Dynamics of a geothermal field traced by noble gases: Cerro Prieto, Mexico

0375- 6505/84 $3.00 + 0.00 Pergamon Press Ltd. © 1984CNR.

Geothermics, Vol. 13, No. 1/2, pp. 91 - 102, 1984.

Printed in Great Britain.

DYNAMICS OF A GEOTHERMAL FIELD TRACED BY NOBLE GASES: CERRO PRIETO, MEXICO E. MAZOR* and A. H. TRUESDELL~" *Weizmann Institute o f Science, P.O. Box 26, Rehovot, Israel, and tU.S. Geological Survey, 345 Middlefield Road, Menlo Park, CA 94025, U.S.A. (Received 17 November 1982)

Abstract--Noble gases have been measured mass spectrometricallyin samples collectedduring 1977 from producing wells at Cerro Prieto. Positive correlations between concentrationsof radiogenic (He and '°Ar) and atmospheric noble gases (Ne, Ar and Kr) suggest the followingdynamicmodel: the geothermal fluids originated from meteoric water that penetrated to more than 2500 m depth (below the level of first boiling) and mixed with radiogenic He and '°Ar formed in the aquifer rocks. Subsequently, small amounts of steam were lost by a Raleigh process (0 - 3°70)and mixing with shallow cold water occurred ( 0 - 3007o).Noble gases are sensitive tracers of boiling in the initial stages of 0 - 3070steam separation and complement other tracers, such as CI or temperature, which are effective only beyond this range. PRINCIPLES The atmosphere is a well-mixed reservoir with known concentrations and isotopic compositions of He, Ne, Ar, Kr and Xe. Infiltrating surface waters dissolve atmospheric noble gases in amounts dependent on the ambient temperature (Fig. 1) and altitude (barometric pressure) (Mazor, 1972). The relevant conditions are those at the base of the unsaturated zone, i.e. practically average local annual temperatures (Herzberg and Mazor, 1979). Once water descends below the water table the dissolved atmospheric noble gases are kept in closed-system conditions by hydrostatic pressure. Radiogenic He (from the decay of U and Th) is formed in aquifer rocks and introduced into thermal waters in excesses of 1 0 0 - 1000 times the original atmospheric He. Similarly, radiogenic '°Ar (from the decay of '°K) is added, increasing the '°Ar/~6Ar ratio above the original atmospheric value of 295.5. The ratio of (He/Ar) radiogenic ranges from 1 to 6 in thermal waters of c o m m o n aquifer rocks (Mazor, 1977), whereas much higher values are expected in water in contact with U and Th ores. Where steam separation occurs in geothermal systems, the noble gases partition between the liquid and steam. At high temperatures (>280°C) solubilities (Henry's Law constants) for the different noble gases converge and under these conditions little fractionation results from boiling. Two modes of steam separation may occur: (a) Raleigh, or (b) single-stage steam removal. In a single-stage (or batch) process, the products of a reaction remain in equilibrium with the reactants. In a Raleigh process, the products of a reaction are removed continuously as formed so that the process is driven to completion. These processes have been applied to the depletion of gases from geothermal waters during steam separation by Ellis (1962) and Glover (1970) and to the effects of steam separation on isotope compositions by Truesdell e t al. (1977). Which of the two modes of steam loss dominate in a specific case may be indicated using salinity and temperature data along with gas or isotope concentrations. The following equations describe changes in the gas concentrations of steam separating by a Raleigh process, C / C o = B I / A - 1~

and by a single-stage or batch process, C/Co

= 1/[F + (1-r'-)/A]

91

92

E. Mazor and A. H. Truesdell

i

iG F -~m

! fi

i

12~__ °

q 4

9

a# 7~

Xe xlO 9

1

6~-

,o 5E-

°

i

,&

i

I

I

i

15 12

I

I

I

I

8

o

7

I I

I I

I4

-!

ee V

Kr x I08

E 5~

5

co

I

I

I

t

i

I

,

I

I

I

I

I

.~

Ar xlO 4

ee 2 i 24

i

~ ~

I

I

1

I

=

I

1

I

~

L T

4

e'107•

fi

15

f

I

j

I

I

L.

t

i

,t

He x 108

5

4L--

I

I

l

I

i

I

~0

20

30

40

50

60

Temperoture

,1

70

(*C)

Fig. 1. Solubility of atmospheric noble gases in fresh water. Sources of data: ( • )

K o n i g ( 1 9 6 3 ) , multiplying his seawater

data by his given ratios for solubility in distilled water; (zx) Douglas (1964); (©) Morrison and Johnstone (1954); (+) W e i s s (1971) a n d ( - - ) from Benson and K r a u s e (1976). A l l data were multiplied by the atmospheric abundances. In the present w o r k w e a p p l i e d t h e v a l u e s of Benson and Krause's line.

