Energy to extract a 1000-meter drillcore from the Högbo area in order to test the site for potential geothermal energy production. The site was chosen because of the high amount of RA-granite in the west of the area, which was believed to be beneficial for the potential of having deep geothermal energy. This is because of the increased amount of radioactive decay, increasing the geothermal gradient and allowing the borehole to be shallower and still receive the desired heat. The following thesis aim was to continue the previous work done by Julia Ladefoged (2021) and Anna Hynynen (2021) together with Co-student Johan Lindell (2022). Whereas this thesis’ focus is on the petrological aspects the work done by Anna Hynynen and Johan Lindell instead concentrated on the structural properties of the sampled rocks. In addition to logging the core, compositional measurements were done with a handheld XRF to determine the concentrations of Potassium, Iron, Silica, Calcium, Titanium and Aluminium. The Uranium, Thorium, and Potassium content were also measured with Mikael Tillberg's help, with a Gamma-ray spectrometer. Other properties such as density and relatively heat production rate were then calculated. Another aim was to do fieldwork at Änggårdsbergen in order to correlate the petrological aspects found in the field with the ones in the core. Furthermore, the long-term goal of this project is to evaluate the possibilities of achieving geothermal energy for Gothenburg. The primary lithologies found in the core between the 556- and 1001.45-meter intervals were granitic gneiss, tonalitic gneiss, granodioritic gneiss, and biotite zones, all of which ranged from weakly to strongly foliated. Additionally found were different amounts of accessory minerals such as Pyrite, Fluorite, Hematite, Magnetite within the main rock types. When correlating the core with the field results it could be seen that red stained quartz veins are present in both the core and in field. Together with the sign of hydrothermal processes occurring because of the presence of chlorite. Different types of intrusions with mafic minerals such as biotite, amphiboles and chlorite are causing sudden increases in density. The granodiorite found in field is believed to be the result of a low density of vegetation which allowed the overlaying RA-granite to be more withered compared to other mountain tops. Furthermore, that those samples found in field require more testing to know for certain if they are RA-granites or not and that the potassium concentration was not enough of an indication. In conclusion the heat production rate can be seen as relatively low but that further investigation is required to discussed if Geothermal energy is suitable for Gothenburg or not.