Proceedings 20th NZ Geothermal'Workshop 1998

SOKORIA, EAST INDONESIA: A CLASSICVOLCANO-HOSTED HYDROTHERMAL SYSTEM

C.C. E.B. R.D. AND

Division of Designpower Ltd., Auckland,

RuralElectrification Divi. of P.T.PLN (Persero) Kantor Jakarta 12160,Indonesia

SUMMARY - A geoscientific survey was out over the Sokoria region of Flores Island, EastIndonesia to assess its potential for small scale geothermal development to supply the electricalrequirements of the region. The Kelimutu volcanic complex is a classic example of a volcano-hostedhydrothermal system. The complex hosts both an active volcanic component and a high temperaturehydrothermal system. Geochemical surveys that at highest elevations, two of the KelimutuLakes contained a magmatic component while the third had characteristics of high elevation steamcondensates from the geothermal system. With decreasing elevation, the chemistries of fumaroles andhot springs extending over an area of 80 sq indicated mixing of both the magmatic and condensatecomponents with neutral chloride outflows. This geochemical model was supported by resistivity datawhich indicated high temperatures north of the Mutabusa fumarolic area and an outflow along theLawongalopoloRiver Valley in the south west of the prospect. .

1. INTRODUCTION

A geoscientific survey was carried out over theSokoria region, north east of Ende on FloresIsland, East Indonesia (Figure 1) to assess itspotential for small scale geothermaldevelopment to supply the electricalrequirements of the region. This survey wasfunded by the New Zealand Ministry of ForeignAffairs and Trade (MFAT) in collaborationwith the Government of the Republic ofIndonesia. A detailed review of existinggeological, geochemical and geophysical datawas supplemented by air photo interpetation,and additional geochemical sampling. Thesedata were interpreted to produce a field modelof the resource and a seriesof recommendationsfor ongoing work.

2.THERMAL FEATURESOF THEAREA

There are over 40 thermal featuresin the projectarea (Figure 2). The dominant features are thethree spectacular high-elevation craterlakes at Keli Mutu (Danau Alapolo, DanauKootainuamuri and Danau Abutu) which haveintermittent fumarolic activity and temperatures

around 30 At times, temperaturesof have been noted. The chemistries oftwo of these lake waters indicate a magmaticcomponent while the chemistry of the westernlake, Danau Abutu, is characteristic of a steamcondensate above an active hydrothermalsystem. The two eastern lakes outflow into thedrainages to the south-and east. High-elevationfumaroles are located south-west of the lakes atMutubusa and north-east at Mutulo'o.

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Figure 1 Location Map

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I

Figure 2 Distribution of ThermalFeatures

The Mutubusa and Nuasida acid hot springs (92to 97 "C) and the Sokoria neutral warmsprings (34 "C) are located downstream fromMutubusa while the Waturaka and Liasembewarm neutral springs (38 to 43 "C) aredownstream from Mutulo'o. To the north-west,neutral warm springs at Detusoko andWoloveo (44 to 76 have chemistriesconsistent with their being condensates from ageothermal system. At lower elevations,neutral springs with a mixedcondensate - reservoir character are located tothe south - east at Landakura, Detu Petu andWolobora (44 to 54 "C). To the south at Rogaand south-east at Jopu, neutral and acidsprings (44 to 51 "C) are found which may alsohave a component of outflows from the KeliMutu lakes. A number of cooler dilute springssuch as at Wolojita (36 in the south-eastand Saga (28 to the west complete theinventory.

3. GEOLOGICAL SETTING

Flores Island lies on the Banda Arc section ofthe Sunda - Banda Island Arc system where theIndia - Australia crustal plate slides northwardunder the Eurasian plate, subducting at a rate ofabout 6 (Hamilton 1979). In thissection of the arc the Eurasian plate consists ofoceanic crust with the resulting volcanismbeing dominated by basaltic andesite andandesite with tholeitic affinities. The lateQuaternary volcanism is located on the southside of Flores with eight active volcanoes overthe 500 length (east to west). The Keli

Mutu Volcanic Complex covers an area ofbetween 300 and 400 extending 23 kmfrom the SW coast in a SSW - NNE direction(Suwarna et 1989).

