Occurrence and Distribution of High Temperature Geothermal Systems in Kenya

Proceedings 15th Geothermal Workshop 1993 241

OCCURRENCE AND DISTRIBUTION OF HIGH TEMPERATURE GEOTHERMAL SYSTEMS IN KENYA

P.A. OMENDA, S.A. ONACHA W.J. Olkaria Geothermal Project, Kenya Power Company, Naivasha, Kenya

SUMMARY - The temperature geothermal Occurrences in Kenya are all located in the tectonically active Rift Valley associated with regions of Quaternary volcanism and more so calderas. The sizes of the geothermal fields, productivity of wells and reservoir characteristics vary but are mainly controlled by secondary permeability caused by tectonics and geologic setting. The geophysical structure is controlled by NNE and oblique E-W trending faults which also control hydrology and fluid flow patterns although this has not been established conclusively.

36 1. INTRODUCTION

High temperature geothermal systems that are considered in this paper are those having temperatures of more than at the top of the reservoir. In Kenya, their Occurrence and distribution has a correlation with the geologic and tectonic evolution of the Rift Valley where all the systems are located. Preliminary geothermal investigation shows that at least eleven geothermal systems exist within the Rift Valley. All the eleven prospects are associated with Quaternary volcanoes, nine of which have calderas. The caldera volcanoes are: Suswa, Longonot, Eburru, Menengai, Korosi, Paka, Silali, Emuruangogolak and possibly - Olkaria (Fig. 1). Namarunu and Barrier are the caldera volcanoes with geothermal potential. The other possible hi& temperature geothermal prospects are the Lakes Baringo and Bogoria areas which have high temperature fumaroles. Out of the above prospects, Olkaria and Eburru have been explored by deep drilling With Olkaria producing 45 since 1981 but the Greater Olkaria is capable of producing more than 200 The Eburru system is capable of producing - for 25 years 1991).

