Borehole geology and hydrothermal alteration of well KJ-29 in the Krafla high-temperature area, NE-Iceland

Abstract

KJ-29 is a 2103 m deep well in the Leirbotnar field of the Krafla high-temperature area, NE-Iceland. It was drilled this year to mine steam for electrical production. The discharge of the well is very high. This study deals with the upper 1000 m of the well. The rock formation dissected by the well consists of basaltic hyaloclastite formations and basaltic lava series which are believed to have accumulated during the last glacial and interglacial periods. Intrusive rocks of basalt/dolerite composition occur below 530 m depth and predominate from 990 m down to the bottom of the well. Aquifers were confirmed at 100-155, 275, 600, 680, 730, 800 and 930 m depth, mostly related to intrusion contacts and fractures. Five alteration zones are identified including a smectite-zeolite zone (<200°C) down to 212 m depth, a mixed-layer clay zone (200-230°C) down to 420 m depth, a chlorite zone (230-250°C) down to 822 m depth, a chlorite-epidote zone (250-280°C) down to 998 m depth and an epidote-actinolite zone (>280°C) from there down to the bottom of the well. Measured temperatures in the lower part of the well are roughly comparable with the alteration temperatures. These evidences indicate a stable high-temperature environment. The early occurrence of wairakite and quartz may be related to an earlier hydrothermal event, presumably within the last glacial period. Hydrothermal evolution, as seen through alteration, indicates an initial low-temperature condition followed through intermediate to a high-temperature environment. The geology of the well confirmed that it is situated inside the horst system in the Krafla caldera, close to a fault related upflow zone causing an elevation of alteration zones to shallow depths.