| Summary: | A 70-year history of continuous hot water production from low temperature fields within and around the city of Reykjavík, has resulted in a vast database of downhole temperature data. Systematic analysis of this database, containing around 200 deep wells which yield 30-50 million tons of hot water annually, provides valuable information on the subsurface temperature distribution in an area covering some 500 km2. A dominant feature is vertical fracture permeability and fluid convection, where the fracture strike is between N and NE. The fracturing is associated with an active fissure swarm of the Krýsuvík volcanic center in the south. Temperatures of the fracture systems range 20ºC to 140ºC, depending on recharge mechanism and length/depth of the convection cells. The same fracture system may thus have two separate inflows, consisting of a hot recharge, coming from the north and rising to shallow depths, and deep and cold underlying recharge coming from the south. Hot springs are commonly found on surface above such mixing areas. It appears that maximum depth of vertical convection cells increases with distance from the mountain Esja, from 600 to more than 2500 m. Two of the fractured geothermal reservoir systems extend laterally over 15-20 km distance.
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