The deep geothermal potential of the radiogenic Løvstakken Granite in western Norway

Abstract

Frey, M. & Ebbing, J. 2020: The deep geothermal potential of the radiogenic Løvstakken Granite in western Norway. Norwegian Journal of Geology 100, 202004. https://dx.doi.org/10.17850/njg100-1-4. © Copyright the authors. This work is licensed under a Creative Commons Attribution 4.0 International License. We developed a first detailed 3D crustal model of the Bergen Region by combining geological information with gravity and magnetic data in order to estimate the geothermal potential of the Løvstakken Granite, which shows a particularly high concentration of radioactive elements. The geometry of the near-surface horizons in the model is consistent with geological observations and interpretations. The basement structure is, in contrast, associated with greater uncertainties due to the lack of land seismic or deep borehole data. To improve the geological model, we performed stochastic inversions of the gravity and magnetic fields resulting in three plausible models for the Løvstakken Granite. Based on these modelling results, the subsurface temperatures were predicted by numerical simulation and the various influencing factors were investigated, whilst the 516 m-deep Fyllingsdalen borehole provided important thermal constraints. Especially the radiogenic heat production affects the thermal structure of the crust in the Bergen Arcs and we show that the concentration of radioactive elements in the entire basement is on average 50% smaller than at the surface. The geometry of the Løvstakken Granite also influences the geothermal gradient, but the differences between the three crustal models of 3°C/km are rather moderate. Furthermore, a correction for the paleoclimatic conditions in western Norway is applied which has an effect of up to 7 ± 4°C. Lastly, the groundwater influence was determined by creating a coupled fluid flow and heat transport model. Local temperature changes of up to ± 5°C are found, but there is no significant cooling at the Fyllingsdalen borehole due to groundwater. According to the calculated geothermal gradient in the Bergen Arcs System, the potential for the generation of electricity is relatively low. In comparison, there is an intermediate potential for the extraction of heating energy, in particular in the urban area of Bergen.