where C / C o is the ratio o f the concentration to the initial concentration, F is the fraction of liquid remaining, and A is the distribution coefficient, C, quiJCvapor. Distribution coefficients were calculated from the Henry's Law constants K h (Fig. 2) measured by Potter and Clynne (1978) using the equation o f Ellis and Mahon (1977): A = R T/Kh V

Dynamics of a Geothermal Field traced by Noble Gases

93

oC k

100 I I

t

200 I

"T"

3~ i

i

I

15

:,Lf \ \

7 0

3.5

3.0

2,5

2.0

1.5

1000 T

Fig. 2. Henry's Law constants, from Potter and Clynne(1978). The solubilityconstants differ largelyin the low (intake) temperature range but convergeat high temperatures, resulting in little significant fractionation. where R is the gas constant, T the absolute temperature, and V the specific volume of steam. The equation given by Potter and Clynne (1978) was used to calculate Henry's Law constants from 20 to 300°C; higher temperature values were obtained by extrapolation to the theoretical value of 221 for all gases at the critical point. The effect of dissolved salts has not been studied experimentally for noble gas solubilities, but approximate corrections may be made by analogy with other gas solubility data as discussed by Ellis and Mahon (1977). In this paper, experimental data for pure water were used without corrections for salt effects. Table 1 shows values of Kh and A, and Table 2 shows gas depletions calculated for Raleigh, singlestage, and multiple-stage steam separation. Figure 3 shows the depletion of He with Raleigh and single-stage steam loss. Thus, the noble gases serve as potential tracers of the origin, mode and degree of steam loss and admixture with cold water. The following characteristics are observed. (a) There are five noble gases, allowing redundancy in the tracing. (b) The atmospheric noble gases have an isotopic signature that enables their identification in geothermal fluids even if modified by depleting processes. (c) The initial concentration of noble gases in recharging meteoric waters can be calculated for each case with remarkable accuracy. (d) Radiogenic noble gases are formed in aquifer rocks and added to geothermal fluids, providing deep-seated tracers. (e) The noble gases take part in no secondary chemical or biogenic processes. (f) The noble gases partition between liquid and steam phases. RESULTS FROM CERRO P R I E T O Samples were collected from producing wells at Cerro Prieto (Fig. 4) and analysed mass spectrometrically using methods described by Mazor (1972, 1979). Precleaning was difficult in

E. M a z o r and A. H. Truesdell

94

T a b l e 1. S o l u b i l i t y o f n o b l e g a s e s in w a t e r as a f u n c t i o n o f t e m p e r a t u r e ( f r o m d a t a o f P o t t e r a n d C l y n n c , 19781 H e n r y ' s L a w c o n s t a n t s , Kh (P/X) Temp. - .

(0°C) 20 40 60 80 100 120 140 160 180 200 220 240 260 280 300 320 340 360

.

. . He

141,000 141,700 132,300 118,600 103,100 87,580 73,130 60,210 49,510 40,850 34,050 28,350 23,670 18,670 14,330 10,000 5887 2213

.

.

. . Ne

.

.

.

.

.

.

Ar

118,600 127,700 131,200 130,600 127,000 121,100 113,600 104,900 95,450 85,440 75,110 64,610 54,060 43,520 33,020 23,800 15,190 6528

36,310 48,150 53,210 54,840 54,250 52,580 50,280 47,600 44,940 42,150 39,130 35,370 29,750 24,000 18,120 12,500 7312 2748

. . Kr

.

19,490 29,010 34,710 38,160 39,890 40,460 40,070 38,840 37,000 34,500 31,290 27,180 21,900 17,870 13,500 9375 5568 2148

.

. . Xe

.

10,420 18,170 22,750 25,330 26,490 26,720 26,270 25,350 24,140 22,690 21,030 19,080 16,750 13,370 10,250 7000 3933 1357

L o g o f d i s t r i b u t i o n c o e f f i c i e n t s , .4 (CtiqL, d."C',~p,,r) . . . . He Ne Ar Kr -6.785 -6.288 -5.825 - 5.400 - 5.006 -4.640 - 4.296 - 3.972 -3.668 - 3.383 -3.116 -2.860 -2.614 -2.349 -2.075 - 1.758 - 1.357 -0.727

--6.710 -6.242 -5.822 - 5.442 - 5.097 -4.780 - 4.487 -4.213 -3.953 - 3.703 -3.460 -3.218 -2.973 -2.717 -2.438 -2.135 - 1.769 - 1.196

- 6.196 -5.819 5.430 - 5.065 - 4.727 -4.418 - 4.133 -3.870 -3.626 - 3.396 -3.176 -2.956 -2.714 -2.458 -2.177 - 1.855 1.451 -0.821

-5.926 -5.599 -5.244 - 4.908 - 4.594 -4.304 - 4.035 -3.781 -3.541 - 3.309 -3.079 -2.842 -2.581 -2.330 -2.049 - 1.730 - 1.333 -0.714

Xe .....5 . 6 5 4 .... 5 . 3 9 6 - 5.061 - 4.730 ....4 . 4 1 6 4.124 3.851 - 3.596 -3.356 - 3.127 - 2.907 - 2.688 - 2.464 - 2.204 - 1.930 - 1.603 - 1.182 -0.514

I00

80 .42

°11

N

• IgA

I --

,,-if,

40

",~ ",, ~ R a l e i g h

-i-

I 20 !

