Geology, hydrothermal alteration and geological structures of well HE-59, Hellisheidi geothermal field, SW-Iceland

Abstract

Well HE-59 is located in Hellisheidi geothermal field within the Hengill hightemperature geothermal area. It was drilled directionally to a total depth of 2400 m with a western bearing into Mt. Reykjafell. Binocular microscopy, petrographic analysis, X-ray diffraction analysis and fluid inclusion analysis were the major tools used to interpret the cutting samples that were collected from the upper 902 m, but subsequently total circulation loss occurred and cuttings were not retrieved at the surface. Beside these, the study was supported by borehole geophysical data and a surface structural study. The stratigraphy of well HE-59 is represented by dominant sub-glacial eruptions of hyaloclastite and thin layers of interglacial and postglacial basaltic lava flows and intrusions. The hyaloclastite formation is basaltic in composition and characterized by the inter-layering of basaltic tuff, breccia and pillow or glassy basalt. Structural correlation of the well path with the surface faults and fissures shows that the well crosses a nearby, narrow, postglacial volcanic fissure at 900 m and this is evidenced from the gyro survey, circulation losses and lithological and intrusion findings. The alteration zones in this well are challenging, where the hydrothermal mineral distribution and clay analysis show different outlines. However, the combination of the two approaches infers the presence of four alteration zones in the well.

These zones are unaltered zones (0-150 m), a smectitezeolite zone (150-616 m), a chlorite-epidote zone (616-716 m) and an epidotewollastonite zone (716-902 m). From the temperature logs and circulation losses, four minor and five major aquifers were identified, which are related to geological boundaries and structures. The formation temperature, which is based on the temperature logs before the recovery of the well, together with fluid inclusions study and alteration temperatures indicate a cooling down of the system in this well. However, further temperature logs are necessary as they may change this assumption.