Borehole geology and hydrothermal mineralisation of well OW–916, Olkaria Domes geothermal field, Naivasha, Kenya

Abstract

Well OW-916 is one of the production wells located in the Olkaria Domes Geothermal field in Naivasha, Kenya. The well is vertical. A geological analysis from the surface to 3000 m depth is presented in this report. Comprehensive binocular and petrographic analyses indicate that the lithology of the well is comprised of five different formations and minor intrusive rocks. These formations/units host secondary hydrothermal mineral assemblages which are dependent on temperature, permeability and rock type, and they show evolution of the hydrothermal system from low- to high-temperature conditions with depth, as observed from alteration minerals in veins and vesicles. The relationship between depth and hydrothermal mineral formation indicates low-temperature minerals at shallow depths and high-temperature minerals at greater depths. Three alteration zones were identified: a zeolite-illite zone, an epidote-chlorite-illite zone and an actinolite-epidote-chlorite-illite zone. Several feed zones were encountered and classified into major and minor feed zones, and they are associated with fractures, lithological contacts and circulation losses encountered during drilling.

The larger feed zones are between 650 and 1300 m and they show little cooling when comparing the alteration and formation temperatures. Fluid inclusion analyses indicate that the system is in equilibrium since the homogenisation temperatures of the inclusions are close to the current measured formation temperature of the well. Alteration and formation temperatures, however, indicate that the past geothermal conditions were at higher temperatures than current conditions. A comparison with other wells drilled in this area shows similarities in the lithology but alteration mineralogy shows that, in this well, alteration is high between 600 and 1500 m. The well produces 16 MW and epidote and actinolite are observed at shallower depth in this well than in the three other wells in this field to which it was compared, indicating that this area might be a possible up flow zone. The comparison also stresses the role and importance of temperature in all geothermal systems.