Subsurface Stratigraphy and Hydrothermal Alteration of the Eastern Section of the Olkaria Geothermal Field, Kenya

Abstract

Petrological examination of cuttings from Well numbers and 13 show that the rocks comprising the drilled eastern part of the Olkaria reservoir below about consist of subhorizontal intercalated rhyolite, trachyte and basalt lavas plus occasional thin pyroclastic beds of similar compositions. Cuttings of microgabbro were also recovered from some wells; these probably derive from dykes o r sills too t h into be other than local ephemeral heat sources. There is no evidence that faults with significant vertical displacement occur within the area drilled. Many of the cuttings, especially those deriving from below show the effects of complete o r interactions which have produced a suite of hydrothermal minerals including quartz, hematite, albite, adularia, chlorite, illite, pyrite and calcium bearing phases such as c a l c i t e, montmorillonite, wairakite, epidote, sphene, fluorite, mordenite, prehnite and anhydrite. The identity and distribution of the minerals have been used to reach the following conclusions about prc- drilling conditions prevailing in the Olkaria reservoir. Rocks down to a depth of about are nearly impermeable, but there poor to good lateral permeability in most rocks occurring below this depth; there is not a close correlation between rock type and permeability, but generally the thin pyroclastic beds have the highest permeability and a sequence of widespread basal t lava flows occurring between 500 and usually, but not invariably, also have relatively good permeability. None of the penetrated dykes or sills of microgabbro serve as channels for thermal fluids, nor is there any evidence of significant vertical permeability in this part of the field. Fluids probably move laterally along very narrow, interconnected joint channels, contacts between lava flows or boundaries between separated blocks within brecciated lavas; i t is probable t h at thermal fluids do not pervade, or perhaps even fully saturate, a l l the rocks in the reservoir.