Borehole geology and hydrothermal mineralisation of well HN-08, Hellisheidi geothermal field, SW-Iceland

Abstract

Well HN-08 is located in the Hellisheidi high-temperature field, within the Hengill geothermal area in SW-Iceland. It is a directional reinjection well reaching a total depth of 2580 m. The uppermost 1000 m are analysed here. The well was drilled in May 2007 targeting a series of large faults at the western margin of the Hengill graben. The lithology of well HN-08 comprises basaltic hyaloclastite formations, basaltic lavas flows and dyke intrusions. Six alteration zones were identified: a zone of no alteration at <210 m depth, smectite-zeolite zone (<200°C) from 210-1004 m, mixed-layer clay zone (200-230°C) from 1004 to 1264 m, chlorite zone (230-240°C) from 1264 to1410 m, chlorite-epidote zone (>240°C) from 1410 to 1700 m and the epidote-amphibole zone (>260°C) from 1700 to the bottom. The alteration zones correlate with measured formation temperature, indicating that the geothermal system at this location is not affected by cooling. Just five insignificant circulation losses were identified in the production part and two very small aquifers above the production part. The correlation of hydrothermal alteration with well HN-05 at Gráuhnúkar shows that the lithology is similar, and the hydrothermal alteration indicated by epidote-wairakite and prehnite appears almost at the same depth. In addition, the intensity of hydrothermal alteration increases moderately with depth in the two wells.