Geothermal energy from impact craters? : The Björkö study

The suspected ca 10 km diameter Bjorko impact structure has been studied for its potential to retrieve geothermal energy. It has formed in crystalline rocks ca 1.2 Ga ago and is located relatively close to the district heating infrastructure of the Stockholm region, which the largest district heating system in Europe. The region is within a crystalline shield with low geothermal gradient of 15-20 Kkm -1 . Impact structures typically contain fractured rock volumes in the form of allogenic and authigenic breccia formations. This brecciation is the target for geothermal investigations. The low geothermal gradient requires target depths of 3-5 km to obtain sufficiently high temperatures for use in district heating systems. In the Bjorko structure therefore both gravity and magnetotelluric (MT) measurements were made to assess the extent of brecciation at such large depth. A relation between the 3-d-fracture frequency and the electric resistivity was derived based on surface sampling in 19 test areas and two > 900 m deep drill holes. MT measurements were made at 40 stations and a dense net of gravity measurements was located in the area. Modelling of the resistivity structure indicated that at least 15 km 3 of connected low resistive rock occurs at the target depth between 3 and 5 km. Flow tests made in the wells show a very low permeability confined to a few larger fracture zones. The fracture fill minerals (mainly calcite) are relatively weak and can be re-fractured at relatively low stress. Before further investigations of the energy potential can be made, more comprehensive hydraulic fracturing experiments must be performed. If it is possible to regenerate the vast fracturing, a very large energy potential can be envisaged for the entire 250 km 3 structure.