2. GEOLOGY

The development of the Rift Valley started during early Miocene with initial volcanism the flanks of the rift but the activity later migrated to the rift floor with continued volcanism of flood basaltic and trachytic rocks (Baker, 1987). The central sector experienced the most intense volcanic activity with the development of the Pbteou phonolites thickest pile in the region of more than 3 km in the Miocene sector. The products, however, are likely 50 to be thinner to the north and south of the central sector 0 where Precambrian metamorphic rocks outcrop the Figure 1- Kenya Rift Valley and Quaternary volcanoes 242 flanks. calderas also cap geothermal fluids and manifestations The present structure of the rift is dominated by only where the rocks are fractured. steeply dipping ( normal faults which have a trend varying from NNW to NNE but some oblique faults also 3. GEOTHERMALACTIVITY occur of which the major ones ate related to shear (accommodation) structures that occur along the rift. The The most common geothermal manifestations that occur strike of the most important rift forming faults broadly in all the prospects are: fumaroles, hot grounds, steaming follow the trends in the east dipping Precambrian grounds and steam jets. The manifestations strongly metamorphic rock foliations and defines striking follow the structural pattern of faults and fractures in northern and southern segments and a to areas of recent volcanism, more so, Quaternary volcanic striking central segment (Fig. 1). The central segment, centers. In most of the prospects, the manifestations however, also has younger rift floor faults trending Occur mainly along N to NNE faults and ring structures between N and NNE directions. Although the structural but in some areas, the manifestations are also associated pattern and faulting of the Kenyan rift system was, to with oblique structures. These manifestations have hence some extent, influenced by the grain in the Precambrian been used as indicators of the existence of geothermal rocks, the main driving force could have been convection systems associated with shallow magma chambers. The currents separating the crustal plates and hence is a zone only exception where manifestations are very strong but of asthenospheric upwelling. This the high heat not associated with any volcanic centre is the L. Bogoria flaw that occurs within the Rift Valley. system which has spouting springs (Geysers) and steam Large Quaternary caldera volcanoes occur within jets discharging at temperatures close to the local boiling the floor at intervals of less than 50 and are all point considered as promising geothermal prospects which Fumaroles and hot grounds are associated with include: Suswa, Longonot, Olkaria, Eburru, Menengai, both strong and weak surface alteration in which the Paka, Korosi Silali and Emuruangogolak. Namarunu and dominant secondary minerals are kaolinite, Barrier, the far north, are the other smaller volcanoes montmorillonitic clays ,silica residue, alunite, alunogen, with geothermal potential. The common rock types in alumina and occasional native sulphur. This alteration these prospects include trachytes, basalts, rhyolites, assemblage occurs mainly over the presumed upflow trachy-andesites, phonolites, trachy-phonolites and regions of the high temperature geothermal prospects basaltic-andesites. Trachytes are the most abundant rock which are, in nearly all cases, located the top part of type, more so, the Plio-Pleistocene plateau group which the volcanoes. On the slopes of the mountains, alteration are extensively faulted and the base for the of the rocks still occur but are often weaker or absent in Quaternary volcanoes. some cases implying that the steam is devoid of In the Eburru-Olkaria sector where deep drilling magmatic gases and The assemblage usually has been carried out, trachytes and pantelleritic rhyolites consists of kaolinite, silica and montmorillonites but and their pyroclastic equivalents dominate both on the sulphur, alunite, alumina and alunogen are typically surface and sub-surface. Minor welded tuffs occur in the absent. These regions outflow from the systems as sub-surface. The Occurrence of these rhyolites is a unique in the case of Eburru Field. feature of the Olkaria and Eburru geothermal areas and Most of the prospects, however, are characterised the volcanic centre. A syenitic intrusive rock by concealed outflows of condensates and deep thermal was encountered in Eburru geothermal Field during fluids. The only exceptions are possibly the Kapedo and drilling and the shallowest dyke was penetrated at about Lorusio springs which appear to tap condensate mixed 800 Such intrusives have not been encountered in with ground water from the nearby Silali volcano. Other Olkaria Field but they could occur under the other major springs occur to the of Emuruangogolak and they volcanoes within the rift since syenitic clasts have been are likely to obtain part of the water from condensate observed within their pyroclastics. from the volcano. The springs deposit travertine on the Suswa volcano which is the most southern surface (Dunkley and Smith, 1990). Chemical geothermal prospect has a geology that consists of geothermometry of the spring waters, however, indicate undersaturated trachytes, trachy-andesites and minor temperatures of less than (Tole, 1988). Apart basaltic-andesites. These lavas form a cover of more than from fumarolic emissions, ground temperature is also a 200 m within the caldera and this has been observed to useful indicator of the existence of a geothermal system. significantly prevent the geothermal fluids from reaching The ground temperatures (1 m depth) measured within the surface and hence the reason for reduced surface these geothermal fields and prospects are close to boiling geothermal activity in the area. Mt. Longonot volcano is point within the calderas and on the high grounds similarly dominated by undersaturated trachytes but of the volcanoes but are usually much lower outwards. pumice fall is quite abundant in the caldera and on the flanks of the mountain and thus preventing geothermal GEOPHYSICAL STRUCTURE fluids from reaching the surface except around the summit crater. Menengai and the other northern The geophysical