Borehole geology and thermal history of well OW-737, Olkaria geothermal field, Kenya

Abstract

OW-737 is a wildcat well drilled in the periphery of the Olkaria Northeast sub-sector of Olkaria geothermal field for the purpose of appraising the extent of the Olkaria geothermal system beyond the boundaries of the Northeast sub-sector. It is a vertical well drilled in 2016 to a total depth of 2998 m CT. Lithostratigraphy established from the top to the bottom of the well comprises five lithological series, namely pyroclastics, rhyolites, basalts, trachytes and syenitic intrusions series with thin basaltic and rhyolitic dikes that occur within the trachytic series. Hydrothermal alteration minerals identified provide information on the paleotemperature of the geothermal reservoir as denoted by the presence of alteration minerals that represent low temperature (e.g. smectite, scolecite, mesolite), intermediate (e.g. quartz, chlorite) and high temperature (e.g. prehnite, epidote, wollastonite and actinolite) with increased depth. Five alteration mineral zones defined by the first appearance of alteration minerals are established: unaltered (0- 208 m), zeolite-illite (208-604 m), chlorite-illite (604-856 m), epidote-chlorite-illite (856-1610 m) and actinolite-epidote-illite (1610-2990 m). This shows that high temperatures are indicated below 1000 m depth as defined by the epidote-chloriteillite zone marking the top of the geothermal reservoir.

The paragenetic sequences of alteration minerals are categorized into alteration zones, indicating two temperature regimes where initially the temperature was high and later a cooling regime occurred, exemplified e.g. by calcite overprinting epidote at 1526 m depth. Calculated formation temperature, alteration mineral temperature and homogenization temperature (Th) from fluid inclusion analyses were compared. The results show great variation between formation and alteration temperatures, suggesting a cooling of the well. The homogenization temperature to some extent shows slight cooling in the well. Presence of minerals such as hematite at various horizons in the well could indicate the incursion of cooler fluids in the well. This seems to support the possibility of the well being located in a downflow zone, also considering its close proximity to the Gorge farm fault that is known to be the recharge conduit of the Olkaria geothermal reservoir. The injectivity of the well could not be determined as the well could not take in water during injection tests due to its current impermeability. Therefore, the permeability zones identified in relation to geological interpretations are presumably fossil.