Reservoir hydrology of Sibayak field

Iso-contour map of ¹⁸O and chloride can be seen at Figure 14 and 15. This ¹⁸O iso-contour shows that the water recharge/fluid moves from along Rim Circle of Singkut Caldera to the system. This is clearly shown by isotope contour from hot spring near to crater which has ¹⁸O value ~ -10 ^o/_oo. This value is more depleted if it is compared to the isotope concentration from Sibayak reservoir. Although the recharge water moves along Singkut fault toward north direction, ¹⁸O shift reactions do not take place progressively. This might be caused by the rock properties that have undergone alteration processes (because of the mature rock). Figure-14 also shows that potential reservoir is located at point A, B and D toward north direction where there is a considerable developing border in the North. This is appointed by borderline located at 1700 m height.

Figure 12. Trilinear diagram for gas samples from wells, hot springs and fumaroles.

Figure 13. δ18O versus δ 34S in SO4 for geothermal fluids in Sibayak.

Figure 15 shows that chloride content in the up flow zone ranges from 800–1300 ppm. The content of chloride in southern or northern Singkut fault gives the lower value because of dilution influence from recharge water. On the other hand, the chloride content gives the higher value in the direction to crater.

Both sources show that reservoir in crater vicinity is potential (called as up flow zone). Hydrology of temperature iso-contour in reservoir can be seen in Figure 16. This figure gives information of

temperature iso-contour through cross-section that is completed by production wells of SBY-1, 2, 3, 5 and 6.

The iso-contour line for 250 ^oC and 300 ^oC is based on gas, chemical and isotope geothermometer, whereas the iso-contour line for 150 ^oC and 200 ^oC is based on the temperature gained from Kuster measurement of SBY-3, SBY-2 and SBY-1. Based on hydrology of temperature iso-contour, it is estimated that the up flow zone is located around the crater, and also it is known that the temperature is lower more southern of Rim caldera in Singkut. This figure shows that fluid movement is from the up flow zone to caldera Rim whereas the recharge water (originated from ground water) moves from Rim caldera. The model hydrology also shows the possibilities of fluid origin whether they come from production well or steam vent.

Figure 14. Iso-contour map for δ 18O in geothermal fluids of Sibayak.

Figure 15. Iso-contour map for Cl in geothermal fluids of Sibayak.

Figure 16.A schematic hydrothermal model of Sibayak geothermal field.

Fluid from steam vent is influenced by volcanic system whereas fluid from production well, mostly it comes from deep fluid has undergone some interaction/reaction with host rock.

8. Conclusions

Based on analysis of ¹⁸O, D, ³⁴S isotope, chemicals, gases and also study of results as well as evaluation of water recharge, isotope, concentration in reservoir, geothermometer and reservoir hydrology problems, the reservoir condition of Sibayak can be concluded as follows:

Reservoir fluid of Sibayak geothermal field originates from meteoric water and does not indicate the volcanic system influence. This reason is supported by geochemistry observation showing that neutralization process because volcanic influence in host rock system has occurred.

Reservoir hydrology shows that a up flow zone or potential reservoir is located around the crater of Sibayak Mountain related to the developing border at points A, B and D. It seems that the reservoir can be exploited properly.

The result of some geothermometer studies shows that geothermal field of Sibayak has temperature 250^oC up to 270^oC. The reservoir fluid originates from parent fluid having the temperature ~ 350^oC.

Recharge of Sibayak geothermal field occurs at the elevation of 1100–1300 m above sea level. Location of this area is estimated around Rim of Singkut Crater. The recharge water flows through Singkut, enters into the reservoir and then moves to the north.

The fluid exiting the crater through steam vents at the elevation of 1900–2000 m shows that it is influenced probably by volcanic system.

Acknowledgements

This study has been carried out through cooperation between the International Atomic Energy Agency and Center for the Application of Isotopes and Radiation, National Nuclear Agency under the IAEA research contract No. 9717. A. Truesdell is thanked for his comments on an earlier version of the manuscript.

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PRELIMINARY NOTES ON THE ACID FLUIDS OF THE MIRAVALLES