structure of the known geothermal fields volcanoes have a lithology dominated by trachytic and is related to the tectonic and geological episodes. basaltic flows and pyroclastics that occur both within the The method that has frequently been used is the DC caldera and on the flanks. The young lavas within these Schlumberger electrical resistivity sounding method 243 augmented by gravity, magnetics and magnetotellurics. shows that low resistivity 30 Qm) may be caused by Most of the resistivity sounding surveys have been either high tempera tures, good permeability or carried out in the region between Menengai and Suswa hydrothermal alteration of the reservoir rocks or surface craters. The and Eburru Fields are the most alteration of pyroclastics by steam condensate. explored. Correlation of the resistivity anomalies in Olkaria The aeromagnetic surveys carried out within the shows that the resistivity structure at 800 (Fig. 2) Rift Valley show that the anomalies have a dipole nature is a good indicator of reservoir properties in the which make interpretations difficult. However, the geothermal’ systems. In most cases, low resistivity 30 anomalies have E-W trends probably related to old pre- correlates with reservoir temperatures of more than rift structures which may be zones of weakness that below this elevation. influenced the formation of the volcanic complexes. Geological evidence also indicates that rocks with different magnetic signatures were formed during different episodes. This further makes interpretation of the magnetic properties in terms of geothermal potential difficult. Some of the positive anomalies as observed in Olkaria have been mistakenly associated with demagnetization but the rocks encountered during drilling are essentially non-magnetic. In some areas, low magnetic field strength anomalies may be attributed to basaltic lavas emplaced within the present magnetic epoch, so that they have high magnetic susceptibility. The available regional teleseismic data indicates that the western part of the central rift region is associated with a large travel time residual anomaly which seems to indicate the presence of low density materials towards the Mau escarpment. The accommodation zones (Fig. 1) are associated with low residual travel time indicating high density bodies which are possibly related to shallow magmatic intrusions. From the seismic data, it has been postulated that the Figure 2- Resistivity map at 800 for the Greater major detachment faults extend to the Lithosphere- Olkaria area Asthenosphere boundary (Green et al, submitted). Decompression melting has also been suggested within The regions of highest temperatures and permeability are the crust and where deep faults extend to the zone, they found in areas defied by resistivity of less than 20 may form important channels of convective flow of hot Olkaria West Field has the lowest resistivity 5 fluids which may form other geothermal systems that are anomaly which has been correlated with higher degree of not associated with volcanic centres. hydrothermal alteration due to a thicker deposit of Gravity data has also been used together with as compared to either Olkaria East, NE (Fig. 3) and resistivity data in modelling the structures and possible Eburru Fields. Areas of low resistivity in the zone above sources of heat for the geothermal systems. The regional 1400 are related to outflows and presence of acid data, though in details, indicate that the central steam condensate. The regions of intermediate resistivity part of the Rift Valley is associated with an axial high (30-100 Qm) at depth represent areas of low gravity anomaly trending in a general N-S direction but temperatures of between depending on the is often truncated by oblique structures. permeability distribution. Most of the wells located The central volcanoes, fault zones and calderas are within these areas often show temperature inversions at often associated with different gravity signatures. For depth. The boundaries of the low and intermediate instance, the Suswa caldera has a high gravity anomaly resistivity anomalies at a depth below 1000 are while the Longonot caldera has a low gravity anomaly. sharp indicating that the geothermal reservoirs are The differences are expressions of the geological mainly controlled by vertical permeability and therefore episodes and modes of formation of the calderas rather essentially difficult to explore. than indicators of the source of heat. The low gravity Resistivity data from Olkaria East Field indicates a anomaly calderas are associated with thick deposits of near surface low resistivity associated with hydrothermal volcanic ejecta while the high gravity anomaly calderas alteration of pyroclastics and tuffs, a resistive caprock are caused by accumulation of dense mafic lavas within associated with steam zone and a deeper liquid the calderas. High gravity localized anomalies as in dominated reservoir associated with low resistivity. This Eburru may also be due to dense cold syenitic intrusives sequence might be an important indicator of the 1990) and therefore not all high density bodies reservoir characteristics in other unexplored areas. The can be associated with heat sources. main conductive zones are usually found within narrow Resistivity data has been the most useful in the N-S graben structures and sometimes bound by ring exploration of geothermal systems. Analysis of the data structures of the volcanoes as for the case of Eburru 244 (Fig. 4). Upflow areas seem to be at intersections of any 5. HYDROLOGY of the following structures: N-S, NE,NW and oblique W structures probably related to the older basement The hydrological structure may be deduced from structures. measured and interpreted water levels in shallow and deep wells in the Rift valley. The deep wells are restricted to Olkaria and Geothermal Fields while the shallow ones are mainly boreholes in the areas around lakes, Naivasha, Nakuru, Bogoria and Longonot volcano. These studies indicate that there exists several hydrological zones with different fluid transmission properties which are controlled by rift tectonic structures. These zones can be divided into shallow and deep regimes that seem to have no obvious hydrological connectionsbetween them.

