Subsurface geology, hydrothermal alteration and geothermal model of Northern Skarðsmýrarfjall, Hellisheiði geothermal field, SW Iceland

Abstract

Skardsmýrarfjall is located in northern part of Hellisheidi, which in turn is situated in the southern sector of the 110 km2 Hengill low-resistivity anomaly, one of the high-temperature geothermal fields in Iceland containing economically promising geothermal prospects. Two wells drilled in this area with the aim of understanding the geothermal system beneath were studied; HE-24 a vertical well drilled to a depth of 2587 m and HE-37 a directional well having a depth of 3111.5 m. The lithology of the wells comprises hyaloclastites and lavas with intrusions of basaltic and intermediate composition. The hyaloclastite formations have been further classified into seven different formations based on their texture, crystallinity and compositional variation. In addition to these the different hyaloclastite formations have been identified for another two wells HE-39 and HE-27 in Skardsmýrarfjall and correlated with the above wells. Permeability in the wells is related to lithological contacts, intrusive boundaries, major faults and fractures. Aquifers in the top part of the wells are related to stratigraphic boundaries while sources of permeability in the bottom part being mostly along intrusive boundaries. Hydrothermal alteration in the wells is controlled by temperature, rock type and permeability. The mineral assemblage showed the hydrothermal system to have evolved from low- to high-temperature conditions followed by cooling evidenced by the precipitation of calcite ......