Borehole geology, hydrotermal alteration and temperature evolution of well KR-2, Krýsuvík SW-Iceland

Abstract

KR-2 is a 1220 m deep well in the Krýsuvík high-temperature field, situated on the Reykjanes Peninsula. Today, this well is used to heat one hut. The rock formation disected by the well consists of basaltic lava series, basaltic hyaloclastite formations and minor intrusive rocks, which is believed to have accumulated during the last few hundred thousand years. Four alteration mineral zones have been identified including a smectite-zeolite zone (< 200°C) down to 140 m depth, a mixed layered clay zone (200-230°C) down to 380 m depth, a chlorite zone (> 230°C) down to 737 m depth and a chlorite-epidote zone (240-260°C) down to at least 1220 m depth. This zonation indicates a past and/or present high-temperature environment. Aquifers were confirmed at 130, 160-166, 920-924 and 1120 m depth, and possibly also at 400, 505-515, and 850 m depth. Hydrothermal evolution as seen through alteration indicates an initial low-temperature condition (< 200°C) followed by a high-temperature condition (240-260°C) and then a cooling as indicated by zeolites (<200°C). Present temperature shows an inverse gradient from about 550 m depth where temperature drops from about 200°C to nearly 150°C at around 800 m depth. A conceptual model indicates an upflow zone to the southwest from well K-2 and a cold water influx from the northeast.