Shallow Hydrology.

The shallow hydrology is deduced from bore hole data, especially around Lake Naivasha and the area between Lakes Nakuru and Bogoria. Although piezometric levels show major fluctuations in response to infiltration, the mean water level in wells shows a gentle gradient from the flanks to the rift floor and a north-south gradient also exists around Lake Naivasha (McCann, 1972, 1974). The amount of water runoff that drains into the Lakes during rainy seasons is also large and seem to be a major recharge for the surrounding boreholes. Figure 3- Structural map of Olkaria Geothermal Field In Olkaria Field, shallow perched aquifers showing well sites corresponding to lake level have been intercepted at depth interval of 80-200 m. The chemical characteristics The intersections of the structures may form zones of of the fluids, however, show that the waters are weakness where both magmatic material and geothermal condensate and are therefore most likely to be derived fluids are injected closer to the surface than other areas from the deeper geothermal system. A major within the Rift Valley. Further evidence from data discontinuity seem to exist to the south where several also indicate that these anomalous areas are also wells drilled have not intercepted water at equivalent associated with second deep (5-6 depth) conductive elevations. anomaly. However, due to the limited data, this has not In the area around Lakes Nakuru and Bogoria, been determined conclusively. It should also be noted contours from bore hole data indicates a that there are some areas of low resistivity anomaly gradient towards the central part that includes Menengai which are not associated with any volcanic centres and caldera (Geotermica Italiana, 1987).Hydrological data is, because of that, they have been given low preferences however, scarce for the area north of L. Bogoria. In and no deep drilling has been carried out to explore area, hot springs flow rather them. abundantlyfrom open fractures on the ground and

