Feasibility of Enhanced Geothermal Systems in the Precambrian crystalline basement in SW Scania, Sweden

Abstract: The ability to evaluate the geothermal potential at a depth of five to six kilometres in the crystalline bedrock is crucial for the EGS-technology to be utilized in the future. This work focuses primarily on presenting expected geological conditions at great depths in southwestern Scania. The study is based on information from E.ON’s deep drilling FFC-1 in Malmö and data and investigations of the crystalline bedrock at Romeleåsen and in the Dalby Quarry. The work presents qualitative and quantitative analyses from optical microscopy, SEM-EDX analysis, chemical analyses, density measurements and analyses of the thermal properties of the bedrock. Geophysical information from borehole logging has been used to assess rock type distribution in the rock mass. The survey shows that the bedrock in FFC-1 is mineralogically and chemically comparable to the bedrock on Romeleåsen. Gneiss of different colors and with varying foliation and quartz content make up approx. 80% of the rock mass. The rest is dominated by mafic rocks such as amphibolite and metabasite. Mineralogy shows that the bedrock in FFC-1 was metamorphosed under amphibolite facies conditions, which is evident in a relative higher frequency of garnet in comparison with the bedrock in Dalby. The two dominated rock types show clear differences in density (2.67 respectively 2.98 g/cm3) and thermal conductivity (3.62 respectively 2.57 W/m K), which gives very different geothermal conditions. The bedrock is similar to Romeleåsen, strongly fractured with different types of fractures, open and closed and presenting corresponding fractures fillings. The predicted elevated fracture frequency of the FFC-1 bedrock at greater depths is explained by pressure relief that created horizontal fractures, and tectonics related to the Tornquist zone. The tectonics and metamorphic history of the Fennoscandian Border Zone also indicates a high prevalence of latent fracture systems with good prospects for hydrostatic stimulation and EGS.