Title: Sub-Surface Petrochemistry, Stratigraphy and Hydrothermal Alteration of the Domes Area, Olkaria Geothermal Field, Kenya

Abstract

Wells OW-905A, OW-910 and OW-917 have been drilled in the Domes area of the
Olkaria geothermal field. Wells OW-905A and OW-910 have been drilled inside the
caldera while OW-917 has been drilled on the eastern margins of the caldera rim.
The caldera rim is characterised by a ring of rhyolitic domes to the east, south and
southwest margins. Plans are underway to expand production drilling to the east of
the Domes field. This study presents an in-depth look at the variation in reservoir
characteristics across the Domes field and to the east of the ring structure. The study
also delineates magma differentiation processes involved in the evolution of the
GOVC. Wells OW-905A and OW-910 cut across seven stratigraphic units namely
basalt, trachybasalt, basaltic trachyandesite, trachyandesite, trachyte, rhyolite and
tuff. Well OW-917 only cuts across trachyte and rhyolite. Intrusives cut by the wells
include micro-granite, syenitic, trachytic and basaltic dykes. Well OW-910 has a
high abundance of high-temperature alteration minerals, OW-905A has moderate
abundance while OW-917 has low abundance of these minerals. Higher
temperatures are observed at shallower depth in OW-910, at relatively greater depth
in OW-905A and at greater depth in OW-917. High permeability is observed in wells
OW-905A and OW-910. Well OW-917 shows poor permeability. Well OW-905A is
located in a recharge zone, OW-910 in the up-flow zone close to the heat source and
OW-917 in a down-flow zone. The main magma differentiation processes involved
in generation of the basalt-trachyte suite are fractional crystallisation and magma
mixing. Rhyolites have been generated through fractionation of trachytes and
anatexis of syenites. The GOVC has had different episodes of eruptions. Its
plumbing system consists of independent discrete magma chambers and conduits in
which magma underwent modification before eruption.