Borehole geology and hydrothermal alteration of well HE-24, Hellisheidi geothermal field, SW-Iceland

Abstract

Well HE-24 is located in the Hellisheidi high-temperature field, Hengill geothermal area in SW-Iceland. It is a vertical well reaching a total depth of 2587 m. The uppermost 1200 m are analysed here. The well was drilled in August 2006 targeting a 5000 years old NE-SW trending volcanic fissure. The lithology of well HE-24 comprises basaltic hyaloclastite formations and dyke intrusions. Five alteration zones were identified in the well: a zone of no alteration at 12-244 m, a smectite-zeolite zone at 244-616 m, a mixed layer clay zone at 616-672 m, a chlorite zone at 672-1074 m, a chlorite-epidote zone at 1074-1172 m and an epidote-actinolite zone below 1172 m depth. These zones depict a trend from a cold groundwater system down to 244 m and passing through a cap rock and into a high-temperature system. Based on mineral sequence deposition and a comparison between hydrothermal alteration and formation temperatures, the geothermal system appears to have cooled in the upper 700-800 m of the well. Around twenty-three feed points were found in the well and categorized into weak, moderate and large aquifers. Seven of these are located in the production part and are mostly associated with intrusions while sixteen aquifers are located above the production part, and are mostly associated with stratification boundaries. The correlation of hydrothermal alteration minerals of wells HE-23, HE-24 and HE-25 at Skardsmýrarfjall shows that the alteration mineral temperature has a similar increase in gradient with depth in the three wells, although it is only in well HE-24 that the epidote-actinolite zone is found above 1200 m depth.