Borehole geology and hydrothermal mineralisation of well HE-32, Hellisheidi geothermal field, SW-Iceland

Abstract

Well HE-32, is an exploration/production well at the base of Mt. Skardsmýrarfjall mountain in the Hellisheidi high-temperature field, within the Hengill volcanic system, in SW-Iceland. It is a directional well drilled to 2465 m depth. The lithology of the uppermost 1116 m has been analysed through petrographic examination of cuttings (binocular and optical microscopy) and geophysical logs. It shows that the lithology is predominantly a hyaloclastite formation comprised of pillow basalts, basaltic breccia and basaltic tuff. Additionally, a few thin interglacial lava flows are present. A couple of thin layers of fine- to mediumgrained tholeiitic basalt intrusions occur around 10, 200 and 550 m. A prediction of the lithology for the interval 1116-1900 m was undertaken demonstrating how geophysical logging can be of value where cuttings are missing due to circulation losses, yet giving an indication of the lithology is vital, e.g., in the production (liner) part of the well. Eight boundaries within 4 rock units were inferred from these logs. Hydrothermal alteration is controlled by temperature, rock type and permeability. The mineral assemblages showed that the alteration temperature increased with depth, first with the appearance of minerals such as low-temperature zeolites, quartz, and smectite. The alteration was high below 388 m with the appearance of quartz (≥180°C), wairakite (≥200°C) and later epidote (≥230°C) at 690 m.

The alteration increased below 850 m (with the appearance of actinolite, ≥280°C) and localized at around 1084 m (+garnet, ≥300°C). Abundant prehnite at 700-1100 m depth may be due to low permeability. Five temperature related alteration zones were characterised: smectite-zeolite zone (0-275 m), mixed-layer clay zone (275-375 m), chlorite zone (375-690 m), chlorite-epidote zone (690-850 m) and epidote-actinolite zone (850-1116 m). Five feed zones/aquifers were identified using temperature logs and drillers’ logs. These show an association with lithological boundaries and intrusive bodies. Comparison of alteration mineral temperature and formation temperature show that the geothermal system is overall in equilibrium, with a reservoir temperature of 230-270°C down to ~850 m, although temperatures of up to 300°C were recorded in a few fluid inclusions. At deeper levels, a localized ~40°C cooling at the aquifer zones between 1000-1100 m was observed. A comparison of the three wells HE-3, HE-32 and HE-51, all located on the same well pad, has enhanced our understanding that while relationships in overall subsurface geology and hydrogeology may exist in adjacent wells, each well is independent and behaves as an individual.