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

Trachytic and basaltic volcanic material erupted from similar to 390 ka to Holocene coalesced to form the 136 km(2) active volcanic edifice of Paka on top of an older similar to 580 ka plateau eruptive sequence. The latter constrains the maximum age for the Paka geothermal system manifested mainly as surface fumamles 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 similar to 180 degrees C at 400 m depth where quartz was first observed in the drill cuttings to >220 degrees C within the chlorite-illite zone at 700 m and 280 degrees C below 1450 m in the actinolite-epidote zone. Fluid inclusion homogenization temperatures (T-h) range widely between 211 and 286 degrees C at 1390 m and 209-271 degrees 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 degrees C at 1800 m depth where a reversal occurs and becomes nearly isothermal at 230 degrees C, then increases slightly below 2200 m before an inversion to about 220 degrees 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.

Automated summary

Trachytic and basaltic volcanic materials erupted from around 390,000 years ago to the Holocene, forming the 136 km(2) active volcanic edifice of Paka. The geothermal system of Paka mainly manifests as surface fumaroles and hot altered grounds. Fluid inclusion analysis and temperature data from exploration wells indicate that the most suitable area for geothermal development lies within a 12 km2 area in the summit area and Northeast of the Paka caldera.