geothermal exploration methods

1. GEOCHEMISTRY OF
GEOTHERMAL SYSTEMS

2. WATER CHEMISTRY
• Chemical composition of waters is expressed in terms of major anion and
cation contents.
• Major Cations: Na+, K+, Ca++, Mg++
• Major Anions: HCO3
- (or CO3
=), Cl-, SO4
=
• HCO3
-  dominant in neutral conditions
• CO3
=  dominant in alkaline (pH>8) conditions
• H2CO3  dominant in acidic conditions
• Also dissolved silica (SiO2) in neutral form
as a major constituent
• Minor constituents: B, F, Li, Sr, ...

3. WATER CHEMISTRY
• concentration of chemical constituents are expressed in units of
mg/l (ppm=parts per million)
(mg/l is the preferred unit)
Molality
Molality = no. of moles / kg of solvent
No.of moles = (mg/l*10-3) / formula weight

4. • Errors associated with water analyses are expressed in terms of CBE
(Charge Balance Error)
CBE (%) = ( z x mc - z x ma ) / (z x mc + z x ma )* 100
where,
mc is the molality of cation
ma is the molality of anion
z is the charge
• If CBE  5%, the results are appropriate to use in any kind of interpretation
WATER CHEMISTRY

5. The constituents encountered in geothermal fluids
TRACERS
Chemically inert, non-reactive, conservative constituents
(once added to the fluid phase, remain unchanged allowing their origins to be traced back to
their source component - used to infer about the source characteristics)
e.g. He, Ar (noble gases), Cl, B, Li, Rb, Cs, N2
GEOINDICATORS
Chemically reactive, non-conservative species
(respond to changes in environment - used to infer about the physico-chemical processes
during the ascent of water to surface, also used in geothermometry applications)
e.g. Na, K, Mg, Ca, SiO2

6. • In this chapter, the main emphasis will be placed on the
use of water chemistry in the determination of :
underground (reservoir) temperatures : geothermometers
boiling and mixing relations (subsurface physico-chemical
processes)
WATER CHEMISTRY

7. HYDROTHERMAL REACTIONS
• The composition of geothermal fluids are controlled by :
temperature-dependent reactions between minerals and
fluids
• The factors affecting the formation of hydrothermal
minerals are:
 temperature
 pressure
 rock type
 permeability
 fluid composition
 duration of activity

8. • The effect of rock type --- most pronounced at low temperatures &
insignificant above 280C
• Above 280C and at least as high as 350C, the typical stable mineral
assemblages (in active geothermal systems) are independent of rock
type and include
 ALBITE, K-FELDSPAR, CHLORITE, Fe-EPIDOTE, CALCITE, QUARTZ, ILLITE &
PYRITE
• At lower temperatures, ZEOLITES and CLAY MINERALS are found.
• At low permeabilities equilibrium between rocks and fluids is seldom
achieved.
• When permeabilities are relatively high and water residence times are
long (months to years), water & rock should reach chemical
equilibrium.

9. At equilibrium, ratios of cations in solution are controlled by temperature-dependent exchange reactions such as:
NaAlSi3O8 (albite) + K+ = KAlSi3O8 (K-felds.) + Na+
Keq. =  Na+ /  K+
Hydrogen ion activity (pH) is controlled by hydrolysis reactions, such as :
3 KAlSi3O8 (K-felds.) + 2 H+ = K Al3Si3O10(OH)2 (K-mica)+ 6SiO2 + 2 K+
Keq. =  K+ /  H+
where,
Keq. = equilibrium constant,
square brackets indicate activities of dissolved species (activity is unity for pure solid phases)

10. ESTIMATION OF RESERVOIR TEMPERATURES
The evaluation of the reservoir temperatures for
geothermal systems is made in terms of
GEOTHERMOMETRY APPLICATIONS

11. GEOTHERMOMETRY
APPLICATIONS

12. GEOTHERMOMETRY APPLICATIONS
•One of the major tools for the exploration
& development of geothermal resources

13. GEOTHERMOMETRY
estimation of reservoir (subsurface)
temperatures
using
Chemical & isotopic composition of
surface discharges from
 wells and/or
 natural springs/fumaroles

