Borehole geology and alteration mineralogy of well OW-914A, Domes area, Olkaria geothermal field, Central Kenya Rift

Abstract

Well OW-914A is a deviated production well drilled on the eastern margins of the Domes field in the Greater Olkaria geothermal area. This well is 2996 m deep with an inclination of 17.85° and azimuth N77.14°E. The well was drilled in order to confirm the resource east of the Domes area and to gather steam for the construction of the 140 MWe Olkaria IV power plant. The drilling was completed within 48 days, beginning 25th June 2010 and ending 11th August 2010. This report presents an analysis of the cuttings collected at 2 m interval from the surface to 2996 m depth in the well. According to binocular and petrographic analyses, it was observed that OW-914A cuts through lithological units mainly composed of pyroclastics, tuff, rhyolite, basalt and trachyte. Trachyte forms the bulk of the stratigraphy, predominantly below 900 m. One fine-grained basaltic dyke and seven fine-grained rhyolitic intrusions were observed. Alteration in the Domes area is mainly controlled by permeability, temperature and the primary rock type. Abundance of alteration minerals is highly related to the permeability and porosity of the formations. Higher temperature alteration minerals appear systematically with increasing depth in OW- 914A. Five alteration zones were identified: - an un-altered zone (0-90 m), a zeoliteillite zone (90-480 m), a chlorite-illite zone (480-732 m), an epidote-chlorite-illite zone (732-1218 m) and an actinolite-epidote-chlorite-illite zone (1218-2996 m).

Comparison of the hydrothermal mineral evolution sequences and fluid inclusion analysis show that the system has been in a state of equilibrium with only small temperature changes. However, there may have been a short lived boiling condition in the well at around 836 m depth, indicated by the fluid inclusion analysis. Thirteen feed zones were identified in this well, with the first one being identified at 90 m and the last one at 2940 m. The location of the feed zones correlates with fractures, fractured rhyolite rock, vesicular tuff, intrusions and lithological contacts. The main reservoir rock in the Domes area is trachyte. The trachyte has tuff, rhyolite and basalt intercalations. It is highly fractured but less porous, as indicated by the relative abundance of vein fillings but fewer vesicles in the drill cuttings.