Borehole geology and hydrothermal mineralisation of well HE-22, Ölkelduháls field, Hengill area, SW-Iceland

Abstract

Well HE-22 is the third exploratory well drilled in the Ölkelduháls area, situated at the eastern flank of the Hengill volcanic and geothermal complex. The intercepted rock succession consists of hyaloclastite formations and lava flows of basaltic composition as well as minor intrusive rocks. The formations host a wide variety of secondary hydrothermal assemblages from low- to high-temperature minerals. Five main alteration zones have been identified: smectite-zeolite (<200°C) at <170 m, mixed-layer clay (200-230°C) from 170 to 206 m, chlorite (230-240°C) from 206 to 364 m, chlorite-epidote (>240°C) between 364 and 740 m, and the upper boundary of epidote-actinolite (>280°C) at 740 m. The relationship of time and mineral crystallisation indicate formation from low-temperature at shallower levels to high-temperature in deeper portions of the well. However, the wide deposition of calcite as the end member of the assemblage may indicate cooling of the geothermal system. Seven feed zones were encountered and categorized into weak and moderate aquifers. These aquifers can be correlated to lithological formations, intensity of alteration, abundance of veins and vesicles, stratigraphic boundaries, and the presence of intrusive bodies. The comparison of hydrothermal mineralisation with other wells drilled in Ölkelduháls geothermal field indicates progressive crystallisation from low-temperature minerals at shallow depth to high-temperature minerals at deeper levels; there are, however, indications of overprinting of these minerals. The current geothermal formation temperature is in disequilibrium with the hydrothermal alteration temperatures.