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Oxygen and hydrogen isotopes in deep thermal waters from the south meager creek geothermal area, british columbia, canada
0375-6505/93 $6.00 + 0.00 Pergamon Press Ltd (~) 1993 CNR.
Geothermics, Vol. 22, No. 2, pp. 79-89, 1993. Printed in Great Britain.
OXYGEN AND HYDROGEN ISOTOPES IN DEEP THERMAL WATERS FROM THE SOUTH MEAGER CREEK GEOTHERMAL AREA, BRITISH COLUMBIA, CANADA M O R T E Z A M. GHOMSHEI* and IAN D. C L A R K t *Canadian Radon and Geothermal Research Ltd., P.O. Box 4301, Vancouver, B.C. V6B 3Z7 Canada, and tUniversity of Ottawa, Ottawa-Carlton Geoscience Centre, 161 Louis Pasteur, Ottawa, Ontario, KIN 6N5 Canada (Received April 1992; accepted for publication October 1992) Abstract--Deuterium and oxygen-18 (180) have been measured in deep thermal, shallow thermal and non-thermal water samples collected at various times between 1982 and 1989 from the Meager Creek area, with the aim of assessing the origin of the thermal waters. The isotopic composition of the reservoir waters (6180 = -13%o and 6D = -114.8%o) was calculated from data on post-flash deep thermal waters, using a two-stage steam loss model. The reservoir composition shows an oxygen shift of 2.4%o relative to the local meteoric water line. The composition of the recharge, obtained by removing the oxygen shift, is isotopically heavier than the average local meteoric waters, suggesting that the recharge may be from an area to the west of Mt Meager where isotopically heavier ground-waters are likely to be found. The small 180 shift of the deep high-temperature waters is indicative of dominance of fracture-related permeability in the reservoir. Analysis of the chemistry and the temperature of the waters from hot springs and shallow thermal wells suggests that these waters have evolved from the deep geothermal waters through dilution by meteoric waters and about 40°C adiabatic cooling (steam loss).
INTRODUCTION Meager Mountain in SW British Columbia, Canada, is the northernmost volcanic complex of the Cascade chain, extending over a 16 km-diameter area and rising about 2000 m above surrounding valleys. It includes late Tertiary to Quaternary volcanic rocks of silicic to intermediate composition, superimposed on the granodiorites of the Coast Plutonic Complex (Souther, 1977; Read, 1979). About 3 m of precipitation per year at higher altitudes and melting of permanent glaciers supply water to the hydrologic system. The elevation of Meager Creek is approximately 430 m, the highest point in the area of geological interest is Plinth Peak at 2679 m and the site of the deep holes is at 820 m above sea level (Fig. 1). The South Meager Creek waters can be classified into four categories (Table 1, Fig. 2): (1) meteoric waters; (2) moderately saline thermal waters from shallow boreholes and hot springs (products of mixing); (3) saline thermal waters from the deep geothermal reservoir (data from deep test-hole MC1); and (4) cool high-chloride waters, in the South Fork area (Fig. 1). The major structure responsible for the seepage of thermal waters to the surface is the Meager Creek fault, which strikes east-west and dips 55° north (B.C. Hydro, 1983). Major-ion data from the deep thermal waters have been reported previously (Ghomshei et al., 1986). Stable isotopes from the shallow boreholes, hot springs and surface meteoric waters have been reported by Clark et al. (1982). In their review of the hydrothermal alteration and fluid geochemistry of the Meager Mountain, Adams and Moore (1987) presented data on the chemical and isotopic composition of rocks and some water samples from the area. The present work complements these studies by presenting isotope data from the Meager Creek deep 79
M. M. Ghomshei and I. D. Clark
80
Y
~
LILLOOET
~ ,.
"~~"~__PEBBLECREEK HOT SPRINGS \
\
pLINTH
p¢
A
A
\ \
MT MEAGER
N PYLON
APK
w
fMEAGER CREEK /t HOT SPRINGS PLACID HOt SPRINGS CItK
NO GOOD ==~ HOT SPRI~I~)/°~M12 J~
0
I
1
2 ....
HoLE
3km Logging
road
~il Fault Fig. 1. Site map of the Meager Creek geothermal area, showing the location of the different waters sampled for this study. For location of the area, see Fig. 4. thermal waters discharged from the well MC1, which is the only discharging (flowing artesian) well drilled into a high-temperature geothermal reservoir in Canada; two other deep exploration wells drilled in the Meager Creek area (MC2 and MC3) did not sustain flow (Jessop et al., 1991).
Sampling and analysis Twenty-seven flashed water samples from MC1 have been collected during a period of 7 years from 1982 to 1989 (Table 2). MC1 is a 2.5 km deep test well, with the production zone at depth 1300-1800 m (wellhead at 817 m above sea level). The measured temperature at this depth is about 200°C. However, the wellhead discharge is a two-phase 145°C fluid comprising 15-18%
81
South Meager Creek Geothermal Area Table 1. Major-ion chemistry of representative waters from South Meager Creek. Concentrations are in ppm T Non-geothermal (meteoric) AngelCreek Hot springs, shallow thermal Meager Creek Hot Springs Deep exploration wells WeirboxMC1 Cool high-chloride waters M12 (shallow well)
pH
Li
4.0 9.6 1
Na
K
Ca
Mg
2.0
1.46
26.1
370
38
69
53
7.1
96
8.3 3.3
1260
97
40
11
7.1
3600
136
490
7.6
F
CI
SO4
11.2 0.05
3.2
41
22 0.21
550
B
SiO 2 HCO 3 Br 4.9
86.4
230
415
ND
120 12.8 370
72
4.2
240 0.32 4230 182037.8 16.4
3534
18.0
0.8
2.1
1990
120 2.3
Note: HCO 3 is total CO 2 expressed as HCO3, T is temperature of sampled water (°C). Analyses performed by B.C. Hydro R&D Laboratory.
steam (measured) and 82-85% brine of 96°C after flashing to the atmosphere (Ghomshei and Stauder, 1989; B.C. Hydro, 1983, 1985). The MC1 samples are post-flash (weirbox), collected at 96°C (local boiling temperature). An additional 17 samples from shallow waters have been collected and analyzed for this study. These include hot springs, borehole fluids, meteoric waters and the waters in the cellars built around the dry wells MC2 and MC3 (evaporated water). More data on the shallow waters can be found in Clark et al. (1982). Most samples were analyzed at the Ottawa-Carlton Geoscience Centre. Ratios of 180/160 in H20 were measured by a VG SIRA-12 gas source mass spectrometer, on CO2 gas prepared by standard equilibration with water samples at 25°C. 2H/1H ratios were measured in hydrogen gas extracted from water by reduction on uranium metal at 800°C. Stable isotope contents are expressed as delta per mil (%0) differences from SMOW. Cation concentration data were generated by ICP spectrographic analysis of filtered (0.45/tm) and acidified water samples. Anion concentrations were determined by high-pressure liquid chromatography and alkalinity by micro-titration with 1.6 N H2SO 4.
161412-
BIB Chloride
[7'0 O.
C
BB TDS
200
Temperature
ge_. ~ DEEP THERMAL FOST-FLA3H
4-
100
20-
TYPE 1
TYPE 2
TYPE 3
TYPE 4
Fig. 2. Chemistry (TDS and CI) and temperature of different types of water in the South Meager Creek geothermal area. Type 1 represents surface waters of meteoric origin (Angel Creek). Type 2 represents hot springs and shallow thermal waters (Meager Creek Hot Springs). Type 3 represents the deep thermal waters (MC1 post-flash), and type 4 represents the cool high-chloride waters (M12).
