Isotopic evidence for magmatic and meteoric water recharge and the processes affecting reservoir fluids in the palinpinon geothermal system, Philippines

Geothermics, Vol. 22~ No. 5/6, pp. 521-533, 1993.

0375~505/93 $6.00 + 0.00 Pergamon Press Ltd © 1993 CNR.

Printed in Great Britain.

ISOTOPIC E V I D E N C E FOR M A G M A T I C A N D METEORIC W A T E R R E C H A R G E A N D THE PROCESSES AFFECTING RESERVOIR FLUIDS IN THE PALINPINON G E O T H E R M A L SYSTEM, PHILIPPINES J A N E Y. G E R A R D O , * S E R G I O N U T I , t F R A N C O D ' A M O R E , t JOSI~ S. S E A S T R E S JR* and R O B E R T O G O N F I A N T I N I + *Geothermal Division, Philippine National Oil Company-Energy Development Corporation, Fort Bonifacio, Makati, Philippines; tlstituto lnternazionale per le Ricerche Geotermiche, Piazza Solferino 2, 56126 Pisa, Italy, and ~lsotope Hydrology, International Atomic Energy Agency, Wagramerstrasse 5, A-1400 Vienna, Austria.

Abslrael--Stable isotopic compositions of meteoric and geothermal waters indicate that the Palinpinon geothermal system of Southern Negros is fed by a parent water that originated from a mixture of local meteoric (80%) and magmatic (20%) waters. The meteoric water has an isotopic concentration of -8.5%,, and -54%0 in ~O and 2H, respectively, which corresponds to an average infiltration altitude of about 1000 m above sea level. With exploitation of the system and injection of wastewaters to the reservoir, the stable isotopic composition became heavier due to significant mixing of geothermal fluids with injection waters. Incursion of cooler meteoric waters, which is confirmed by the presence of tritium, also leads to the formation of acid-sulfate waters. Stable isotopes are effective as "natural tracers" to determine the origin and mixing of different fluids in the reservoir. Key words: Philippines, Palinpinon geothermal field, magmatic waters, meteoric waters, isotope

analyses.

INTRODUCTION T h e isotopic investigation of the Palinpinon g e o t h e r m a l system was c o n d u c t e d from 1980 until 1983, prior to exploitation of the reservoir (Clemente, 1986). Stable isotope (6180 and 62H) data were o b t a i n e d for cold springs, rivers and discharges from g e o t h e r m a l wells, the latter then being tested. A f t e r seven years of exploitation, the isotopic investigation, which included tritium m e a s u r e m e n t s , was r e s u m e d in 1989 and continues to the present. Its main p u r p o s e is to determine the location of meteoric water recharge to the g e o t h e r m a l system, the origin of the parent geothermal water and the chemical and physical processes affecting the fluids in the reservoir as a result of exploitation. Background

T h e Palinpinon system is located within the O k o y Valley, in southeastern Negros island (Fig. 1). Early exploration results are discussed by M a u n d e r et al. (1982). T h e field is b o u n d e d by three Late P l e i s t o c e n e - R e c e n t andesite volcanoes: C u e r n o s de N e g r o s to th~ south; Mt. G u i n s a y a w a n to the northwest; and B a l i n s a s a y a o - G u i n t a b o n D o m e to the northeast. T h e g e o t h e r m a l area is drained to the north by the O k o y River and to the south by the D u m a g u e t e River, with tributaries originating at elevations as high as 1700 m a b o v e sea level (a.s.l.). T h e average annual rainfall is 2200 m m , with the rainy season c o n c e n t r a t e d f r o m June to August. T h e g e o t h e r m a l area is geographically divided into two contiguous sectors, namely, P u h a g a n 521

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and Nasuji-Sogongon (Fig. 2). The Puhagan sector includes thirty production wells located at an average elevation of 740 m a.s.I, and drilled to depths of 1500 to 2000 m below sea level (b. s. I.). Wastewater is alternately injected through fifteen injection wells located on the northeastern margins of the reservoir. This sector has supplied the steam requirement of the 112.5 MWe power plant since the plant began commercial production in 1983. The Nasuji-Sogongon sector is located upstream and west of Puhagan, at an average elevation of 960 m a.s.I. Development in this area is at an advanced stage, with thirteen production and three injection wells drilled to 2000 to 2500 m b.s. 1. By 1994, four modular power plants with a total generating capacity of 80 MWe will be completed and using production wells in this sector. ISOTOPIC COMPOSITION OF THE LOCAL METEORIC WATERS

