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

Skarðsmýrarfjall is located in northern part of Hellisheiði, 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 haver 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 Skarðsmýrarfjall 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 at later stages. The mineralogical examination also revealed five zones of hydrothermal alteration beneath a zone of unaltered rocks. These zones are zeolite-smectite, mixed layer clay, chlorite, chlorite-epidote and epidote-actinolite. Fluid inclusion studies have shown three distinct ranges of homogenization temperatures indicating two or a third probable phases of geothermal activity in well HE-37; an earlier one with high rather anomalous temperature up to 3200C and a lower temperature range of 215-230°C which conforms to present formation temperature at 734 m but is higher than the formation temperature at 1162 m. A third stage may be present, showing temperatures as low as 175°C at 734 m, which is lower than formation temepratures at that depth but conforms to the temperature at 1162 m. In well HE-24 a similar wide range of fluid inclusion temperatures show probable three phases and boiling is proposed at 700 m. A more shallow hydrothermal alteration is found in the northern part of Skarðsmýrarfjall which diminishes to the south observed in well HE-37 in the north and well HE-24 in the south. This shallow hydrothermal alteration in this area together with the scanty surface manifestations in the whole of Skarðsmýrarfjall on the one hand and the presence of very extensive fossil surface alteration in Hengill mountain on the other could indicate that an upflow channel of the geothermal system may underlie Hengill mountain, which also further indicates that the studied wells are located in the outflow zone of the system. In general the evidences from this study has shown that there are three successive stages within the history of the geothermal system. A progressive heating, a later cooling episode and finally a probable renewed heating phase, which may relate to the two Holocene eruptive fissure eruptions.United Nations University, Geothermal Training Pregramme, Government of Icelan