Borehole geology and hydrothermal mineralisation of well HE-27, Hellisheidi geothermal field, SW-Iceland

Abstract

Well HE-27 is an exploration/production well located on Skardsmýrarfjall Mountain situated in the Hellisheidi high-temperature field, within the Hengill volcanic system, SW Iceland. This directional well is 2116 m deep. An analysis of the uppermost 1000 m is presented in this report. Following binocular and petrographic analysis, it can be concluded that the lithology of well HE-27 comprises six hyaloclastite units based on textural differences. The rock types within these hyaloclastite units are: pillow basalts, basaltic breccia, basaltic tuff and a minor basaltic scoria layer between 854 and 860 m. Fine- to mediumgrained tholeiitic basalt intrusions occur below 700 m and a sandstone sediment formation is present between 262-278 m. Eight feeder and water receiving zones were identified using temperature, neutron and caliper logs. These zones show an association with circulation loss zones and lithological boundaries in the upper levels and occur in relation to intrusive bodies at depth. Resistivity and gamma logs generally designate the alteration zones.

Alteration within HE-27 is primarily influenced by the temperature of the formation which is approximately 248°C, based on the occurrence of epidote and wollastonite as alteration products. Five alteration zones were identified: unaltered, smectite/zeolite, mixed-layer clay, chlorite and chlorite/epidote zone, established by the alteration mineralogy in the uppermost 1000 m of the well. Based on the sequencing of the alteration minerals, there is a strong indication that the area experienced a heating period. Nonetheless, the occurrence of calcite as a late deposit in the sequences implies subsequent cooling. Fluid inclusion analyses, however, indicate a renewed heating episode after the period of cooling. A brief comparison of hydrothermal minerals in well HE-27 and OW-908A in Olkaria, Kenya emphasise the role of temperature in all hydrothermal systems.