Isotopic sampling and analysis Meteoric water samples were collected from the Okoy River and Dumaguete River watersheds, Lake Belendepaldo, perennial springs and water wells (Fig. 1), to establish the Southern Negros meteoric water line. Rainwater samples were also taken at four different elevations (350,760,980, 1100 m a.s.l) to determine the isotopic altitude gradient in the region. Separated thermal waters were collected from geothermal wells using a Webre separator. Repeated sampling during production showed no significant variation in the isotopic concentration beyond the analytical uncertainty range, and values are averaged over time. The samples were stored in polyethylene double-capped bottles and analyzed at the Isotope Hydrology

Isotopic Evidence for Magmatic and Meteoric Water Recharge, Palinpinon

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Laboratory, I A E A , Vienna, except for the baseline samples, which were analyzed at the Institute of Nuclear Sciences, New Zealand. The analytical result of the geothermal waters is corrected to the total discharge composition according to the equation:

~td =

~1 --

Y 1000 In a

(1)

where Otd is the total discharge composition (equivalent to the composition of the single phase reservoir), ~= is the isotope composition of the liquid, y is the steam fraction at the sampling pressure, and 1000 In a is the fractionation factor at the saturation temperature for the sampling pressure. The results are reported as per mille (%0) deviation from the Vienna Standard Mean Ocean Water (VSMOW), with an uncertainty of +0.1%o and +1.0%o in lSo and 2H, respectively. Tritium is reported as TU, where 1 TU has an isotopic ratio 3HflH = 10-~s. The average analytical error is 0.34 TU.

Southern Negros meteoric water line The isotope analyses of the meteoric water samples are reported in Table 1. The sixteen samples have a broad range of 6180 values, from - 8 . 1 to -7.0%0, with a deuterium-excess (Dansgaard, 1964) of 14 + 2. Deviations from the world meteoric water line defined by Craig (1963) may be due to differences in the mean relative humidity of the air masses overlying the oceans (Merlivat and Jouzel, 1979) or due to contribution of ocean water vapor and vapor produced from terrestrial evaporation and transpiration (Ingraham and Matthews, 1990).

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524

l'ablc I. Meteoric walcl samples. Palmpinoii gcothcr)mll licld ,) t,':( )

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1000 ~15() t)00

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Isotopic altitude gradient in the region T h e weighted averages of rainwater for each raingauge station (Table 2) are defined by the following equations having correlation coefficients (r 2) of 0.93 and 1/.96, respectively': h = ( - d ~ 8 0 - 6.5)

{2)

(}.{}019

h - (-dZH - 44) 0.0114

(3)

where h is the mean elevation of meteoric water recharge (Gonfiantini et al., 1976) for d 1'SOand d2H. The equations indicate that the average rainfall at sea level would have an isotopic composition of -6.5%0 61~O and -44%o OZH. They correspond to a decrease of(). 19 and 1.14%o, respectively, for every 100 m increase in altitude, with an uncertainty of +53 m and _+88 m, respectively. P R E - E X P L O I T A T I O N H Y D R O L O G Y OF THE G E O T H E R M A L SYSTEM AT PUHAGAN AND NASUJI-SOGONGON The Palinpinon reservoir is liquid-dominated with localized two-phase zones at shallow levels (500 to 1000 m b.s.l.) in the central Puhagan and Sogongon sectors (Amistoso et al., 1990). The baseline enthalpy-CI diagram (Fig. 3) suggests a reservoir temperature of 323°C with parent C1 concentration of 4100 mg/kg (Jordan, 1983). A maximum temperature of 320°C was measured in PN-20D, while pre-exploitation well measurements indicate that the hottest fluids ascend in the vicinity of Lagunao Dome (Amistoso et al., 1990) (Fig. 2). Similar CI/B ratios

Isotopic Evidence for Magmatic and Meteoric Water Recharge, Palinpinon

525

Table 2. Rainwater samples from raingauge stations, Palinpinon geothermal field

Location

Code

Ticala 35(1 m a.s.I.

