Borehole geology and hydrothermal mineralisation of wells MW-09 and MW-12, Menengai geothermal field, Kenya

Abstract

Wells MW-09 and MW-12 are exploration/production wells located in the caldera of the Menengai volcano in the Menengai geothermal field. They are both vertical wells. Well MW-09 was drilled to a depth of 2077 m while Well MW-12 was drilled to a depth of 2042 m. This report presents data obtained from analysis of drill cuttings from both wells. Analytical techniques applied, include binocular analysis, petrographic analysis, XRD analysis, and fluid inclusion analysis. The rock units encountered in both wells consist predominantly of trachyte, tuffs, pyroclastics, and a syenitic intrusion at depth. The top section of the well consists of blocky postcaldera trachyte lavas overlain by a thin layer of pyroclastic deposits. A thick layer of tuff encountered at 200-300 m marks the boundary between pre- and post-caldera lavas. Hydrothermal minerals in Menengai occur as replacements of primary minerals or as vein and vesicle fillings. The main hydrothermal minerals observed in both wells include: zeolite, chalcedony, calcite, pyrite, quartz, wollastonite, and actinolite. Based on alteration mineral assemblages and clay analysis, three alteration mineral zones were identified. They are: smectite zeolite zone, quartzillite zone, and actinolite-wollastonite-illite zone. The appearance of actinolite at 1364 m in Well MW-09 and at 1922 m in Well MW-12, indicates formation temperatures over 280°C.