°

-14

*5 ,,20

• 25

(R)

"26

,,29 •

,30

s i n g l e s t a g e (ss)

*21A .27

• 31 "'-.

*

*

"8

-35

oL 0

2

4

6

8

10

0 m

L,

500 i

I000 I

STEAM SEPARATION , % Fig. 3. D e p l e t i o n o f H e d u e to b o i l i n g and steam separation b y the Raleigh and single-stage processes. Data from Table 2.

Fig. 4. L o c a t i o n o f sampled C e r r o P r i e t o wells.

these samples, and a cold trap had to be applied at the mass spectrometer inlet, preventing the measurement o f Xe. The data are given in Table 3 in ml STP noble gas/ml dry gas and calculated in Table 4 to ml S T P / g total fluid, applying measured steam fractions and steam/total gas ratios (Nehring and D ' A m o r e , 1984) (STP: 0°C and 760 m m Hg). The elemental abundances are plotted in Fig. 5. The following features are seen.

Dynamics of a Geothermal Field traced by Noble Gases

95

Table 2. Calculated noble gas depletion from steam separation at 290°C by Raleigh and single-stage processes with multiple-stage separation for He (0.5 and 1% steps) Fraction of gas remaining in liquid Single-stage Raleigh

Fraction residual liquid 1.000 0.999 0.998 0.997 0.996 0.995 0.990 0.985 0.980 0.975 0.970 0.965 0.960 0.955 0.950 0.945 0.940 0.935 0.930 0.925 0.920 0.915 0.910 0.900 0.850 0.800 0.750 0.700 0.650 0.600 0.500 0.400 0.300 0,200

He multiple-stage

He

Ne

Ar

Kr

Xe

He

Ne

Ar

Kr

Xe

1.000 0.861 0.755 0.673 0.607 0.553 0.382 0.292 0.236 0.198 0.171 0.150 0.134 0.121 0.110 0.101 0.093 0.087 0.081 0.076 0.072 0.068 0.064 0.058 0.040 0.030 0.024 0.020 0.017 0.015 0.012 0.010 0.009 0.008

1.000 0.726 0.570 0.469 0.399 0.347 0.210 0.150 0.117 0.096 0.081 0.070 0.062 0.056 0.050 0.046 0.042 0.039 0.037 0.034 0.032 0.030 0.029 0.026 0.017 0.013 0.010 0.009 0.008 0,007 0.005 0,004 0.004 0,003

1.000 0.829 0.707 0.617 0.547 0.492 0.326 0.244 0.195 0.162 0.139 0.121 0.108 0.097 0.088 0.081 0.075 0.069 0.065 0.061 0.057 0.054 0.051 0.046 0.031 0.024 0.019 0.016 0.014 0.012 0.010 0.008 0.007 0.006

1.000 0.867 0.765 0.684 0.619 0.565 0.394 0.303 0.245 0.207 0.178 0.157 0.140 0.126 0.115 0.106 0.098 0.091 0.085 0.080 0.075 0.071 0.067 0.061 0.042 0.032 0.025 0.021 0.018 0.016 0.013 0.011 0.009 0.008

1.000 0.896 0.812 0.742 0.684 0.634 0.464 0.366 0.302 0.257 0.224 0.198 0.178 0.161 0.147 0.136 0.126 0.117 0.110 0.103 0.098 0.092 0.088 0.080 0.054 0.041 0.033 0.028 0.024 0.021 0.017 0.014 0.012 0.011

" 1.000 0.850 0.723 0.615 0.523 0.444 0.196 0.087 0.038 0.017 0.007 0.003 0.001 0.001 0.000 0.000 0.000 0.000 . . . . . . . . . . . . . . . .

1,000 0.686 0.470 0.322 0.221 0.151 0.023 0.003 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 . . . . . . . . . . . . . . . .

1.000 0.813 0.661 0.537 0.437 0.355 0.125 0.044 0.015 0.005 0.002 0.001 0.000 0.000 0.000 0.000 0.000 0.000 . . . . . . . . . . . . . . . .

1.000 0.857 0.735 0.630 0.540 0.463 0.213 0.098 0.045 0.020 0.009 0.004 0.002 0.001 0.000 0.000 0.000 0.000 . . . . . . . . . . . . . . . .

1.000 0.891 0.793 0.706 0.629 0.560 0.313 0.174 0.097 0.053 0.029 0.016 0.009 0.005 0.003 0.001 0.001 0.000 . . . . . . . . . . . . . . . .

. . . . . . . . . . . . . . . .

