| Abstract |
Well, OW-710C, is a directional well drilled in the Northeast Olkaria geothermal field to a depth of 3136 m from cellar top (CT). Drilling and analysis revealed the following lithological units; pyroclastics, rhyolite, basalt, intermediate rock, trachyte, tuff, breccia and intrusions. Hydrothermal alteration shows evidence of water-rock interaction, fluid pathways, chemical and physical conditions of the system, whose products occur as replacement and/or deposition in vesicles and veins. Prograde alteration is evident in the well with low temperature alteration minerals encountered at shallow depth and progressively increasing with depth. There is, however, evidence of retrogression at depth where calcite is overprinting epidote thus indicating probable cooling condition. Five alteration zones were established with first appearance of temperature dependent minerals. Below the unaltered zone, fine-grained clays- zeolites are found succeeded by course-grained clays, epidote, and actinolite-epidote zones. Evidence of small and medium feed zones are postulated below the production casing. However, mineralization especially with calcite may have sealed the once permeable feed zones. Intersection of the Ololbutot fault zone at 1174 m CT may have caused an overall increase in permeability, however, it may have contributed to the relative cooling due to influx of cooler fluids. The reservoir temperature measurements appear to be stable between 1200-1450 m CT of around 256°C but then gradually lower to 215℃ at 3100 m CT after 29 months of heat recovery. |