M. M. Ghomshei and I. D. Clark
82
T a b l e 2. 180, d e u t e r i u m a n d c h l o r i d e in t h e r m a l , m i n e r a l a n d m e t e o r i c w a t e r s in S o u t h M e a g e r C r e e k a r e a Sample
6 iSO%o
6D%o
C1 ( p p m )
Post-flash d e e p t h e r m a l w a t e r s ( M C 1 ) MC87-9 -ll.77 CM87-10 -12.72 MC89-1 -12.84 MC89-3 -11.61 MC89-4 -12.85 MC1-83-1 -12.03 MC1-83-2 -11.98 MC1-84-1 -12.25 MC1-84-2 -12.16 MC1-84-3 -12.2 MC1-84-4 -12.29 MC1-84-5 -10.89 MC1-84-6 -12.05 MC1-84-7 -11.54 MC1-84-8 -11.8 MC1-84-9 -12.15 MC1-84-10 -12.25 MC1-84-11 -11.94 MC1-84-12 -12.12 MC1-84-13 -11.91 MC1-84-14 -12.08 MCL-H1 -12.78 MCL-H2 -12.8 MCL-H3 -12.77 MCL1-SEP -12.66 MCL2-SEP -12.54 MCL3-SEP -12.64
-113.7 -115.8 -118.8 -115.5 -114.2 -104.37 -100.39 -112.28 -101.69 -120.18 -104.7 -110.15 -108.65 -114.89 -103.69 -108.56 -114.4 -113.42 -108.58 -112.67 -109.82 -112.2 - 115.71 - 116 -111.47 -111.79 -115.17
2056 1914 2021
M e a n post-flash
- 12.2
- 111.4
Pre-flash w e l l h e a d , c a l c u l a t e d : simplified solution e x a c t solution
- 12.64 - 12.62
- 113.3 - 113.2
R e s e r v o i r , calculated: simplified solution e x a c t solution
- 13.05 - 13.05
- 114.8 - 114.9
Shallow thermal waters MC87-5 MC87-12 MC89-7 MC89-6 MC-87-11 ADMS-MC CLRK-MC CLRK-PLC CLRK-N-G2
- 16.18 - 16.22 - 16.02 - 16.94 -16.27 - 16.4 - 16.5 - 16 - 17.2
- 123.6 - 124.8 - 123.8 - 123.3 -123.8 - 126 - 125 - 124 - 128
Cool high-Cl w a t e r s MC87-7 MC87-2 HYDR-NSFS HYDR-SFCS ADAMS-M12 HYDR-M1 *
-
-
1932 2030 2010 2030 2010 2020 2010 2140 2150 2140 2150 2150 2140 2140 2140 2140
Description
F l a s h e d b r i n e 96°C . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1987 1987 1989 1989 1989 1983 1983 1984 1984 1984 1984 1984 1984 1984 1984 1984 1984 1984 1984 1984 1984 1985 1985 1985 1985 1985 1985
2070
145°C ( m e a s u r e d )
200°C ( m e a s u r e d )
16.56 16.65 15.9 16.1 14.2 15.5
125.8 125.3 119 123 120 129
1640
553 603
661 528 674 196
2350 1740 4230 2420
E M R - 3 0 1 - 1 , 58°C M e a g e r C r e e k H o t Springs, 45°C M e a g e r C r e e k H o t Springs, 50°C N o - g o o d hot springs N o - g o o d h o t springs M e a g e r C r e e k H o t Spring M e a g e r Creek H o t Spring Placid C r e e k , 45. I ° C N o - g o o d hot spring, 29.5°C
South Fork swamps S o u t h F o r k C r k , 10°C U p p e r South Fork swamps South Fork swamps M 1 2 shallow hole M1 shallow h o l e , 56°C
83
South Meager Creek Geothermal Area Table 2. Continued Sample Non-thermal MC87-3 MC-85-Al MC-85-A2 MC87-4 MC87-6 MC87-8 MC-89-5 MC89-8 MC89-2 MC87-1 PROB-1
61X0%0
6D%0
-17.10 -17.98 -17.9 -16.78 -16.77 -17.57 -9.95 -14.44 - 16.09 -17.07 -17.7
-126.9 -132.93 -130.22 -119.0 -124.7 -129.2 -100.8 -118.0 -120.0 -130.06 -127
CJ (ppm)
Description
waters
Notes: CLRK samples are from Clark from Adams et al. (1985). The calculated scatter of 20/w in 6 ‘0 and 20%. in 6D. Ghomshei et al. (1986). *The chemistry of this sample matches
0.8
Angel Creek Angel Creek Angel Creek Meager Creek Meager Creek Hot Springs, Boundary Creek 8°C Rain #l June 1989 Rain #2 June 1989 Rain #3 June 1989 Hot Springs Creek Problem Creek # 1
9°C
et al. (1982). HYDR samples are from B.C. Hydro (1983). ADMS samples are reservoir values are based on the mean post-flash data, which demonstrate a Complete chemical composition of Meager Creek waters can be found in that of the cool high-Cl waters,
despite
its high temperature.
