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, low contents of iron in the rocks and thermal fluids which are important for epidote formation.