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Pang and Wang
APPLICATION OF ISOTOPE GEOCHEMICAL TECHNIQUES TO GEOTHERMAL EXPLORATION
----THE ZHANGZHOU CASE
Pang Zhong- He and Wang Ji-Yang
Laboratory for Institute of Geology, Academia Sinica. P. 9825, 100029, CHINA
ABSTRACT
Zhangzhou Geothermal system, an inter-medium temperature
geothennal system of convective type in the southeast of China is
used as an example to demonstrate the problems and improvements
in the application of isotope and geochemical techniques to the
exploration of the saline geothermal waters in a granite terrain. It is
found that deuterium, and tritium contents of water are good
tracers for the origin of the water and of the salinity in it. Fluid
mineral equilibrium modeling is an important data interpretation
scheme as to assist the selection and use of chemical
geothermometers. Isotopic composition of dissolved sulfate was
used not only as a tracer of the origin of salinity, particularly in the
case of marine sulfate, but also as a geothermonmeter and a dating
method. Carbon-14 is a good dating tool for thermal waters in
granite terrain, but for thermal water of marine origin,
arises ftom the limited amount of carbon in it. Alteration minerals
found the drill-holes helped the reconstruction of the thermal
history of the system.
Key words: Isotope hydrology; Hydrogeochemistry; Geothermal
systems; Methodology; Southeast China; Zhangzhou Basin
1. INTRODUCTION
Hot springs are widely distributed in the southeastern coastal area of
China including Fujian, and Provinces, which is a
granite terrain. Zhangzhou geothermal system is the highest in
temperature (122 at a depth of 90 meters, 114 at the wellhead) and one of the highest in flow-rate and salinity, There has long
since been intensive dispute on the genesis and energy potential of
these systems. In this context, since 1985, the present authors and
others have carried out a multi-discipline program (Wang, et al.,
1989) to study Zhangzhou geothermal system and its surrounding
systems (Figure 1) in order to improve the understanding of similar
geothermal systems in other places of the region. In this process, a
number of geochemical and isotope methods have been applied, most
of them proved to be very but some presented problems. This
paper is a brief review of the hydrogeochemical and isotope studies
Figure 1. Map of the geothermal zone showing the sampling locations in the southeast of China
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and Wang
of these geothermal systems and a discussion on the and
effectiveness of the methods used as well as the problem
encountered in the work. This work includes investigations on the
chemical, gaseous contents and isotopic composition of the thermal
water and of the dissolved species in it. A very short review paper
was presented earlier by Pang and Wang (1991); but this paper
includes progress made in the last a few years.
2. GEOLOGICAL BACKGROUND THE PROBLEMS
Zhangzhou geothermal field is located in the central part of the
Zhangzhou which is a fault basin formed in the late Quaternary
by tectonic activity affecting the granite basement , which was
formed in the Mesozoic era. The Quaternary cover, some 20 meters
in thickness, is composed of inter-bedded continental and marine
unconsolidated sediments, which contain a major aquifer and some
small water-bearing lenses. Fractures are densely developed in the
bedrock. Two groups of major faults have been identified, the NE,
group and the NNE, group, which form a
network that provides high permeability and porosity for water
circulation and occurrence (Figure 2).
Attempts were made to investigate major problems such as, the
origin and recharge of the thermal fluids and their salinity, the
residence time of the thermal fluids, and the reservoir temperature ,
etc. Items covered in the investigation mainly include the major,
trace and gaseous constituents in the water and isotope compositions
of the water and of the dissolved carbon and sulfur. Computer
modeling of fluid mineral equilibrium was also camed out in
combination with other techniques in the study and seems to have
potential in improving conventional chemical geothermometers.