Results
The 6D and 6180 relationships of the thermal and meteoric waters are illustrated in Fig. 3. The stable isotope values from meteoric waters of this study lie along the local meteoric water line established by Clark et al. (1982). The isotopic measurements of the MC1 post-flash samples cluster around the mean composition of: 6r80 = -12.2 f 0.46%0 and 6D = - 111.4 + 4.7%,,. The variation in the post-flash compositions does not show any correlation with time. The isotopic composition of a single MC1 sample reported by Adams et al. (1985) and Adams and Moore (1987), falls significantly outside the cluster of our data. This sample was collected shortly after completion of drilling (J. N. Moore, personal communication, 1990), and may have been affected by extensive disturbances (e.g. drilling water, airlifting, enriched 6D and 6180 due to well bore boiling). In order to determine the isotopic composition of pre-flash reservoir waters, the mean postflash 6r*O and 6D values were corrected for both continuous steam separation (flashing outside the well) and single-step steam separation (in-well flashing). In-well steam separation reduces the temperature from a starting point of 200°C (at the reservoir level) to 145°C (at the wellhead). Correction for in-well flashing is simple, as the steam separated during ascent remains with the liquid and separates at a single temperature. Assuming a rapid isotopic equilibration between liquid and vapour during ascent, the steam separated in this temperature interval will be in isotopic equilibrium with liquid at the wellhead temperature (Giggenbach and Stewart, 1982). During the flash to the atmosphere, steam separates continuously as soon as it is formed. This process reduces the temperature from the wellhead temperature (145°C) to the local boiling temperature (96°C). Changes in the isotopic composition in this process were calculated using the following equations (Truesdell et al., 1977; Truesdell and Hulston, 1980): exp ‘?I [(l - lla)l(H, - &)I dH, = 6’ HI,
(1)
8 = (1000 + 6,i)/(lOOO + 6,~)
(2)
where 6,, is 6t80 or 6D of water at the final temperature (after steam separation), 6,i is 6180 or 9D of water at the initial temperature, Hii is enthalpy of water at the initial temperature, ZY,,is enthalpy of water at the separation temperature, and a(T) is the R1/Rv ratio at the temperature T
84
M. M. Ghomshei and I. D. Clark
(Majoube, 1971; Henley et al., 1984), RI and Rv being the ratio of the heavy isotope to the light isotope in liquid and vapour, respectively. In the temperature range of interest (96-200°C), equation (1) can be simplified based on the assumptions that the numeric value of enthalpy of water in cal g-I is approximated by the temperature in °C and the latent heat of vaporization of water is constant and equal to 500 cal g-1. Equation (1) can therefore be expressed as: 0 = exp
0.002(1 - 1/a) dT
(3)
Ti
where Tli is the initial water temperature, Tls is the final water temperature (after steam separation). Equation (3) can be easily solved by dividing the temperature interval into smaller sub-intervals in which a linear function can be adopted for a(T). The isotopic values of the preflash wellhead fluid (145°C) were calculated using both simplified (equation (3)) and exact (equation (1)) solutions; the results were sufficiently similar (Table 2) to justify the simplified approach. The changes in the isotopic composition of water in single-stage steam separation (in-well flashing) were calculated from the equation (Truesdell et al., 1977; Truesdell and Hulston, 1980): 0 = L + (1 - L)/a
(4)
where L is the liquid fraction. By applying the same simplifying assumptions used for continuous steam separation one obtains: 0 = 1 - (Vii- rls)(1 - 1/a)/500
(5)
Pre-flash reservoir isotopic values, calculated according to this two-stage model, are given in Table 2. Again the simplified (equation (5)) and exact (equation (4)) solutions give similar results. DISCUSSION The calculated reservoir isotope values show an oxygen isotopic shift of about 2.4%0 relative to the local meteoric water line. Considering the high reservoir temperature (>200°C) and the low porosity (<1%) (B.C. Hydro, 1983), this shift is relatively small and therefore implies that the fluid flow in the reservoir may be confined to fracture systems. Knowing the initial and final water composition, the initial rock composition and the reservoir temperature, a water/rock ratio integrated over the life of the reservoir can be calculated, assuming conditions of water/ rock isotopic equilibrium (Taylor, 1977; Henley et al., 1984). This ratio is about 1.8 (in weight units) for Meager Creek, using a reservoir temperature of 210°C (maximum temperature measured at the bottom of the production zone) (B.C. Hydro, 1983, 1985), and an initial rock composition of 6aSo = 5.3%o (Adams and Moore, 1987). It was assumed that oxygen exchange is controlled by feldspar (Taylor, 1977; Henley et al., 1984). The calculated time-integrated w/r ratio of Meager Creek fits within the range of (w/r) weight values for geothermal systems (Henley et al., 1984). Considering the low porosity of the reservoir rocks, the high water/rock ratio is, most probably, related to a low volume of rock participating in isotope exchange (i.e. only the wall rocks next to the fractures are in isotope equilibrium with water). Note that our calculated w/r ratio is based on assuming equilibrium conditions in a closed system. At present no experimental data on the final rock isotopic composition are available. Detailed isotopic studies on the deep reservoir rocks are necessary to
85
South Meager Creek Geothermal Area -80
x
x -90"
Adams et
al.
~/xx % *o'
•
~
•
,?~'/
~J
m
-130- / -140-
>~
.........
J " CelLar (evaporatedra,n)
.,-~"" x
Recharg
-120-
-I-
Shallow thermal
-100-110-
MC1 Post-flash
Cool High-CI []
Meteoric
Water line
~
.... ~ / ~
[]
--
×
•
(Calculated)
/
- 150 -20 -1'9 -1'8 -1'7 -1'6 -1'5 -1'4 -1'3 -1'2 -1'1 -1'0 -9 -8 -7 6180 %,
Fig. 3. Deuterium and [80 compositions of meteoric, thermal and mineral waters in the South Meager Creek area. The local water line is from Clark et al. (1982). The reservoir composition is calculated from the average post-flash data. Shallow thermal waters fall very close to the local meteoric water line, whereas the deep thermal waters demonstrate a positive oxygen shift. The composition of recharge waters on the local water line is estimated by removing the oxygen shift from the reservoir composition. Samples from the well cellars are evaporated rain water and carry information on the local evaporation line.
provide a more reliable assessment of the w/r ratio and a better understanding of the nature of the Meager Creek geothermal reservoir. The composition of the recharge waters can be estimated from extrapolation of the calculated reservoir isotopic composition to the local water line (i.e. by removing the oxygen shift). The composition thus obtained is notably heavier than the average local meteoric waters (Fig. 3) and precludes a local recharge. This suggests a regional origin for these deep thermal waters, dominated by recharge at lower altitude and in closer proximity to the source of water vapour than local waters. The possibility that the MC1 waters originated as very local low-altitude (isotopically enriched) recharge is incompatible with ground-water flow in mountainous regions. Shallow ground-waters are generally dominated by very local recharge due to active circulation driven by steep gradients in high-relief terrains. Deep ground-waters are generally dominated by regional scale circulation, and recharge will not be affected by the local altitude effects and seasonal variations. Water flows according to hydraulic gradients are established by regional topography and can also be strongly affected by thermal convection where strong temperature gradients exist. Limited regional isotopic data are available for comparison with the Meager Creek area. Thermal waters from the Pebble Creek area on the north side of Meager Mountain (Clark et al., 1982) are considerably lighter than those of the south side. Fritz et al. (1987) show the distribution of 6180 values in locally recharged ground-waters across Canada. For southwestern B.C., these data show a strong correlation with the distance from the ocean. Clark (1985) presents 6180 values for geothermal and non-thermal waters from Nass River near the Alaska border, southeast to Bella Cools. Two effects emerge from the compilation (Fig. 4) of these and other data (Clark et al., 1989): an increase in 6180 contents with proximity to the coast and an increase with decreasing latitude. The coastal effect reflects proximity to the vapour source, where initial precipitation from a vapour mass is enriched in heavy isotopes, and the clouds evolve towards lighter isotope contents along the trajectory inland. The latitude effect reflects
86
M. M. Ghomshei and 1. D. Clark
decreasing temperatures at higher latitudes, where t80 in ground-water recharge is depleted in cooler temperature environments. Based on the limited available regional isotopic information, one can suggest that the recharge of the South Meager Creek reservoir is from an area towards the coast (west) or south, where isotopically heavier waters are likely to be found. More detailed regional isotope data is needed to discuss more clearly the recharge area. The isotopic composition of shallow thermal waters can be reconstructed if a deep water composition is known as a starting point (Giggenbach and Stewart, 1982). Accepting a single origin for the thermal waters in the area, the important processes contributing to generation of shallow thermal waters are: (1) steam loss (adiabatic cooling); and (2) mixing with cold meteoric waters (Fournier and Truesdell, 1974; Fournier, 1977). Assuming that geothermal waters boil near the surface prior to mixing with shallow waters, the hot end-member in the mixing process should be isotopically and chemically similar to the post-flash samples from MC1. In this scenario, the chloride and 6180 values agree with a hot/cold mixing ratio of 1/4 (Fig. 5). Nevertheless the relatively high temperature (above 50-60°C) of the hot springs will not agree with this mixing rate, as the mixing of the 96°C post-flash thermal waters with the 7°C meteoric waters, by the chemically calculated ratio of 1/4, will give a final product of less than 30°C. The elevated hot spring temperature, therefore, implies that the deep thermal fluids were not exposed to total steam loss during ascent and mixing. The chemical data, also, agree with partial steam loss. The amount of steam lost from the system was estimated using iterative calculations (trial-
120 ° W 60 ° N
BRITISH COLUMBIA
"":-2o
N
-18
l
\ -le~W~ \ \
19
Meager
Creek
-1
-19
~
- lz - Is " 16
49 ° N U.S.A.
