Geochemistry of thermal water from the Theistareykir geothermal field in NE-Iceland compared to that of the geothermal prospects in NW-Rwanda

Abstract

Geochemical methods were applied to the Theistareykir field in NE Iceland and the geothermal prospects in NW Rwanda. The results were then compared. Theistareykir was subjected to production drilling a decade ago, while in NW Rwanda, exploratory drilling started in August 2013. In Theistareykir field, Wells ThG-2, ThG-3, ThG-4 and ThG-5b discharge the characteristic enthalpy of a mixture of liquid and vapour (917 - 2661 kJ/kg). The discharged fluid was classified as mature Na-Cl-HCO3 water and the maximum subsurface temperature calculated by the WATCH programme was 300°C. The chemical composition of the wells and warm springs suggests the presence of more than one aquifer drawing fluid from a single base reservoir; warm springs are the result of super-heated shallow water. When boiled, the deep liquid becomes super-saturated with amorphous silica; calcite saturation has been reached in most wells. The geochemistry of the warm springs in the geothermal prospects in NW Rwanda suggests that the water is Na-HCO3 water that has not attained full equilibrium. The thermal water might be steam heated groundwater, not necessarily reflecting subsurface properties. Both in the Theistareykir field and the geothermal prospects in NW Rwanda there are warm springs on the surface which do not necessarily reflect the deep reservoir properties. Their presence simply indicates that the prevailing reservoir of the geothermal system is composed of many aquifers, some shallow, others deep.