| Abstract |
In geothermal resource assessment a multidisciplinary approach is adopted. The aim is to ascertain and map the size of the reservoir, determine reservoir temperature and chemical composition of the reservoir fluids. An initial conceptual model guides on primary exploration well sites and is updated as and when new data is available. 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 N-S, NNW-SSE and NNE-SSW. The N-S and NNW-SSE are older regional structures while the NNE-SSW are younger structures and has huge influence on reservoir fluid flow based on measured temperature contours. Borehole geology data infers a syn-caldera tuff marker horizon between 300-400 m CT is present in all 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 intrusive have been encountered in the wells within the caldera summit area during drilling. Gravity data collected so far presents an anomaly at the centre of the caldera, this is related to the magmatic body forming the dike intrusion. Furthermore, the aforementioned summit area exhibits shallow seismic movements confirming shallow magmatic activity. Measured temperature contours shows a marked NNE-SSW anomaly pattern inferring fluid flow pattern in the reservoir. The reservoir geometry is inferred from resistivity indicating that the reservoir is marked zone of 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 fluids is evident whereby some wells discharge one phase (i.e. steam) while the others discharge two phase. |