Borehole geology and hydrothermal mineralisation of well OW-911A, Olkaria Domes geothermal field, Central Kenya Rift Valley

Abstract

Well OW-911A is a production well located in the Olkaria Domes, situated within the Greater Olkaria geothermal system. This directional well was drilled to a depth of 3006.3 m. Sampling of the cuttings was fairly good, except where circulation losses occurred at 78-222, 660-664, 720-722, 1742-1744, and at 2876-2886 m. The rock types encountered consist of pyroclastics, rhyolites, tuffs, trachytes and basalts. The top 78 m are made up of unconsolidated pyroclastics overlying the rhyolite, basalt, trachyte and tuff. At depth trachyte was found to dominate the lower part, intercalating with tuff and occasionally basalt. The first occurrence of crystalline epidote was at 670 m depth, indicating an alteration temperature of >240ºC below that depth. The well showed some permeability, as indicated by temperature logs, circulation losses and hydrothermal alteration mineralogy patterns. This report discusses the likely causes as to why OW-911A is a lower producer than the surrounding wells: OW-909, OW-908A, OW-910A and OW-912A. It also gives a subsurface view of the extent of the Domes since it appears to be located at the boundary of the Domes area.