Borehole geology and alteration mineralogy of well HE-11, Hellisheidi geothermal field, SW-Iceland

Abstract

HE-11 is a 1652 m deep well in the Hellisheidi high-temperature field in the Hengill geothermal area in SW-Iceland. It was drilled in June 2004 as a production well for the utilization of electric power generation. The well is directional, aimed to intersect a 5000 years old northeast trending Postglacial volcanic fissure. The lithology consists predominantly of volcanic rocks of basaltic composition that includes lava flows and sub-glacial formations of hyaloclastites. Several basaltic intrusions intrude the hyaloclastites. The hyaloclastite formations make up about 90% of the whole stratigraphic section and include tuff, breccia and pillow lava. Hydrothermal alteration indicates four alteration zones; a zone of no alteration down to 486 m, the smectite-zeolite zone (486-700 m), mixed-layer clay zone (700-756 m), chlorite zone (756-950 m) and a chlorite-epidote zone (>950 m), indicating temperatures of <200°C, 200-230°C, 230-250°C and >250°C, respectively. The time related mineral depositional sequences show a geothermal system undergoing a progressive temperature increase over time. A correlation between the formation and alteration temperatures indicates that recent heating has occurred from about 500 m down to 1000 m but is in equilibrium from there to about 1200 m. Many of the aquifers in the production part of the well appear to be related to dyke intrusions, indicating the dominance of fracture permeability in the geothermal reservoir.