14. GEOTHERMOMETERS
• CHEMICAL GEOTHERMOMETERS
 utilize the chemical composition
 silica and major cation contents of water discharges
 gas concentrations or relative abundances of gaseous components in
steam discharges
• ISOTOPIC GEOTHERMOMETERS
 based on the isotope exchange reactions between various phases (water, gas,
mineral) in geothermal systems

15. Focus of the Course
CHEMICAL GEOTHERMOMETERS
As applied to water discharges
PART I. Basic Principles & Types
PART II. Examples/Problems

16. CHEMICAL
GEOTHEROMOMETERS
PART I. Basic Principles &Types

17. BASIC PRINCIPLES
Chemical Geothermometers are
• developed on the basis of temperature dependent
chemical equilibrium between the water and the
minerals at the deep reservoir conditions
• based on the assumption that the water preserves its
chemical composition during its ascent from the
reservoir to the surface

18. • Studies of well discharge chemistry and alteration
mineralogy
the presence of equilibrium in several
geothermal fields
the assumption of equilibrium is valid
BASIC PRINCIPLES

19. • Assumption of the preservation of water chemistry may
not always hold
Because the water composition may be affected by
processes such as
cooling
mixing with waters from different reservoirs.
BASIC PRINCIPLES

20. •Cooling during ascent from
reservoir to surface:
CONDUCTIVE
ADIABATIC
BASIC PRINCIPLES

21. CONDUCTIVE Cooling
Heat loss while travelling through cooler rocks
ADIABATIC Cooling
Boiling because of decreasing hydrostatic head
BASIC PRINCIPLES

22. •Conductive cooling
does not by itself change the composition of
the water
but may affect its degree of saturation with
respect to several minerals
thus, it may bring about a modification in the
chemical composition of the water by mineral
dissolution or precipitation
BASIC PRINCIPLES

23. •Adiabatic cooling (Cooling by boiling)
causes changes in the composition of
ascending water
these changes include
degassing, and hence
the increase in the solute content as a result
of steam loss.
BASIC PRINCIPLES

24. MIXING
• affects chemical composition
• since the solubility of most of the compounds in waters increases
with increasing temperature, mixing with cold groundwater results in
the dilution of geothermal water
BASIC PRINCIPLES

25. • Geothermometry applications are not simply inserting
values into specific geothermometry equations.
• Interpretation of temperatures obtained from
geothermometry equations requires a sound
understanding of the chemical processes involved in
geothermal systems.
• The main task of geochemist is to verify or disprove the
validity of assumptions made in using specific
geothermometers in specific fields.

26. TYPES OF CHEMICAL GEOTHERMOMETERS
•SILICA GEOTHERMOMETERS
•CATION GEOTHERMOMETERS (Alkali
Geothermometers)

27. SILICA GEOTHERMOMETERS
• based on the
experimentally determined
temperature dependent
variation in the solubility of silica in water
• Since silica can occur in various forms in geothermal fields (such as
quartz, crystobalite, chalcedony, amorphous silica) different silica
geothermometers have been developed by different workers

28. Geothermometer Equation Reference
Quartz-no steam loss T = 1309 / (5.19 – logC) - 273.15 Fournier (1977)
Quartz-maximum steam loss at
100 oC
T = 1522 / (5.75 - logC) - 273.15 Fournier (1977)
Quartz T = 42.198 + 0.28831C - 3.6686 x 10-4C2 + 3.1665 x 10-7C3 + 77.034 logC Fournier and Potter
(1982)
Quartz T = 53.500 + 0.11236C - 0.5559 x 10-4C2 + 0.1772 x 10-7C3 + 88.390 logC Arnorsson (1985) based
on Fournier and Potter
(1982)
Chalcedony T = 1032 / (4.69 - logC) - 273.15 Fournier (1977)
Chalcedony T = 1112 / (4.91 - logC) - 273.15 Arnorsson et al. (1983)
Alpha-Cristobalite T = 1000 / (4.78 - logC) - 273.15 Fournier (1977)
Opal-CT
(Beta-Cristobalite)
T = 781 / (4.51 - logC) - 273.15 Fournier (1977)
Amorphous silica T = 731 / (4.52 - logC) - 273.15 Fournier (1977)
SILICA
GEOTHERMOMETERS