PHI-3002 PHI-3027 PHI-3033

JuI-Scp Puhagan 760 m a.s.I.

PHI-399 PHI-3028 PHI-3035

Jul-Scp Balas-Balas 98(1 m a.s.I.

PH1-3001 PH1-3029 PHI-3036

Jul-Sep Nasuji 11011 m a.s.l.

PHI-3000 PHI-3030 PHI-3034

JuI-Scp

blXo (%0)

6D (%0)

-8.1 -6.3 -7.1 WM# -7.3 Jul 91 -8.0 A u g 91 -7.0 Sep 91 -7.g WM# -7.7 Jul 91 -9.1 A u g 91 -7.5 Scp 91 -8.4 WM# -8.4 Jul 91 -9.0 A u g 91 -7.6 Sep 91 -9.1 WM# -8.6

-52 -41 -49 -48 -53 -46 -54 -52 -59 -49 -58 -56 -59 -47 -62 -56

mm rain

Date

303 197 223 sum 723 326 220 224 sum 771) 297 245 275 sum 817 338 293 316 sum 947

Jul 91 A u g 91 Sep 9l

WM#

WM#

WM#

WM#

D-excess*

10

I0

11

13

Legend:

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=

6D

-

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(ranging from 24 to 29) for the two sectors suggest that Puhagan and Nasuji-Sogongon are fed by a common parent geothermal water (Jordan, 1983). Localized boiling with subsequent vapor loss occurs within the reservoir as the geothermal fluid ascends through fractures. This produces a more concentrated brine, such as that in PN-26 (Fig. 3). The CI contents of reservoir fluids generally decrease linearly with enthalpy from Puhagan to Nasuji-Sogongon, indicating that dilution is greater to the west. The dilution trend extrapolates to nil CI at a low temperature (Fig. 3), consistent with cold groundwater being the diluent. Measured reservoir temperatures also decrease from the upflow zone towards the

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526

J. ~'. (;erardo ct al.

Fig. 4. Isotherms (°C) at 1500m b.s.l, prior to exploitationof the Puhagan and Nasuji-Sogongonsectors (aftcr I 1rhino et al., 1986). The negat ve numbers reflect the baseline I~Ovalues of the reservoir fluid. northeast and northwest (Fig. 4) as the geothermal fluids lose heat during flow along permeable structures (Amistoso et al., 1990). ISOTOPIC C O N C E N T R A T I O N S OF W A T E R S IN T H E G E O T H E R M A L PRODUCTION WELLS PRIOR TO EXPLOITATION The baseline isotopic compositions of geothermal well waters range from - 6 . 0 to -3.6%0 61SO and - 4 9 to -41%o 62H in the Puhagan sector, and from - 6 . 8 to -5.7%0 6~SO and - 5 1 to -45%0 c~2H in the Nasuji-Sogongon sector (Table 3, Fig. 5). Baseline geochemistry of the Puhagan wells (Jordan, 1983) and bore output measurements (Amistoso et al., 1990) suggest that some wells, such as PN-24, are tapping fluids that have direct access to the region of high temperature. In this set of samples, PN-24D has the highest geothermometer temperature (299°C, Table 3). Fluids from this well may most closely represent those from the deepest part of the reservoir and have a 61SO and 62H composition of -4.5%0 and -42%o, respectively. The trend of the data defines the line AB (62H = 2.55 61SO - 32.5) with a correlation coefficient of 0.88 (Fig. 5). This trend can be explained by two possibilities: 1. Removal of isotopically light steam; and 2. Mixing between meteoric waters and isotopically heavier waters. The trend of line AB may result from a single-step steam separation similar to that observed in El Tatio (Giggenbach, 1978) and Wairakei (Giggenbach and Stewart, 1982). If the water that undergoes steam separation has a composition similar to the upflowing fluid of well PN-24D, the residual waters would have an isotopic composition that is more enriched than the original fluid, as observed in PN-26 (Fig. 5). This phenomenon is further illustrated in Fig. 6, where PN-26 is enriched in both C1 and 180 relative to the parent water, due to separation of steam at about 220°C.

