Borehole geology and alteration mineralogy of well HE-52, Hellisheidi geothermal field, SW-Iceland

Abstract

Well HE-52 is located in the northwestern part of Hellisheidi, which in turn is situated in the southern sector of the Hengill high-temperature field. It is drilled to a depth of 2516 m as a directional well and deviated northwards (N) to reach the high-temperature reservoir situated at the root of the Skardsmýrarfjall Mountain. The lithology of this well comprises alternating successions of hyaloclastite formations and lava flows of basaltic composition, as well as minor successions of sedimentary basaltic tuff. The hyaloclastite formations make up about 80% of the stratigraphy, and include breccia, pillow lava and tuff. The permeability in the well is related to lithological contacts, intrusions, faults and fractures. Four hydrothermal alteration zones were identified below the unaltered zone: a smectite-zeolite zone (120-386 m), a mixed-layer clay zone (386-586 m), a chlorite zone (586-634 m) and a chlorite-epidote zone (634-1190 m). Based on the intensity of alteration, the abundance of veins and vesicles and stratigraphical boundaries, seven feed zones were recognized in the uppermost 1000 m and categorized as weak aquifers. Based on a study of the mineral assemblage and the mineral sequence, the hydrothermal system in well HE-52 is increasing from lowto high-temperature sequence with increasing depth. This condition is followed by cooling evidence envisaged by precipitation of calcite at a later stage. Fluid inclusions studied have shown a wide range of homogenization temperatures which range from 180 to 285°C and, after comparing it with the formation temperature, indicate large temperature variation in the geothermal system at 874 m.