3. ANALYTICAL TECHNIQUES
3.1 Chemical Analyses
Most of the thermal water samples were analyzed for their chemical
compositions. Table 1 is a summary of selected chemical analytical
results of the samples. The anions were analyzed on the Dionex
20201 Ion Chromatography; the cations by ICP spectrometer and
other items by conventional chemical analytical methods. was
measured both in the field and at the laboratory and results showed
difference of approximately one unit lab field). Titration of
A geological map of the central part of basin where geothermal field is located
1) 2) 3) Granite bedrock; 4) Fractures; 5) Hot springs; 6) The geothermal field
1038
Pang and Wang
Selected chemical analyses of thermal and other natural. waters from Zhangzhou and surroundings
No 1 ZGF, W-B 98
2 W-B 97
3 73
4 105
5 52
6 20
7 W-Q 20
8 73
9 W-B 80
81
7.59
6.52
6.64
8.43
6.92
6.55
7.58
7.40
6.82
136.4
114.4
91.3
140.0
93.9
22.5
33.6
74 0
100.0
82.2
Na 1981.0 171.9
2061.2 128.0
1500.0 85.0
2175.0 89.0
4573.3 283.9
54.2 15.6
37.3 13.8
111.1 5.4
3837.6 97.8
892.8 41.0
1382 1 123
12520 220
14050 9 9
15182 173
28564 450
593 7 5
2 7
1 9
13752 993
5472 0 6
2145 31 8
1848 370
2370 390
2243 21 9
3390 373
815
104 389
1260
5780 71 7
2334 31 0
4895.1 0.015
5420.0 0.242
52900 0.198
5973.7
10573.5
90.6
25 9
7971.5 0.355
2057.0 0.156
Fe 0035 -550-372 21
0 154 -5 84 -41 1
2 057 -5 58 -35 6
-567 -379
0 504 -3 85 -26 3
002 -4 28 -30
0 021 -5 57 -36 4
0 105 -769-506 462
0 550 -4 65 -33 4
0 030 -6 60 -45 2
Notes: ZGF: geothermal field; sample; Q: Quaternary aquifer; B: bedrock aquifer; The unit of and is
SMOW, for is nd: not determined.
Table 2. Dissolved gases in thermal water and other natural waters in Zhangzhou basin
No Type Temp He Total
56 5 86) 5 25) 19
2 Well 41 5 90) 80) 0 85 (4 24) 20
3 Well 270 89) 61) 11) 036 (275) 13
4 Tianbao 21 0 35) 122 (5 93) 20
Notes: 1 of gas contents is per L ofwater.
2. Data in brackets are the percentage of each gas in the total gases dissolved in the water.
3. Samples were collected in 1987
HCO, at the laboratory does not very much fiom that in the
field.
3.2 Gas Analyses
Thermal water and other natural water samples were collected for
analysis of dissolved gases. Table 2 is a list of the results. Water
samples were collected on glass bottles of 10 liters in volume, which
were tightly sealed. The extraction of the gases and gas
chromatographic analysis were done in the laboratory soon after
collection.
3.3 Isotope Analyses
Many samples of both thermal and non-thermal natural waters have
been collected and 18, Deuterium and tritium
contents (Pang and Wang, and oxygen isotopes of
aqueous sulfate were also studied (Pang, et al., 1995). Carbon - 14
is used as a dating tool. Both Carbonate and Sulfate were separated
fiom the water through precipitation in the laboratory. See Table 2
for the data.
3.4. Alteration Mineral Study
Since tens of wells have been drilled in the geothermal field, their
drill cores are used for the study on alteration minerals. Products of
metasomatism were sampled from the cores and observations include
mineralogy, inclusions in alteration minerals were made in the
laboratory.
- 2 0
-40 -
-
A
xo -12 -6 -2 0 2 4 6
( SMOW)
Figure 3. Comparison of isotope composition of the thermal water
(circles) with that of the non-thermal meteoric waters (triangles and
rectangles).
3.5 Fluid - mineral Modeling
The fluid mineral equilibrium modeling approach first proposed by
Reed and Spycher (1984) has been used. This was done on both the
hot spring and well waters from the Zhangzhou zone (Pang
and 1989). The WATCH program (Amorsson et at.,
1982) was used for the calculations. The samples were collected and
completely analyzed chemically for this purpose.
1039
and Wang
4 DISCUSSIONS
4.1 About the System
of the thermal water and the recharge of the system
A local meteoric water line was determined based on compositions of
different types of meteoric waters including rainwater, river water
and shallow ground waters from the study area. The line is found to
be identical to that of the global meteoric water line. The thermal
waters plot very close to the meteoric water line, which indicates a
meteoric origin (Figure 3). According to the "elevation effect"
principle, the elevation of the recharge area for Zhangzhou
geothermal system was found out to be 800 to 1000 meters higher
that that of the discharge area. In most cases, recharge areas are the
periphery of the basins. With reference to modeling of the mass and
heat transport and conservation in the systems (Hochstein, et al.,
1990; it is concluded that these hot spring systems are
formed by the deep circulation of ground water within the basins by
forced convection induced by the topography.
