Prospecting for geothermal energy in the Iddefjord granite, Østfold, Norway

The Iddefjord granite in Østfold, Norway, which is a continuation of the Båhus granite in Sweden, was studied with respect to its possib1e use as a geothermal hot water reservoir. Two hoies (123 & 128 m long) drilled specifically for this project and a total of 22 deep water wells were measured. The temperature gradient of the granite increases towards the east where a maximum of 21.5°C/km is reached. The average gradient near the border of the granite is as low as 15°C/km. Conductivities from the two drilled hoies were measured. The heat flow in the thickest part of the granite is 1.88 hfu (78.6 mW/m'), which is high compared to the Norwegian average of 1.02 ±0.21 hfu.