Title: Subsurface geology, hydrothermal alteration and geothermal model of Northern Skardsmýrarfjall, Hellisheidi 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 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 temperatures 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
Skardsmýrarfjall 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 Skardsmýrarfjall 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.