Borehole geology and hydrothermal alteration of well HE-58, Hellisheidi geothermal field, SW-Iceland

Abstract

Well HE-58 is located in the Hellisheidi high temperature field within the Hengill geothermal area in SW-Iceland. This directional well was drilled to a total depth of 2531 m. An analysis of the uppermost 1200 m is presented in this report. Binocular microscope, petrographic microscope, XRD-analysis, a fluid inclusion analysis and geophysical logs were used to gather data which are published and interpreted in the paper. Following binocular and petrographic analyses, it can be concluded that the lithology of well HE-58 mostly comprises hyaloclastite units. The rock types within these hyaloclastite units are: pillow basalts, basaltic breccia, and basaltic tuff. There are also basaltic intrusions at depths of 1050-1060 m and 1142-1148 m. Four feeder zones were identified using temperature, neutron and caliper logs. These zones show an association with circulation losses and lithological boundaries in the upper levels but occur in relation to a basaltic intrusion at depth. Four alteration zones were identified in this well: a zone of no alteration at 0-100 m depth; a zeolite-smectite zone (<200°C) from 200 to 450 m; a chlorite zone (230- 250°C) from 450 to 760 m and a chlorite-epidote zone (250-280°C) from 760 to 1200 m. Fluid inclusion studies show a wide range of homogenization temperature which range from 220-386.7°C. The homogenization temperature appears to be in equilibrium with the alteration mineralogy. However, it was not possible to compare the alteration with the formation temperature, because the well was still heating up at the time of the study.