Borehole geology and hydrothermal alteration of well OW-30, Olkaria geothermal field, Kenya

Abstract

This report presents data from the geological well logging of OW-30 in the East producing field, Olkaria. Sampling of the cuttings was fairly good as most of the drilled depth of 1596 m had good circulation returns except for the depth intervals 50-196, 202-268, 840-920, and 1122-1354 m where total circulation losses were experienced. The rock types penetrated include pyroclastics, tuffs, rhyolites, trachytes and basalts. The upper part of the well is mainly tuffs and rhyolitic lava flows blanketed by loose unconsolidated pyroclastics. Basaltic to intermediate lavas with tuff intercalations make up most of the well, with trachytes dominating at deeper levels. Hydrothermal alteration mineral assemblages indicate a high temperature area, with certain depths showing evidence of cooling probably due to incursion of cold fluids. Four hydrothermal alteration zones are present in this well and are represented by the first appearance of the index minerals representing each zone. They are the smectite-zeolite, the mixed-layer clays, the illite-chlorite and the chlorite-epidote zones representing temperatures less than 200, 200-230, 230-250, and over 250°C, respectively. Appearance of garnet at 1354 m and disappearance of calcite at 1410 m indicates a temperature of over 280°C. Completion and temperature recovery tests show major permeability at around 600-700, 900-1050 and 1300-1400 m, and probably at the well bottom.