| Abstract |
Three geothermal exploration wells OW-901, OW-902 and OW-903 were drilled in Olkaria Domes field to evaluate its geothermal potential. The three wells were drilled to a depth of 2200 m and all encountered a high temperature system and discharged on test. Rocks encountered in the wells include pyroclastics, rhyolite, tuff, trachyte, basalt and minor dolerite and microsyenite intrusives. Little or no hydrothermal alteration is observed in the upper parts of the wells but in the deeper parts, hydrothermal alteration ranged from high to extensive. The most important hydrothermal alteration controls in the field are temperature, rock types and permeability. Four hydrothermal alteration zonations can be recognized in the field based on the distribution of the hydrothermal alteration minerals. Hydrothermal alteration temperatures correlate well with the measured formation temperatures in wells OW-901 and OW-903 indicating probable equilibrium conditions with the geothermal system in that sector of the field. In well OW-902, however, a high temperature alteration mineral (garnet) was observed at depths where current measured formation temperature is 246?C. This indicates that cooling must have occurred in that part of the field. Fluid inclusions in quartz and calcite veins from well OW-901 and OW-903 indicate that heating must have occurred in that sector of the field with present temperatures being higher than the average fluid inclusions homogenization temperatures. In well OW-902, however, homogenization temperatures reflect more or less present condition with measured temperatures being close to the average fluid inclusion homogenization temperatures. Feeder zones in the wells are mainly confined to faults, fractures, joints and lithologic contacts. |