Borehole geology and hydrothermal alterations of well HE-39, Hellisheidi geothermal field, SW-Iceland

Abstract

Well HE-39 is located on Mt. Skardsmýrarfjall in the northern part of the Hellisheidi high-temperature field in the southern sector of the Hengill central volcano in SW-Iceland. It is a directional well with a total measured depth of 3055.5 m. The well was drilled with the aim of targeting 2000 and 5000 year old NE-SW trending volcanic fissures and to explore the geothermal system northeast of Mt. Skardsmýrarfjall. Drilling started on 13th October 2007 and was completed on 15th April 2008. The lithology of the well consists of basaltic hyaloclastite formations, basaltic lava flows and dyke intrusions. This report describes the uppermost 1000 m of the well, analysed as a part of this UNU-GTP project. Geophysical and drilling data indicate that permeability in the well is related to lithological contacts, intrusives, boundaries, and major faults and fractures.

Aquifers in the top 1000 m of the well are related to stratigraphic and intrusive boundaries. Hydrothermal alteration in the well is controlled by temperature, rock type and permeability. The mineral assemblage showed the evolution of the hydrothermal system from low- to high-temperature conditions followed by cooling, which is evidenced by the precipitation of calcite at later stages. Mineralogical examination revealed five zones of hydrothermal alteration beneath a zone of unaltered rocks, associated with zeolite-smectite (<200°C), mixed layer clays (200-230°C), chlorite (230-240°C), chlorite-epidote (>240°C) and epidoteactinolite (>260°C). In general, this study shows 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 fissure eruptions. An attempt to compare alteration sequences of wells OW-909, OW-901, OW-902 and OW-903 from Olkaria, Kenya revealed four hydrothermal alteration zones beneath an unaltered zone, the zeolite-chlorite zone, illite-chlorite zone, epidote-illite-chlorite zone and garnet-biotite-actinolite zone.