Borehole geology of well Rn-9, Reykjanes, SW-Iceland

Abstract

Well Rn-9 is the largest producer in the Reykjanes high temperature geothermal field. Analyses of drill cuttings were done and results used with other downhole measurements to interpret some conditions of the well. Ten probable locations of aquifers have been identified on the basis of circulation loss measurements, temperature profile anomalies, variations in hydrothermal mineral intensity and intensity of veining. The aquifers above 600 m are controlled by permeability along stratigraphic units whereas those below are mostly controlled by permeabilities adjacent to basaltic intrusions. Petrographic and XRD studies of the cuttings show four alteration zones in this well; a smectite-zeolite zone, a mixed layer clay zone, a chlorite belt and a chlorite-epidote zone. Interpreted formation temperatures based upon hydrothermal alteration minerals indicate temperatures of about 110 C at shallow depths of 82 m increasing rapidly to about 200 C at 310 m depth and over 250 C at about 600 m depths. Temperatures of 270 C at about 700 m depth are indicated by occurrence of wollastonite. Disappearance of calcite and presence of garnet indicate temperatures above 290 C at about 1000 m depth. Although the maximum measured temperature in the well is 295 C, occurrence of calcite as the latest deposition about 1100-1250 m depth may indicate lower temperatures at this depth interval. Correlations of occurrence of hydrothermal alteration minerals in the other wells in Reykjanes show that the geothermal system was more extensive in the past, as seen by occurrences of high temperature minerals at shallow and cold parts of some of the wells.