187 Proceedings 9th Geothermal Workshop 1987 GEOLOGY AND HYDROTHERMAL ALTERATION OF THE NORTH AND WEST EXPLORATION WELLS IN THE OLKARIA GEOTHERMAL FIELD, KENYA T.M. LEACH* AND G.G. (* Chartered Mineral Services and GENZL) (** Kenya Power Company) ABSTRACT No large intrusive body or "basement" rock was encountered, although rarely micro-to porphyritic syenites, granites and diorites Petrographic, x-ray diffraction and fluid inclusion studies were were intersected in a few wells, and are interpreted to be sub- camed out cores and selected cuttings from ten exploration wells in volcanic to surface extrusives. Tracesyenite lithic fragments the north and western sectors of the Olkaria geothermal field. The were observed in some of the pyroclastics. wells penetrated a thick volcanic pile of alkali and minor lavas and pyroclastics, with rare intersections of high level PERMEABILITY intrusives. The main permeability in the formation occurs in widely spaced faults with little fracturing occurring within the fault blocks. Fracturing in the volcanic pile is not common, with only rare thin Lithological permeability is restricted to shallow levels, with veinlets observed in a minority of the cores. Fault intersections, hydrothermal mineral deposition sealing the primary porosity at based on core shearing, fracturing, brecciation and veining, were depth. The distribution of illitic and chloritic clays helps to determine noted in only a few samples. These cores had undergone intense the extent of the bicarbonate, and the alkaline chloride fluids alteration indicative of a high fluid to rock ratio. The depths encounteredin the reservoir. The distribution of kaolinite, smectite of these fault intersections are usually zones of major production and carbonates is used to delineatethe extent of cool recharge fluids. from downhole measurements core at 1353 m amsl in 701). INTRODUCTION The pyroclastics are typically intensely altered indicatingexcellent The OlkariaGeothermal Field is situated approximately 70 km north- primary porosity. However, from correlations with downhole west of Nairobi within the East African Rift Valley (see Fig la). The measurements, only the near surfacepyroclastics now permeable, geothermal fieldis located in a complex of Pliocene to Recent acidic the deeper ones apparently being sealed by alteration. Some of the to alkali volcanic centres at the south west end of lake Naivasha. lavas are vesicular, especially the basalts; however these are Except for Longonot volcano and Crater Lake, these centres are generally sealed suggesting these lithologies are no longer associated with a NNW-trending belt of volcanism and substantial permeable. Joints in basaltic lavas exhibit multiple phases of sealing faulting, along the western rift valley (Fig Ib). This volcanic and reopening, suggesting that these zones of weakness are sources belt includes Mount Suswa to the south and the Eburru volcanic of some permeability. Permeability at the contact of intrusivescould complex to the north. not evaluated because of the absence of samples from these regions. However no production zones were found to exist at the Although N-S and NNW-trending normal faults predominate in the depths of the known intrusives. Olkaria Geothermal area, several striking linements and E-W postulated faults also cut the rift valley floor (Figure A number It is interpreted that the most permeable zones are at depths of fault of phreatic eruption craters, rhyolitic fissure flows and steam vents intersection, and that these faults are widely spaced with very little are aligned along the N-S trending Olbutot Fault. Steam vents and movement and subsequent fracturing taking place between the faulted solfataras (in the west) are also aligned along the E-W trending blocks. The abundant permeable zones encountered at shallow Olkaria Fault. elevations is interpreted to be caused by the intersection of that have not undergone the alteration sealing exhibited Twenty-six wells have been drilled in the production field located in in the deeperequivalents. eastern portion of the Olkaria field. The geology and alteration mineralogy in these wells has been described by Browne (1984). A HYDROTHERMAL ALTERATION deep exploration drilling programme to better define the resource potential of the Olkaria geothermal field was implemented in 1981. a) Distribution of Alteration Minerals: To date, ten wells to a depth of 1 have been drilled in the The alteration minerals encountered in the Olkaria exploration northern and western portionsof the Olkariafield. wells and the approximate temperature distribution of these minerals are indicated in Figure 2. The distribution of someof This paper reviews the geological logging of these wells (carried out these minerals have been used