Title: Sub-Surface geology, petrochemistry and hydrothermal alteration of Wells MW-03, MW-09 and MW-20 from Menengai geothermal field, Kenya

Abstract

The Menengai geothermal field is one of the high temperature geothermal systems in Kenya. It is
seated within the Great East Africa Rift System in the Central Kenyan Rift Valley and covered by
Quaternary volcanics. The study wells, MW-03, MW-09 and MW-20, were drilled inside the Menengai
caldera, which is characterized by ring faults, the Molo TVA which trends NNW-SSE and the Solai TVA
which trends NNE-SSW direction. The volcano formed about 200,000 years ago and the prominent 12 x 8
km caldera about 8000 years ago. Binocular and petrographic microscopes, XRD-analysis, ICP analysis
and temperature logs were applied for the research. In the study wells, fine-coarse grained
trachytes, pyroclastics, tuffs, basalts and intrusives (syenite) were observed. Based on Al₂O₃
concentration and total K₂O
+ Na₂O, the analysed rocks are metaluminous compared to the neighbouring wells MW-04, MW-06 and
MW-07. The geochemical evolution of Menengai rocks seems to be mainly controlled by fractional
crystallization. However, more than one process is involved. The relationship between trace
elements with depth in wells MW-03, MW-09 and MW-20 depict four volcanic episodes which may be
related to Menengai caldera formation.
The study defined seven hydrothermal alteration zones; Unaltered, zeolite-smectite (40-180°C),
quartz (above 180°), illite (220°C), chlorite (above 230°C), epidote (240°C) and
wollastonite-actinolite zone (above 280°C). Chlorite was noted in MW- 03 and MW-20, illite zone was
defined in MW-09 and MW-20. In MW-03, illite and actinolite occur at the bottom of the well and
wollastonite-actinolite zone is thinning towards MW-03. Based on the alteration minerals and
formation temperatures, the wells show indications of heating, where MW-03 appears heating
from approximately 1800m. Nine aquifers/feed zones were identified in MW-03, eight in MW-09 and six
in MW-20. These feed zones are linked to lithological boundaries, fractures and faults and
permeable formations.

Calcite looks to be the dominant alteration mineral in the wells and tends to deposit later than
higher temperature alteration minerals at some depths. Calcite forms by; replacing primary minerals
such as feldspar, pyroxene and volcanic glass; boiling of the reservoir fluid that causes to the
loss of CO₂ leading to calcite precipitates in veins or open spaces and calcite precipitates in
veins or fractures when hotter fluid mixes with circulating ground water. Epidote appears to be a
rare alteration mineral in the Menengai geothermal field, which could be due to high concentrations
of CO₂, ontents of iron in the rocks and thermal fluids which are important for epidote
formation.