Permeability, Geology and Hydrothermal Alteration Mineralogy of OW-39A, Olkaria Geothermal Project, Naivasha, Kenya

Authors: Joyce OKOO
Keywords: alteration, mineralogy, permeability, cooling
Conference: World Geothermal Congress Session: Geology
Year: 2015 Language: English
Abstract: Well OW-39A is a directional well drilled to the south at azimuth of 180°and inclination of 20°to a measured depth (MD) of 3066 m adjacent to the N-S trending Ololbuttot fissure, which is a N-S trending eruptive fissure. Comprehensive binocular and petrographic analyses of cuttings from the well indicate that the lithology of the well comprises five rock units i.e. pyroclasts, rhyolites, tuffs, basalts, and trachytes and then intrusions of rhyolitic, syenitic, basaltic and granitic composition. Trachyte forms the main reservoir rock and it is the dominant rock below 900 m. These rock units host secondary hydrothermal mineral assemblages which are dependent on temperature, permeability and rock type. Mineral deposition sequences in the well show systematic evolution from low to high temperature conditions with depth, as observed from alteration minerals in veins and vesicles. Five alteration zones were identified: unaltered zone (0-134 m), zeolite- smectite-illite zone (134-578 m), chlorite-illite zone (578-748 m), epidote-chlorite-illite zone (748-896 m) and actinolite-epidote-wollastonite zone(896-3066).Appearance of epidote at 750 m and actinolite at 896 m indicate reached 250°C and 280°C temperature conditions respectively at these depths. Comparison of fluid inclusion analyses, alteration and formation temperature indicate two geothermal episodes; one of a high temperature geothermal system below 700-800 m depth and a recent second phase of cooling. Permeability is observed in the cuttings by the intensity of oxidation, veining, alteration intensity, circulation losses, presence of calcite and abundance of pyrite. Eight feed zones are identified in the well starting from 750 down to 2750 m deduced from the shape of temperature profiles, and their locations correlate with the cutting data as mentioned above. The intermediate feed zone at 1100 and the major feed zone at the 2100 m are considered the dominant ones in the well. Alteration mineral correlation with neighbouring wells of OW-37A and OW-35 suggests that the well is nearer to an up flow zone than the latter, while the stratigraphic correlation between the wells reveal the existence of normal faults between OW-39A/OW-37A and OW-37A/OW35.
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