The complex is predominantly andesitic incomposition with minor dacites. A number oferuptive centres have been identified, forming acomplex inter - fingering sequence of lavas andpyroclastics. There are a number of collapsefeatures that provide additional complexity to

volcanic stratigraphy. The eruptive historyof the Keli Mutu complex included a series oferuptive phases; periods of structural collapseand possible caldera formation, with filling ofthe collapse features by later eruptions.

Surrounding the young Quaternary Keli MutuVolcanic Complex on the northern and easternsides, and presumably underlying it, areMiocene volcanics, sediments and intrusiverocks. While andesitic lithologies dominate thevolcanic rocks, dacitic - rhyolitic lavas andpyroclastics are also found on the NE side ofthe Keli Mutu complex and possiblyunderlie itin this quadrant. The sediments are dominatedby limestones and sandstones which commonlyhave a tuffaceous component. A 10diameter granodiorite body intrudes Miocenerocks to the east of Keli Mutu.

4. GEOHYDROLOGY

The regional hydrological setting is likely to benorth to south from the divide approximatelymidway across the island of Flores, towards thesouthern coast. Local drainage is controlled bythe recent volcanic centres of Keli Mutu andKeli Bara (Soetrisno, 1983). The pyroclasticnature of Keli results in high

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transmissivity and a radial pattern of highlyproductive groundwater aquifers. The lavas ofKeli Mutu are likely to be less permeable butthere is strong geochemical evidence ofsubsurface flow from the lakes that flow downslope to the Toba springs and possibly Jopu.

5. STRUCTURAL GEOLOGY

The major structural feature within the KeliMutu complex is the NNE - SSW trendingLawongalopolo Fault which is traced from thesouth coast for about 19 across the volcaniccomplex 2). The fault has a linear traceacross about 1300 m of relief cutting relativelyyoung rocks, suggestingthat the fault is close tovertical and, has been recently active.

This fault intersects the inferred calderastructure of the earliest Sokoria volcanics in thevicinity of Sokoria where several of thecurrently active thermal features are located.The most western crater lake Danau Abutu isclose to the extension of the LawongalopoloFault. The association of these thermal featureswith this fault indicates that it may have highpermeability associated with it, and thereforeshould have high priority as a drilling target.

6. HEAT SOURCE

The close association of thermal activity withthe young volcanism on the KeliMutu VolcanicComplex indicates that the heat source for thegeothermal system is local magmatic activity.

7. PERMEABILITY

Keli Mutu Volcanic Complex variespyroclastics to lavas. The pyroclastics mayprovide high permeability for the movement ofthermal fluids while overlying lavas may act aslow permeability If condensates areforming at high elevations in the vicinity of theKeli Mutu Lakes, then they may move downslope through these high permeability units.Enhanced permeability may also occur atformation boundaries. In the centre of theprospect, the Roga Springs are located at theboundary between the Keli Bara pyroclasticsand the Keli Nabe volcanics.

The NNE - SSW aligned Lawongalopolo Faultextends across the volcanic complex and mayprovide fault-controlled permeability at depth.The location of several of the major, active,thermal features within this valley is consistentwith this interpretation. Several springs, withfluid characteristics indicating a componentfrom a deep chloride reservoir, are found at lowelevations along the fault.

8. GEOCHEMISTRY

A selection of analyses of thermal, stream andlake waters are presented in Table 1. Thelocations of these features are shown in Figure2. Geochemical surveys of the Sokoria areahave been undertaken on at least five occasionssince 1974. Three analyses of the KelimutuCrater Lake waters collected in 1992 arepresented in Table 1 after Pastemak andVarekamp (1994); Danau Alapolo1382 m), Danau Kootainuamuri (Keli-TIN,1394 m), and Danau Abutu (Keli-TAM.

Four and fumaroleanalyses areincluded fiom the most recent (1997)programme. Cation-anion balances andgeothennometry calculations are presented inTable 1 (Giggenbach, 1991).Gas analyses fiomthe Mutubusa and Mutulo'o fumaroles (Figure2) are presented in Table 2, which includes theresults of the two samples collected in 1997.

Stable isotope analyses of and D arepresented in Tables 1 and 2.

