情報地質 第28巻 第2号 092-093頁 2017年
Geoinformatics, vol.28, no.2, pp.092-093, 2017

Oxygen Isotope and Ranking Faults Analyses to Delineate Water-rockInteraction Processes in a High-Temperature Geothermal System
空白
Riostantieka Mayandari*, Yohei Tada*, Koki Kashiwaya*, Katsuaki Koike*,
Irwan Iskandar**, M.N. Heriawan**, S. Notosiswoyo** and Dwiyoga Malik***

空白*Graduate School of Engineering, Kyoto University, Katsura C1-2-215, Kyoto 615-8540, Japan.
E-mail: shoedarto.mayandari.56r@st.kyoto-u.ac.jp**Faculty of Mining and Petroleum Engineering, Institute Technology of Bandung, Bandung, Indonesia*** Star Energy Geothermal (Wayang Windu) Ltd., Wisma Barito Star Energy Tower, Jakarta, Indonesia

空白Key words: Geothermal, Hydrochemistry, Oxygen-18 isotope, Upflow zone, Wayang Windu

1. Introduction
According to Lumb (1981), one of the main objectivesof geothermal explorations is to locate productive zones orupflow zones to ascertain an existence of usefulgeothermal production field. Wayang Windu geothermal
field is a recognized high-temperature field in Indonesia
and characterized by a transitional liquid to vapor systemthat has installed the capacity of 135 MWe.
The purpose of this study is to clarify the possible
correlated signatures between productive zones andoxygen isotopes of the reservoir fluid in the northern partof the area, due to its numerous high-ranked faults
compared to the southern part.

2. Methods

According to previously study from Star Energy, themost productive zones were specified based on theappearances of the best-fitted faults from the integrationof surface and subsurface structures (Star Energy, 2015).
An integrated study of geological structures has beenconducted by Star Energy Geothermal (Wayang Windu)
Ltd. for almost 15 years. Star Energy used image data
analyses such as surface lineaments from satellite images
and aerial photos. These lineaments were then verified by
the field data on thermal manifestations, surface
structure evidences, and stream conductivities. Before
they arranged a surface structure identification, theysuggested structure rankings from their analyses onstructure identifications. In addition, some production
data such as feed zones, steam-kick spots, loss zones, andopen fractures were selected to verify the subsurface
structure
identification.
Finally,Star
Energyconfidentially proposed the best fitted of the fault ranking
based on the integrated data from major surface andsubsurface structures. They were generated from series ofcross-sections to identify the presence and location offaults. Several faults were then identified as the best
ranked faults such as Haneut, Kijang, Cibitung, Cibitung
1, and Gambung Selatan Faults. The second best-ranked
faults was also recognized as well as the third and fourth
best-ranked faults. These results will be integrated withgeochemistry interpretations.
Additionally, methods used in this study include field
work, water and isotope sampling, laboratory analyses,and their interpretations. There are two-types data as:

A. Primary data including water geochemistry andisotopes sampling from 7 hot springs, 3 fumaroles, and16 well heads.
B. Secondary data consist of Wayang Windu geothermal
field map from Bogie et al. (2003).
Chemical properties were analyzed to obtain isotopesof δ18O and δ2H ratios by a Picarro instrument. All thesesurface data were used to calculate the values of isotopes
in the reservoir isotopes fractionation calculation (Eq. 2and Eq. 3) from Truesdell (1977) op cit. Nuti (1992).

3. Calculation
Such subsurface processes like boiling, mixing, steam
heating, surface evaporation, conduction, and water-rockinteraction can change the isotopic content of thegeothermal waters to somewhat lower or higher than thatof the local precipitation because the oxygen contents areexchanged between host rocks (Nicholson, 1993).
Calculation of stable isotope values in the reservoir is
obtained by heat and mass balance approach:Hl,res = Hl,t (1-y) + Hv,t (y)(1)δl,res = δl,t (1-y) + δv,t (y)(2)
Hl,res refers to liquid entalphy in the reservoir; Hl,tdenotes liquid entalphy in the given temperature; Hv,tdenotes vapor entalphy in the given temperature; δl,resdenotes liquid isotopes ratio in the reservoir; δl,t denotes
liquid isotopes ratio in the given temperature; δv,tdenotes vapor isotopes ratio in the given temperature; yis vapor fraction; and (1-y) or x is liquid fraction.
Equation 2 can be reformed to δl,t = δl,res + (y) 1.000
ln α, by changing the value factor isotopic fractionation δlδv to ≈ 1.000 ln α where 1.000 ln α is a function oftemperature. This function was derived from Horita andWesolowski equation (1995) in 0C to the criticaltemperature 374C (T in K):
1.000 ln −= −7,685 + 6,7123− 1,6664+(3)0,350411.000 ln −H = 1.158,8794.84− 161.04 + 2,9992− 1.620,1+(4)Those equations mean when the parent fluid movestowards a shallower depth, the temperature will bedecreased and the δ18O fractionation factor will beincreased slightly. Changes in fractionation factors will

