Borehole geology and hydrothermal mineralisation of wells MW-01 and MW-03, Menengai geothermal field, Central Kenya Rift valley

Abstract

MW-01 and MW-02 are geothermal exploration wells located on the Menengai caldera floor. They are both vertical wells, drilled to a total measured depth of 2206 m (MW-01) and 3200 m (MW-02). This report presents a study of the drill cuttings which were taken at 2 m intervals from the 2206 m in well MW-01 and the upper 2156 m in MW-02. The Menengai lithology is largely peralkaline silica saturated trachytes. MW-01 penetrated a massive fine- to- medium grained pantelleritic trachyte sequence while MW-02 cut more evolved rocks, comprising largely of rhyolitic trachytes, minor pantelleritic trachytes, comendites and comenditic trachytes. A pantelleritic intrusive body exists between 850-1082 m and at 2196 m in MW-01 whereas a possible comendite intrusion exists below 1952 m. No apparent stratigraphic correlations between the two wells are evident. Temperature logs from the wells indicate two main feed zones in MW-01 at 750- 800 m and at 1900-2000 m, but in MW-02 minor feed zones are found at 400-600 and 1100-1300 m. Feed zones associated with circulation loss zones appear at lithological boundaries and adjacent to intrusive bodies. The upper feed zones in each well seem to have low temperatures leading to cooling effects at the bottom of the wells. Hydrothermal alteration is influenced by the rock chemistry, temperature and permeability.

The main index mineral is epidote, indicating minimum alteration temperatures of 240-250°C. Epidote was first noted at 1090 m in MW-01 and at about 1500 m in MW-02. Smectite, chlorite and mixed-layer clay minerals are conspicuously minor or completely absent, while illite is found in both wells. More research is, therefore, required to outline the indicators for the alteration zone system and the mineralogical evolutionary sequence. A comparison of fluid inclusion homogenization temperatures with the estimated formation and alteration temperatures indicates reservoir temperatures between 230 and 280°C and a state of equilibrium. The present Menengai caldera is the result of collapses of mini-caldera segments along criss-crossing fractures on the floor of the main caldera. These fractures could contribute greatly to the localised recharge of the geothermal system, originating from the underlying intra-caldera lake situated in the northeast part of the caldera.