[S-VC33] 火山の熱水系

[SVC33-06] Study of near-surface geothermal activity in Olkaria, Naivasha Kenya

*Gilbert Kipngetich Bett¹、Yasuhiro Fujimitsu² (1.Graduate School of Engineering, Department of Earth Resources Engineering, Kyushu University、2.Department of Earth Resources Engineering, Faculty of Engineering, Kyushu University)

キーワード：hydrothermal, geochemical facies, fumaroles

Olkaria geothermal complex has gained world recognition for its renewable energy production. This has enhanced Kenya’s interest in investing in green sustainable technologies as it targets to increase energy capacity and reliability. At Olkaria complex, surface development reveals hidden active geothermal systems where geofluid activity contributes to mass balance that has encouraged further expansion. In this study, a parametric analysis of geological units and geochemical tracers were carried out to assess near-surface geothermal occurrences' flow patterns and stability such as fumaroles and their geological contacts. Surface geology (fig.1) outlay the basaltic lava, pyroclastic sediments and pumice associated with post-rift volcanic (buried caldera) occurrences. Previous studies of geochemical and seismic hypocenters mapped hydrological circuits to be associated with fault network which acts as permeable flow channels for recharge (N-S and NNE-SSW) and upflow (NW-SE, NNW-SSE) of deep hydrothermal fluid. Fig. 3 show cation values were clustered near similar values which indicate an outflow area of the system. Na-K-Mg diagram (fig.2) show that majority of fumarole fluid occurs at the Mg corner point which indicates that the source of the fluid is meteoric water that cooled down the geothermal source. It also occurs in immature equilibrium which indicates the source as shallow depths and low temperatures. Based on CO2/H2 and Na/K geothermometers, reservoir temperatures in the Olkaria geothermal system is estimated to be 250-280°C. K-Mg geothermometer describes the chemical input caused by water-rock interactions and temperature at equilibrium. Fig. 4 illustrates that fumarole water has more sulphate which indicates an association with a deeper boiling reservoir. The structural setting of the Ololbotut fault could be the conduit for active geothermal that rises to the surface whereas the sealing tuffaceous cap rock act as a conductive body. This shows that shallow aquifer determines the rate and extent of surface manifestations and their trends.