| Abstract |
Reservoir rock characterization for any geothermal field under both exploitation and exploration provide rationale for further utilization programs. The Olkaria reservoir rocks within the Northeast production field (NEPF) are largely typified by trachytes and rhyolitic intercalation. In attempt to characterise the reservoir geometry under the influence of hydrothermal alteration three wells; OW-706, OW-717 and OW-725 located in the NEPF are studied. The key petrophysical parameters studied include; porosity, relative permeability and lithological characteristics under influence of hydrothermal metamorphism. Effects of hydrothermal alteration on geochemical mobility were also evaluated selected elements mainly on bulk chemistry (K2O, Al2O3, CaO, FeO SiO2 etc.) and trace elements (Nb, Y, Rb, Zr, Ba, etc) dispersion through empirical comparisons between the protolith samples and subsurface samples. The former are considered relatively fresh and perhaps least affected by hydrothermal alteration while the latter are assumed to have undergone some changes due to the effects of hydrothermal alteration. The findings of this research reveal that, the reservoir rocks (trachytes and rhyolites) lack primary porosity and permeability is mainly controlled by the secondary porosity. The broad mechanism controlling porosity and permeability distribution include; dissolution, deposition and replacement. These mechanisms has direct bearing on elemental transfers where K2O and Al2O3 show significant depletion trends from protolith samples while CaO and FeO show enrichment of subsurface samples. On trace elements, there are no major shifts, however positive correlation of Nb, Y and Rb against Zr suggest possible enrichment. Based on the findings, the research concludes that, variability of petrophysical properties have direct bearing on mobility of chemical elements. In addition, the reservoir geometry from study wells is well enhanced along the NW-SE fault and less propagated to the eastern periphery of the field. The effect is largely attributed to major structural controls which augments the vertical permeability within the field sector. For optimal management of exploration programs, future production drilling should aim at targeting the NW-SE fault inferred to be the possible upwelling zone and where the reservoir geometry appear well developed. Correspondingly, re-injection wells should be sited to the eastern periphery of the field adjacent to the Gorge farm fault which seemingly channels cooler fluids and equally considered to be a possible recharge zone at shallow depth. |