Isotopic Evidence for Magmatic and Meteoric Water Recharge, Palinpinon

527

Table 3. Pre-exploitation baseline isotope and reservoir chemistry of production wells, Palinpinon geothermal field Well name

Date

A. Puhagan PN-14 05 Oct 82 12 Oct 82 PN-17D 12 Jan 83 PN-18D 11 Mar 83 15 Mar 83 PN-19D 03 Oct 82 13 Oct 82 21 Oct 82 22 Sep 83 PN-20D 19 Jan 83 28 Sep 83 PN-21D 28 A u g 82 31 A u g 8 2 (18 Sep 83 PN-22D 24 Nov 82 20 Nov 83 PN-23D 09 Mar 83 PN-24D 24 Dec 82 29 Dec 82 04 Jan 83 PN-26 28 Dec 82 PN-27D 03 Sep 83 PN-30D 11 Oct 83 OK-10D 23 Sep 83 B. Nasuji-Sogongon OK-6 (18 Jan 82 12 Mar 83 OK-8 15 Jan 82 SG-ID 26 Dec 82 SG-3D 21 Apr 83 NJ-1D (17 Nov 83 NJ-3D 10 A u g 83 NJ-4D 10 A u g 83

618Otd

6D td

Clre~

TsiI

61sO td

6D td

mean

mean

mg/kg

°C

%0

%0

%0

%0

4337 4274 2692 4195 3411 4321 6290 536l 4787 4140 3487 4117 4116 4136 3553 4211 3485 4471 4812 5014 6846 2840 3137 3638

277 280 247 283 287

-3.6 -3.7 -6.0 -4.5 -4.6 -4.1 -4.0 -4.2 -4.4 -4.3 -4.7 -4.6 -4.3 -4.4 -4.8 -4.7 -5.0 -4.4 -4.5 -4.5 -3.9 -5.3 -4.4 -4.7

-45 -43 -48 -44 -43 -43 -43 -43 -45 -49 -46 -44 -41 -42 -46 -46 -46 -43 -41 -42 -42 -49 -46 -44

-3.7

-44

-4.5

-43

-4.2

-44

-4.5

-47

-4.4

-42

-4.8

-46

-4.5

-42

-6.7 -6.0 -6.8 -5.6 -6.4 -5.9 -5.9 -5.7

-50 -49 -51 -45 -48 -47 -48 -47

-6.4

-49

1017 2983 2250 2735 2670

3048

272 277 290 289 229 234 247 278 271 282 298 293 299 275 273 277 257

270

262

267

Legend: res--at reservoir condition. td--total discharge composition calculated from equation 1. Tsil--temperature by silica (quartz) geothermometry (Fournier and Potter, 1982).

However, steam removal alone cannot account for the general trend in isotopic (Fig. 5) and CI (Fig. 6) composition. Rather, the trends can be explained by mixing of compositionally distinct waters, one being local meteoric water with isotopic composition indicated by the intercept of the regression line AB with the Southern Negros meteoric water line (-8.5%0 6180 and -54%0 62H) (Fig. 5). The meteoric water originates from an average recharge altitude of 1000 m a.s.l., based on equations (2) and (3). The deep end-member fluid is similar in isotopic con~position to magmatic water associated with andesitic volcanism. This magmatic water (+10 _+ 2%00180 and - 2 0 + 10%o62H; Giggenbach, 1992) is typical of Japanese, New Zealand, Italian and other high temperature volcanic discharges (Mizutani et al., 1986; Giggenbach, 1992; Bolognesi and D'Amore, 1993), and is a component of geothermal systems in the Philippines, Japan and elsewhere (Reyes and Giggenbach, 1992; Seki, 1991; Yoshida, 1991). The 180-C1 relation (Fig. 6), which further defines this mixing trend, can be extrapolated to the composition of magmatic

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Isotopic Evidence for Magmatic and Meteoric Water Recharge, Palinpinon