Fig. 4. Distribution of mean annual 6z80 values (%0) of modern meteoric waters in western British Columbia. See text
for explanation.
87
South Meager Creek Geothermal Area 4500-
x
•1• /
MC1 Post-flash
Mixing line (High-Cl Waters)
Shallow thermal
M12
40003500/
3000-
/
+
!
Cool High-CI E3
!
Meteoric
i=.
=" 2500-
k M1
/
2000-
/
15001000500-
(9600)
/
/
11.0 ( ~ )
HOT-END IN MIXING
................ ,,. ,."Mixi
line
RESERVOIR
/ / ...."
020 -1'9 -l'a -1'7 -1'6 -1's -1'4 -1'3 -1'2 -1'1 -1'0 -9 -B -7 ~%,~,
Fig. 5. Chloride-lSo relations for meteoric, thermal and mineral waters in the South Meager Creek geothermal area (see text for explanation). The chemistry of the waters from the M1 hole matches that of the cool high-Cl waters, despite its abnormally high temperature of 58°C (see text for explanation).
and-error-procedure). The isotope, chloride and temperature values of the hot end-member were calculated for incremental values of steam loss. A mixing rate compatible with the chemistry (6180 and CI) of the hot and cold end-members and that of the hot springs (product of mixing) was calculated at each iteration. The temperature of the hot springs was then calculated on the basis of the chemically calculated mixing rate and the temperatures of the end-members. The calculated hot spring temperature was then compared to the measured temperature. Iteration proceeded until the calculated hot spring temperature values converged to the measured value (average temperature of the Meager Creek Hot Spring and the shallow hole E M R 301-1). The resultant steam loss value of about 8% (i.e. hot end-member temperature of about 160°C) and the mixing rate of about 0.35 (ratio of the hot-to-cold end-members) thus matches both the chemistry and the temperature conditions (Fig. 5). Note that the calculated 8% steam loss indicates the total amount of steam escaped from the system. A higher degree of steam loss at an early stage followed by some steam capture at a later stage should not be ruled out, as the high bicarbonate and sulphate concentrations of the shallow thermal waters (3471470 ppm HCO3 and 120-1980 ppm SO4) relative to post-flash MC1 discharge (72-98 ppm for HCO3 and 120-410 ppm for SOn) (Ghomshei et al., 1986) suggest the possibility of some steam capture. It should be mentioned that in the northern area of Meager Mountain (Pebble Creek area) vigorous steam capture is indicated for the low-salinity, high-temperature acid hot springs of this area (Adams et al., 1985; Adams and Moore, 1987). The cool high-chloride waters (found mostly in the South Fork area, Fig. 1) are isotopically lighter than MC1 waters (Fig. 3). Their oxygen shift is also relatively small. This can be explained by: (a) rock/water equilibrium at relatively low temperatures; or (b) a high water/rock ratio (high porosity). Accepting the first scenario, the high salinity of these waters may indicate dissolution of evaporites (present in the local rocks) or presence of a connate water component originating in the metasediments in the South Fork area (B.C. Hydro, 1983). In the second scenario, the low temperature of the waters (except M1) can be explained by conductive cooling near the surface. The first scenario or a combination of the two may be more justified, considering the comparatively low silica concentration of these waters (Ghomshei et al., 1986).
88
M . M . G h o m s h e i a n d I. D . C l a r k
T h e distinct origin of the cool high-chloride waters is also a p p a r e n t from the 180-C1 c o r r e l a t i o n (Fig. 5). This c o r r e l a t i o n shows that the cool high-chloride waters result from the mixing of a single p a r e n t m e m b e r f o u n d in M12 shallow hole (14,100 p p m T D S a n d l l ° C t e m p e r a t u r e ) with local m e t e o r i c waters. T h e a b n o r m a l l y high t e m p e r a t u r e (58°C) of the highCI waters f o u n d in the M1 hole m a y be related to steam heating. Concluding remarks
E x t r a p o l a t i o n of the isotopic c o m p o s i t i o n of the deep t h e r m a l waters to the local m e t e o r i c water line suggests a regional origin for the M e a g e r C r e e k g e o t h e r m a l system, d o m i n a t e d by recharge at low altitude a n d in closer proximity to the source of w a t e r v a p o u r . T h e o b s e r v e d small oxygen isotopic shift of the deep t h e r m a l waters a n d the calculated high t i m e - i n t e g r a t e d w/r ratio are suggestive of d o m i n a n c e of f r a c t u r e - r e l a t e d p e r m e a b i l i t y in the reservoir rocks. A n a l y s e s of the t e m p e r a t u r e a n d the chemistry of the shallow-circulating t h e r m a l waters suggest that d e e p t h e r m a l waters have b e e n subjected only to partial steam loss ( a b o u t 8 % ) d u r i n g ascent a n d mixing. Acknowledgements--This work was supported jointly by NSERC (operating grant OGP0042590 to I. D. Clark) and the
Canadian Crew Energy Corp. Dr John Gray from the Department of Physics of the University of Alberta and Dr Michael Maxwell from the Department of Geophysics and Astronomy of the University of British Columbia assisted in the analysisof part of the samples. Our gratitude extends to the reviewers of this paper, Dr Lisa Shevenell and Dr Alfred Truesdell, for their most useful comments on the manuscript.
REFERENCES Adams, M. C. and Moore, J. N. (1987) Hydrothermal alteration and fluid geochemistry of the Meager Mountain geothermal system, British Columbia. Am. J. Sci. 287, 720-755. Adams, M. C., Moore, J. N. and Forster, C. (1985) Fluid flowin volcanic terrains - - hydrogeochemistry of the Meager Mountain thermal system. Geotherm, Resources Counc. Trans. 8,377-382. B. C. Hydro (British Columbia Hydro and Power Authority) (1983) Report on Meager Creek Geothermal Project, 1981-1982. Report No. P7010/2 March 1983, prepared by KRTA Kingston Reynolds Thom and Allardice, New Zealand. B. C. Hydro (British Columbia Hydro and Power Authority) (1985) Report on activities for 1983/84-1984/85. Meager Creek Project staff, Vancouver. Clark, I. D. (1985) Oxygen isotope data from Nass River to southeast Bella Coola. Unpublished report. Geological Survey of Canada. Clark, I. D., Fritz, P., Michel, F. and Souther, J. G. (1982) Isotope hydrogeology and geothermometry of the Mount Meager geothermal area. Can. J. Earth Sci. 19, 1454-1473. Clark, I. D., Fritz, P. and Souther, J. G. (1989) Isotope hydrology of Mount Edziza-Mess Creek geothermal area. Can. J. Earth Sci. 26, 1160-1171. Fournier, R. O. (1977) Chemical geothermometers and mixing models for geothermal systems. Geothermics 5, 41-50. Fournier, R. O. and TruesdeU, A. H. (1974) Geochemical indicators of subsurface temperature, 2. Estimation of temperature of fraction of hot-water mixed with cold water. U.S. Geol. Surv. J. Res, 2,263-270. Fritz, P., Drimmie, R. J., Frape, S. K. and O'Shea, K. (1987) The isotopic composition of precipitation and groundwater in Canada. In: Proc. Isotope Techniques in Water Resources Development, Int. Atomic Energy Agency Symposium SM299, Vienna, pp, 537-550. Ghomshei, M. M., Croft, S. A. S. and Stauder, J. J. (1986) Geochemical evidence of chemical equilibria in the South Meager Creek geothermal system, British Columbia, Canada. Geothermics 15, 49--61. Ghomshei, M. M. and Stauder, J. J. (1989) Brief review of the Meager Creek Geothermal Project: a second look at the data. Geotherm. Resources Counc. Bull. 18, 3-7. Giggenbach, W. E. and Stewart, M. K. (1982) Processes controlling the isotopic composition of steam and water discharges from steam vents and steam-heated pools in geothermal areas. Geothermics 11, 71-80. Henley, R. W., Truesdell, A. H. and Barton, B. P. Jr (1984) Fluid-mineral equilibria in hydrothermal systems. Revs econ. Geol. 1. Published annually by the Society of Economic Geologists, 267 p. Jessop, A. M., Ghomshei, M. M. and Drury, M. J. (1991) Geothermal energy in Canada. Geothermics 20, 369--385. Majoube, M. (1971) Fractionnement en oxygene-18et en deuterium entre l'eau et sa vapeur. J. Chim. phys. 68, 14231436. Read, P. B. (1979) Geology, Meager geothermal area, British Columbia. Geol. Surv. Can. Open File 603.