sw EB-18 EB-108 EB-77 EB-109 EB-6

-1 EBURRU CRATER r

ESURRU STATION

Geothermal

Figure 4- 2-D Resistivity section across Eburru Geothermal Field. 245 combine into a large stream that flows lateral variations, the deep reservoirs are probably in northwards towards Lake Turkana. The source of the communication with the recharge areas through the rift water is doubtlessly the shallow water system which is faults and the older oblique heated by a surface heat source but modified by The systems have temperatures in excess of geothermal fluids from the nearby prospects. in most parts of the reservoirs with the highest measured temperature in the Greater Olkaria Field being 5.2 Deep Hydrology while in a value of has been recorded. In the Field, the main reservoir lies from about The hydrology has been deduced from well data 1000 below the surface while the Olkaria system in the Olkaria and Eburru geothermal Fields. The data occurs below about 400-600 m. Their vertical extents, shows that there significant variation the deep however, have not been established but are thought to be system and that more regimes exist at depth where the in the range of few hundred metres (1000-2500 m) and known water level is not continuous. In area, the shows substantial underpressure. A caprock composed of water level only exists the North East Field where the basalts and trachytes has been to cover most of upflow of the eastern sector of the system is believed to the East and NEFields but none has been identified for exist. The water rest level constructed from pressure the West and Eburru Fields. pivot points corresponds to a level of 1600-1650 The areal extent of the fields are remarkably which is about 200 m below the mean annual level of different with the Greater Olkaria covering an area of Lake Naivasha (Ambusso and Ouma, 1991). This about 50-80 while Eburru is 2-5 (Fig. 5). therefore, suggests that no hydrological connections exist between the Olkaria system and the shallow basin around the Lake. In the East Olkaria Field, which is to the south of the North East Field, no free water level exists as the upper part of the reservoir has a steam cap which extends to the south of the field. However, the system at depth are lower than those of the North East Field and a gradient of 9 bars per has been deduced (Bodvarsson and Pruess, 1981). In the Olkaria West Field, a steam zone exists over much of the area and disguises the water However, the water level occurs OW-601 (Non-productive) to the far North west of the field and is within the 1600-1650 range which shows continuity of deeper hydrology. In the Eburru geothermal Field which is located 40 north of Olkaria, no shallow water levels were intercepted during drilling. However, the interpreted deep water level is somewhat difficult to establish since all Eburru wells, apart from EW-02, have a common pivot point at around 750 The water rest level in EW-01 and EW-06 are at 1863 and 1814 respectively which are within the range of the lake level. The water level in the other wells are lower by upto 100 m. These lower levels are consistent with a northerly Figure 5- Structural and Resistivity map of Eburru flow as deduced from measurements in shallow wells GeothermalField. around the lake. The coverage appears to be controlled by the size of the 6. DISCUSSION heat source and confining structures. The Eburru system is restricted to the eastern part of an caldera of The analysis of the available geologic, geophysical and about 3 diameter while the Olkaria system occurs in reservoir data from the geothermal prospects, it is northern and western parts of a ring structure evident that the physical characteristics and production thought to be a large caldera having a long diameter of capacity of the geothermal prospects depend mostly on about 11 tectonic and geological evolution of the various segments The fields show strong evidence of fault controlled of the Rift Valley. The prospects differ permeability with the Olkaria system being closely terms of size, reservoir characteristics and structural associated with Olkaria fault which strides both Olkaria setting. Although localized heat sources are a main NE and West Fields in a SW-NE direction and is the feature of consideration exploration, vertical presumed upflow of hot water the NE Field which permeability is a critical factor. This means that the then flows mainly to the south and recharges most of the prospects are essentially difficult to explore. Although East Field. The high permeability associated with Olkaria the near surface cold water reservoirs show fault is thought to result from the intersection of the fault 246 and the abundant N-S faults which are covered by the generally low in the fields but higher occur Quaternary volcanic products. within zones. The average permeability thickness in Olkaria, as in Eburru, is about 2 Darcy-metres. This is also indicated 8. ACKNOWLEDGEMENT by the low mass output from the Olkaria wells whose average mass flow is about however, Eburru We thank the management of the Kenya Power well has a mass flow of about 76 More Company for allowing to publish this paper. locally, however, significant variations occur and values higher than 10 Darcy-metres have been determined for REFERENCES some wells along the fault the North East Field where the reservoir rocks are most fractured. Ambusso, W. J. and Ouma, P. A. (1991). The production zones in these fields, as identified Thermodynamic and permeability structure of Olkaria from well tests, are distributed at various parts of the NE Field: Olkaria fault. GRC Transactions v 15 p. wells especially at lithological contacts with the main 242 zones occurring the Plio-Pleistocene trachytes which underlie the Quaternary This Baker, B. H. (1987). of the petrology of the zoned production is responsible for cyclic behaviour in a Kenya rift alkaline In: Alkaline Igneous Rocks, number of wells. Most wells in Olkaria discharge fluids J. G. and B. G. J. (eds.),Geol. Spec. with enthalpies greater 2000 J/g which are always in NO.30 293-311. excess of corresponding liquid enthalpies at measured temperatures. This effect is attributed to the low Bordvasson, G. S and Pruess, D. (1981). Olkaria permeability of the reservoir rocks and relative mobility Geothermal Field numerical studies of the generating of the phases. the Eburru system, the main production capacity of the reservoir. The Kenya Power Co. report zones occur above the syenite intrusive body at about (Unpublished) 2200 m and 1000 m drilled depth. In contrast to the Olkaria system, the discharge enthalpy is low and Browne, P. R. L. Hydrothermal alteration compares to the enthalpy at measured temperature. This active geothermal fields. Annual Reviews of Earth and shows that less phase separation has occurred in the Planetary Science V. 6,p 229-250. reservoir than at Olkaria. Long term production in Olkaria has shown that P. N. and M. The geothermal reservoir response to production is quite varied and activity and geology of the northern part of the Kenya depends on both local and regional features. The overall Rift Valley between L. Baringo and Emuruangogolak. change in production has been gradual with field average Draft report. BGS and GOK. decline rate of 4% per year. As compared to other fields over a similar period of production this value is low and Geotermica Italiana srl, (1987). Geothermal reflects the effect of high storage but low withdrawal reconnaissance survey in the Menengai - Bogoria area of rates from the reservoir. Changes in physical parameters the Kenyan Rift Valley. Final report DTCD con 7/85 - in the fields have shown strong dependance on the region KEN UN DTCD and of Ener. Kenya. of the reservoir in question since cooler fluids surround the reservoirs. In the central part of the reservoir, Green V. W. and Robert, P. M. (Submitted). Array minimal changes in temperatures have occurred and observations of phases across the Kenya Rift. measured temperatures are upto below the pre- Implicationsfor structure and tectonics. Tectonophysics. production values. This reflects near isothermal depletion of the reservoir. However, in the peripheral and less (1991). Discussion papers for Eburru scientific central wells, interference due to entry of cooler water review meeting, Nairobi. The Kenya Power Co. internal has been detected through development of inverted report. temperatures; increase in well instability and decline in discharge enthalpy. However, on the overall, there has McCann, D. (1972). Preliminary hydrogeologic been little detriment on field production. evaluation of the long term yield of catchments related to geothermal prospect areas in the Rift valley of Kenya. 7. CONCLUSIONS of Kenya (Unpublished)

Kenya has an abundance of high temperature geothermal McCann, D. L. (1974). Hydrologic investigation of the systems which are all located within the Rift Valley Rift Valley catchments. of Kenya Geoth. associated with Quaternary volcanic centres. The systems Expl. (Unpublished) are difficult to explore since most of them occur on high grounds (mountains) but the most useful geophysical S. M. (1990). The gravity structure of Eburru, method is the DC Schlumberger resistivity sounding. The Kenya. UNU Iceland report 13.39 pp. systems vary in size between 2 to more than 50 and the reservoirs occur in narrow zones by Tole, M. P. (1988). Low enthalpy geothermal systems faults, fractures and caldera structures. Permeability is Kenya. Geothermics V. p. 777-783.