Borehole stratigraphy and alteration mineralogy of well HE-6, Hellisheidi, SW-Iceland

Abstract

Well HE-6 is located in the Hellisheidi high temperature geothermal field in southwest Iceland. Drilled in 2002 to explore the conditions of the geothermal system, well HE-6 is now discharging at a rate of 29.4 kg/s with an enthalpy of 1950 kJ/kg. The stratigraphic units in well HE-6 are comprised of 4 distinct hyaloclastite formations, 3 lava units and 2 possible intrusions, one being of intermediate composition. There are four alteration zones: smectite-zeolite (376-730 m), smectitemixed layer clay-chlorite (730-800 m), chlorite (800-1000 m) and chlorite-epidote (1000-1018 m). Fourteen aquifers were located along lithological contacts, intrusive boundaries, fractures and faults, indicating good, permeability zones. The distribution of hydrothermal alteration minerals, such as pyrite and calcite, also depict valuable permeable zones. Calcite forms a platy structure at boiling temperatures and the homogenization temperatures range from 240 to 275°C. This homogenization temperature of the platy calcite represents the highest alteration temperature range and conforms to the present formation temperatures in the well. However, the alteration temperatures are actually lower than the present formation temperatures in the well, indicating that the well may be heating up.