29. The followings should be considered :
• temperature range in which the equations are valid
• effects of steam separation
• possible precipitation of silica
before sample collection
(during the travel of fluid to surface, due to silica oversaturation)
after sample collection
(due to improper preservation of sample)
• effects of pH on solubility of silica
• possible mixing of hot water with cold water
SILICA GEOTHERMOMETERS

30. Temperature Range
• silica geothermometers are valid for temperature ranges up to 250
C
• above 250C, the equations depart drastically from the
experimentally determined solubility curves
SILICA GEOTHERMOMETERS

31. SILICA GEOTHERMOMETERS Temperature Range
Fig.1. Solubility of quartz (curve A) and amorphous silica
(curve C) as a function of temperature at the vapour pressure
of the solution. Curve B shows the amount of silica that would
be in solution after an initially quartz-saturated solution
cooled adiabatically to 100 C without any precipitation of
silica (from Fournier and Rowe, 1966, and Truesdell and
Fournier, 1976).
At low T (C) 
qtz less soluble
amorph. silica more soluble
Silica solubility is controlled by amorphous silica at
low T (C) quartz at high T (C)

32. SILICA GEOTHERMOMETERS Effects of Steam Separation
• Boiling  Steam Separation
• volume of residual liquid
• Concentration in liquid
• Temperature Estimate
e.g.
T = 1309 / (5.19 – log C) - 273.15
C = SiO2 in ppm
increase in C (SiO2 in water > SiO2 in reservoir) decrease in
denominator of the equation increase in T
for boiling springs boiling-corrected geothermometers
(i.e. Quartz-max. steam loss)
SiO2
liquid V1
(1)
V2 V1
<
SiO2 (2) SiO2 (1)
>
liquid V2
SiO2 (2)
steam

33. SILICA GEOTHERMOMETERS Silica Precipitation
• SiO2 
• Temperature Estimate
e.g.
T = 1309 / (5.19 – log C) - 273.15
C = SiO2 in ppm
decrease in C (SiO2 in water < SiO2 in reservoir)
increase in denominator
decrease in T

34. SILICA GEOTHERMOMETERS Effect of pH
Fig. 2. Calculated effect of pH upon the solubility of quartz at various
temperatures from 25 C to 300 C , using experimental data of Seward
(1974). The dashed curve shows the pH required at various temperatures to
achieve a 10% increase in quartz solubility compared to the solubility at
pH=7.0 (from Fournier, 1981).
• pH 
• Dissolved SiO2  (for pH>7.6)
• Temperature Estimate
e.g.
T = 1309 / (5.19 – log C) - 273.15
C = SiO2 in ppm
increase in C
decrease in denominator of the equation
increase in T

35. SILICA GEOTHERMOMETERS Effect of Mixing
• Hot-Water  High SiO2 content
• Cold-Water  Low SiO2 content
(Temperature  Silica solubility )
• Mixing (of hot-water with cold-water)
• Temperature
• SiO2 
• Temperature Estimate 
e.g.
T = 1309 / (5.19 – log C) - 273.15
C = SiO2 in ppm
decrease in C
increase in denominator of the equation
decrease in T

36. SILICA GEOTHERMOMETERS
Process Reservoir Temperature
• Steam Separation  Overestimated
• Silica Precipitation  Underestimated
• Increase in pH  Overestimated
• Mixing with cold water  Underestimated

37. CATION GEOTHERMOMETERS (Alkali Geothermometers)
based on the partitioning of alkalies between solid
and liquid phases
e.g. K+ + Na-feldspar = Na+ + K-feldspar
majority of are empirically developed
geothermometers
 Na/K geothermometer
 Na-K-Ca geothermometer
 Na-K-Ca-Mg geothermometer
 Others (Na-Li, K-Mg, ..)