93

substitute δ18O values in boiling production wells intolighter δ18O values.
Stable isotope compositions of surface manifestations
are ranged from -8.76‰ to -5.93‰ for δ18O/16O, and are 58.81‰ to -47.29‰ for δ2H/1H. Meanwhile stable isotope
compositions in each wells are ranged from +0.4‰ to 4.57‰ for δ18O/16O and -34.55‰ to -44.75‰ for δ2H/1H. By
using deep well liquid data from Bogie et al. (2003), whereT is 300°C, the values of δ18O/16O parent fluid is calculated
as 1.97‰ and -0.87‰ for δ2H/1H.

on integrated data from major surface and subsurface
structures from Star Energy are correlated to the resultsof stable isotope compositions in the reservoir. Subsurface
processes from isotope composition especially water-rockinteraction process could be a supportive signature of thequality of the faults, whether they are surficial orsubterraneously connected.

4. Results and Discussion

As host rocks have relatively heavy isotopic ratios, itcauses the oxygen shifting of the well fluids as we can see
from the values of Wayang fumarole and production wells
in Figure 1. These exchange processes are greatly
accelerated in high temperature zones, where less oxygen
shift is anticipated in low to intermediate temperaturezones. Parent fluid of deep liquid reservoir was generated
from meteoric water which was heated by steam fromboiling processes in the depth (yellow line). Meanwhile
fluids from production wells were initiated from meteoric
water, which had undergone water and rocks interactions(green line), and were also resulted from the condensationfrom deep seated parent fluid steam (purple line).
Figure 2. Those first-ranked faults (red-colored lines) are
confirmed with oxygen isotope analyses.

Acknowledgement: We sincerely thank Star Energy
Geothermal (Wayang Windu) Ltd. for the collaboration on
the Science and Technology Research Partnership for
Sustainable Development (SATREPS) Program with JST,JICA, and Institute Technology of Bandung.

References

Figure 1. Meteoric water is the source of surface manifestations
and the reservoir, meanwhile Wayang fumarole gained thesteam from condensation process of boiled production fluids.

Figure 2 introduces the 1st ranked faults in red lines,
and 2nd ranked faults in yellow lines. Meteoric water ispresumably seeped through the 1st ranked faults such as
Haneut Fault which is responsible for water-rockinteraction processes in MBD5 and Wayang fumarole aswell as Kijang Fault for MBA2, MBA3, also MBD1; andCibitung 1 Fault for MBD4 (Figure 2). Meanwhile, thewater-rocks interaction processes of MBB3 and MBB5 areunder control of the intersections of 2nd ranked in thenorthern part. Wayang fumarole sits in Haneut Fault
which possibly develops its steam that connected withMBD5 and MBB5, since they are located in the samefault.

5. Conclusion

Wayang Windu reservoir fluids were derived from its
local meteoric water. Predominant subsurface processes
based on isotopic analyses are water-rocks interaction,condensation, meanwhile boiling process need to besupported by other analyses. The best-fitted faults based

Bogie, I., and Mackenzie, K.M. (1998) The Application ofa Volcanic Facies Model to an Andesitic Stratovolcano
Hosted Geothermal System at Wayang Windu, Java,Indonesia. Proc. 20th NZ Geothermal Workshop.
Ellis, A.J., and Mahon, W.A.J. (1997) Chemistry andgeothermal systems. Academic Press, New York, 392p.
Hingerl, F. F. (2012) Geothermal electrolyte solutions:

thermodynamic model and computational ﬁtting
framework development. Dissertation Doctor ofSciences in Geochemistry, ETH Zürich, Switzerland.
Horita, J., Cole, D.R., and Wesolowski, D.J. (1995). The
activity-composition relationship of oxygen andhydrogen isotopes in aqueous salt solutions: III. Vaporliquid water equilibration of NaCl solutions to 350°C.
Geochimica et Cosmochimica Acta, Vol. 59,.1139-1151.
Lumb, J. T. (1981) Prospecting for geothermal resources.

In: Rybach, L. and Muffler, L.J.P., eds., Geothermal
Systems, Principles and Case Histories. J. Wiley &Sons, 77—108p.
Nicholson, K. (1993) Geothermal Fluids, Chemistry andExploration Techniques. Springer-Verlag, Berlin
Heidelberg.
Nuti S. (1981). Isotope techniques in geothermal studies.

In: Proc. Application of Geochemistry in Geothermal
Reservoir Development. UNITAR, 215-251.

Star Energy Geothermal
unpublished report, 2016.

(WayangWindu)

Ltd.,