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Abstract:
During the last decades, more than 30 relatively deep boreholes were drilled onshore in Norway. According to thermal logging, the highest temperatures (up to 28.2 °C at 800 m depth) were observed in the boreholes, located on the Svalbard archipelago which has signs of Quaternary magmatic activity. In contrast, the tectonically quiescent northernmost Norway is characterized by the lowest measured temperatures (slightly more than 9 °C at 650 m depth). The highest temperature in Central Norway (22.2 °C at 800 m depth) was measured on the small island close to the deep sedimentary basins in the Norwegian Sea. The southwestern part of Norway shows relatively low temperatures (e.g., 17.6 °C at a depth of 800 m near Stavanger). These low temperatures can be a result of groundwater flow that cools down the uppermost crust in this region. The area around the Oslo Fjord shows a complicated thermal pattern in the subsurface. There, at 800 m depth, the measured temperature ranges from 16 to 24 °C. The described differences in the measured temperatures reflect the tectonic differentiation of the areas. A variation in the content of radioactive elements is also reflected by the measured temperatures. Consequently, the superposition of tectonic structure/evolution and compositional features controls the subsurface thermal pattern of Norway which is locally disturbed by the groundwater flow. Moreover, the paleoclimatic thermal effect is still present in these 300 to 1500-m-deep boreholes. Therefore, the above-mentioned factors must be evaluated in detail before planning the first deep geothermal site in Norway.
