第31回日本情報地質学会講演会 講演要旨集 13-14頁 2020年

Identification of meteoric water incursion to a high-temperaturegeothermal reservoir by rare earth elements and isotope oxygen-18
analyses
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Riostantieka M. Shoedarto, Yohei Tada, Koki Kashiwaya, Katsuaki Koike
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Graduate School of Engineering, Kyoto University, Katsura C1-2-215, Kyoto 615-8540, Japan.
E-mail: riostantieka.mayandari.7x@kyoto-u.ac.jp

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Key words: Rare earth element, oxygen-18, geothermal monitoring

1. Introduction
In this study, we use Principal Component Analysis(PCA) to establish a relationship among rare earth
elements (REE), δ2H, and δ18O for clarifying mixing
process in the reservoir from previous recharge study by
Shoedarto et al. (2020). PCA has been frequently applied
to processing geochemical and other types of geoscience
data to enhance the interpretability of the components
as combinations of multiple variables (Grunsky, 1997;
Cheng et al., 2009). Amongst several geochemistry
methods, the H2 and O18 are widely known to identify
meteoric mixing in the reservoir. On the other hand,
REEs have been well studied as geochemical tracers inaqueous environments (e.g., Willis and Johannesson,2011).
The samples were obtained from seven active
geothermal wells in a high-temperature transitionalliquid to vapor reservoir, that is located geothermal fieldin the Southern Bandung, West Java, Indonesia. Thispower plant has been generating 275 MWe over the past
twenty years. Monitoring of the reservoir performance iscritical to minimize any undesirable effects in a timely
manner.

2. Methods

All of the samples were taken from the transitionalzone except for the GA-12 (Fig. 1) which was sampledfrom the liquid zone. They were analyzed for the REEs
using
an
InductivelyCoupled
Plasma-Mass
Spectrometer (ICP-MS, Agilent 7500cx, Agilent
Technologies, U.S in Research Institute for Humanityand Nature, Kyoto) and for δ2H and δ18O analyses inKyoto University using water isotope analyzer based on
cavity ring-down spectroscopy (L2130-i, Picarro Inc.,U.S.). The δ denotes the ratio of the sample to thestandard VSMOW.
The variations in Ce, Eu, and total REEs are theresult of inherited source rock signatures and water rockinteraction (WRI) through the recorded sorption andfrom their REEs anomalies. To identify the anomalies
by considering the geological setting, normalization toknown materials such as Post Archean Australian Shale(PAAS; McLennan, 1989) is indispensable.
The REEs, δ2H and δ18O data compositions wereconverted into real number space by log-ratio transform.
By applying PCA function in the Origin program, thedata were reoriented to the greatest axis of variances
which becomes the 1st and 2nd principal components(PCs) in a bivariate plot (Fig. 3).

3. Calculation

The isotopic shift ratios for both δ2H and δ18O fromthe surface to the reservoir, δi, can be calculated asfollow (D’Amore et al., 1993):
δi = δl – xv  1,000 ln α
where xv and α denote vapor fraction and fractionation
factor of the isotope at a specific temperature between
liquid and vapor, respectively.
The δ2H and δ18O fractionations between liquidand vapor in the reservoir can be expressed as:δ2Hres = δ2Hsep – xv  1,000 ln α (δ2Hsep)
δ18Ores = δ18Osep – xv  1,000 ln α (δ18Osep)

where the subscripts “res” and “sep” denote thereservoir and separator, respectively. In thetemperature range from 0 to 374.1 C, ln α values forδ2H and δ18O are approximated by polynomial
equations of temperature T (in K, Horita andWesolowski 1995) as:
1,000 ln α (δ2H) = 1,158.8 (T 3/109) – 1,620.1 (T 2/106)+ 794.84 (T /103) – 161.04 + 2.9992 (109/T 3);
1,000 ln α (δ18O) = −7.685 + 6.7123 (103/T )– 1.6664 (106/T 2) + 0.35041 (109/T 3).

