| Title | Conceptual Model of the Menengai Geothermal Field |
|---|---|
| Authors | Geoffrey Mibei, Joseph Mutua, Lucy Njue and Calistus Ndongoli |
| Year | 2016 |
| Conference | ARGeo |
| Keywords | Reservoir, Intrusion, Marker horizon, Metamorphism, Lavas |
| Abstract | This paper describes findings and current status of Menengai geothermal exploration and resource assessment respectively. Deep drilling program in Menengai field has so far resulted in 31 geothermal wells within the Menengai Caldera. Geoscientific data acquired is continually reviewed and integrated to bring out an updated geothermal model of the field. Current assessment from the surface geology indicates that Menengai caldera has been volcanically active in recent geological time as evidenced by widespread eruption of lavas and pyroclastic. Regional structures are oriented in the N-S, NNW-SSE and NNE-SSW. The N-S and NNW-SSE are older regional structures. The NNE-SSW structures are younger structures and have huge influence on reservoir fluid flow jointly with the N-S structures as inferred from the measured temperature contours. Borehole geology data infers a syn-caldera tuff marker horizon between 300-400 m CT which is present in almost all wells. High temperature alteration minerals like actinolite are present in wells drilled within the summit area indicating zones of contact metamorphism related to system of hot dike intrusions. In addition syenitic intrusions have also been encountered in the wells within the caldera summit area and form the crown of a hotter body below. Gravity data collected so far presents an anomaly at the centre of the caldera; this is related to a magmatic body developing the dike intrusion. Furthermore, the aforementioned summit area exhibits shallow seismic movements confirming shallow magmatic activity. Measured temperature contours show a marked N-S to NNE-SSW anomaly pattern inferring fluid flow pattern and the probable geometry of the reservoir. The reservoir is marked by resistivity values between 30-70 ohm-m. Reservoir fluids are of Na-HCO3 type with a high pH and moderately high chloride concentrations (> 400 mg/kg). A marked variation in the fluids is evident whereby some wells discharge one phase (i.e. steam) while the others discharge two phase. Calcite scale deposition is real as indicated by results of saturation indices. |