Borehole geology and hydrothermal alteration of wells MW-08 and MW-11, Menengai geothermal field, Kenya

Abstract

In order to understand the subsurface geology in any geothermal field, drilling is essential. The borehole geologists use the cuttings obtained during drilling to study the subsurface formation so as to identify the lithology, hydrothermal alteration mineralogy and probable location of aquifers. However, it is from the drilled wells that the temperature of the reservoir is measured. With such data, the dynamics of the geothermal system can be evaluated. This paper presents the stratigraphic sequence and the hydrothermal alteration of Wells MW-08 and MW-11, drilled in the Menengai geothermal field. The stratigraphic column is made up of four rock units. They include: pyroclastics, trachyte, tuff and basalt. These dissected rocks conform to the Pre-, Syn- and Postcaldera volcanic activities that started shortly before 0.18 Ma. The highly altered basalt penetrated by the wells at greater depths is presumed to be older than the caldera and is linked to the pre-caldera volcanics. The main hydrothermal alteration minerals encountered include: zeolites, chalcedony, quartz, pyrite, calcite, smectite, illite, chlorite, epidote, wollastonite, and actinolite. From the hydrothermal alteration, four alteration zones have been identified below a zone of no alteration: a zeolite-illite zone appears followed by a quartz-illite zone, an epidote-quartz-illite, zone and finally an epidote-illite-wollastonite-actinolite zone. Aquifers in the wells are associated with fractures, lithological contacts and loss of returns. A correlation of the hydrothermal alteration, fluid inclusion analysis and measured temperatures of the wells indicates that Well MW-08 is at equilibrium or slightly heating below 1700 m depth, whereas cooling is observed in Well MW-11.