Kamojang Geothermal Field

Kamojang geothermal field is situated over an area of 14 km², in the East-West trending Rakutak - Guntur volcanic chain. The area is composed of undifferentiated deposits and pyroxene andésite. There are about 30 wells producing dry steam and they are exploited for electric power generation, while the other three wells are used for reinjection of steam condensate. All the wells are located within the boundary of low resistivity zone and their depth is in the range from 600 to 2,100 m. The average temperature of the reservoir is 243°C. The purpose of the isotope study in Kamojang geothermal field is to know the origin and hydrological processes of the fluids.

86

-1O-S

-3O--3O—

-7O

weN v

Steam heated pool

A

wed X

T I I I I I I I I j I I I I I I I I I f^

-1C -5 O OXYGEN-18 (o/oo)

FIG. 2. Oxygen-18 vs deuterium, Dieng geothermal field, central Java.

-3O-—4O—

i

-50—

-«0—

-70—-n

WELL /V HOT SPRINGS

i | i i i i | i i i i | T i i r ] - r i i i 7 n r 2SO 50O 75O 1OOO 125O

CHLORIDE (PPM)

FIG. 3. Chloride vs deuterium, Dieng geothermal field, central Java.

87

-1O—

-2O—

-30--4O—

—SO—

-GO—

-7O-Geothermol Une

-1O I

-B

I I | I I I I [ I -5 -3 OXYCEN-18 (o/oo)

O

r

FIG. 4. Oxygen-18 vs deuterium, Kamojang geothermal field, west Java.

' s _io

8

^-^ S;

Legend

Oxygen-18(%o) 12o : Wells

FIG. 5. Distribution of oxygen-18, Kamojang geothermal field.

TABLE in. STABLE ISOTOPE COMPOSITION OF WELLS OF KAMOJANG FIELD

The average value of deuterium content of the wells is - 46.8 ± 2 %o. It corresponds to the variation of deuterium in precipitation at altitudes 1,200 to 1,700 m. The variation of oxygen-18 is from -8.3 %o until -5.3 %o (Fig. 4). The geothermal line intercepts the local meteoric water line at the point where the value of oxygen-18 is -8 %o and deuterium is -45.8 ±1.7 %⁰. This composition represents the composition of precipitation infiltrating into the ground as an input for geothermal fluids. The oxygen shift that is due to water-rock interaction is from 0 to 2.7 %o. The oxygen-18 content of wells at the Western part of the field ( KMJ 27, 28, 30, and 42 ) are around -8 %o, it means that the oxygen shift is 0. While the wells at North-East and South-West ( KMJ 11, 12, 14, 17, 18, 45, 38, and 31 ) their oxygen shift is around 1, and the wells at the North ( KMJ 24, 25, 39, 36, 46, 44 and 51 ) their oxygen shift is around 1.5. The longest oxygen shift is found at KMJ 41 and 35. Since the type of rock throughout the production area is relatively similar, the difference in oxygen shift is due to the difference in the path of its circulation. The flow direction of the geothermal fluids is indicated by distribution of oxygen-18 of the wells in Kamojang area.

In the Western part the flow is from West to East, while in the production area the fluids flow from South West to North East ( Fig. 5 ). This circulation informs that there is a mixing between meteoric water with original steam at the reservoir boundary and keeping a depletion of oxygen-18 at the production wells.

At the Western region the flow velocity is higher than at the Eastern region. This phenomenon is supported by the fact that the oxygen shift of wells in the Northern region is higher than those in the Southern region. The slower velocity in the Northern region is also probably due to the higher value of permeability of rock in that region.

4. SUMMARY

1. The isotope study in Dieng field has provided informations of the circulation of geothermal fluids. The data on the deuterium content of the reservoirs indicate that the fluids originated from precipitation infiltrating at 1,600 -1,800 m. The difference of the value of positive oxygen shift at South East area (2.3 %⁰) and at North East area (9.8 %⁰) is probably due to the difference of the path of circulation.

2. The geothermal fluids originated from precipitation infiltrating at elevations of 1,200 - 1,500 m. The fluids flow from South West to North East. Since the value of oxygen-18 at the South West area is more depleted than at the North East area, this indicate that there is an interaction of deep fluids with meteoric water at the boundary. The oxygen shift is relatively very small, namely 2.3 %o.

Acknowledgement - The authors are grateful to Dr. R. Gonfiantini who has supported the Centre for Application of Isotope and Radiation, BATANin the upgrading laboratory facilities for doing activities in geothermal research and providing valuable consultations to the Staff

members of Hydrology Laboratory at B AT AN. We also would like to thank to Dr. C. Panichi for his constructive comments on this work. Appreciation is also addressed to Mrs. Nazly

Hilmy, Director of the Centre for Application of Isotope and Radiation BAT AN, who has put the programmes of isotope hydrology as one of the priority to be developed. And last but not least, to colleagues in Isotope Hydrology Laboratory ofCAIR BATAN, we are grateful for their time to time help and cooperation.

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References

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[2] VAN BEMMELEN,R.W.,The Geology of Indonesia, Vol IA, Government Printing Office , The Hague, (1949)

[3] PINANTUN,M.," Geothermal Development of Indonesia", Geothermics, 17, 2/3, (1988),415-420

[4] VINCENT,T.RADJA.,"Indonesia Geothermal Development in the National Energy Scenario, Development Program to the year 2000", Geothermics, 15, 5/6,(1988), 600.

[5] MUZIEL,A.,"Geothermal Energy Potensial Related top Active Volcanic in Indonesia", Géothermie, 15, 5/6, (1986), 601-607

[6] FAUZI, A., Mineralogy and Fluid Composition at Dieng Geothermal Field, Indonesia, Master Thesis, Victoria University, Wellington (1987)

[7] GIGGENBACH, W.F., "The Isotopic Composition of waters from the El Tatio Geothermal Field, Northern Chile", Geochim. Cosmochim. Acta, 42, (1987),979 [8] FOURNIER, R. O., SAREG, M.L.,"Chemical and Isotopic Prediction of Aquifer Temperatures in the Geothermal System at Long Valley California", J.of Volcanology and Geothermal Research, 5, (1979), 1-16

THE USE OF GAS CHEMISTRY AND ISOTOPE GEOTHERMOMETRY