South Meager Creek Geothermal Area
89
Souther, J. G. (1977) Volcanism and tectonic environments in the Canadian Cordillera--a second look. In: Volcanic Regimes in Canada (Edited by Baragar, W. R. A., Coleman, L. D. and Hall, J. M.). Geol. Assoc. Can. Spec. Paper 16, 3-24. Taylor, H. P., Jr (1977) Water/rock interactions and the origin of 1-12O in granitic batholiths. J. Geol. Soc. (London) 133, 509-558. Truesdell, A. H. and Hulston, J. R. (1980) Isotopic evidence of environments of geothermal systems. In: Handbook of Environmental Isotope Chemistry (Edited by Fritz, P. and Fontes, J. C.), Vol. 1, pp. 179-226. Elsevier, New York. 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, 3694-3704.
Geothermics, Vol. 22, No. 2, pp. 79-89, 1993. Printed in Great Britain.
OXYGEN AND HYDROGEN ISOTOPES IN DEEP THERMAL WATERS FROM THE SOUTH MEAGER CREEK GEOTHERMAL AREA, BRITISH COLUMBIA, CANADA M O R T E Z A M. GHOMSHEI* and IAN D. C L A R K t *Canadian Radon and Geothermal Research Ltd., P.O. Box 4301, Vancouver, B.C. V6B 3Z7 Canada, and tUniversity of Ottawa, Ottawa-Carlton Geoscience Centre, 161 Louis Pasteur, Ottawa, Ontario, KIN 6N5 Canada (Received April 1992; accepted for publication October 1992) Abstract--Deuterium and oxygen-18 (180) have been measured in deep thermal, shallow thermal and non-thermal water samples collected at various times between 1982 and 1989 from the Meager Creek area, with the aim of assessing the origin of the thermal waters. The isotopic composition of the reservoir waters (6180 = -13%o and 6D = -114.8%o) was calculated from data on post-flash deep thermal waters, using a two-stage steam loss model. The reservoir composition shows an oxygen shift of 2.4%o relative to the local meteoric water line. The composition of the recharge, obtained by removing the oxygen shift, is isotopically heavier than the average local meteoric waters, suggesting that the recharge may be from an area to the west of Mt Meager where isotopically heavier ground-waters are likely to be found. The small 180 shift of the deep high-temperature waters is indicative of dominance of fracture-related permeability in the reservoir. Analysis of the chemistry and the temperature of the waters from hot springs and shallow thermal wells suggests that these waters have evolved from the deep geothermal waters through dilution by meteoric waters and about 40°C adiabatic cooling (steam loss).
INTRODUCTION Meager Mountain in SW British Columbia, Canada, is the northernmost volcanic complex of the Cascade chain, extending over a 16 km-diameter area and rising about 2000 m above surrounding valleys. It includes late Tertiary to Quaternary volcanic rocks of silicic to intermediate composition, superimposed on the granodiorites of the Coast Plutonic Complex (Souther, 1977; Read, 1979). About 3 m of precipitation per year at higher altitudes and melting of permanent glaciers supply water to the hydrologic system. The elevation of Meager Creek is approximately 430 m, the highest point in the area of geological interest is Plinth Peak at 2679 m and the site of the deep holes is at 820 m above sea level (Fig. 1). The South Meager Creek waters can be classified into four categories (Table 1, Fig. 2): (1) meteoric waters; (2) moderately saline thermal waters from shallow boreholes and hot springs (products of mixing); (3) saline thermal waters from the deep geothermal reservoir (data from deep test-hole MC1); and (4) cool high-chloride waters, in the South Fork area (Fig. 1). The major structure responsible for the seepage of thermal waters to the surface is the Meager Creek fault, which strikes east-west and dips 55° north (B.C. Hydro, 1983). Major-ion data from the deep thermal waters have been reported previously (Ghomshei et al., 1986). Stable isotopes from the shallow boreholes, hot springs and surface meteoric waters have been reported by Clark et al. (1982). In their review of the hydrothermal alteration and fluid geochemistry of the Meager Mountain, Adams and Moore (1987) presented data on the chemical and isotopic composition of rocks and some water samples from the area. The present work complements these studies by presenting isotope data from the Meager Creek deep 79
M. M. Ghomshei and I. D. Clark
80
Y
~
LILLOOET
~ ,.
"~~"~__PEBBLECREEK HOT SPRINGS \
\
pLINTH
p¢
A
A
\ \
MT MEAGER
N PYLON
APK
w
fMEAGER CREEK /t HOT SPRINGS PLACID HOt SPRINGS CItK
NO GOOD ==~ HOT SPRI~I~)/°~M12 J~
0
I
1
2 ....
HoLE
3km Logging
road
~il Fault Fig. 1. Site map of the Meager Creek geothermal area, showing the location of the different waters sampled for this study. For location of the area, see Fig. 4. thermal waters discharged from the well MC1, which is the only discharging (flowing artesian) well drilled into a high-temperature geothermal reservoir in Canada; two other deep exploration wells drilled in the Meager Creek area (MC2 and MC3) did not sustain flow (Jessop et al., 1991).
