DRILLHOLE GEOLOGY AND HYDROTHERMAL ALTERATION OF WELL KJ- 28 KRAFLA HIGH-TEMPERATURE AREA, NE-ICELAND

Abstract

The report describes the study of the drill cuttings of a 1003 m deep drill hole located in the Krafla high-temperature area. The strata penetrated by the drillhole comprise fine to medium grained basalts (olivine tholeiites and tholeiites), altered glassy basalts and basaltic tuffs and breccias which are referred to as hyaloclastites. Intrusions in the well appear at 520, 635, 700, 725, and 780 m depth. There are more than 15 aquifers. Both high temperature (>200oC) and low temperature (40-200oC) hydrothermal minerals are present in the well. According to the distribution of deposition minerals four alteration zones have been identified . They are smectite-zeolite zone (<200oC) down to 180 m depth, mix layer clay zone (200-230oC) down to the depth of 525 m, chlorite zone (230-250oC) down to 600 m depth and chlorite-epidote zone (250-280oC) which is continuous down to the depth of 808 m. No cuttings were collected below 808 m to the bottom of the well because of the total circulation loss (>40 l/s) . The increase in the temperature is indicated by the transformation of low grade clays to relatively coarse clays. With increasing depth the smectite becomes inter-layered with chlorite and high-temperature minerals such as wairakite appear. With the further increase in depth and temperature, epidote albite and sphene identified . They are smectite-zeolite zone (<200oC) down to 180 m depth, mix layer clay zone (200-230oC) down to the depth of 525 m, chlorite zone (230-250oC) down to 600 m depth and chlorite-epidote zone (250-280oC) which is continuous down to the depth of 808 m. No cuttings were collected below 808 m to the bottom of the well because of the total circulation loss (>40 l/s) . The increase in the temperature is indicated by the transformation of low grade clays to relatively coarse clays. With increasing depth the smectite becomes inter-layered with chlorite and high-temperature minerals such as wairakite appear. With the further increase in depth and temperature, epidote albite and sphene were identified Calcite pyrite and quartz are identified in all the alteration zones. Zeolites are most common above 250 m depth. Comparison of well KJ-28 with another drillholes in the area shows that it is located in a major upflow zone.