38. CATION GEOTHERMOMETERS Na/K Geothermometer
•
Fig.3. Na/K atomic ratios of well
discharges plotted at measured downhole
temperatures. Curve A is the least square
fit of the data points above 80 C. Curve B
is another empirical curve (from
Truesdell, 1976). Curves C and D show the
approximate locations of the low albite-
microcline and high albite-sanidine lines
derived from thermodynamic data (from
Fournier, 1981).

39. CATION GEOTHERMOMETERS Na/K Geothermometer
Geotherm. Equations Reference
Na-K T=[855.6/(0.857+log(Na/K))]-273.15 Truesdell (1976)
Na-K T=[833/(0.780+log(Na/K))]-273.15 Tonani (1980)
Na-K T=[933/(0.993+log (Na/K))]-273.15
(25-250 oC)
Arnorsson et al.
(1983)
Na-K T=[1319/(1.699+log(Na/K))]-273.15
(250-350 oC)
Arnorsson et al.
(1983)
Na-K T=[1217/(1.483+log(Na/K))]-273.15 Fournier (1979)
Na-K T=[1178/(1.470+log (Na/K))]-273.15 Nieva and Nieva
(1987)
Na-K T=[1390/(1.750+log(Na/K))]-273.15 Giggenbach (1988)

40. CATION GEOTHERMOMETERS
Na/K Geothermometer
gives good results for reservoir temperatures above
180C.
yields erraneous estimates for low temperature
waters
temperature-dependent exchange equilibrium between
feldspars and geothermal waters is not attained at low
temperatures and the Na/K ratio in these waters are
governed by leaching rather than chemical equilibrium
yields unusually high estimates for waters having
high calcium contents

41. CATION GEOTHERMOMETERS
Na-K-Ca Geothermometer
Geotherm. Equations Reference
Na-K-Ca T=[1647/ (log (Na/K)+  (log (Ca/Na)+2.06)+ 2.47)]
-273.15
a) if logCa/Na)+2.06 < 0, use =1/3 and calculate TC
b) if logCa/Na)+2.06 > 0, use =4/3 and calculate TC
c) if calculated T > 100C in (b), recalculate TC using =1/3
Fournier
and
Truesdell
(1973)

42. CATION GEOTHERMOMETERS
Na-K-Ca Geothermometer
• Works well for CO2-rich or Ca-rich environments provided that
calcite was not deposited after the water left the reservoir
in case of calcite precipitation
Ca 
1647
T = --------------------------------------------------------- - 273.15
log (Na/K)+  (log (Ca/Na)+2.06)+ 2.47
Decrease in Ca concentration (Ca in water < Ca in reservoir)
decrease in denominator of the equation
increase in T
• For waters with high Mg contents, Na-K-Ca geothermometer yields
erraneous results. For these waters, Mg correction is necessary

43. CATION GEOTHERMOMETERS
Na-K-Ca-Mg Geothermometer
Geotherm. Equations Reference
Na-K-Ca-Mg T = TNa-K-Ca - tMg
oC
R = (Mg / Mg + 0.61Ca + 0.31K) x 100
if R from 1.5 to 5
tMg
oC = -1.03 + 59.971 log R + 145.05 (log R)2 – 36711
(log R)2 / T - 1.67 x 107 log R / T2
if R from 5 to 50
tMg
oC=10.66-4.7415 log R+325.87(log R)2-
1.032x105(log R)2/T-1.968x107(log R)3/T2
Note: Do not apply a Mg correction if tMg is negative
or R<1.5.
If R>50, assume a temperature = measured spring
temperature.
T is Na-K-Ca geothermometer temperature in Kelvin
Fournier
and Potter
(1979)

44. CATION GEOTHERMOMETERS
Na-K-Ca-Mg Geothermometer
Fig. 4. Graph for estimating
the magnesium temperature
correction to be subtracted
from Na-K-Ca calculated
temperature (from Fournier,
1981)
R = (Mg/Mg + 0.61Ca + 0.31K)x100

