Borehole geology and hydrothermal alteration mineralogy of well MW-13, Menengai geothermal field, Kenya

Abstract

Rock cuttings provide a realistic picture on down-hole stratification and alteration. Therefore, the study of cuttings is essential in order to evaluate the dynamics of the geothermal system, such as permeability, temperature, size and depth of the reservoir. In this study, four different analytical methods (binocular, petrographic, microthermometry and X-ray diffractometry) were applied to analyse cuttings from well MW-13, located in the Menengai geothermal field, Kenya. This is a vertical production well, drilled to a depth of 2001 m. Well MW-13 was aimed at tapping NNW-SSE structures associated with the Molo and Solai Tectono-Volcanic Axes (TVA) which cut the Menengai caldera. Four rock types are encountered which include: trachyte (most dominant), tuff, pyroclastics and syenite. From lithostratigraphic correlation of well MW-13 with other Menengai wells, several inferred faults have been identified, while a complicated tectonic scenario exists between some of the wells. The shallow section of the well is characterized by lowtemperature hydrothermal alteration minerals which evolve to form hightemperature ones at greater depths.

Four alteration zones, based on mineralogical assemblages, were identified: smectite-zeolite, quartz-smectite, epidote-quartz, and illite-wollastonite-actinolite-quartz. The uppermost 30 m section is unaltered. A comparison on the inferred alteration temperature, homogenization temperature (Th) and the boiling point curve shows that the system was in boiling conditions at some point in time. The measured temperatures do not reflect the current formation temperature. On correlating temperature logs to circulation losses, penetration rates (ROP), and lithological characteristics, eight permeable zones have been identified. The main sources of permeability are faults and lithological contacts. A brief comparison of clays in Menengai and Olkaria geothermal systems indicates that high-temperature clays appear at shallower depths in Olkaria wells than in Menengai wells. It is speculated that the different ages of the two systems are accountable for the dissimilarities noted.