0.5

1.0

1.000 ----0.553 0.305 0.169 0.093 0.051 0.028 0.015 0.008 0.004 0.002 ----

1.000 -----0.382 -0.145 -0.056 -0.021 -0.008 ----

. . . . . . . . . . . . . . . .

Atmospheric isotopic compositions Those of Ne, Ar (mass 36 and 38) and Kr (Table 3), indicate meteoric origin for the fluid. Atmospheric elemental abundances of

Ne, Ar,

and

Kr

T h e y r e s e m b l e w a t e r s a t u r a t e d w i t h a i r a t a b o u t 1 0 ° C ( F i g . 5). T h i s is i n g o o d a g r e e m e n t w i t h current ideas of major recharge from the Colorado

River (Truesdell

et ai.,

1981).

Steam losses They

are

atmospheric

revealed

by

depletions

noble gas concentrations

of

the

atmospheric

are lower than

noble

gases.

In

the original content

most

samples,

in t h e m e t e o r i c

recharge water and indicate that steam has been lost. The percent retention of the atmospheric n o b l e g a s e s is g i v e n i n T a b l e 5 a n d F i g . 6. S t e a m s e p a r a t i o n is d e a l t w i t h i n m o r e d e t a i l l a t e r .

Lack of mass-dependent fractionation during noble gas depletion T h i s is r e f l e c t e d i n t h e g e n e r a l s i m i l a r i t y o f t h e s l o p e s o f t h e N e - A r - K r l i n e s i n F i g s 5 a n d 6, d e s p i t e t h e large v a r i a t i o n s in t o t a l c o n c e n t r a t i o n or p e r c e n t r e t e n t i o n . T h i s i n d i c a t e s t h a t

96

E. Mazor and A. H. Truesdell T a b l e 3. N o b l e g a s e s in C e r r o P r i e t o fluids, 10 -8 ml S T P * / m l S T P d r y gas

Well

He

Ne

M-5 66 ± 7 M-8 57 ± 6 M-11 1 6 7 0 ± 170 M-I1 1710_+ 170 M-14 1600 ± 160 M - 1 9 A 1970 ± 2 0 0 M-20 980 ± 100 M - 2 1 A 1290 ± 130 M-25 1670 ± 170 M-26 960 ± I 0 0 M-27 1120 ± 120 M-29 2 5 2 0 ± 250 M-30 1940±200 M-31 1620 ± 160 M-35 140 ± 30 M-42 102 ± 30

,'\r

0.73 ± 0 . 0 6 <0.03 8.8 ± 0 . 8 7.9±0.7 7.4 ± 0.7 9.9 ± 0.9 6.2 ± 0 . 6 5.0 ± 0.5 6 . 6 ± 0.6 8.6 ± 0.8 5.5 ± 0.5 15.4 ± 1.4 1 1 . 8 ± 1.0 8.6 ± 0.8 0.7 ± 0.1 1.8 ± 0.2

'" Ne :~Ne ~°Ar ............................ •' "Ne : ' N e '~Ar

Kr

3 3 0 0 _+ 4 0 0 -. . . . -3.7±0.4 9 3 0 0 ± 1600 2 . 5 ± 0 . 3 13,400 ± 1600 3.3 ± 0.3 13,400 ± 1600 5.3 ± 0.5 10,700 _+ 1400 2.8 ± 0.3 7800 ± I 0 0 0 1.9 ± 0 . 2 12,200 ± 1500 -11,300 ± 1400 2 . 0 ± 0.2 7700 ± 1000 1.6 ± 0.2 22,900 ± 2800 7.2 ± 0.7 -5 . 4 ± 0.5 13,600 ± 1700 3.5 ± 0.3 1400 ± 180 0 . 3 4 ± 0.3 -1.50 ± 0.2

Air saturated

11.6 . 10.2 10.2 10.3 10.0 11.1 10.6 10.6 10.4 I0.0 10.4 10.7 10.2 9.7 11.0

-. 41 ------34 -------

10.3

34.3

'~Ar

":Kr

*~kJ

"KJ

'~Ar

~'Kr

~Kr

"~Kr

291.4 5.39 . . . --321.9 5.31 300.0 5.61 291.1 5.18 301.0 5.32 307.3 5.46 295.7 5.62 310.1 5.78 297.5 5.39 284.2 5.23 301.2 5.66 289.6 5.35 295.0 5.17 287.7 5.22

. . . . . . . . 0.203 0.206 0.297 0.202 0.191 0.305 0.202 0.199 0.313 0.205 0.203 0.292 0.201 0.200 0.304 0.197 0.199 0.310 . . . . 0.196 0.188 0.296 0.195 0.198 0.304 0.205 0.199 0.303 0.187 0.191 0.298 0.202 0.204 0.307 ---0.207 0.200 0.308