The springs and can be divided intoseveral groups on the basis of their location andgeochemistries (Figure 2). At the highestelevations m) condensation of

gases into the crater lakes hasproduced high chloride high sulphate waters.Gas eruptions, hydrothermal eruptions andvariable fumarolic activity are recorded withinthese features. Lake Keli-TIN has the lowestrecorded and the highest chloride

ppm) and sulphate (47,000 ppm)Water samples collected from

the flanks of Keli Mutu indicated possibleoutflows to the south and east along the Mboeli,Watu Gana and Ai Mutu rivers.

The western Kelimutu lake (Keli TAM) has asignificantly different ratio fiom the twoeastern lakes and a significantly higher boroncontent which may be associated with thecondensation of high temperature steam ahigh temperature geothermal resource.

At lower elevations, downstreamfrom the lakesthe majority of the fumaroles and warm springsexhibit features that are typical of a volcanicallyhosted hydrothermal system. The Mutubusaand Mutulo'o fumaroles have gas chemistrieswhich are typical of fumarolesassociated with ahigh temperature hydrothermal system. Thechemistries of the Detusoko springs classifiesthem as neutralised acid condensates, that havebeen neutralised by water-rock interaction.

The Toba located at 940 m elevation,down slope from the Keli Mutu Lakes, areclassified as chloride sulphate waters.Their origin is unclear but they are almostcertainly associated with outflows from the Keli

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aa

a

>

C

C

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Mutu Lakes. Based on molar ratio the highelevation Roga Springs and the eastern Jopusprings may have a minor component of theKeliMutu lake-type waters.

At lower elevations a series of springssuch as Detu Petu, Landakura, and Wolobora,(38 -5 1 are chloride-sulphatesprings whichappear to be mixed reservoir fluid condensateoutflows. Finally there are a number of dilutewarm springs (Sokoria, Saga, Wolojita,Waturaka and Liasembe) which have beententatively classified as condensatewaters.

Overall, the distribution and geochemistry ofthe springs are consistent with four types ofwater present at Sokoria :1. a input which is evident as a

minor component at the highest elevationsin the KeliMutu Lakes

2. a condensate from a high temperaturehydrothermal system

3. a deep neutral high temperaturereservoir fluid.

4. groundwater with no geothermal ormagmatic component.

The thermal features are mixtures of two ormore of these components. Close to the coastextreme dilution by the surface groundwatermay mask any surface outflows of the deepreservoir fluid, which are diagramaticallyillustrated in Figures 3 and 4.

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Li

Dilute CondensateCondensateReservoir CondensateReservoir Magmatic

+ Magmatic

Figure 3 Li-Cl-B Diagram

9. RESERVOIR CHARACTERISTICS

On the basis of the gas thereservoir temperature may be as high as 300°Cwhile, cation geothermometry indicatesreservoir temperatures of about

The high boron concentrations in the DanauAbutu crater lake are consistent with hightemperature steam separation at or above 300

The gas chemistry fkom the fumaroles atMutubusa and have similarto many developed hydrothermal systems.However, no assessment can be made of thelikely gas concentrations of the reservoir fluid.

Based on the composition and silicaconcentrations in the low elevation Landakurasprings, and assuming a deep reservoirtemperature of about and adiabaticisoenthalpic cooling, the lower elevation springchemistries indicate reservoir chlorideconcentrationsof about 5,000 ppm.

10. RESERVOIR BOUNDARIES

The thermal features around the Keli Mutucomplex extend over an area of approximatelySO Interpretation of the various typesfluids indicate the presence of a condensatecomponent at higher elevations in the northeast, north west and south west of the Keli

lakes, and a component of the deepfluid along the Lawongalopolo Valley

and possibly east at Roga and Jopu.

30

25

20

a

E

Dilute Condensate5

00

Water (ppm)

Figure 4 ChlorideBoron Trends

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Figure 5Elevation of Resistive

Basement the

Lowongolopolo Valley

Cold47

7

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?

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Deep cold ?

Modeled Resistivity (ohmm)Layer Boundary

61700 Sounding Name

5500

Figure 6 Modelled Resistivity and Inferred Temperatures, Lowongolopolo Valley

-200

Distance(m)

?

