Borehole geology, hydrothermal alteration and structural mapping of well HE-30, Hellisheidi, SW-Iceland

Abstract

Well HE-30 is located in the Hellisheidi high-temperature geothermal field, SWIceland. It is a directional production well which was drilled to 2318 m to explore faults at the western margin of the Hengill fissure system as well as to investigate the Reykjafell mini-graben. HE-30 is composed of hyaloclastites (basaltic tuff, basaltic breccia and pillow basalt) and interglacial lavas, occasionally intruded by fine- to medium-grained crystalline basalts. Permeability is provided through faults, formation boundaries and intrusions. Thirteen feed zones were identified in HE-30. Five intrusions were observed and four alteration zones: smectite-zeolite (116 – 456 m), mixed layer clay (456 – 718 m), chlorite - epidote (718 – 1014 m), and epidote – actinolite (1014 – 1328 m). Fluid inclusion homogenization temperatures are lower than in the formation and alteration temperatures, which may indicate an old formation temperature. Alteration and formation temperatures indicate that the system is in thermodynamic equilibrium.