to help determine the depth at by G Muchemi, and A J Ryder of GENZL) and follow up which production casings were set. work (done by T M Leach and G G Muchemi). Of the silica minerals, opaline silica and cristobalite are LITHOLOGIES restricted to near surface conditions; whereas chalcedony and secondary quartzoccur at higher temperatures, generally above The exploration wells encountered a very thick volcanic pile (at least of predominantly alkali to silicic subaerial lavas and pyroclastics with minor intercalated basaltic volcanics. There is a The Mg and Fe rich carbonates, siderite and ankerite, were continuous range in composition of the alkali-silicic volcanics from encountered at moderately high temperatures (100- spherulitic rhyolites to silica poor trachytes and alkali trachytes. whereas calcite was ubiquitous throughout the reservoir. The rhyolites(comendites), containingminor sodic amphiboles and manganese carbonate, was noted as vitreous biotites were encountered in the upper sections of the volcanic coatings in a few from western Olkaria field. sequence, and outcrop as spectacular columnar jointed cliffs within It was restricted to fairly high temperature conditions the Hell's Gate gorge. The basaltic volcanics by volume are very from fluidinclusion analyses). minor, ranging in composition from clinopyroxene bearing alkaline basalts, through more alkali trachybasalts to rare are rare in Olkaria, indicative of the low sulphur undersaturated, nepheline bearing phonolites. activity of the reservoir fluid. The hydrated calcium sulphate gypsum was noted at very shallow levels in OW-501, whereas The volcanics appear to be essentially horizontal. The stratigraphic anhydritewas found at depth in well OW-303A. succession of volcanic units acrossthe field is very variable with no widespread and distinctive marker horizons, making well to well The calcium-hydrated zeolites heulandite and stilbite were correlationsvery difficult. encountered in near surface cores; whereas the less hydrated wairakite was observed in one core in OW-701 at temperatures 188 Leach FIGURE la FIGURE lb location of Recent Rhyolite Flows in the Location of Olkoria Geothermal Field in Olkaria - Eburru Region African Rift Zone FIGURE WELL LOCATIONS AND MAJOR STRUCTURES. OLKARIA GEOTHERMAL FIELD KEY Completed .mopped. 2 3 Km I 189 Leach of (from fluid inclusion analyses on co-existing temperatures less than about is relict of higher calcite). Although the hydrated sodium Al-silicate analcite is temperature conditions. not a zeolite, both optically and genetically it closely resembles zeolites. Analcite was commonly found as Fluorite occurs in the formation at temperatures greater than a and vesicle filling at temperatures less about and is absent under conditions. Under than about the very low salinity conditions at Olkaria, fluorite solubility is directlyrelated to temperature up to and inversely The amorphous Ti-mineral leucoxene, gives way to the proportional at higher temperatures (Richardson and Hollands, crystalline Ca-Ti-silicate sphene at temperatures greater than in Barnes 1979). It is therefore interpreted that fluorite has about Epidote is encountered in some wells at deposited in the Olkaria formation by the downward movement temperatures less than however from the distribution of of cool fluorine rich fluids, which, upon heating up to clay minerals in these wells, it is interpreted that epidote at temperatures greater than start to deposit fluorite. 50 so chlorite to illite onito biot it MINERALS quartz CARBONATE an rhodochrorito to onhvdr R to garnet t tito fluor ito FIGURE 2 r a tion Mincrols and their Ranges in Olkaria Exploration I 190 Leach Bicarbonate fluids appear to be almost ubiquitous in the west, b) Clay Minerals and Reservoir Hydrology: suggesting condensation of and possible mixing with These suites of minerals are ubiquitous in the Olkaria formation alkaline chloride fluids (see distribution of zone) and the range of clays with is outlined in Figure 2. even at depth in the reservoir. A purely condensate layer In the northern wells the clay minerals grade from smectite to to present at shallow depths in the vicinity of OW- interlayered chlorite-smectite to chlorite with increasing 302. The bicarbonate - condensate fluid is almost totally absent Detailed clay analyses on cuttings from OW-703 in the easternreservoir, except for a minor mixed revealed the presence of an interlayered chlorite-illite co- chloride at shallow levels in the region of OW 702. existing with chlorite at temperatures greater than about In the western wells smectite grades to interlayered The data used in Figure 3 is from clay analyses on cores only. illite-smectitethen to illite at increasing depth and temperature. A detailed analyses of clays