Origin of the High Salinitv in the Thermal Water
Most of the hot spring waters in the southeast of China are dilute.
However, thermal waters from some geothermal systems close to the
seashore are saline. The total dissolved solids in thermal waters in
Zhangzhou geothermal field i s as high as 10,000 to 12,000 mg L.
The mixing of meteoric water with sea water is evidenced by the
close correlation of the chloride concentration with bromide contents
(Figure 4). This is also supported by the close linear correlation
between chloride concentrations and oxygen - 18 (Pang, et al.,
1995).
70
611
The sea water in most of the geothermal systems seems to be connate
water that entered the systems during a period of high sea level,
which occurred, for the southeast of China, 6,000 7,000 years ago
according to some geological evidences. The dilute waters from
other systems have chemical features derived from the dissolution of
the granite country rocks. Knowledge of the mixing processes has
deepened the understanding of the genesis and origin of the systems.
For instance, for Zhangzhou Geothermal Field, it is found out that
two mixing phases took place: Primary mixing at a depth of
2. Secondary mixing: the mixing between the ascending saline
thermal water with the dilute ground water in the shallow Quaternary
aquifer.
Reservoir Temperature and Thermal History of the System
An attempt was made by the authors to combine the conventional
geothermometers with the chemical equilibrium modeling in order to
the applicability of the conventional geothermonmeters in
different conditions (Pang, 1991). this consideration, the
selected geothermometers may be applied better to other geothermal
systems with similar geochemistry. By using a reference geothermal
a 0
20
100°C
1 5 0 ° C
-6 -4 -2 0
in Water ( SMOW)
Figure 5. Reservoir temperature predicted by water-sulfate oxygen18 geothermometer. The circles represent those samples from
Zhangzhou geothermal
0
Chloride
system, this has been achieved for the Southeast China Geothermal
Belt (Pang, which includes the Zhangzhou Xiamen
geothermal zone.
Figure 4. Linear relation between chloride and bromide in thermal
mixing of meteoric water sea water.
An approximate range of reservoir temperatures could be obtained
the data of the thermal water, which implies limited waterrock exchange of isotopes due to the low-medium temperature
environment of the systems. The water and dissolved sulfate show
reservoir temperature
good equilibrium which serves as a very good indicator of the
sampling area for a detailed investigation on this sea water mixing
processes. Samples at increasing distance the sea and with
different salinity were collected for chemical and isotopic analyses. reservoir temperatures thus predicted for most of the geothermal
1040
Pang and Wang
systems in the SE China are between 100 -- 120 C with the highest
temperature (140 -- 150 C) at Zhangzhou Geothermal Field (Pang,
Sulfate-water geothermometer yields very similar
temperature (Pang, et al., 1995) (Figure 5).
Alteration minerals found frequently in the field are: epidote, quartz,
calcite, laumontite, prehnite, pyrite, opal, fluorite, and chlorite, etc.
The sequence of occurrence of these minerals represent a thermal
evolution from high temperature (-300°C) to an inter-medium
temperature (-150°C) ofthe present.
Residence Time of the Thermal Water
The residence time of the geothermal fluid is defined as the time since
it was last isolated from atmosphere. Radioactive isotopes tritium
t Zhangzhou Basin
.
Temperature ("C)
Figure 6. Tritium contents of the thermal water and other natural
waters from Zhangzhou basin.
data for most of the low temperature minerals, especially the clay
minerals, are not which prevents the method from wider
application in this type of systems.
Most of the gothermometers are empirical methods and involve may
assumptions 1989, 1990). temperatures can be
indicated using different geothennometers with the same set of
geochemical data. A particular may be suitable to
one place but not another. This deficiency prevents geothermometers
from more effective application and causes considerable problems
sometimes.
A theoretical calibration approach was proposed by Pang b).