45. UNDERGROUND MIXING OF HOT AND COLD
WATERS
Recognition of Mixed Waters
• Mixing of hot ascending waters with cold waters at shallow depths
is common.
• Mixing also occurs deep in hydrothermal systems.
• The effects of mixing on geothermometers is already discussed in
previous section.
• Where all the waters reaching surface are mixed waters,
recognition of mixing can be difficult.
• The recognition of mixing is especially difficult if water-rock re-
equilibration occurred after mixing (complete or partial re-
equilibration is more likely if the temperatures after mixing is well
above 110 to 150 C, or if mixing takes place in aquifers with long
residence times).

46. UNDERGROUND MIXING OF HOT AND COLD
WATERS
Some indications of mixing are as follows:
• systematic variations of spring compositions and
measured temperatures,
• variations in oxygen or hydrogen isotopes,
• variations in ratios of relatively *conservative elements
that do not precipitate from solution during movement
of water through rock (e.g. Cl/B ratios).

47. SILICA-ENTHALPY MIXING MODEL
• Dissolved silica content of mixed waters can be used to
determine the temperature of hot-water component .
• Dissolved silica is plotted against enthalpy of liquid water.
• Although temperature is the measured property, and
enthalphy is a derived property, enthalpy is used as a
coordinate rather than temperature. This is because the
combined heat contents of two waters are conserved when
those waters are mixed, but the combined temperatures are
not.
• The enthalpy values are obtained from steam tables.

48. SILICA-ENTHALPY MIXING MODEL
Fig. 5. Dissolved silica-
enthalpy diagram showing
procedure for calculating the
initial enthalpy (and hence the
reservoir temperature) of a
high temperature water that
has mixed with a low
temperature water (from
Fournier, 1981)

49. SILICA-ENTHALPY MIXING MODEL
A = non-thermal component
(cold water)
B, D = mixed, warm water
springs
C = hot water component at
reservoir conditions
(assuming no steam
separation before mixing)
E = hot water component at
reservoir conditions
(assuming steam separation
before mixing)
Boiling
T = 100 C
Enthalpy = 419 J/g
(corresponds to D in the graph)
Enthalpy values (at corresponding temperatures)
are found from Steam Table in Henley et al.(1984)
419 J/g
(100 C)
0

50. SILICA-ENTHALPY MIXING MODEL
Steam Fraction did not separate before mixing
• The sample points are plotted.
• A straight line is drawn from the
point representing the non-
thermal component of the mixed
water (i.e. the point with the
lowest temperature and the lowest
silica content = point A in Fig.),
through the mixed water warm
springs (points B and D in Fig.).
• The intersection of this line with
the qtz solubility curve (point C in
Fig.) gives the enthalpy of the hot-
water component (at reservoir
conditions).
• From the steam table, the
temperature corresponding to this
enthalpy value is obtained as the
reservoir temperature of the hot-
water component.
419 J/g
(100 C)
0

51. SILICA-ENTHALPY MIXING MODEL
Steam separation occurs before mixing
• The enthalpy at the boling
temperature (100C) is obtained
from the steam tables (which is
419 j/g)
• A vertical line is drawn from the
enthalpy value of 419 j/g
• From the inetrsection point of this
line with the mixing line (Line AD),
a horizantal line (DE) is drawn.
• The intersection of line DE with the
solubility curve for maximum
steam loss (point E) gives the
enthalpy of the hot-water
component.
• From the steam tables, the
reservoir temperature of the hot-
water component is determined. 419 J/g
(100 C)
0