529

waters (about +8 to + 10%o 6~80), indicating an end-member C1 concentration of about 19,500 to 22,000 mg/kg. Therefore, the deep geothermal waters (e.g. PN-24D) are largely meteoric water. As the meteoric water is heated, it mixes with a 20% component of magmatic water. This mixture ascends south of Puhagan and, as it flows to shallower depths, is diluted by shallow meteoric ground waters originating from about 1000 m a.s.1. EFFECTS OF E X P L O I T A T I O N ON T H E ISOTOPIC COMPOSITION OF GEOTHERMAL WATERS The changes in the isotopic composition of reservoir waters since the beginning of exploitation is shown in Table 4. Eight years of exploitation (up to 1991) at Puhagan has resulted in: 1. An increase in reservoir C! from 4000 to 9000 mg/kg, caused by injection of waste water that has a C1 concentration of 9000 to 11,000 mg/kg; 2. A temperature decline of at most 50°C due to rapid return of cool (~165°C) injection water, and 3. A pressure drawdown. The latter has resulted in the formation of gas-rich steam at shallow depths, leading to an increase in the concentration of non-condensable gases from 1.0 to 1.3 wt.%, as well as an increase in field enthalpy, from 1500 to 1640 kJ/kg. Pressure drawdown also induced the inflow of cooler groundwaters, and the entry of steam-heated acid-sulfate fluids from shallower depths into some wells. The isotopic concentrations of water in the Puhagan sector were enriched in ~SO and 2H by approximately 1%o and 5%0, respectively. The enrichment in 1SO is most prominent in the northeastern and southern sector of Puhagan in wells PN-15D, PN-16D, PN-17D, PN-18D, PN-21D, PN-23D, PN-26, PN-27D, PN-28, PN-29D and OK-7 (Fig. 2). This is due to the proximity of the injection sector and rapid flow of injection fluids through fractures back to the production wells. The 61SO, however, was depleted by 1%o in wells OK-10D, PN-13D, PN- 14D, PN-15D, PN-20D, PN-24D, PN-30D and PN-31D (Fig. 2) due to inflow of cooler, isotopically lighter fluids. Although Nasuji-Sogongon is not yet exploited, the pressure drawdown in Puhagan is known to have induced inflow of cooler waters in some wells in Nasuji, particularly in OK-6, where l~O and 2H decreased by 1%o and 8%0 respectively.

Effects of injection The injection waters are the residue after separation of isotopically light steam (~40%) at the wellhead (average separation pressure of 0.70 MPa). This results in isotopic enrichment of the separated water relative to the original water, and allows the injected water to be used as a natural tracer (e.g. Nuti et al., 1981; D'Amore et al., 1987, 1988). Injection waters have averaged 6JSO concentrations of -2.8%o. The majority of the wells in Puhagan now have a component of injection waters in their discharge. The trend of the mixing line (Fig. 6) can be fit by the regression equation (r = 0.88): 6180 = 3 × 10-4C1 - 5.8

(4)

The fraction of injected water (RI) in the wells can be calculated by the following: %RI =

(6well- oh) (6iw -- 6upflow)

(5)

where 6well is the isotopic concentration of the well during exploitation; 6b is the baseline isotopic composition of the well; 6~w, the isotopic composition of the injected water, is -2.7%o 61SO; and 6up~ow is -4.5%o 6~SO. The calculated RI percentages are listed in Table 4. Some

53{}

.I.Y. (;erardo ctal. Table4. Isotopic and chloride d a t a o t s c l c c t e d w c l l s s i n c c s t a r t { ) l c x p I o i t a t l o n a t l}uhagan, Palirlpimm geothcrmal licld

Source

Date

A. Puhagan PN-13D 15 May 9{~ {}5 Aug 91 PN-14 12 Sep 89 03 Apt 9{J {}6 Fetl 91 12 Jun 91 (}6 Aug 91 PN- 15D {)4 Apr 9{7 23 Aug 9{1 08 Feb 91 I1} Jun 91 13 Aug 91 PN-16D 20 Feb 9) {}5 Aug 91 04 Oct 91 PN-17D 19 Apr 90 23 Aug 9{} 28 Feb 91 PN-18D 11 Jun 91 {15 Aug 91 PN-19D 06 Feb 91 06 Aug 91 11 Oct 9l PN-2(}D 15 May 9{) I1 Jun 9I 21 Jun 91 {}7 Aug 91 PN-21D 04 A p t 9{1 18 Feb 91 1(7 Jttn 91 02 Aug 91 PN-23D 19 Apt 9(} 05 Feb 91 13 Jun 91 {78 Aug 91 PN-24D 12 Sep 89 06 Apr 90 t9 Feb 91 13 Jun 91 l0 Aug 91 PN-26 (72 May 91 PN-27D 20 Feb 91 I I Jun 91 15 Aug 91 PN-28 03 Apr 90 {77 Feb 9I 11Jun 91 02 Aug 91 PN-29D 17 Apr 9{} 12 Fcb 91 1{1 Aug 91 14 Jun 91 PN-30D 16 Feb 91 08 Aug 91 17)7Oct 91