Sampling and analysis Twenty-seven flashed water samples from MC1 have been collected during a period of 7 years from 1982 to 1989 (Table 2). MC1 is a 2.5 km deep test well, with the production zone at depth 1300-1800 m (wellhead at 817 m above sea level). The measured temperature at this depth is about 200°C. However, the wellhead discharge is a two-phase 145°C fluid comprising 15-18%
81
South Meager Creek Geothermal Area Table 1. Major-ion chemistry of representative waters from South Meager Creek. Concentrations are in ppm T Non-geothermal (meteoric) AngelCreek Hot springs, shallow thermal Meager Creek Hot Springs Deep exploration wells WeirboxMC1 Cool high-chloride waters M12 (shallow well)
pH
Li
4.0 9.6 1
Na
K
Ca
Mg
2.0
1.46
26.1
370
38
69
53
7.1
96
8.3 3.3
1260
97
40
11
7.1
3600
136
490
7.6
F
CI
SO4
11.2 0.05
3.2
41
22 0.21
550
B
SiO 2 HCO 3 Br 4.9
86.4
230
415
ND
120 12.8 370
72
4.2
240 0.32 4230 182037.8 16.4
3534
18.0
0.8
2.1
1990
120 2.3
Note: HCO 3 is total CO 2 expressed as HCO3, T is temperature of sampled water (°C). Analyses performed by B.C. Hydro R&D Laboratory.
steam (measured) and 82-85% brine of 96°C after flashing to the atmosphere (Ghomshei and Stauder, 1989; B.C. Hydro, 1983, 1985). The MC1 samples are post-flash (weirbox), collected at 96°C (local boiling temperature). An additional 17 samples from shallow waters have been collected and analyzed for this study. These include hot springs, borehole fluids, meteoric waters and the waters in the cellars built around the dry wells MC2 and MC3 (evaporated water). More data on the shallow waters can be found in Clark et al. (1982). Most samples were analyzed at the Ottawa-Carlton Geoscience Centre. Ratios of 180/160 in H20 were measured by a VG SIRA-12 gas source mass spectrometer, on CO2 gas prepared by standard equilibration with water samples at 25°C. 2H/1H ratios were measured in hydrogen gas extracted from water by reduction on uranium metal at 800°C. Stable isotope contents are expressed as delta per mil (%0) differences from SMOW. Cation concentration data were generated by ICP spectrographic analysis of filtered (0.45/tm) and acidified water samples. Anion concentrations were determined by high-pressure liquid chromatography and alkalinity by micro-titration with 1.6 N H2SO 4.
161412-
BIB Chloride
[7'0 O.
C
BB TDS
200
Temperature
ge_. ~ DEEP THERMAL FOST-FLA3H
4-
100
20-
TYPE 1
TYPE 2
TYPE 3
TYPE 4
Fig. 2. Chemistry (TDS and CI) and temperature of different types of water in the South Meager Creek geothermal area. Type 1 represents surface waters of meteoric origin (Angel Creek). Type 2 represents hot springs and shallow thermal waters (Meager Creek Hot Springs). Type 3 represents the deep thermal waters (MC1 post-flash), and type 4 represents the cool high-chloride waters (M12).
M. M. Ghomshei and I. D. Clark
82
T a b l e 2. 180, d e u t e r i u m a n d c h l o r i d e in t h e r m a l , m i n e r a l a n d m e t e o r i c w a t e r s in S o u t h M e a g e r C r e e k a r e a Sample
6 iSO%o
6D%o
C1 ( p p m )
Post-flash d e e p t h e r m a l w a t e r s ( M C 1 ) MC87-9 -ll.77 CM87-10 -12.72 MC89-1 -12.84 MC89-3 -11.61 MC89-4 -12.85 MC1-83-1 -12.03 MC1-83-2 -11.98 MC1-84-1 -12.25 MC1-84-2 -12.16 MC1-84-3 -12.2 MC1-84-4 -12.29 MC1-84-5 -10.89 MC1-84-6 -12.05 MC1-84-7 -11.54 MC1-84-8 -11.8 MC1-84-9 -12.15 MC1-84-10 -12.25 MC1-84-11 -11.94 MC1-84-12 -12.12 MC1-84-13 -11.91 MC1-84-14 -12.08 MCL-H1 -12.78 MCL-H2 -12.8 MCL-H3 -12.77 MCL1-SEP -12.66 MCL2-SEP -12.54 MCL3-SEP -12.64
-113.7 -115.8 -118.8 -115.5 -114.2 -104.37 -100.39 -112.28 -101.69 -120.18 -104.7 -110.15 -108.65 -114.89 -103.69 -108.56 -114.4 -113.42 -108.58 -112.67 -109.82 -112.2 - 115.71 - 116 -111.47 -111.79 -115.17
2056 1914 2021
M e a n post-flash
- 12.2
- 111.4
Pre-flash w e l l h e a d , c a l c u l a t e d : simplified solution e x a c t solution
- 12.64 - 12.62
- 113.3 - 113.2
R e s e r v o i r , calculated: simplified solution e x a c t solution
- 13.05 - 13.05
- 114.8 - 114.9
Shallow thermal waters MC87-5 MC87-12 MC89-7 MC89-6 MC-87-11 ADMS-MC CLRK-MC CLRK-PLC CLRK-N-G2
- 16.18 - 16.22 - 16.02 - 16.94 -16.27 - 16.4 - 16.5 - 16 - 17.2
- 123.6 - 124.8 - 123.8 - 123.3 -123.8 - 126 - 125 - 124 - 128
Cool high-Cl w a t e r s MC87-7 MC87-2 HYDR-NSFS HYDR-SFCS ADAMS-M12 HYDR-M1 *
-
-
1932 2030 2010 2030 2010 2020 2010 2140 2150 2140 2150 2150 2140 2140 2140 2140
Description
F l a s h e d b r i n e 96°C . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1987 1987 1989 1989 1989 1983 1983 1984 1984 1984 1984 1984 1984 1984 1984 1984 1984 1984 1984 1984 1984 1985 1985 1985 1985 1985 1985
2070
145°C ( m e a s u r e d )
200°C ( m e a s u r e d )
16.56 16.65 15.9 16.1 14.2 15.5
125.8 125.3 119 123 120 129
1640
553 603
661 528 674 196
2350 1740 4230 2420
E M R - 3 0 1 - 1 , 58°C M e a g e r C r e e k H o t Springs, 45°C M e a g e r C r e e k H o t Springs, 50°C N o - g o o d hot springs N o - g o o d h o t springs M e a g e r C r e e k H o t Spring M e a g e r Creek H o t Spring Placid C r e e k , 45. I ° C N o - g o o d hot spring, 29.5°C
South Fork swamps S o u t h F o r k C r k , 10°C U p p e r South Fork swamps South Fork swamps M 1 2 shallow hole M1 shallow h o l e , 56°C
83
South Meager Creek Geothermal Area Table 2. Continued Sample Non-thermal MC87-3 MC-85-Al MC-85-A2 MC87-4 MC87-6 MC87-8 MC-89-5 MC89-8 MC89-2 MC87-1 PROB-1
61X0%0
6D%0
-17.10 -17.98 -17.9 -16.78 -16.77 -17.57 -9.95 -14.44 - 16.09 -17.07 -17.7
-126.9 -132.93 -130.22 -119.0 -124.7 -129.2 -100.8 -118.0 -120.0 -130.06 -127
CJ (ppm)
Description
waters
Notes: CLRK samples are from Clark from Adams et al. (1985). The calculated scatter of 20/w in 6 ‘0 and 20%. in 6D. Ghomshei et al. (1986). *The chemistry of this sample matches
0.8
Angel Creek Angel Creek Angel Creek Meager Creek Meager Creek Hot Springs, Boundary Creek 8°C Rain #l June 1989 Rain #2 June 1989 Rain #3 June 1989 Hot Springs Creek Problem Creek # 1
9°C
et al. (1982). HYDR samples are from B.C. Hydro (1983). ADMS samples are reservoir values are based on the mean post-flash data, which demonstrate a Complete chemical composition of Meager Creek waters can be found in that of the cool high-Cl waters,
despite
its high temperature.
