Borehole geology and hydrothermal alteration mineralogy of well OW-39A, Olkaria geothermal project, Naivasha, Kenya

Abstract

Well OW-39A is a directional well drilled to the south at an azimuth of 180° and inclination of 20° to a measured depth (MD) of 3066 m, adjacent to the Ololbuttot N-S trending eruptive fissure. Comprehensive binocular and petrographic analyses of cuttings from the well indicate that the lithology of the well comprises five rock units, i.e. pyroclasts, rhyolites, tuffs, basalts, and trachytes and then intrusions of rhyolitic, syenitic, basaltic, and granitic composition. Trachyte forms the main reservoir rock and it is the dominant rock below 900 m. These rock units host secondary hydrothermal mineral assemblages which are dependent on temperature, permeability and rock type. Mineral deposition sequences in the well show systematic evolution from low to high temperature conditions with depth, as observed from alteration minerals in veins and vesicles. Five alteration zones were identified: an unaltered zone (0-134 m), a zeolite- smectite-illite zone (134-578 m), a chlorite-illite zone (578-748 m), an epidote-chlorite-illite zone (748-896 m) and an actinolite-epidote-wollastonite zone (896-3066). The appearance of epidote at 750 m and actinolite at 896 m indicates temperatures of 250 and 280°C, respectively, at these depths. A comparison of fluid inclusion analyses, alteration and formation temperature indicates two geothermal episodes, one of a high temperature geothermal system below 700-800 m depth and a recent second phase of cooling.

Permeability is observed in the cuttings by the intensity of oxidation, veining, alteration intensity, circulation losses, the presence of calcite, and an abundance of pyrite. Eight feed zones are identified in the well starting from 750 down to 2750 m, deduced from the shape of temperature profiles, and their locations correlate with the cutting data as mentioned above. The intermediate feed zone at 1100 and the major feed zone at the 2100 m are considered the dominant ones in the well.