Sub-surface geology, petrology and hydrothermal alteration of Menengai geothermal field, Kenya

Abstract

Menengai is a trachytic central caldera volcano in the Kenya rift valley with abundant high-temperature geothermal activity. The field is currently in its initial stages of development for geothermal energy in Kenya following Olkaria and Eburru fields. Regional surface geology of Menengai is largely composed of late Quaternary volcanics. The building of a 200,000 year old trachyte shield volcano was followed by piecemeal subsidence through two paroxysmal eruptions 29,000 and 8,000 B.P to produce a caldera of about 84 km2 in size that has subsequently been largely filled by recent trachyte lavas. Twenty four exploration and production wells, some of them hotter than 390oC, have been drilled in Menengai caldera by Geothermal Development Company (GDC). The aim of this study is to reveal the evolutionary history of the Menengai volcano and to describe its igneous lithostratigraphy and secondary mineralization in order to characterize hydrothermal processes within the field. Analytical methods used include binocular microscope analysis, petrographic analysis, X-ray Diffractometer analysis and inductively coupled plasma (ICP-ES) analysis. Studies of drill cutting from wells MW-02, MW-04, MW-06 and MW-07 have provided information on the stratigraphy, hydrothermal alteration and present state of the geothermal system. Petrochemistry of these wells revealed subsurface lithostratigraphy that includes, consolidated pyroclastic tuff, pyroclastics, basalt, trachybasalt, phonolite, trachy-andesite, syenitic intrusive units, while trachyte constitutes over 90% of the total.

Reaction of geothermal fluids with the host rocks has resulted in a progressive hydrothermal alteration with increasing depth. A number of studies have recognized characteristic alteration zones at Menengai based on distribution of key index alteration minerals. These zones, in order of increasing alteration grade, are zeolite-smectite, quartz-illite and wollastonite-actinolite zone. The study reveals that Menengai evolved a complex compositional variation with time as a result of magma fractionation, possibly in combination with crustal contamination and hydrothermal activities en route to the surface. The study defines the geology of the Menengai into, post-caldera, syn-caldera, upper and lower pre-caldera volcanics. Petrography and mineral chemistry of the basaltic and trachytic end members of the Menengai rocks indicate that two or more distinct magma types were involved in the formation of the volcano. This study reveals that magma exists beneath the caldera floor, at depth slightly over 2000 m within the summit area. This is evidenced by fresh glassy, quenched cuttings at these depths in wells MW-04 and MW-06. This implies that the thickness of the geothermal reservoir above the magma-containing domain may be no more than 1.5 km from the hydrostatic surface at ~400 m.