295.5

0.203

5.35

0.203

0.305

• A t 0 ° C , 760 m r n H g . T a b l e 4. N o b l e g a s e s in C e r r o P r i e t o fluids, 10 -8 ml S T P / g t o t a l fluid ml d r y g a s * Well M-5 M-8 M-11 M-11 M-14 M-19A M-20 M-21A M-25 M-26 M-27 M-29 M-30 M-31 M-35 M-42

T ( ° C ) * g t o t a l fluid 289 290 288 288 274 293 266 288 290 276 283 264 286 280 293 281

1.5 3.6 2.0 2.0 2.3 2.1 2.8 4.3 2.3 1.6 4.5 0.7 1.6 2.5 2.9 1.7

A q u i f e r CI? (rng/kg) 9370 7550 9480 9480 8310 9780 7890 9680 9390 7980 7160 8800 9705 7600 8740 8470

Air saturated water, 10°C Air saturated water, 15°C

He 100 200 3300 3350 3600 4100 2800 5500 3900 1500 5000 1750 3200 3980 410 170

Ne

___ 10 ± 20 ± 300 ± 300 ± 400 ± 400 ± 300 ± 500 ± 400 ± 200 ± 500 __. 180 ± 320 ± 400 ± 40 ± 50

Ar

1.1 ± 0.1 <0.1 17.2 ± 1.5 15.4 ± 1.5 16.8 ± 1.5 2 0 . 8 ± 1.8 17.5 ± 1.5 21.2 ± 1.9 15.4 ± 1.4 13.5 ± 1.2 2 4 . 6 ± 2.2 10.7 ± 0 . 9 19.4 ± 1.7 21.1 ± 2 . 0 2.1 ± 0.2 3.1 ± 0.4

4.8 4.6

4900 ± . -19,000 ± 30,400 ± 28,100 ± 30,200 ± 33,100 ± 28,500 ± 17,700 ± 34,400 ± 15,900 ± -33,400 ± 4100 ± --

20.5 19.8

(At) rad:~

Kr 600 . 4000 3800 3500 3800 4100 3600 2300 4300 2000 4000 500

.

. . ± 0.7 ___ 0 . 6 ± 0.7 ___ 0.1 ± 0.3 ± 0.8 . . 3.2 ± 0.3 7.3 ± 0.7 5.0 ± 0.5 8.9 ± 0 . 9 8.6 ± 0 . 9 1.0 ± 0.1 2.5 ± 0.3

. . --8.9 1700 ----1.8 540 4.0 1330 . . 4.9 870 ----------. . . .

(Ar) atm¢

.

. 7.2 5.1 7.5 11.1 7.9 8.1

39,000 35,000

(Ar) rad:~

-17,300 --29,700 31,800 16,900 ------

9.4 8.0

* T r u e s d e l l et al. (1981) a n d N e h r i n g a n d D ' A m o r e (1984). 1 U s e d to c o n v e r t the v a l u e o f T a b l e 1 to this table. ; t C a l c u l a t e d f r o m ' ° A r a n d ' ° A r / 3 6 A r o f T a b l e 1. O n l y v a l u e s o f "°Ar/36Ar o v e r 300 w e r e r e g a r d e d to be a n a l y t i c a l l y s i g n i f i c a n t . boiling Henry's

took Law

place

at

high

constants

for

temperatures the different

where gases

there (Fig.

is l i t t l e

difference

in the

values

of

the

the angle

by which

the

Ne-Ar

2).

Excess Ne This

is o b s e r v e d ,

lines of most

samples

relative

to atmospheric

are inclined

Ar,

as seen

to the air-saturated

from

water

line in Figs

5 and

6. Atmospheric

97

Dynamics of a Geothermal Field traced by Noble Gases 21A 27 19A 14 11

I

I

29 26

1000

35

:3

'\

'4-

\

1 O0 0 4-'

\

/

\

(3.. I-U)

\

\

\

. ..""

_.

'i\

......

...'"

10 --

ASW 10°C

.....°"

.~°°

.J

~j L He Xl0 s

Ne Xl08

. -

I Ar X l 0 s

I Kr X l 0 9

Fig. 5. Noble gas patterns in the geothermal fluid of producing Cerro Prieto wells (Table 4). Well numbers are indicated• N e - A r - K r abundance lines resemble air-saturated water at 10°C (ASW) but the absolute concentrations reveal various degrees of depletion• Radiogenic He, in excess over the ASW l0 ° value, is positively correlated to the atmospheric Ne, Ar, and Kr.

A r / N e values (Table 5) range from 1200 to 2000, as compared to 1900 in 10°C air saturated water. Thus, a Ne excess o f up to 66°7o is observed. A similar Ne excess, of 1 2 - 250°7o, has been reported from Larderello (Mazor, 1979), and two possible origins have been discussed there: primordial additions or fractionation during boiling. More data are needed for the final answer.

Radiogenic He This is encountered in all samples. It is recognizable as an excess over the amount carried by the meteoric recharge water (5 x 10 -8 ml S T P / g , Table 4, Fig. 5).

