Geology and Hydrothermal Alteration in the Reservoir of the Hellisheidi High Temperature System, SW-Iceland

Abstract

The Hengill central volcano is situated in the Western Volcanic Zone in Iceland on a triple junction where two active rift zones meet a seismically active transform zone. The area is a high temperature geothermal field which Reykjavík Energy has been exploring and exploiting. Presently the drilling of well number 55 is taking place in the Hverahlíð field at the southeastern sector of the Hengill area. Aside from those the drilling of 12 reinjection wells has already taken place.
The dominant rock formation in the Hellisheiði field is hyaloclastite (tuffs, breccias and pillow lavas) formed sub-glacially. This is to be expected as the area is a part of the Hengill central volcano where sub-glacial rock formations pile up. Lava successions from interglacial periods flow to the lowlands and are therefore less common. Hverahlíð field is, however, different from the Hellisheiði field in respect of the build up of lavas since the dominant rock formation in Hverahlíð wells is lava series. This would suggest that Hverahlíð has been outside the domains of the Hengill central volcano.
Aquifers in 57 wells at Hellisheiði have been located using down-hole temperature logs. Aquifers in the wells were assessed and placed at 100 m depth intervals and normalised with respect to the number of wells reaching each depth interval showing that large aquifers are not found below 2000 m depth.
Hydrothermal alteration ranges from totally fresh rocks in the overlying cold groundwater system through zeolite assemblage and into high-temperature mineral assemblage including epidote, wollastonite and actinolite. The comparison of alteration and formation temperatures seems to indicate minor cooling at the western side of Skarðsmýrarfjall as well as a cooling front from the east between Skarðsmýrarfjall and Hverahlíð. The Gráuhnúkar area, at the south western sector of the Hellisheiði field seems to be heating up and the same can be said about a certain part of the Hverahlíð field. Formation temperature and hydrothermal alteration indicate three upflow zones beneath Gráuhnúkar, Reykjafell and Hverahlíð.