187

Proceedings 9th Geothermal Workshop 1987

GEOLOGY AND HYDROTHERMAL ALTERATION OF THE NORTH AND WEST EXPLORATION WELLS IN THE GEOTHERMAL FIELD,

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 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 . 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 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

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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

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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 in the cuttings would lead to a better understanding of the distribution of the fluid types at The distribution of the clay minerals within the reservoir give Olkaria, and therefore a knowledge of the hydrology of the importantinsights into the hydrology of the system. system (currently being undertaken by G Muchemi). There is a significant in the clay mineralogy from the Detailed petrology on cores and cuttings in the north-eastern north-easternand the western sectors of the field. Illitic clays wells was undertaken in order to aid in setting production are prevalent, and almost totally restricted to the reservoir rocks casing in this sector of the field. The distribution of kaolinite, in the west whereas chloritic clays are dominant in the north- smectite and carbonatein this sector are illustratedin Figure 4. east. The occurrence of illitic versus chloritic clays is a An influx of carbonates (calcite, ankerite and siderite) were function of The follow relationship of and clay observed at the piezometric surface of the hydrothermal minerals, at temperatures of about has been system, and to persist at depth below this level. This determined from thermodynamic data (Robie et al1978): distributionis interpreted to have derived in response to cool bicarbonate fluids moving down into hotter reservoir illitic illitic chloritic chloritic conditions (refer Leach et 1985). Kaolinite is formed by 5-6 rock reaction with low fluids, interpreted to have formed by condensation of and possibly some oxidation Chemical sampling of discharge fluids from the exploration of above the geothermal system, as evidence by the wells has encountered two distinct fluid types with a distinct presence of gypsum in OW 501. The smectite forms under distribution across the field (see Barnett et al, this proceedings): low temperature ( conditions. The distribution of an alkaline NaCl fluid in the north east; and a lower these minerals in the northern sectors is therefore interpreted to bicarbonate fluid in the western wells. It is interpreted indicate the influx of cool, low bicarbonate fluid into the therefore that the illitic clay has been formed by rock reaction reservoir. This recharge is deepest in the north (OW 501) and with the bicarbonate fluid, and the clays with alkaline has penetrated progressively shallower depths to the south. Na-Cl fluids. The bicarbonate fluid is interpreted (Barnett The first occurrence of epidote illustrates that the reservoir 1987) to represent condensation of gases which have conditions throughout this region were similar at some stage in evolved off the reservoir fluid. the past, however cool recharge fluids are now into the reservoir from the north.

West Eost

OW-302 OW-702

0

8 -

FIGURE 3 Distribution of Clays in the Olkario + Chloritic Cloys Exploration Area Chloritic Cloys

Fluid Inclusions inclusiontemperatures unexpectedly decrease with depth in both wells. OW 703 has not stablised after drilling, and OW inclusion homogenisationdeterminationswere carried 302 has a strongcool downflow inside the wellbore; therefore on quartz, calcite and rhodochrosite crystals picked from veins it is not possible, at the time of writing, to compare the fluid and vesicles in selected cores from some of the exploration inclusion temperatures with the downhole temperatures. Two wells. Histogramsof homogenisation temperatures in crystals explanations for the coolerfluid inclusions with depth from wells OW 302 and 703 are shown in Figure 5. Most of crystals contained vapour, vapour and rich phases indicative of entrapment of two phase fluids. This would The crystals deposited during different account for the wide in homogenisation temperatures periods in the evolution of the geothermal system, ie the encountered in most of the samples. In these cases the lower cooler temperatures represent deposition while the system more representative of conditions. was heating up. 191

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North

.--- Eo

------First of

--- Isotherms I A of Production FIGURE 4

Distribution of Cool low Recharge Fluid in North - East Olkoria from tho of Kaolinite, Smectite and Carbono

The lower temperatures at depth indicate current influx of ACKNOWLEDGEMENT cool fluids deep into the reservoir. Deep temperature inversions have already been observed in OW 201 and 401, The authors would like to thank the Kenya Power Company for and cool inflows have been encountered in OW 302. It is having the foresight to encourage the petrological studies that have interpreted that these inflows are due to the intersection of made this possible, for the data for publication, and faults channelingdownward moving recharge fluids. the paper to be The fine geological logging by Andy Ryder of GENZL gratefully acknowledged. The authors Fluid inclusion work is currently been undertaken on a wide also indebted to the Instituteand far helping range of cores from different wells and sectorsof the field. It the detailed petrological work to be carried out in Auckland, and to is hoped that this study will determine which of the above the GENZL typists for preparing the manuscript, Most of the data scenarios is likely; and if the second one is proposed, which for this paper are from unpublished KPC and GENZL reports faults are channelingcool fluidsdeep into the reservoir. by the authors. CONCLUSIONS REFERENCES exploration wells at Olkaria have intersected a thick volcanic pile 1. Barnes HL (1979): Geochemistry of Hydrothermal Ore of alkali and minor basaltic extrusives. The best permeability in the formation is found at widely spaced fault intersections, with very little permeability occurring within the fault blocks. Primary 2. Barnett P et (1987) Recent Exploration and Development, porosity in the lavas and pyroclastics provide good permeability at Olkaria Geothermal Field (this shallow levels, however these have been sealed by mineral deposition at depth. 3. Browne, PRL (1984) Subsurface stratigraphy and Hydrothermal Alteration of the Eastern Sectionof the Olkaria The Occurrence of some key hydrothermal minerals can be used to Geothermal Field. 6th Geothermal Workshop. understand the hydrology of the geothermal system. The distribution of illitic and chloritic clays can determine the extent of the low 4. Leach, TM (I985) Mineral Zonation in Island Arc bicarbonate and the alkaline chloride fluids within the reservoir. The Settings: The Bacon-ManitoGeothermal Field, Philippines. distribution of kaolinite, smectite and carbonates can be used to determine the regions and extent of cool recharge fluids into the 5. Robie, RA, Hemingway, B and Fisher, R (1978): system. Thermodynamic Properties of Minerals and Related Substances at and 1 Bar Pressure and at Higher Fluid inclusion analyses suggest the presence of cool inflows deep Temperatures. U.S.G.S. Bull into the reservoir,probably via faults. Future detailed fluid inclusion studies will be aimed at defining these in order to aid further development. 192

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I

0 50

Depth Depth OW -703 (me OW -302

\

\ \ \

e

, rich

1500

FIGURE Fluid Inclusion Histograms with Depth in Cores from OW-302 and OW- 703