Exploration of geothermal fluids in the Reykholt area, Borgarfjörður, W-Iceland

Abstract

The Reykholt geothermal system in Borgarfjördur is the largest low-temperature thermal system in Iceland. The natural discharge of boiling water is 400 l/s. This system comprises the largest low-temperature geothermal area in Iceland and all the largest ones in Borgarfjördur, including the Deildartunga-Kleppjárnsreykir, Hurdarbak-Sídumúli and Vellir fields. After studying the chemistry of the fluid, the ratio of Cl/B in the range of Icelandic basalt and the D/ O18 values both represent the origin of thermal fluid as meteoric. These meteoric waters originate from the Arnarvatnsheidi highlands percolating through faults and permeable rocks. The faults are intersected by active fractures in the lowlands. The fluids then flow along the fractures to the surface in the form of hot springs, usually in lines. The Giggenbach Cl-SO4-HCO3 triangular diagram classifies the thermal fluid (with the exception of a few carbonate localities) as sulphate water. The change in chemical composition and temperature with time is negligible. There is no evidence of mixing with cold ground water in the area and the mineral equilibria are consistent, as reflected from the saturation index diagrams and also from the cation geothermometer temperature results. The geothermometers give quite reliable results for the reservoir temperature in the investigated area. These are in the range 70-150°C for the chalcedony geothermometer.