4. Results and Discussion
The results of δ2H and δ18O analyses reveal that
there are two groups of well fluids in Fig. 1. WY-1, GA11, and GA-12 wells had undergone water-rockinteraction (WRI), with GA-12 at the most advanced
stage. The waters of PB-1, GA-3, GA-13, and WY-2 areon the mixing line with PB-1 as the most diluted well as
it is located near to the Local Meteoric Water Line(LMWL).

Figure 1. Two groups of water-rock interaction (WRI) andmixing are characterized by the δ2H and δ18O analyses.

14Fluids that had been contacted with oxic
environment (recent meteoric recharge) usually show
negative Ce anomalies. From the spider diagram in Fig.
2, surprisingly the most diluted PB-1 and GA-3 have nosign of negative Ce anomalies. Normalizing to PAAS
gives a clear anomaly of HREE (heavy REE)
enrichments relative to LREE (light REE) as well as
negative Ce anomalies and positive Eu anomalies formost of the well samples (Fig. 2). Enrichments in HREE
are resulted from stable complexes forming with someligands and stay longer in the solutions. This implies
that all the fluid samples had interacted longer withreservoir rocks.
100Log Sample/PAAS

10-110-2

10-3

10-4
GA-3
GA-13

PB-1
WY-2

GA-11
WY-1

GA-12

10-5
La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb LuFigure 2. All samples showing HREE enrichments relative toLREE, negative Ce (except for PB-1, GA-2, GA-3, and GA-5wells) and positive Eu anomalies.

GA-12, GA-3, and PB-1 are well spread out along thePC1 axis in Fig. 3A, meanwhile a factor-loading plot inFig. 3B shows that the most significant variables for thecomponent 1 are the HREE and wells from the WRIgroup. Gd, Dy, Er, Tm, Yb, and Lu are better distributed
along the PC1 axis. Eu including the rest of the REEs
and δ18O are spread out along the PC2 axis in Fig. 3B.

Positive Eu anomalies in the WRI group from Fig. 2
show that reservoir fluids had undergone hydrothermal
alteration and removal of Eu from the hot fluids during
the precipitation of secondary minerals. This is inagreement with the finding that Eu and Tm account forthe most influential elements in the WRI group. Fromthe score plot in Fig. 3B, the mixing group GA-3, GA-13,and PB-1 are positively correlated to only the middle
REEs (Sm, Tb, and Ho). Conclusively, the mixing andWRI groups are responsible for clustering their REEsand δ18O samples.

5. Conclusion
Even though the δ2H and δ18O analysis present
signs for probable meteoric incursion for the wells in themixing line, there is still no correlation with the oxic
environments from Ce anomalies. It seems that themixing process in the δ2H and δ18O diagram aredominated by non-meteoric water fluids such ascondensate fluids. Mixing of meteoric recharge in PB-1
and GA-3 was confirmed with the score plot of PC1 thatcovered 77% of the variation. For this reason, the linear
mixing in Fig. 1 is not supposed to pass through the GA13 and WY-2 as there are more complex process otherthan mixing with colder fluid in those wells.
On the other hand, oxic environment from thenegative Ce is oppositely correlated with the WRI group.
Hence, wells in the WRI group have no contact withrecent meteoric water in the great depth. The HREE
enrichments from the spider diagram suggest solid hintof hydrothermal alteration and WRI for all the samples.
The WRI process in the reservoir that is reflected theWRI group have positive correlation with not only Eu,
but also Gd, Dy, Er, Tm, Yb, and Lu. Those HREE could
be significant for WRI in the next research. The
approach using PCA for reservoir monitoring toascertain whether colder water incursion could bepredicted from the classification groups was explored.
Acknowledgement: We sincerely thank Star Energy
Geothermal (Wayang Windu) Ltd. for the collaborationon the Science and Technology Research Partnership for
Sustainable Development (SATREPS, Grant No.
JPMJSA1401) Project with JST, JICA, and InstituteTechnology of Bandung team.

ReferencesFigure 3. The mixing wells PB-1 and GA-3 are well plotted in
the PC1 of the bivariate 2H and 18O plot (A), meanwhile theWRI wells, GA-13 and WY-2 are located along the PC2. Basedon the REEs, the loading plot of the WRI group strongly
influences PC1, while the mixing wells have more influences in
PC2 (B).

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