UIR,, i')l>'() Id hi) ld mg/kg %, %

4297 4179 9105 7510 6870 6772 7311 6467 6711 7563 7160 5648 53ll 50{}8 4878 8956 86110 8698 6682 6149 5939 5223 4272 2583 665 486 851 6771} 6688 6196 6194 5092 5495 4859 4708 6958 3919 4666 4{765 4590 8425 41}82 3736 3857 8222 8821 7578 7311 8394 7828 61173 641{7 5314 1138 1624

(~Js() td ()l) td lllCall mean %, 'i,,~

4.2 ~ 4.8 3.6 3.8 -4, I 3.9

44 -48 43 -42 ~ 44 43

4.-~ -4.8

43 -48

3.9

43

3,6 -3.7 -3,6 3,3

42 -42 -42 41

-3,6

-42

4.4

45

Trilimn T17T

RI {% )

{}. I

0,2 17

{}.1 (L2 -3.3 -3.0 -3.{I 3.3 3.6

-4(7 -41 -4{) -40 41

-3.1

-4(}

88

51 {}.3

- 4,4 4,4 --4.5 -5.2 -3.6 -5.9

-44 -44 -45 -46 -39 -47

-4.4

--44

-5.2 -3.6 -5.9

-46 -39 -47

-3,8 -3.9 --3.9

-42 -43 -42

-3.9

-42

-4.3 4.3 -4,6

-44 46 -44

-4.4

- 45

-4.8 -4.8 -4.6 -4.7

-46 -42 -44 -43

-4.7

-44 -43

-3.2 -4.7 4.7

-39 -44 44

-3.3 -3.4 -3.3

-42 -41 -41

-3.7 -3.7

-42 -4I

-3.8 -4.2 -5.5 -5.7

-44 -44 -47 -48

{7.2

5{}

0,3 32

{}.2

25

{}.6

1.2 -4.7

22

44 -0.2

0. -3.7

42 3.9

(}.6 (continued }

Isotopic Evidencefor Magmatic and

Meteoric Water

Recharge, Palinpinon

531

Table 4. Continued

ClreS d l s O t d d D t d mg/kg %0 %0

Source

Date

PN-31D

18 A p r 90 08 Feb 91 14 Jun 91 12 A u g 91 08 Feb 91 08 A u g 91 05 Oct 91 18 Apr 90 05 A u g 91 04 Oct 91 10 Apr 90 02 A u g 91

4178 4794 4360 4082 1060 296 444 6527 6905 6200 2245 2889

-4.3 -4.3 -4.6 -4.5 -6.3 -5.4 -4.0 -3.9 -3.6 -3.9 -5.4 -5.2

-43 -45 -44 -45 -45 -44 -44 -41 -42 -42 -47 -43

B. Nasuji-Sogongon OK-5 21 A u g 9 1 OK-6 21 A u g 91 OK-9D 05 Feb 91 08 A u g 91 17 A u g 91 OK-12D 15 A u g 91 NJ-4D 21 Oct 91 NJ-8D 08 Oct 91 BL-1D 19 Mar 91 08 Jun 91 17 Jun 91

1480 1485 4394 4160 3994 7848 3450 3098 971 72l 621

-6.1 -7.3 -4.7

-50 -57 -45

-4.6 -3.4 -5.7 -5.8 -6.1 -6.3 -5.9

-45 -41 -47 -47 -46 -46 -45

C. Injection waters R1317U 12 Sep 89 23 May 90 05 Mar 91 RI318U 21 A u g 91 05 Mar 91

10799 8608 9347 8180 8942

-3.2

-40

-2.7 -3.0 -2.9

-38 -40 -35

PN-32D

OK-7

OK-10D

d180td mean %

dDtd mean %0

-4.4

-44

Tritium TU

RI (%)

