Borehole Geology and Hydrothermal Alteration of Well KJ-28 Krafla High-Temperature Area, NE-Iceland

Abstract

The report describes the study of drillcuttings of 100 3 m deep drillhole located in the Krafla high-temperature area. The strata penetrated by the drillhole consist of fine to medium grained basalts (olivine tholeii tes & tholeiites), altered glassy basalts, basaltic tuff and breccia which are referred as hyaloclastites. Intrusions appear at various depth levels, and there are more than 15 aquifers encountered in the well. Both high-temperature(>2000C) and low temperature (40-20 00C) hydrothermal alteration minerals are reported fr om the well. According to the distribution of alteration minerals four alteration zones have been identified viz.smectite zeolite zone (<2000C) down to 180 m depth, mix layer clay zone (200-2300C) > 180 to 525 m, Chlorite zone (230-2500C) from 525-600m depth and Chlorite-epidote ( 250-2800C ) continuous down to the depth of 808 m. No cuttings were collected be low 808 m to the bottom of the well because of total circulation loss (>40 l/s ). The increase in temperature is indicated by transformation of clays from fine to coarse grained. Zeolites are most common above 250 m depth. With the increased depth the smectite becomes ineterlayered with chlorite and high-temperature minerals such as wairakite appear. With further increase in depth and temperature minerals such as epidote, albite and sphene appear. Calcite, pyrite, and quartz are distributed in all alteration zones. Comparison of the well KJ-28 with other wells in the area shows that it is located in a major upflow zone.