Geology and hydrothermal alteration of Menengai geothermal field : case study: wells MW-04 and MW-05

Abstract

This study details the geological aspects of Menengai geothermal field. Surface geology is critically reviewed while the subsurface geology is reported based on the analysis of cuttings from wells MW-04 and MW-05 (has also been named MW-06). The field comprises three sectors: the Ol'rongai area, the Solai area and the Menengai caldera. The geology is divided into post-caldera, syn-caldera and precaldera stages while, tectonically, there are two systems: the Ol’rongai and the Solai fault systems. The faults and fractures in the Ol’rongai area have a NW direction and are older than the ones trending to the NE associated with the Solai system. Borehole data confirms that the Menengai stratigraphy is composed of a predominately trachytic lava sequence with intermittent tuffs and occasional pyroclastic layers. Different lava units are separated from each other by pyroclastic layers and sometimes by reddened soils, especially in the pre-caldera lava sequence. At shallow depths the trachyte formation consists of unconsolidated blocky lavas which are a challenge during drilling. The main hydrothermal alteration minerals are zeolites, chalcedony, quartz, pyrite, calcite, epidote, wollastonite, illite, actinolite and albite. The reservoir temperatures are between 200 and 280°C based on the presence of illite, epidote, wollastonite and actinolite. A correlation of hydrothermal alteration and fluid inclusion geothermometry indicates that the geothermal system around the caldera summit area is generally heating up.