Borehole geology and alteration mineralogy of well HE-9 in Hellisheidi geothermal field, SW-Iceland

Abstract

Well HE-9 was drilled down to 1604 m in Hellisheidi geothermal field in southwest Iceland in the summer of 2003. The goal of drilling was production of steam and to improve the existing conceptual model of the geothermal system. The main content of this report is to reveal how useful borehole geology is in the investigation of a geothermal field. The study emphasizes the uppermost 800 m of the well. Lithology of the well consists of hyaloclastite formations and basaltic lava flows. Hydrothermal alteration indicates four alteration zones: a smectite- zeolite zone, a mixed-layer clay zone, a chlorite zone, and a chlorite-epidote zone, respectively, down to 806 m depth. Calcite and pyrite are common in most parts of the well. Siderite is very common at 180-490 m depth. Alteration above 450 m is very low, indicating temperature less than 120C. Below that there is a sharp change indicating a high-temperature geothermal system characterized by chlorite, prehnite and wollastonite. The heating up of the geothermal system is deduced from correlation between alteration mineral temperatures and measured temperatures. Intrusions were identified at depths of 152-160, 180-186, 300-328, 388-430, 784-794, and 764-770 m. Aquifers were found at depths of 100, 120, 430 and 800 m where they are related to fractures along intrusion boundaries. Alteration mineral time sequences indicate heating up in the high-temperature geothermal system when prehnite and wollastonite have deposited after mixed layer clay.