Reservoir characterization of the Paka geothermal system in Kenya: Insights from borehole PK-01

Abstract

Trachytic and basaltic volcanic material erupted from ∼390 ka to Holocene coalesced to form the 136 km2 active volcanic edifice of Paka on top of an older ∼580 ka plateau eruptive sequence. The latter constrains the maximum age for the Paka geothermal system manifested mainly as surface fumaroles and hot altered grounds. To date, three exploration wells have been drilled to >2500 m depth. Results from analyses of well PK-01 show that the upper part of the well (0–1050 m) is exclusively a trachyte lava sequence while the lower part (1050–2552 m) penetrates sequences of trachyte, intermediate, basaltic and syenite intrusive rocks. Temperature attained in the well based on hydrothermal alteration assemblage vary from ∼180 °C at 400 m depth where quartz was first observed in the drill cuttings to >220 °C within the chlorite-illite zone at 700 m and 280 °C below 1450 m in the actinolite-epidote zone. Fluid inclusion homogenization temperatures (Th) range widely between 211 and 286 °C at 1390 m and 209–271 °C at 2550 m. Fluid inclusion indicate that the reservoir fluid from the paleo system was bicarbonate-rich and of very low salinity according to ice-melting temperature, which is comparable to aqueous fluid discharge from the well. Stabilised well temperature increases with depth to a maximum of 245 °C at 1800 m depth where a reversal occurs and becomes nearly isothermal at 230 °C, then increases slightly below 2200 m before an inversion to about 220 °C. The main high-temperature geothermal system is hosted within the lower part of the well (1050–2552 m). It is probably steam-dominated above 900 m depth where calculated formation temperatures intersect the boiling point curve for pure water, and liquid-dominated below 900 m. Suitability modelling from surface data integration indicates that the most suitable area (P90) for geothermal development is within an area of 12 km2 in the summit area and Northeast of the Paka caldera.