52. SILICA-ENTHALPY MIXING MODEL
• In order for the silica mixing model to give accurate results, it is vital
that no conductive cooling occurred after mixing. If conductive cooling
occurred after mixing, then the calculated temperatures will be too
high (overestimated temperatures). This is because:
• the original points before conductive cooling should lie to the right of
the line AD (i.e. towards the higher enthalpy values at the same silica
concentrations, as conductive cooling will affect only the temperatures,
not the silica contents)
• in this case, the intersection of mixing line with the quartz solubility
curve will give lower enthalpy values (i.e lower temperatures) than that
obtained in case of conductive cooling.
• in other words, the temperatures obtained in case of conductive
cooling will be higher than the actual reservoir temperatures (i.e. if
conductive cooling occurred after mixing, the temperatures will be
overestimated)

53. SILICA-ENTHALPY MIXING MODEL
• Another requirement for the use of enthalpy-silica model is that no silica deposition
occurred before or after mixing. If silica deposition occurred, the temperatures will
be underestimated. This is because:
• the original points before silica deposition should be towards higher silica contents
(at the same enthalpy values)
• in this case, the intersection point of mixing line with the silica solubility curve will
have higher enthalpy values(higher temperatures) than that obtained in case of
silica deposition
• in other words, the temperatures obtained in case of no silica deposition will be
higher than that in case of silica deposition (i.e. the temperatures will be
underestimated in case of silica deposition)

54. CHLORIDE-ENTHALPY MIXING MODEL
• Fig.6. Enthalpy-chloride
diagram for waters from
Yellowstone National Park.
Small circles indicate Geyser
Hill-type waters and smal dots
indicate Black Sand-type
waters (From Fournier, 1981).

55. CHLORIDE-ENTHALPY MIXING MODEL
ESTIMATION OF RESERVOIR
TEMPERATURE
• Geyser Hill-type Waters
A = maximum Cl content
B = minimum Cl content
C = minimum enthalpy at
the reservoir
• Black Sand-type Waters
D = maximum Cl content
E = minimum Cl content
F = minimum enthalpy at
the reservoir
Enthalpy of steam at 100 C =
2676 J/g (Henley et al., 1984)

56. CHLORIDE-ENTHALPY MIXING MODEL
ORIGIN OF WATERS
• N = cold water component
• C, F = hot water components
• F is more dilute & slightly cooler
than C
• F can not be derived from C by
process of mixing between hot
and cold water (point N), because
any mixture would lie on or close
to line CN.
C and F are probably both related
to a still higher enthalpy water
such as point G or H.

57. CHLORIDE-ENTHALPY MIXING MODEL
ORIGIN OF WATERS
• water C could be related to
water G by boiling
• water C could also be related
to water H
by conductive cooling
• water F could be related to
water G or water H by mixing
with cold water N

58. steam
steam
B E
D
C
G
F
H
N
H
cold water reservoir
hot water reservoir
steam
hot water
mixed water
residual liquid from boiling
B
hot water undergoing
conductive cooling
mixed water undergoing
conductive cooling
residual liquid undergoing
conductive cooling

60. ISOTOPES
IN
GEOTHERMAL
EXPLORATION
& DEVELOPMENT

61. ISOTOPE STUDIES IN GEOTHERMAL
SYSTEMS
• At Exploration, Development and Exploitation Stages
• Most commonly used isotopes
• Hydrogen (1H, 2H =D, 3H)
• Oxygen (18O, 16O)
• Sulphur (32S, 34S)
• Helium (3He, 4He)

62. ISOTOPE STUDIES IN GEOTHERMAL
SYSTEMS
Geothermal Fluids
• Sources
• Source of fluids (meteoric, magmatic, ..)
• Physico-chemical processes affecting the fluid comosition
• Water-rock interaction
• Evaporation
• Condensation
• Source of components in fluids (mantle, crust,..)
• Ages
(time between recharge-discharge, recharge-sampling)
• Temperatures (Geothermometry Applications)

63. Sources of Geothermal Fluids
• Sources of Geothermal Fluids
H- & O- Isotopes
• Physico-chemical processes affecting the fluid
composition
H- & O- Isotopes
• Sources of components (elements, compounds) in
geothermal fluids
He-Isotopes (volatile elements)

