Subsurface Geology, Petrology and Hydrothermal Alterations of Menengai field, Kenya; Case Study of Wells MW-02, MW-04, MW-06 and MW-07

Abstract

SUB-SURFACE GEOLOGY, PETROLOGY AND HYDROTHERMAL ALTERATION OF MENENGAI GEOTHERMAL FIELD, KENYA: Case study of wells MW-02, MW-04, MW-06 and MW-07. Mbia P.K, (a), Mortensen A. K (b), Oskarsson N (c) and Bjorn S. Harðarson (b) (a) Geothermal Development Company (GDC) Nairobi, Kenya (b) ISOR, Iceland GeoSurvey, Grensasvegur 9, 108, Reykjavik, Iceland (c) School of Engineering and Natural sciences, University of Iceland ABSTRACT Menengai is a trachytic central caldera volcano in the Kenya rift valley with abundant high-temperature geothermal activity. The regional surface geology of Menengai is largely composed of late Quaternary volcanics. The building of a trachyte shield volcano was followed by piecemeal subsidence to produce a caldera of about 90km2 in area which has been largely filled by recent trachyte lavas. Twenty four exploration and production wells have been drilled in Menengai caldera to date by Geothermal Development Company (GDC), some of which have encountered temperatures of more than 350o C. The aim of this study was to unravel the evolutionary history of the Menengai caldera and to describe lithostratigraphical and hydrothermal processes in order to characterize the secondary mineralization in this field. Analytical methods used include binocular microscope analysis, petrographic analysis, X-ray Diffractometer analysis, inductively coupled plasma (ICP-ES) analysis and XRF analysis. Studies of drill cutting from wells MW-02, MW-04, MW-06 and MW-07 have provided important information on the stratigraphy, hydrothermal alteration and status of the geothermal system. Petrochemistry of these wells revealed subsurface lithostratigraphy which includes trachyte, trachy-phonolites, tuff, pyroclastics, basalt, trachybasalt, phonolite phonotephrite, trachy-andesite and syenitic intrusive units with trachyte constituting over 90% of the total rocks. Reaction of geothermal fluids with the host rocks has resulted in a progressive hydrothermal alteration sequence with increasing depth observed throughout the Menengai geothermal field. A number of studies have recognized characteristic alteration zones at Menengai based on key index minerals. These zones, in order of increasing alteration grade, are zeolite-smectite, quartz-illite, illite-wollastonite and wollastonite-actinolite. Surface geological and geochemical data was incorporated in rebuilding the evolutionary history of the Menengai caldera. This study reveals that magma exists below the caldera floor at just over 2000 m depth as was evidenced by the presence of fresh glassy and quenched cuttings at these depths in wells MW-04 and MW-06. This implies that the thickness of the geothermal reservoir where the magma is located may be no more than 1.5km from the water rest-level at ~400 m to 2000 m. The petrography and mineral chemistry of basaltic and trachytic end member lavas analyzed in this project reveal distinct mineral parageneses that may be ascribed to two distinct magma types from which the different mineral assemblages crystallized. Keywords: Menengai; Evolution; Magma, Petrochemistry; hydrothermal alteration