| Title | Conceptual Model of Suswa Geothermal Prospect, Kenya |
|---|---|
| Authors | Jill HAIZLIP, William CUMMING, Nicholas HINZ, Glenn MELOSH, Mark HARVEY, Keg ALEXANDER, J. Rúnar MAGNÚSSON, Sunna Björg REYNISDÓTTIR, |
| Year | 2020 |
| Conference | World Geothermal Congress |
| Keywords | Suswa, Kenya, East African Rift, conceptual model, gas geochemistry, MT, well target |
| Abstract | Geothermal conceptual models for the Suswa Geothermal Prospect have been developed to support resource assessment and well targeting based on the integration of previous geoscience studies with new geologic, geochemical and geophysical field surveys conducted in 2016 by EFLA and GDC scientists supported by ICEIDA and the Nordic Development Fund. The new field studies include fumarole steam and gas sampling, a CO2 flux survey, geology and structure mapping, MT-TEM resistivity surveys and a gravity survey. The favored conceptual model derived from the integrated analysis of these data includes a magmatically-heated, 250 to 300ºC, neutral, low-gas, partially two-phase reservoir associated with hot volcanic rock overlying recent intrusions that may include magma at 2.5 to 5 km depth within the Suswa inner caldera. However, the configuration and parameters of a geothermal resource at Suswa are relatively uncertain due to challenging field conditions that resulted in ambiguous results from many fumarole gas analyses and a lack of MT-TEM coverage in the inner caldera that precluded the detection of a shallow low resistivity zone corresponding to the clay cap characteristic of analogous geothermal fields. Alternate conceptual models have been developed to accommodate the related uncertainty. Despite the presence of numerous fumaroles between the inner caldera and the north wall of the outer caldera, the preferred and pessimistic conceptual models exclude this area because the gas geochemistry and resistivity pattern are consistent with a ~100ºC steam zone outflowing along tuff-lava contacts near the water table at about 600 m depth and leaking to the surface along fault traces. Shallow ~100ºC steam outflow also likely extends to the south, exemplified by the numerous steam vents leaking from porous tuffs exposed in the inner caldera wall. Conceptual issues encountered in other Kenyan geothermal fields that may also limit the capacity of a Suswa reservoir include the possibility of cold meteoric down flow through recent eruption vents that may depress the top of a high temperature reservoir and the possibility that shallow magma may result in a shallow base of a reservoir. On the other hand, the optimistic model assumes that evidence of magmatism below most of the inner caldera implies heating of a large overlying ~300ºC resource that outflows at more than 250ºC to the north at 2 km depth and possibly continuing to an intermediate depth below the northern outer caldera boundary. Within the overall context of the conceptual models, the key elements that most affect the risk assessment of targets at Suswa are the fumaroles with 250 to 300ºC geothermometry located over intersections of structures likely to form open space permeability at reservoir depths. |