64. Sources of Geothermal Fluids and
Physico-Chemical Processes
STABLE
H- & O-ISOTOPES

65. Sources of Geothermal Fluids
Stable H- & O-Isotopes
1H = % 99.9852
2H (D) = % 0.0148
D/H
16O = % 99.76
17O = % 0.04
18O = % 0.20
18O / 16O

66. Sources of Geothermal Fluids
Stable H- & O-Isotopes
(D/H)sample- (D/H)standard
 D ( ) = ----------------------------------- x 103
(D/H)standard
(18O/16O)sample- (18O/16O)standard
 18O ( ) = -------------------------------------------- x 103
(18O/16O)standard
Standard = Standard Mean Ocean Water
= SMOW

67. Sources of Geothermal Fluids
Stable H- & O-Isotopes
(D/H)sample- (D/H)SMOW
 D ( ) = ----------------------------------- x 103
(D/H)SMOW
(18O/16O)sample- (18O/16O)SMOW
 18O ( ) = -------------------------------------------- x 103
(18O/16O)SMOW

68. Sources of Geothermal Fluids
Stable H- & O-Isotopes
Sources of Natural Waters:
1. Meteoric Water (rain, snow)
2. Sea Water
3. Fossil Waters (trapped in sediments in sedimanary basins)
4. Magmatic Waters
5. Metamorphic Waters

69. Sources of Geothermal Fluids
Stable H- & O-Isotopes
0
0
-40
-80
-120
10 20 30
-10
-20
 O (per mil)
18
D (per mil)
+
SMOW
Field of
Formation
Waters
Magmatic
Waters
Most igneous
biotites &
hornblendes
Metamorphic
Waters

70. Sources of Geothermal Fluids
Stable H- & O-Isotopes
Ocean
Seepage
precipitation
evaporation
River
H, O
1 16
H, O
1 16
D, O
18
D, O
18
D, O
18 H, O
1 16
H, O
1 16 D, O
18
D, O
18
(D/H) < (D/H)
vapor water
vapor
<
18 16
O / O
( ) 18 16
O / O
( )water
precipitation

71. Sources of Geothermal Fluids
Stable H- & O-Isotopes
0
-40
-80
-120
-12 -8 -4 0
del- O (per mil)
18
+
SMOW
Condensation
Evaporation
Water-Rock
Interaction

72. Sources of Geothermal Fluids
Stable H- & O-Isotopes
18
Magmatik
Sular
0
-50
-100
-150
-15 -10 -5 0 +5 +10
Larderello
The Geysers
Iceland
Niland
Lassen Park
Steamboat Kaynakları
 O (per mil)
D (per mil)

73. Physico-Chemical Processes:
Stable H- & O-Isotopes
•Latitute 
•D 18O
•Altitute from Sea level 
•D 18O

74. Physico-Chemical Processes:
Stable H- & O-Isotopes
• Aquifers recharged by precipitation from lower altitutes
higher D - 18O values
Aquifers recharged by precipitation from higher altitutes
lower D - 18O values
Mixing of waters from different aquifers

75. Physico-Chemical Processes:
Stable H- & O-Isotopes
• Boiling and vapor separation 
D 18O in residual liquid
Possible subsurface boiling as a consequence of
pressure decrease (due to continuous exploitation
from production wells)

76. Monitoring Studies in
Geothermal Exploitation
• Aquifers recharged by
precipitation from lower
altitutes higher D - 18O
• Aquifers recharged by
precipitation from higher
altitutes lower D - 18O
• Boiling and vapor
separation 
D 18O in residual
liquid
• Any increase in D - 18O values

due to sudden pressure drop in
production wells
recharge from (other)
aquifers fed by
precipitation from lower
altitutes
subsurface boiling and
vapour separation

77. Monitoring Studies in
Geothermal Exploitation
• Monitoring of isotope composition of geothermal
fluids during exploitation can lead to determination
of, and the development of necessary precautions
against
• Decrease in enthalpy due to start of recharge
from cold, shallow aquifers, or
• Scaling problems developed as a result of
subsurface boiling

