Borehole geology and hydrothermal alteration mineralogy of well MW-19A, Menengai geothermal field, Kenya

Abstract

Well MW-19A in Menengai is a directional well drilled to the north at an azimuth of 339.92° and inclination of 20.19° to a measured depth (MD) of 2355 m. The aim of drilling this well was to tap steam from NNW-SSE trending structures associated with the Molo Tectono-Volcanic Axis (TVA) and also to confirm the extent of the resource to the north inside the Menengai caldera. Comprehensive binocular and petrographic analyses of cuttings from the well indicate that the lithology of the well comprises five rock units i.e. pyroclastics, tuffs, trachytes and intrusions of syenitic and basaltic composition. Trachyte forms the main reservoir rock and it is the dominant rock below 504 m. These rock units host secondary hydrothermal mineral assemblages which are dependent on temperature, permeability, rock type and the chemistry of the circulating fluid. Hydrothermal alteration patterns show prograde alteration with increase in temperature and depth, as observed from alteration minerals in veins and vesicles. Five alteration zones were identified: unaltered zone (0-120 m), zeolite zone (120-502 m), chlorite-smectite zone (502-1100 m), quartzillite-epidote zone (1100-1358 m) and quartz-illite-epidote-wollastonite-actinolite zone (1358-2355).

Appearance of epidote at 1202-1760 m and actinolite at 1358- 1882 m indicates that alteration temperatures have reached 230°C and 290°C respectively at these depths. Correlation of fluid inclusion analyses, alteration and formation temperature indicates two geothermal episodes; one of a high temperature geothermal system below 600 m depth and a recent second phase of cooling between 1140 and 1400 m. The well is also very permeable as observed from the intensity of alteration, oxidation, veining, in addition to circulation losses and the abundance of calcite and pyrite. Eight permeable locations were identified which were further grouped into four major aquifers deduced from temperature logs, and their locations correlate with the cutting data. The major feed zone below 1400 m is considered the dominant one in the well. Alteration mineral correlation of MW-19A with neighbouring wells of MW-22 and MW-11 suggests that the well is nearer to an up flow zone than the latter wells while the stratigraphic correlation between the wells reveals the existence of normal faults between MW-19A / MW-22 and MW-22 / MW-11.