E. Mazor and A. H. Truesdell

98

T a b l e 5. P e r c e n t retention ( c o m p a r e d to intake at 10°C, sea level) a n d noble gas ratios

At

4HC

Ne

....Ar

tad Well M-5 M-8 M-11 M-II M-14 M-19A M-20 M-21A M-25 M-26 M-27 M-29 M-30 M-31 M-35 M-42

Nc

&r

5 0.5 84 75 82 101 85 103 75 66 120 52 95 106 10 15

12 . -47 78 72 77 85 73 45 88 41 -95 11 -

kJ . .

.

.

.

76 53 81 118 83 86 -

-

34 78 53 94 108 II 27

-

/19A ~'~,

-. . . . . .

. .

.

.

.

. 1.5 - -

---5.0 4.2 -1.8 ------

-

I

120 .~z~ I00 --~.c~q--~

. -1200 1800 1400 1700 1600 1800 1300 1400 1500 -1600 2000 --

I

Zl A

. .

.

/ "

- - /. . . .

-

-

ASW IO*C--

6o

2OI35~-

o ~----~ Ne

~_.~5

I Ar

.... s5

I Kr

Fig. 6. P e r c e n t r e t e n t i o n o f the a t m o s p h e r i c n o b l e gases as c o m p a r e d to a i r - s a t u r a t e d w a t e r at 10°C. N o systematic m a s s - d e p e n d e n t f r a c t i o n a t i o n is seen, in a c c o r d a n c e with c o n v e r g e n c e o f the solubility c o n s t a n t s at high t e m p e r a t u r e s (Fig. 2). T h e a n g l e o f the N e - A r lines reveals the presence o f N e excess (see text).

Radiogenic Ar This is present, as revealed by samples with '°Ar/36Ar higher than the atmospheric value of 295.5 (Table 3). Detection of radiogenic '°Ar is often difficult, due to masking by the relatively large amounts of atmospheric Ar commonly present. The radiogenic He is much easier to detect. Common aquifer rocks These are indicated by the (He/Ar) radiogenic ratios of 1 . 5 - 5 . 0 (Table 5j. In light of the discussion in the first section, no U or Th ores seem to occur in the Cerro Prieto geothermal aquifer.

Dynamics of a Geothermal Field traced by Noble Gases

99

Deep recharge of meteoric water into the geothermal field This is revealed by a positive correlation of the radiogenic He and atmospheric Ne, Ar and Kr, as shown in Figs 5 and 7. This correlation indicates that the atmospheric noble gases are well mixed at depth with the radiogenic He and that all steam losses by boiling occur subsequent to mixing and deplete both gas groups together. Pressure and temperature data show that some boiling occurs even at a depth o1 2500 m and 340°C (Bermejo et al., 1979) and most of the He must, therefore, have entered the field even deeper. ,~_deep seated brine

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Fig. 7. Radiogenic He-atmospheric Ar. A positive correlation is observed, indicating the meteoric water circulated deeply and received its radiogenic He before steam losses occurred. Line A passes the highest observed He content, assuming it reflects least losses. Extrapolation up to 4 x 10-' ml STP Ar/g water results in 6.8 x 10-I ml STP He/g fluid, regarded as the value of the original, non-depleted fluid at Cerro Prieto. Steam losses, causing He depletion, will move the values down along line A, whereas dilution with shallow water will shift the values towards ASW.

Geothermal fluids are diluted with shallow cold water at Cerro Prieto Figure 7 shows a positive correlation between radiogenic He and atmospheric Ar but a certain scatter of the values is observed, best explained by various degrees of dilution with cold shallow groundwater (devoid of radiogenic He). Line A of Fig. 7 has been drawn through the origin and the point of well 21A, which has the highest He concentration and is, therefore, apparently relatively little depleted in He. Under this assumption, extrapolation of line A up to the value of 4 × 10-' ml STP A r / g recharge water will give the original He content in the Cerro Prieto geothermal fluid. A value of 6.8 x 10-' ml STP H e / g fluid is obtained. Loss of gas due to steam separation results in lowering gas concentrations in the residual liquid along line A, whereas subsequent dilution with shallow cold water shifts the points toward the value of airsaturated water in Fig. 7. F r o m this interpretation, well 21A has lost about 15°70 He by boiling but has not been diluted with cold shallow water. Chloride concentrations also suggest that well 21A fluid is not diluted (Fig. 9). Corrections for dilution, based on either CI or Ar (Figs 8 and 9), suggest that well 27 fluid is actually the least depleted in He but has been diluted with 1 5 - 25070 cold water. Well 35 has lost most of its He and is moderately diluted with shallow water. Well 20 lost about half its He by steam loss and was subsequently diluted by about 30070 cold water. Each fluid analysis shown in Figs 7 and 8 can be interpreted as showing some degree of depletion and dilution. The two figures do not always agree due to analytical uncertainties and to some uncertainties in calculations of aquifer C1 and gas concentrations.

