Title: Geology, hydrothermal alteration and fluid inclusion studies of Olkaria Domes geothermal field, Kenya

Abstract

Three geothermal exploration wells OW-901, OW-902 and OW-903 were drilled in Olkaria Domes field to
evaluate its geothermal potential. The three wells were drilled to a depth of 2200 m and all
encountered a high temperature system and discharged on test. The highest recorded measured
temperatures in the wells are; 342/C at -290 masl, 248/C at 207 masl and 341/C at -107 masl for
wells OW-901, OW-902 and OW-903 m respectively. Rocks encountered in the wells include
pyroclastics, rhyolite, tuff, trachyte, basalt and minor dolerite and microsyenite intrusives.
Fractures, vesicles, spaces between breccia fragments, glassy rocks and primary minerals exhibit
little or no hydrothermal alteration in the upper parts of the wells with mainly silica, calcite,
zeolites, phyllosilicates, oxides and sulphides being the alteration minerals present. In the
deeper parts of the wells, however, hydrothermal alteration to ranged from high to extensive.
Hydrothermal zeolites, calcite, epidote, phyllosilicates, silica, sulphides, epidote, albite,
adularia, biotite, garnet, fluorite, prehnite, oxides and titanite are the alteration minerals
observed. The most important hydrothermal alteration controls in Olkaria Domes field are
temperature, rock types and permeability. Four hydrothermal alteration zonations can be recognized
in the field based on the distribution of the hydrothermal alteration minerals. They are in the
order of increasing depth and temperature; the zeolite-chlorite, the illite-chlorite, the
illite-chlorite-epidote and the garnet- biotite-actinolite zones. Hydrothermal alteration
temperatures correlate well with the measured formation temperatures in wells OW-901 and OW-903
indicating probable equilibrium conditions with the geothermal system in that sector of the field.
In well OW- 902, however, a high temperature alteration mineral (garnet) was observed at depths
where current measured formation temperature is 246/C. This phase now exist where the present
temperature is much lower than what would be expected from its thermal stability range, which is
over 300/C. This indicates that cooling must have occurred in that part of the field. Fluid
inclusions in quartz and calcite veins from well OW-901 and quartz veins in OW-903 indicate that
heating must have occurred in that sector of the field with present temperatures being higher than
the average fluid inclusions homogenization temperatures by as much as 60/C. In well OW-902,
however, homogenization temperatures reflect more or less present condition with measured
temperatures being close to the average fluid inclusion homogenization temperatures. Feeder zones
in the wells are mainly confined to faults, fractures, joints and lithologic contacts.
Observations from hydrothermal alteration mineralogy, pressure and temperature profiles indicate
that well OW-901 is close to the upflow whereas well OW-903 is in the outflow zone and well OW-902
is in the outflow and
also the marginal zone of the field.