Influence of hydrothermal alteration in permeability of Olkaria North East and Olkaria domes geothermal reservoirs.

Abstract

The geothermal exploration well OW-701 and OW-709 are found in the Olkaria Northeast fields while OW-904 and OW-905A are found in the Olkaria Domes. Rocks encountered in the wells include pyroclastics, ryholite,tuff, trachyte and basalts. Fractures, vesicle, spaces between breccia fragments, glassy rocks and primary minerals exhibit little or no hydrothermal alteration in the upper parts of the wells. Fractures are mechanical breaks in rocks, they provide pathways for fluid flow. The intensity of veining varies with depth. They are most common in basalts and trachytes and less common in rhyolite and tuffs. The common vein filling minerals are calcite, quartz, pyrite, epidote and clays. The secondary mineralization results in decrease in permeability of reservoir rocks. The chemical composition of the host rock determines the availability of components to form alteration minerals as well as possible fugitive components from the presumed magmatic heat source. Temperature dictates the formation of hydrothermal minerals. Hydrothermal mineral deposition is mostly found in vesicles and vein fillings. The vesicles are filled, lined or unfilled by secondary minerals. Hydrothermal mineral deposition can be useful in measuring permeability.