Results
The 6D and 6180 relationships of the thermal and meteoric waters are illustrated in Fig. 3. The stable isotope values from meteoric waters of this study lie along the local meteoric water line established by Clark et al. (1982). The isotopic measurements of the MC1 post-flash samples cluster around the mean composition of: 6r80 = -12.2 f 0.46%0 and 6D = - 111.4 + 4.7%,,. The variation in the post-flash compositions does not show any correlation with time. The isotopic composition of a single MC1 sample reported by Adams et al. (1985) and Adams and Moore (1987), falls significantly outside the cluster of our data. This sample was collected shortly after completion of drilling (J. N. Moore, personal communication, 1990), and may have been affected by extensive disturbances (e.g. drilling water, airlifting, enriched 6D and 6180 due to well bore boiling). In order to determine the isotopic composition of pre-flash reservoir waters, the mean postflash 6r*O and 6D values were corrected for both continuous steam separation (flashing outside the well) and single-step steam separation (in-well flashing). In-well steam separation reduces the temperature from a starting point of 200°C (at the reservoir level) to 145°C (at the wellhead). Correction for in-well flashing is simple, as the steam separated during ascent remains with the liquid and separates at a single temperature. Assuming a rapid isotopic equilibration between liquid and vapour during ascent, the steam separated in this temperature interval will be in isotopic equilibrium with liquid at the wellhead temperature (Giggenbach and Stewart, 1982). During the flash to the atmosphere, steam separates continuously as soon as it is formed. This process reduces the temperature from the wellhead temperature (145°C) to the local boiling temperature (96°C). Changes in the isotopic composition in this process were calculated using the following equations (Truesdell et al., 1977; Truesdell and Hulston, 1980): exp ‘?I [(l - lla)l(H, - &)I dH, = 6’ HI,
(1)
8 = (1000 + 6,i)/(lOOO + 6,~)
(2)
where 6,, is 6t80 or 6D of water at the final temperature (after steam separation), 6,i is 6180 or 9D of water at the initial temperature, Hii is enthalpy of water at the initial temperature, ZY,,is enthalpy of water at the separation temperature, and a(T) is the R1/Rv ratio at the temperature T
84
M. M. Ghomshei and I. D. Clark
(Majoube, 1971; Henley et al., 1984), RI and Rv being the ratio of the heavy isotope to the light isotope in liquid and vapour, respectively. In the temperature range of interest (96-200°C), equation (1) can be simplified based on the assumptions that the numeric value of enthalpy of water in cal g-I is approximated by the temperature in °C and the latent heat of vaporization of water is constant and equal to 500 cal g-1. Equation (1) can therefore be expressed as: 0 = exp
0.002(1 - 1/a) dT
(3)
Ti
where Tli is the initial water temperature, Tls is the final water temperature (after steam separation). Equation (3) can be easily solved by dividing the temperature interval into smaller sub-intervals in which a linear function can be adopted for a(T). The isotopic values of the preflash wellhead fluid (145°C) were calculated using both simplified (equation (3)) and exact (equation (1)) solutions; the results were sufficiently similar (Table 2) to justify the simplified approach. The changes in the isotopic composition of water in single-stage steam separation (in-well flashing) were calculated from the equation (Truesdell et al., 1977; Truesdell and Hulston, 1980): 0 = L + (1 - L)/a
(4)
where L is the liquid fraction. By applying the same simplifying assumptions used for continuous steam separation one obtains: 0 = 1 - (Vii- rls)(1 - 1/a)/500
(5)
Pre-flash reservoir isotopic values, calculated according to this two-stage model, are given in Table 2. Again the simplified (equation (5)) and exact (equation (4)) solutions give similar results. DISCUSSION The calculated reservoir isotope values show an oxygen isotopic shift of about 2.4%0 relative to the local meteoric water line. Considering the high reservoir temperature (>200°C) and the low porosity (<1%) (B.C. Hydro, 1983), this shift is relatively small and therefore implies that the fluid flow in the reservoir may be confined to fracture systems. Knowing the initial and final water composition, the initial rock composition and the reservoir temperature, a water/rock ratio integrated over the life of the reservoir can be calculated, assuming conditions of water/ rock isotopic equilibrium (Taylor, 1977; Henley et al., 1984). This ratio is about 1.8 (in weight units) for Meager Creek, using a reservoir temperature of 210°C (maximum temperature measured at the bottom of the production zone) (B.C. Hydro, 1983, 1985), and an initial rock composition of 6aSo = 5.3%o (Adams and Moore, 1987). It was assumed that oxygen exchange is controlled by feldspar (Taylor, 1977; Henley et al., 1984). The calculated time-integrated w/r ratio of Meager Creek fits within the range of (w/r) weight values for geothermal systems (Henley et al., 1984). Considering the low porosity of the reservoir rocks, the high water/rock ratio is, most probably, related to a low volume of rock participating in isotope exchange (i.e. only the wall rocks next to the fractures are in isotope equilibrium with water). Note that our calculated w/r ratio is based on assuming equilibrium conditions in a closed system. At present no experimental data on the final rock isotopic composition are available. Detailed isotopic studies on the deep reservoir rocks are necessary to
85
South Meager Creek Geothermal Area -80
x
x -90"
Adams et
al.
~/xx % *o'
•
~
•
,?~'/
~J
m
-130- / -140-
>~
.........
J " CelLar (evaporatedra,n)
.,-~"" x
Recharg
-120-
-I-
Shallow thermal
-100-110-
MC1 Post-flash
Cool High-CI []
Meteoric
Water line
~
.... ~ / ~
[]
--
×
•
(Calculated)
/
- 150 -20 -1'9 -1'8 -1'7 -1'6 -1'5 -1'4 -1'3 -1'2 -1'1 -1'0 -9 -8 -7 6180 %,
Fig. 3. Deuterium and [80 compositions of meteoric, thermal and mineral waters in the South Meager Creek area. The local water line is from Clark et al. (1982). The reservoir composition is calculated from the average post-flash data. Shallow thermal waters fall very close to the local meteoric water line, whereas the deep thermal waters demonstrate a positive oxygen shift. The composition of recharge waters on the local water line is estimated by removing the oxygen shift from the reservoir composition. Samples from the well cellars are evaporated rain water and carry information on the local evaporation line.
