Borehole geology and hydrothermal mineral alteration of well HN-3, Hellisheidi geothermal field, SW-Iceland

Abstract

Well HN-3 is located in the Hellisheidi high-temperature field, within the Hengill geothermal area in SW-Iceland. It is a directional reinjection well reaching a total depth of 1864 m. The uppermost 1272 m are presented in this report. A binocular microscope, a petrographic microscope, XRD-analysis, a fluid inclusion analysis and geophysical logs were used to gather data. The lithology of well HN-3 comprises basaltic hyaloclastite formations, tuffaceous sediment and fine- to medium-grained olivine tholeiitic and tholeiitic basalt. The rock types within the hyaloclastite units are: partially crystallized basalts, basaltic breccia, and basaltic tuff. There are also basaltic intrusions at depths of 1026-1032 m and 1234-1244 m. Four alteration zones were identified in this well: a zone of no alteration at 0-92 m depth, a zeolitesmectite zone (<200°C) from 92 to 972 m, a mixed-layer clay zone (200-230°C) from 972 to 1236 m and a chlorite zone (>230°C) from 1236 to 1272 m. Seven very small possible aquifers were identified above the production part of the well and six small aquifers at 440, 550, 660, 725, 1180 and 1245 m depth by using temperature, neutron-neutron, gamma, resistivity and calliper logs. Fluid inclusions studied show a wide range of homogenization temperatures which range from 180 to 330°C. Fluid inclusion temperatures, at 1200-1270 m, show higher values than the present formation temperature, indicating that the well is cooling at that depth.