The result was interesting for it shows reasonable mineral assemblage
at equilibrium with the thermal water and reasonable agreement
between temperatures fiom the equilibrium modeling and ftom some
chemical geothermometers. In this way, conventional
geothermometers can be selected and used later on for the
investigation of other geothermal systems of the same type.
However, attention must be paid to the salinity of the water.
Geothennometers suitable to a dilute geothennal system must be
used for saline thermal waters. As mentioned above, the
high salinity comes from the mixing with sea water. Applicability of
conventional geothermometers to this type of systems is different
from applicability to water systems. For these systems,
modeling of the fluid-mineral equilibrium shows that there is no
overall equilibrium. Most cation geothermometers failed . However,
the influence of the on the silica geothennometers my be
neglected, so they are still useful. In Zhangzhou geothermal field,
with consideration of the mixing of deep saline thermal water with
shallow cold ground water, the reservoir temperature obtained by
using chalcedony geothermometer is 140 C (Pang, et al.,
Oxygen 18 and deuterium are goodtracers for the origin of the
water and for the salinity in the case of marine origin of the thermal
water. Other natural waters as well as the thermal water must be
studied in order to draw a completed picture of the water circulation
system and to extract more comprehensive information on the
hydrology ofthe system (Pang, et al.,
Tritium is more useful as a tracer rather than as a dating tool. The
time range is limited to 100 years, which is usually too short for
most of the geothermal systems. But it is a quite good tool for
distinguishing recent waters old ones. It is also a good tracer of
and carbon - 14 were use for dating of geothermal water and other
natural waters in the study areas. (Figure 6).
Geothermal parent waters are tritium free. The presence of tritium in
the thermal water is an indicator of dilution by the shallow
ground water. Tritium was found to be quite effective for the
investigation on the relationship between different natural waters.
However, it seems not to be a good dating method because the age
of geothermal water is far beyond the limit (about 100 year) of the
tritium method. Thermal water from hot spring to the west of
Zhangzhou Basin was dated using carbon - 14 method and the result
turned out to be 1,755 360 years old. However, carbon 14 dating
could not be applied to the saline thermal water because of its low
carbon content.
4.2 About the Methodology
the flow path of the thermal water and of mixing with other waters
(Pang et al., Carbon - 14 is a good dating tool many
situations, especially in the granite terrain. But conventional methods
require too large amount of carbon, which causes great for
some saline thermal waters of low carbon contents. Accelerator
mass spectrometers should be used particularly in these cases.
34 in sulfate dissolved in thermal waters is a good indicator
of the origin of the salinity of the water. This is particularly true in
the case of marine sulfate, whose isotope ratio is very constant.
Oxygen- 18 in sulfate is a good geothermometer even for low
temperature geothermal system. Chemical methods should be
combined with the isotope studies so as to reconstruct the evolution
history ofthe sulfur species (Pang et al., 1995).
5 . CONCLUSIONS
Modeling has proved useful in drawing conclusions about the fluid
selection of conventional chemical geothermometers. However, the
1) Geochemical and isotopic methods have been found to be very
1041
Pang and Wang
Zhangzhou geothermal field and surrounding systems in the coastal
area of southeast China, including the saline thermal waters close to
the coast.
2) It can be concluded the studies on the thermal water and
gases dissolved in it that the geothermal systems under investigation
are formed by the deep circulation of the meteoric water and that
they all belong to the low-medium temperature category. The
reservoir temperatures may not exceed 150 within the depth of
interest.
3) Fluid mineral equilibrium modeling helped the selection and
improved the use of conventional chemical
4) Deuterium, oxygen - 18 and tritium contents of water are
tracers for the meteoric origjn of the water and the marine origjn of
the salinity in the saline thermal waters.
5) Carbon - 14 is a good dating tool for geothermal systems in the
granite terrain. dissolved carbon is available for analysis.
6) Sulfur and oxygen isotopes of dissolved sulfate are good tracers
of the origjn of the salinity in the case of marine sulfate. The sulfate
water geothermometer gives correct reservoir temperature
prediction even for low temperature systems.
ACKNOWLEDGMENTS
Present investigation has been supported financially by the
National Natural Science Foundation of China (Grant No. 49000034)
and with partial support fiom (International Atomic Energy
Agency). Thanks are also due to Truesdell and another
anonymous reviewer for their comments and suggestions on
the manuscript.
REFERENCES
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