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, a part of the Hengill central volcano in SW-Iceland. It is a vertical well reaching a total depth of 2587 m. The well was drilled with targeting a 5000 years old NE-SW trending volcanic fissure. The lithology of well HE-24 comprises basaltic hyaloclastite formations and dyke intrusions. Different alteration zones were identified in the well; zone of no alteration, smectite-zeolite, mixed layer clay, chlorite, chlorite-epidote and epidote-actinolite zone. These zones depict a trend from a cold groundwater system down to 244 m through a cap rock and into a high temperature system. Based on mineral sequence deposition and comparison between hydrothermal alteration mineral and formation temperature the geothermal system appears to have cooled, particularly in the upper 700-800 m of the well. Feed points were found in the well and categorized into weak, moderate and large aquifers. Some of these are located in the production part and are mostly associated with intrusions while others are located above the production part, and they are mostly associated with stratification boundaries.