L. Mazor and A. H. Truesdell

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Fig. 8. Helium and CI concentration in produced fluids. R line calculated for Raleigh (continuous removal) and SS line calculated for single-stage boiling. R fits Cerro Prieto data much better. Wells 19A, 21A and 30 were lowered along the R line due to small a m o u n t s of steam loss; well 27 lost no steam but was diluted by about 25°70 with shallow water. The combination of steam loss and dilution may be read for each studied well. 12 + 19A

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ARGON x 108mtlg Fig. 9. Krypton and observed Ar in well fluids (Table 4). The calculated R and SS lines are close to each other in the case of the heavier noble gases. Both the original geothermal fluid and diluting shallow water look like A S W 10°C. Steam loss will shift the values down the R line and dilution will bring them up. The data reflect various combinations of both processes.

Raleigh distillation dominates at Cerro Prieto In Fig. 8, He concentrations before dilution are shown as projections from air-saturated water (ASW) to the maximum C1 concentration of the Cerro Prieto I aquifer fluid (about 10,000 mg/kg). Using these corrected values, the percent of He depletion has been calculated in Table 6. Applying the equations mentioned before, accompanying steam losses and temperature changes have been calculated for the R (Raleigh) and SS (single-stage) processes. It is seen that, for up to 70°7o He loss, very low steam losses occur (up to 1 - 207o) in both the R and

101

Dynamics of a Geothermal Field traced by Noble Gases Table 6. Calculated He, steam and temperature loss for 1977, Cerro Prieto fluids (in °7o and °C) Steam loss Well M-27 M-21A M-14, M-19A, M-25, M-31 M-11, M-20, M-30 M-26, M-29 M-35 M-42 M-5, M-8

Temp. loss to 290°C

He loss

R

SS

R

SS

0 15 40 50 71 93 97 98

0 0.14 0.4 0.6 1.0 2.2 3.0 3.3

0 0.15 0.6 0.8 2.0 11 25 40

0 0 1 1.5 2.7 6 8 9

0 0 1.5 2.0 5.4 28 58 79

SS processes. For higher losses, up to 98°70 small steam losses still occur in the R mode but 40°70 steam must separate in the SS mode. Large steam losses would be accompanied by up to 79°C temperature drops (Table 6) and by rise in total CI up to a concentration of 16 g/l. (Fig. 8). Neither has been observed at Cerro Prieto. The R line in Fig. 8 seems to fit the observed data best. Hence, Raleigh type (continuous removal) steam separation seems to dominate at Cerro Prieto.

Only small steam losses occur at Cerro Prieto Steam losses of 0-3°70 are indicated by the He depletions. This is well established in the range 0-70°70 He depletion (found in 11 of 15 wells), equivalent to 0-2°70 steam losses, regardless of the mode of boiling (R or SS). For higher depletions, small steam losses are also indicated for the R mode which is most consistent with the temperature and C1 data. The radiogenic He (together with the atmospheric noble gases) thus serves as a very sensitive tracer for the initial stages of boiling, with up to about 3°70 steam loss. In this range other indicators of boiling, such as salinity increases or temperature changes, are not sensitive. Combined H e - C1 interpretation is effective Helium has been shown to be a sensitive tracer to steam loss, and CI has been shown (Truesdell et al., 1981) to be a sensitive tracer for dilution with shallow water. A H e - C 1 plot (Fig. 8) is, therefore, illustrative and the dynamics of the Cerro Prieto wells might well be read from it. Wells 19A, 21A and 30 lost various amounts of steam but are not diluted by shallow water; wells 5, 8, 35 and 42 lost most of their He (2 - 3°70 steam loss) and are slightly diluted by shallow water; M-20 lost half its He (0.6°70 steam loss) and was diluted by about 20°70 shallow water. Redundant atmospheric tracers provide analytical checks Krypton has not been used for the interpretation so far. It can, therefore, be used as a redundant tracer, e.g. in a K r - Ar plot (Fig. 9). The data reveal a positive correlation and plot along the calculated R and SS distillation lines which are close for these two heavier noble gases. Steam losses will bring the values down along the R and SS lines, and dilution with cold shallow water will bring the values up. Thus, the data fall along the 'permitted pass' of the two processes already revealed by the He and Ar. CONCLUSIONS The noble gases are useful in reconstructing dynamic processes in the Cerro Prieto geothermal field, in terms of water origin, depth of circulation, degree of steam loss and dilution with shallow water.