0.2 0.8 -3.8

-42 -0.5

-5.3

-45 -0.1

-4.7

-45 0.4

-5.8

-47

-2.7 -2.4

-40 -37

Legend: td--total discharge composition calculated from equation 1. RI(%)--injection water returns calculated from equation 5, if baseline isotopic data is available.

production wells (e.g. PN-17D) indicate up to an 88% component of injected fluid, which is comparable to the calculations based on the C1 mass balance relations (Harper and Jordan, 1985) and from gas calculations (D'Amore et al., 1993). Iodine-131 and fluorescein tracer tests indicate that the transit time of injected water to the production sector ranges from less than one day to two weeks (Urbino et al., 1986). As a consequence, there is relatively little residence time for effective reheating of injected waters. Thus the temperature decreases by as much as 50°C in those production wells heavily affected by returns of injection water. Injection was therefore shifted in 1989 further north of Puhagan in an attempt to slow down this thermal deterioration.

Inflow of meteoric and acid-sulfate waters D'Amore et al. (1993) suggest that a gas-rich steam condensate layer is forming at shallow depths, with pressure drawdown further contributing to steam formation and ascent. The steam condenses into groundwaters, and H2S is subsequently oxidised in the vadose zone to sulfate, forming acid-sulfate, steam-heated waters (Hedenquist, 1991). This phenomenon was observed during the discharge of well BL-1D (Fig. 2), which produced a low pH (5.5), high SO4 (200

532

./. Y. Gerordo ct al.

mg/kg) and low CI (1600 mg/kg) fluid. The samples collected during its discharge indicat~~!hat this stearn-heatcd water has a h~SO of -t~.3%. and an cnthalpy of l()()() k,I/kg. This fluid also mixes with unaltered parent geothermal water (Fig. 6), causing acidity in some production wells. PN-20D, for example, suffered a decrease in pH from 5.5 to 4.5, with rcscrvoir St):, and Mg increasing from 50 to 250 and (1.5 to 2.5 mg/kg, respectively. Most wells dominated by steam-heated groundwaters have a measurable tritium content, in contrast to the deep fluids, which have ()TU. The tritium contents in local rainwaters averagc 2.4 TU, while springs and groundwatcrs contain from 1.5 to 4.5 TU. The tritium content of waters discharged from southern Puhagan wells PN-14, PN-20D, PN-24D, PN-30D, PN-31 D, PN-32D and OK-9 (Fig. 2) range from 0.2 to 1.2 TU, which indicate a component of groundwater that fell as rainwater less than 50 years ago. The presence of tritium in these wells indicates that the incursion of shallow groundwater is common in this system. CONCLUSIONS The stable isotope results, supplemented by chemical and engineering data, confirm mixing of geothermal water with groundwater and injection waters as a consequence of production from the Palinpinon geothermal system; steam separation has also occurred due to pressure decrease. These data provide a better understanding of the reservoir prior to and during exploitation. The parent geothermal fluid is a mixture of isotopically light meteoric (80%) and isotopically heavy magmatic waters (20%). This parent fluid ascends and is diluted by groundwater that originates above 1000 m a.s.l. Steam separation also takes place along the flow path, resulting in residual waters that are isotopically enriched. As a consequence of exploitation, isotopically heavy injection fluids mix with the geothermal waters. Incursion of cooler groundwater causes the oxidation of H:S-rich steam and leads to the formation of acid-sulfate waters tapped in well PN-20D. Stable isotopes can thus be successfully used as tracers of different sources of fluids in the reservoir. Acknowledgements--This isotopic investigation was perfl)rmcd undcr the Technical Assistance grant (PHI-8-016) of the International Atomic Energy Agency to PNOC-EDC. The authors wish to express their gratitude to the PNOC-EDC managcmcnt for allowing the publication of data, the staff of the Southern Negros Geothermal Project for painstakingly collecting the samples and the draftsmen for the figures. The authors are also indebted to Jeff Hedenquist, Stuart Simmons and David Sussman for their critical reviews and comments, which greatly improved the interpretation and the

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