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903800

903600

9034001

9032001

9030001

E

9028001

a Dilute Condensate

370000 372000 376000 380000 382000

Easting

Figure 7 Interpreted Boundaries and Thermal Flows at Sokoria

LakesAbutu,

Figure 8 Conceptual Model of the Sokoria Hydrothermal System

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The resistivity mapping and sounding surveysfrom the Mutubusa - Sokoria area have outlineda resistivity structure that can be interpreted asa high-temperature geothermal reservoir

5 and overlainby a cooler clay-richalteration zone, characteristic of geothermalsystems in andesiticterrains of high relief.

Because of the limited extent of the geophysicalsurveys, only the southern margin of thegeothermal reservoir has been delineated, eastand north of the Mutubusa area.These manifestations are located at the veryedge of the system, and do not constitute apotential drilling target.

The resistivity pattern indicates an outflow ofgeothermal fluids south-east (Figure 6) alongthe Lawongalopolo fault, manifested as diluteneutral chloride springs down theLawongalopolo valley. The deep resistivitystructure suggests that the acid springs at Tobawithin the Roga valley to the east, are quiteseparatefrom the Lawongalopolooutflow.

The extent and boundaries of the geothermalreservoir cannot be determined from the presentdata. However, low resistivities reported athigh elevation on the north side of the KeliMutu complex, together with fumarolesand hotsprings at Waturaka, suggest that thegeothermal reservoir could extend for somedistance to the north.

11. CONCEPTUAL FIELD MODEL

The various geochemical trends are presentedin Figure 7 with a conceptual resource model inFigure 8 which presents the possible reservoirboundaries and the relationship with the varioushot springsand fumaroles.

The thermal features at Sokoria include thehighest elevation crater lakes (above 1300 m

in two of which a magmatic component ishighly likely. At high elevations (600 to 1200m acid sulphate condensates are formedfrom the condensation of steam and acid gasesfrom an active hydrothennal system. In somecases (such as Toba) these condensates aremixed with the outflows from the lakes.

At lower elevations m the Rogaand Jopu springs have molar ratios whichindicate mixing of the lake waters with areservoir fluid, while at Detu Petu, Woloboraand Landakura there is evidence of neutralreservoir chloride fluids mixed with a small component of the high elevation condensates.

However, on the basis of gas geothermometryand mixing models a hypothetical 300°Creservoir fluid is proposed with a chlorideconcentrationof approximately 5000 ppm.

The regional distribution of thermal features,and interpretation of the resistivity indicate apreferred drill target at relatively high elevation,in the headwaters of the Lawongalopolo Rivervalley. The reservoir in this area is likely to befrom 600 to 800 m

The Mutubusa fumarolic area is likely to be onthe margins of the geothermal system, and isnot a suitable drilling target. Refinement of adrill target in this area will require additionalresistivity data, in particular a deeply-penetrating magnetotelluric(MT) survey.

In addition, geochemical sampling withhigh precision chemical analyses arerecommended downstream in theLawongalopolo valley and at low elevations asfar as the coast to Kapo One in the east. Such asurvey would assist in characterising thereservoir composition and determine thesignificance of the inferred eastern outflows.

12. ACKNOWLEDGEMENTS

The study team gratefully acknowledgesthe co-operation and assistance of staff of PT PLN(Persero) during the surveys and permission ofthe New Zealand Ministry of Foreign Affairs

'and Trade, and PLN to publish this paper.

13. REFERENCES

Giggenbach, W.F. (1991) Chemical techniquesin geothermal exploration, Application ofGeochemistry in Geothermal ReservoirDevelopment: Centre on SmallEnergy Resources. Ed. F. D'Amore ppl19-144.

Hamilton, W. (1979) Tectonics of the IndonesiaRegion. USGS Prof. Paper 1078,345 p.

and Varekamp, J.C. (1994)The geochemistry of the Keli Mutu crater lakes,Flores, Indonesia. Geochem. Journal, v.28, pp

Soetrisno, S. 1983 Hydrogeological map ofGeologi Tata

Bandung.

Suwarna, N., Santosa, and Koesoemadinata,S. (1989) Geological map of the Endequadrangle, East Nusatenggara. GeologicalResearch and DevelopmentCentre, Bandung.

All features are heavily diluted by near surfaceground waters which has constrained anydetailed interpretation of fluid geothermometry.

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