Proceedings World Geothermal Congress 2005 Antalya, Turkey, 24-29 April 2005 Geology, Hydrothermal Alteration and Fluid Inclusion Studies of Olkaria Domes Geothermal Field, Kenya John Lagat1, Stefan Arnorsson2 and Hjalti Franzson3 Kenya Electricity Generating Company Ltd, Olkaria Geothermal Project, P. O. Box 785, Naivasha 20117, Kenya1 University of Iceland, Science Institute, Dunhagi IS 101 Reykjavik, Iceland2 Icelandic Geosurvey, Grensasvegur 9, IS 108 Reykjavik, Iceland3 [email protected], [email protected], [email protected] Keywords: Kenya Rift, Olkaria Domes, Geothermal, The Greater Olkaria geothermal area is within the Greater Geology, Hydrothermal Alteration, Fluid Inclusion Olkaria volcanic complex. It is subdivided into seven fields for geothermal development purposes namely Olkaria East, ABSTRACT Olkaria Northeast, Olkaria Central, Olkaria Northwest, Olkaria Southwest, Olkaria Southeast and Olkaria Domes Three geothermal exploration wells OW-901, OW-902 and (Figure 2). OW-903 were drilled in Olkaria Domes field to evaluate its geothermal potential. The three wells were drilled to a o depth of 2200 m and all encountered a high temperature 38 E North Island system and discharged on test. Rocks encountered in the 4oN wells include pyroclastics, rhyolite, tuff, trachyte, basalt L. Turkana ETHIOPIA and minor dolerite and microsyenite intrusives. Little or no Central Island hydrothermal alteration is observed in the upper parts of the wells but in the deeper parts, hydrothermal alteration ranged from high to extensive. The most important hydrothermal alteration controls in the field are South Island temperature, rock types and permeability. Four Cenozoic Barrier Volcano volcanics hydrothermal alteration zonations can be recognized in the UGANDA Namarunu field based on the distribution of the hydrothermal alteration minerals. Hydrothermal alteration temperatures Emuruangogolak correlate well with the measured formation temperatures in Mt. Elgon Silali wells OW-901 and OW-903 indicating probable Paka KENYA equilibrium conditions with the geothermal system in that Korosi L. Baringo sector of the field. In well OW-902, however, a high Nyambeni temperature alteration mineral (garnet) was observed at Arus-Bogoria depths where current measured formation temperature is Kisumu Oo 246°C. This indicates that cooling must have occurred in Menengai Mt. Kenya that part of the field. Fluid inclusions in quartz and calcite Eburru veins from well OW-901 and OW-903 indicate that heating L. Naivasha Olkaria must have occurred in that sector of the field with present Longonot temperatures being higher than the average fluid inclusions Suswa Nairobi homogenization temperatures. In well OW-902, however, Ol'Esakut homogenization temperatures reflect more or less present Olorgesaille o condition with measured temperatures being close to the L. Magadi 2 S 0 150 average fluid inclusion homogenization temperatures. 50 100 Shombole Kilometers Chyulu Feeder zones in the wells are mainly confined to faults, Ol'Doinyo Lengai fractures, joints and lithologic contacts. Volcanic center Lake 1. INTRODUCTION Geothermal Prospect Kilimanjaro The Greater Olkaria geothermal area is situated south of Geothermal Field Lake Naivasha on the floor of the southern segment of the Rift Fault o Kenya rift (Figure 1). The Kenya rift is part of the East 36oE 38 E African rift system that runs from Afar triple junction at the Gulf of Eden in the north to Beira, Mozambique in the south. It is the segment of the eastern arm of the rift that Figure 1: Map of the Kenya rift showing the showing extends from Lake Turkana to the North to Lake Natron, the location of Olkaria geothermal field and other northern Tanzania to the south (Figure 1). The rift is part of quaternary volcanoes along the rift axis. a continental divergent zone where spreading occurs resulting to the thinning of the crust hence eruption of lavas and associated volcanic activities. 