Borehole geology and alteration mineralogy in the upper half of well HE-3, Hellisheiði, SW-Iceland

Abstract

The 1887 m deep exploration well HE-3 was drilled at Hellisheidi high- temperature field in the Hengill geothermal area in SW-Iceland during June-August 2001. The well is directionally drilled below 330 m and this project, which deals with the upper 812 m of the well, shows a stratigraphical section with volcanic rocks of basaltic composition that includes lava flows and subglacial formations of hyaloclastites, intruded by a couple of narrow basaltic intrusions. The hyaloclastite formations make up about 90% of the whole section and are composed of pillow lava, breccia and tuff. Four alteration zones are identified: smectite - zeolite zone (20-250 m), mixed layer clay zone (250-370 m), chlorite zone (370-792 m), and chlorite - epidote zone (>790 m), indicating temperatures of <200°C, 200-230°C, 230-250°C, and >250°C, respectively. The time related mineral depositional sequence shows that the geothermal system has evolved from a low-temperature through intermediate- to a high-temperature system. Calcite deposition succeeding the high-temperature assemblage may imply later cooling. The shape of the alteration temperature curve suggests that the well is located near an upflow zone in the geothermal system. Aquifers are generally small and confined to the upper 180 to 500 m depth interval. They are related to lithological boundaries and intrusion contacts. All these aquifers are nonproducing and are excluded by casing and subsequent cementing of the well. Based on the geological findings, hydrothermal alteration analysis and field relationships of geological units and structures, a possible conceptual geothermal model is proposed for the upper part of the geothermal system into which the well was drilled.