102

t:. M a z o r a n d A . H . T r u e s d e l l

It s e e m s t h a t t h e n o b l e g a s e s m a y b e u s e d t o f o l l o w c h a n g e s o c c u r r i n g in t h e f i e l d d u e to e x p l o i t a t i o n . T h e y m a y b e a p p l i e d a l s o in d e c i d i n g t h e l o c a t i o n a n d o p t i m u m s p a c i n g o f n e w wells, as t h e y m a y s i g n i f y w h i c h p o r t i o n s o f a f i e l d h a v e b e e n e x p l o i t e d ( a n d c h a n g e d ) b y a d j a c e n t p r o d u c i n g wells a n d w h i c h p o r t i o n s r e t a i n e d t h e i r o r i g i n a l f l u i d a n d , h e n c e , w a r r a n t e x p l o i t a t i o n ( M a z o r , 1979). Acknowledgements--Sincere thanks are due to Mrs. E. Negreano and Mr. M. Feld of the Isotope Department, Ihe Weizmann lnsititute of Science, for the mass spectrometric measurements; to the Staff of the Comisi6n Federal de Electricidad, Cerro Prieto, for their help in the sample collection at the field; and to the team of scientists at Lawrence Berkeley Laboratory for their constant support. We would like to thank R. O. Fournier and R. H. Mariner of the USGS and M. J. Lippmann of LBL for reviewing the manuscript, and P. C. Bennett and F. R. Portillo for help in producing the paper. The work was partially financed by the Extramural Program of the U.S. Geological Survey, through grant No. 14-08-0001-G-546 and partially by the U.S. Department of Energy and the U.S. Geological Survey as part of the C F E - DOE co-operative agreement technically co-ordinated by M. J. Lippmann. REFERENCES Benson, B. S. and Krause, D., Jr. (1976) Empirical laws for dilute aqueous solutions of nonpolar gases. J. Chem. Phys. 64, 689 - 709. Bermejn, F. J., Navarro, F. X., Castillo, B., Esquer, C. A. and Cortez, C. (1979) Pressure variation in the Cerro Prieto reservoir during production. Proc. Second Syrup. Cerro Prieto Geothermal Field, Mexicali, October 1979, Comisi6n Federal de Electridad, 4 7 3 - 496. Douglas, E. (1964) Solubilities of oxygen, argon and nitrogen in distilled water. J. Phys. Chem. 68, 174- 196. Ellis, A. J. (1962) Interpretation of gas analyses from the Wairakei hydrothermal area, N . Z . J . Sci. 5, 434-452. Ellis, A. J. and Mahon, W. A. J. (1977) Chemistry and Geothermal Systems, 392 pp. Academic Press, New York. Glover, R. B. (1970) Interpretation of gas compositions from the Wairakei field over 10 years. Proc. U.N. Syrup. on Development and Utilization of Geothermal Resources, Pisa. Geothermics (Special issue) 2, 1355 - 1366. Herzberg, O. and Mazor, E. (1979) Hydrological applications of noble gases and temperature measurements in underground water systems: examples from Israel. J. Hydrol. 4 1 , 2 1 7 - 231. Konig, Von H. (1963) Uber die LOslichkeit tier Edelgase in Meerwasser. Z. Naturf. 18a, 363-367. Mazor, E. (1972) Paleotemperatures and other hydrological parameters deduced from noble gases dissolved in groundwaters; Jordan Rift Valley, Israel. Geochim. Cosmochim. Acta 36, 1321 - 1336. Mazor, E. (1977) Geothermal tracing with atmospheric and radiogenic noble gases. Geothermics 5, 21 - 36. Mazor, E. (1979) Noble gases in a section across the vapor-dominated geothermal field of Larderello, Italy. Pure Apl. Geophys. (Pageoph.)117, 262-275. Morrison, T. J. and Johnstone, N. B. (1954) Solubilities of the inert gases in water. J. Chem. Soc. Ill, 3441 -3446. Nehring, N. L. and D'Amore, F. (1984) Gas chemistry and thermometry of the Cerro Prieto, Mexico, geothermal field. Presented at the Third Syrup. Cerro Prieto Geothermal Field, San Francisco, March 1981. Geothermics 13, 75-89. Potter, 11, R. W., Mazor, E. and Clynne, M. A. (1977) Noble gas partition coefficients applied to the conditions of geothermal steam formation. Abstract, Geochemistry of Geothermal Systems Syrup., Annual Meeting, G.S.A.. Seattle, 1977. Potter, I1, R. W. and Clynne, M. A. (1978) The solubility of the noble gases He, Ne, Ar, Kr, and Xe in water up to the critical point. J. Solution Chem. 7, 837-844. Truesdell, A. H., Nathenson, M. and Rye, R. O. (1977) The effects of subsurface boiling and dilution on the isotopic compositions of Yellowstone thermal waters. J. Geophys. Res. 82, 3694- 3703. Truesdell, A. H., Thompson, J. M., Coplen, T. B. Nehring, N. L. and Janik, C. J. (1981) The origin of the Cerro Prieto geothermal brine. Presented at the Second Syrup. Cerro Prieto Geothermal Field, Mexicali, October 1979. Geothermics 10, 2 2 5 - 238. Weiss, R. F. (1971) Solubility of helium and neon in water and sea water. J. Chem. Engng. Data 16, 235-241.