1 Lagat et al. 9907000 2 2050 1 2 2 1 1 0 1 9 9 0 0 1 9 2 5 7 3 11 9 OW-501 0 5 9 0 0 0 1 0 0 0 2110 5 2 0 20 2 20 110 OW-102 50 2 1 7 1990 0 OW-101 OW-704 0 2 1 1 1 OW-R2 1 0 7 2 9 1990 1 0 9 1 0 2 0 3 2 2 9905000 OLKARIA 0 30 7 0 9 0 OLKARIA NORTH 1 0 5 1 199 2 3 20 OW-601 OW-204 2 10 90 2 21 19 20 0 50 5 20 CENTRAL OLKARIA OW-306 2050 50 EAST FIELD 0 2 2 OW-202 2 OW-201 1 170 0 1 93 0 1 NORTH WEST 2 OW-705 110 OW-203 0 OW-721 2110 17 050 2 2 2 2050 0 050 0 0 1 9 3 1 9 9 9903000 2 1 1 OW-308 OW-302 OW-30 2 0 OW-26 0 50 2 3 2 9 3 OLKARIA EAST 1 OW-301 0 50 21 23 70 0 OLKARIA SOUTH 1 FIELD 1 2 0 2 193 290 0 0 OW-303A 2170 OW-27 99 87 1 1 WEST FIELD 2110 2230 2 05 OW-305 0 20 OW-22 50 0 2 9901000 7 0 OW-11 8 OW-901 5 1 0 0 OW-304D 21 3 1 9 0 1 90 OW-307 2 19 2 0 0 5 5 OW-401 0 OLKARIA DOMES 0 2050 0 0 FIELD 2 0 OW-903 9 1 0 1 5 9 1 8 0 1 2 1 0 5 0 OW-801 2 OW-902 0 9 0 0 9 9899000 1 3 7 9 8 1 1 OW-1 2 0 5 OLKARIA SOUTH 0 0 1 8 2 1 0 5 0 EAST FIELD 0 5 1 1 7 99 9 1 1 0 7 1 9 1 5 8 8 1 0 2 9 0 0 1 70 3 50 0 0 9897000 192000 194000 196000198000 200000 202000 204000 206000 Figure 2: Map of the Greater Olkaria geothermal area showing the showing the location of the fields. Dots and adjacent numberings indicate well and corresponding well number. Olkaria East field (Olkaria I) has been producing power 3. GEOLOGY since 1981 when the first of the three 15 MWe units was commissioned. The current generating capacity of the field 3.1 Regional geology is 45 MWe. Olkaria Northeast field (Olkaria II) is The evolution of the East African rift system was generating 70 MWe and Olkaria Northwest field (Olkaria structurally controlled with the rift faults exploiting the III) which is being developed by an Independent Power weak collisional zone at the contact between the Archean Producer (IPP) is currently producing 12 MWe and will Tanzania craton and Proterozoic orogenic belts (Smith and have a capacity of 48 MWe when the construction of the Mosley, 1993). The development of the rifting within power plant is completed in 2005. The rest of the fields are Kenya started during the late Oligocene (30 Ma) in the area at various exploration stages. This paper therefore discus known as the Turkana rift (Baker et al., 1971, 1972; the results obtained from the analysis of samples recovered Kampunzu and Mohr, 1991; Smith, 1994). Volcanism from the exploratory wells drilled in the Olkaria Domes associated with rifting started during the Miocene. The field, which is the field earmarked for development as magmatic activity was accompanied by domal uplift of Olkaria IV. about 300 m on the crest of which erupted phonolites (Baker and Wohlenberg, 1971, Williams, 1972, Hay and 2. OBJECTIVES OF THE STUDY Wendlandt, 1995). Because this volcanism preceded the 1 To identify the rocks penetrated by wells OW-901, major rift faulting events, the rifting process has been OW-902 and OW-903 and thereby gain knowledge of modelled as active with the driving force being provided by the stratigraphy and the structures of Olkaria Domes convecting asthenospheric mantle (Baker et al., 1971, 1972; geothermal field. Baker and Wohlenberg, 1971). 2 To identify the hydrothermal alteration minerals found The Miocene volcanics were subsequently faulted and then in Olkaria Domes and to obtain a better understanding followed by massive and extensive Pliocene eruption of of the water rock interaction processes occurring in trachytic ignimbrites in the central area to form the Mau this active hydrothermal system. and Kinangop Tuffs. A second faulting episode, which followed the ignimbrite eruptions, resulted in the formation 3 To identify the textural relationships of these minerals of the graben structure, as it is known today. In the to provide clues about the past and present conditions developing graben, fissure eruptions of trachytes, basalts, of Olkaria Domes reservoir. basaltic trachyandesites, and trachyandesites occurred. The plateau rocks that filled the developing graben were then 4 To determine the general location of the wells with block faulted to create high angle normal faults within the respect to the upflow, outflow and the marginal zones rift floor. The fractures apparently served as conduits for of the system and the appropriate casing depths in the the Quaternary volcanic activity of mafic to felsic field. composition. The most intense volcanic activity occurred within the central sector of the rift where the volcanic succession is thought to be of the order of 5 km thick. This thickness is estimated from seismic data (Henry et al., 1990, 2 Lagat et al. Simiyu et al., 1997), stratigraphic correlation (e.g. Baker et trending faults (Figure 3). The faults are more prominent in al., 1972), and drilled geothermal wells in Olkaria area. the East, Northeast and West Olkaria fields but are scarce in the Olkaria Domes area, possibly due to the thick 3.1 Geology of Olkaria volcanic complex pyroclastics cover. The NW-SE and WNW-ESE faults are thought to be the oldest and are associated with the The Greater Olkaria volcanic complex is characterized by development of the rift.