provide a more reliable assessment of the w/r ratio and a better understanding of the nature of the Meager Creek geothermal reservoir. The composition of the recharge waters can be estimated from extrapolation of the calculated reservoir isotopic composition to the local water line (i.e. by removing the oxygen shift). The composition thus obtained is notably heavier than the average local meteoric waters (Fig. 3) and precludes a local recharge. This suggests a regional origin for these deep thermal waters, dominated by recharge at lower altitude and in closer proximity to the source of water vapour than local waters. The possibility that the MC1 waters originated as very local low-altitude (isotopically enriched) recharge is incompatible with ground-water flow in mountainous regions. Shallow ground-waters are generally dominated by very local recharge due to active circulation driven by steep gradients in high-relief terrains. Deep ground-waters are generally dominated by regional scale circulation, and recharge will not be affected by the local altitude effects and seasonal variations. Water flows according to hydraulic gradients are established by regional topography and can also be strongly affected by thermal convection where strong temperature gradients exist. Limited regional isotopic data are available for comparison with the Meager Creek area. Thermal waters from the Pebble Creek area on the north side of Meager Mountain (Clark et al., 1982) are considerably lighter than those of the south side. Fritz et al. (1987) show the distribution of 6180 values in locally recharged ground-waters across Canada. For southwestern B.C., these data show a strong correlation with the distance from the ocean. Clark (1985) presents 6180 values for geothermal and non-thermal waters from Nass River near the Alaska border, southeast to Bella Cools. Two effects emerge from the compilation (Fig. 4) of these and other data (Clark et al., 1989): an increase in 6180 contents with proximity to the coast and an increase with decreasing latitude. The coastal effect reflects proximity to the vapour source, where initial precipitation from a vapour mass is enriched in heavy isotopes, and the clouds evolve towards lighter isotope contents along the trajectory inland. The latitude effect reflects
86
M. M. Ghomshei and 1. D. Clark
decreasing temperatures at higher latitudes, where t80 in ground-water recharge is depleted in cooler temperature environments. Based on the limited available regional isotopic information, one can suggest that the recharge of the South Meager Creek reservoir is from an area towards the coast (west) or south, where isotopically heavier waters are likely to be found. More detailed regional isotope data is needed to discuss more clearly the recharge area. The isotopic composition of shallow thermal waters can be reconstructed if a deep water composition is known as a starting point (Giggenbach and Stewart, 1982). Accepting a single origin for the thermal waters in the area, the important processes contributing to generation of shallow thermal waters are: (1) steam loss (adiabatic cooling); and (2) mixing with cold meteoric waters (Fournier and Truesdell, 1974; Fournier, 1977). Assuming that geothermal waters boil near the surface prior to mixing with shallow waters, the hot end-member in the mixing process should be isotopically and chemically similar to the post-flash samples from MC1. In this scenario, the chloride and 6180 values agree with a hot/cold mixing ratio of 1/4 (Fig. 5). Nevertheless the relatively high temperature (above 50-60°C) of the hot springs will not agree with this mixing rate, as the mixing of the 96°C post-flash thermal waters with the 7°C meteoric waters, by the chemically calculated ratio of 1/4, will give a final product of less than 30°C. The elevated hot spring temperature, therefore, implies that the deep thermal fluids were not exposed to total steam loss during ascent and mixing. The chemical data, also, agree with partial steam loss. The amount of steam lost from the system was estimated using iterative calculations (trial-
120 ° W 60 ° N
BRITISH COLUMBIA
"":-2o
N
-18
l
\ -le~W~ \ \
19
Meager
Creek
-1
-19
~
- lz - Is " 16
49 ° N U.S.A.
Fig. 4. Distribution of mean annual 6z80 values (%0) of modern meteoric waters in western British Columbia. See text
for explanation.
87
South Meager Creek Geothermal Area 4500-
x
•1• /
MC1 Post-flash
Mixing line (High-Cl Waters)
Shallow thermal
M12
40003500/
3000-
/
+
!
Cool High-CI E3
!
Meteoric
i=.
=" 2500-
k M1
/
2000-
/
15001000500-
(9600)
/
/
11.0 ( ~ )
HOT-END IN MIXING
................ ,,. ,."Mixi
line
RESERVOIR
/ / ...."
020 -1'9 -l'a -1'7 -1'6 -1's -1'4 -1'3 -1'2 -1'1 -1'0 -9 -B -7 ~%,~,
Fig. 5. Chloride-lSo relations for meteoric, thermal and mineral waters in the South Meager Creek geothermal area (see text for explanation). The chemistry of the waters from the M1 hole matches that of the cool high-Cl waters, despite its abnormally high temperature of 58°C (see text for explanation).
and-error-procedure). The isotope, chloride and temperature values of the hot end-member were calculated for incremental values of steam loss. A mixing rate compatible with the chemistry (6180 and CI) of the hot and cold end-members and that of the hot springs (product of mixing) was calculated at each iteration. The temperature of the hot springs was then calculated on the basis of the chemically calculated mixing rate and the temperatures of the end-members. The calculated hot spring temperature was then compared to the measured temperature. Iteration proceeded until the calculated hot spring temperature values converged to the measured value (average temperature of the Meager Creek Hot Spring and the shallow hole E M R 301-1). The resultant steam loss value of about 8% (i.e. hot end-member temperature of about 160°C) and the mixing rate of about 0.35 (ratio of the hot-to-cold end-members) thus matches both the chemistry and the temperature conditions (Fig. 5). Note that the calculated 8% steam loss indicates the total amount of steam escaped from the system. A higher degree of steam loss at an early stage followed by some steam capture at a later stage should not be ruled out, as the high bicarbonate and sulphate concentrations of the shallow thermal waters (3471470 ppm HCO3 and 120-1980 ppm SO4) relative to post-flash MC1 discharge (72-98 ppm for HCO3 and 120-410 ppm for SOn) (Ghomshei et al., 1986) suggest the possibility of some steam capture. It should be mentioned that in the northern area of Meager Mountain (Pebble Creek area) vigorous steam capture is indicated for the low-salinity, high-temperature acid hot springs of this area (Adams et al., 1985; Adams and Moore, 1987). The cool high-chloride waters (found mostly in the South Fork area, Fig. 1) are isotopically lighter than MC1 waters (Fig. 3). Their oxygen shift is also relatively small. This can be explained by: (a) rock/water equilibrium at relatively low temperatures; or (b) a high water/rock ratio (high porosity). Accepting the first scenario, the high salinity of these waters may indicate dissolution of evaporites (present in the local rocks) or presence of a connate water component originating in the metasediments in the South Fork area (B.C. Hydro, 1983). In the second scenario, the low temperature of the waters (except M1) can be explained by conductive cooling near the surface. The first scenario or a combination of the two may be more justified, considering the comparatively low silica concentration of these waters (Ghomshei et al., 1986).
88
M . M . G h o m s h e i a n d I. D . C l a r k
T h e distinct origin of the cool high-chloride waters is also a p p a r e n t from the 180-C1 c o r r e l a t i o n (Fig. 5). This c o r r e l a t i o n shows that the cool high-chloride waters result from the mixing of a single p a r e n t m e m b e r f o u n d in M12 shallow hole (14,100 p p m T D S a n d l l ° C t e m p e r a t u r e ) with local m e t e o r i c waters. T h e a b n o r m a l l y high t e m p e r a t u r e (58°C) of the highCI waters f o u n d in the M1 hole m a y be related to steam heating. Concluding remarks
E x t r a p o l a t i o n of the isotopic c o m p o s i t i o n of the deep t h e r m a l waters to the local m e t e o r i c water line suggests a regional origin for the M e a g e r C r e e k g e o t h e r m a l system, d o m i n a t e d by recharge at low altitude a n d in closer proximity to the source of w a t e r v a p o u r . T h e o b s e r v e d small oxygen isotopic shift of the deep t h e r m a l waters a n d the calculated high t i m e - i n t e g r a t e d w/r ratio are suggestive of d o m i n a n c e of f r a c t u r e - r e l a t e d p e r m e a b i l i t y in the reservoir rocks. A n a l y s e s of the t e m p e r a t u r e a n d the chemistry of the shallow-circulating t h e r m a l waters suggest that d e e p t h e r m a l waters have b e e n subjected only to partial steam loss ( a b o u t 8 % ) d u r i n g ascent a n d mixing. Acknowledgements--This work was supported jointly by NSERC (operating grant OGP0042590 to I. D. Clark) and the
Canadian Crew Energy Corp. Dr John Gray from the Department of Physics of the University of Alberta and Dr Michael Maxwell from the Department of Geophysics and Astronomy of the University of British Columbia assisted in the analysisof part of the samples. Our gratitude extends to the reviewers of this paper, Dr Lisa Shevenell and Dr Alfred Truesdell, for their most useful comments on the manuscript.
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