78. (Scaling)
Vapour Separation
Volume of (residual) liquid 
Concentration of dissolved components in liquid 
Liquid will become oversaturated
Component (calcite, silica, etc.) will precipitate
Scaling

79. Dating of Geothermal Fluids
3H- & 3He-ISOTOPES

80. Dating of Geothermal Fluids
• Time elapsed between Recharge-Discharge or Recharge-Sampling
points (subsurface residence residence time)
•3H method
•3H-3He method

81. TRITIUM (3H)
• 3H = radioactive isotope of Hydrogene (with a short half-life)
• 3H forms
Reaction of 14N isotope (in the atmosphere) with cosmic rays
14
7N + n  3
1H + 12
6C
Nuclear testing
• 3H concentration
Tritium Unit (TU)
1 TU = 1 atom 3H / 1018 atom H
• 3H  3He + 
• Half-life = 12.26 year
• Decay constant () = 0.056 y-1

82. 3H – Dating Method
3H concentration level in the atmosphere has
shown large changes
• İn between 1950s and 1960s (before and after the
nuclear testing)
• Particularly in the northern hemisphere
Before 1953 : 5-25 TU
In 1963 : 3000 TU

83. 3H – Dating Method
• 3H-concentration in groundwater < 1.1 TU
Recharge by precipitations older than nuclear testing
• 3H-concentration in groundwater > 1.1 TU
Recharge by precipitations younger than nuclear testing
N=N0e-t 3H0 (before 1963)  10 TU
3H= 3H0e-t  = 0.056 y-1
t = 2003-1963 = 40 years
 3H  1.1 TU

84. 3H – Dating Method
• APPARENT AGE
3H= 3H0e-t
3H = measured at sampling point
3H0 = measured at recharge point
(assumed to be the initial tritium concentration)
 = 0.056 y-1
t = apparent age

85. 3H – 3He
Dating Method
• 3He = 3H0 – 3H (D = N0-N)
• 3H= 3H0 e-t (N =N0e -t)
• 3H0= 3H et
• 3He = 3H et - 3H = 3H (et – 1)
t = 1/ * ln (3He/3H + 1)
3He & 3H – present-day concentrations measured in water sample

86. Geothermometry Applications
• Isotope Fractionation – Temperature Dependent
• Stable isotope compositions 
utilized in Reservoir Temperature estimation
• Isotope geothermometers
• Based on: isotope exchange reactions between phases in natural
systems
(phases: watre-gas, vapor-gas, water-mineral.....)
• Assumes: reaction is at equilibrium at reservoir conditions

87. Isotope Geothermometers
12CO2 + 13CH4 = 13CO2 + 12CH4 (CO2 gas - methane gas)
CH3D + H2O = HDO + CH4 (methane gas – water vapor)
HD + H2O = H2 + HDO (H2 gas – water vapor)
S16O4 + H2
18O = S18O4 + H2
16O (dissolved sulphate-water)

1000 ln  (SO4 – H2O) = 2.88 x 106/T2 – 4.1
(T = degree Kelvin = K )

88. Isotope Geothermometers
• Regarding the relation between mineralization and
hydrothermal activities
• Mineral Isotope Geothermometers
• Based on the isotopic equilibrium between the
coeval mineral pairs
• Most commonly used isotopes: S-isotopes

89. Suphur (S)- Isotopes
• 32S = 95.02 %
• 33S = 0.75 %
• 34S = 4.21 %
• 36S = 0.02 %
(34S/32S)sample- (34S/32S)std.
 34S () = -------------------------------------------- x 103
(34S/32S)sample
Std.= CD
=S-isotope composition of troilite (FeS) phase in Canyon Diablo Meteorite

90. S-Isotope Geothermometer
34S = 34S(mineral 1) - 34S(mineral 2)
34S = 34S= A (106/T2) + B

91. Pyrite-Galena
800 400 200 150 100 50
0
4
8
12
4
8
0
0
0 2 4
4
2
6 8 10 12
Sphalerite-Galena
Pyrite-Sphalerite
Temperature C
0
10 / T ( K )
6 2 -2
0