Transcript GEOTHERMOMETRY APPLICATIONS - Middle East Technical University

GEOCHEMISTRY OF
GEOTHERMAL SYSTEMS
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, ...
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
WATER CHEMISTRY
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
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
WATER CHEMISTRY
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)
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
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.
At equilibrium, ratios of cations in solution are controlled by temperaturedependent 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)
ESTIMATION OF
RESERVOIR
TEMPERATURES
The evaluation of the
reservoir temperatures for
geothermal systems is made
in terms of
GEOTHERMOMETRY
APPLICATIONS
GEOTHERMOMETRY
APPLICATIONS
GEOTHERMOMETRY
APPLICATIONS
One of the major tools for the 
exploration & development
of geothermal resources
GEOTHERMOMETRY
estimation of reservoir (subsurface)
temperatures
using
Chemical & isotopic composition of
surface discharges from
 wells and/or
 natural springs/fumaroles
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
Focus of the Course
CHEMICAL GEOTHERMOMETERS
As applied to water discharges
PART I. Basic Principles & Types
PART II. Examples/Problems
CHEMICAL
GEOTHEROMOMETERS
PART I. Basic Principles &Types
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
BASIC PRINCIPLES
Studies of well discharge chemistry and
alteration mineralogy
the presence of equilibrium in several
geothermal fields
the assumption of equilibrium is valid
BASIC PRINCIPLES
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
Cooling during ascent from
reservoir to surface:
CONDUCTIVE
ADIABATIC
BASIC PRINCIPLES
CONDUCTIVE Cooling
 Heat loss while travelling through cooler
rocks
ADIABATIC Cooling
 Boiling because of decreasing hydrostatic
head
BASIC PRINCIPLES
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
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
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
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.
TYPES OF CHEMICAL
GEOTHERMOMETERS
SILICA GEOTHERMOMETERS
CATION GEOTHERMOMETERS
(Alkali Geothermometers)
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
SILICA GEOTHERMOMETERS
Geothermometer
Equation
Reference
Quartz-no steam loss
T = 1309 / (5.19 – log C) - 273.15
Fournier (1977)
Quartz-maximum
steam loss at 100 oC
T = 1522 / (5.75 - log C) - 273.15
Fournier (1977)
Quartz
T = 42.198 + 0.28831C - 3.6686 x 10-4 C2 + Fournier and
3.1665 x 10-7 C3 + 77.034 log C
Potter (1982)
Quartz
T = 53.500 + 0.11236C - 0.5559 x 10-4 C2 + Arnorsson
0.1772 x 10-7 C3 + 88.390 log C
(1985) based on
Fournier and
Potter (1982)
Chalcedony
T = 1032 / (4.69 - log C) - 273.15
Fournier (1977)
Chalcedony
T = 1112 / (4.91 - log C) - 273.15
Arnorsson et al.
(1983)
Alpha-Cristobalite
T = 1000 / (4.78 - log C) - 273.15
Fournier (1977)
Opal-CT
(Beta-Cristobalite)
T = 781 / (4.51 - log C) - 273.15
Fournier (1977)
Amorphous silica
T = 731 / (4.52 - log C) - 273.15
Fournier (1977)
SILICA GEOTHERMOMETERS
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
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
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)
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)
steam
SiO2
SiO2 (2)
liquid
(1)
liquid V1
V2 < V1
SiO2 (2) > SiO2 (1)
V2
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
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
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
SILICA GEOTHERMOMETERS
Process
Reservoir Temperature
Steam Separation  Overestimated
Silica Precipitation  Underestimated
Increase in pH
 Overestimated
Mixing with cold water  Underestimated
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, ..)
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).
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)
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
CATION GEOTHERMOMETERS
Na-K-Ca Geothermometer
Geotherm.
Na-K-Ca
Equations
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
Reference
Fournier
and
Truesdell
(1973)
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 = --------------------------------------------------------log (Na/K)+  (log (Ca/Na)+2.06)+ 2.47
- 273.15
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
CATION GEOTHERMOMETERS
Na-K-Ca-Mg Geothermometer
Geotherm.
Na-K-Ca-Mg
Equations
T = TNa-K-Ca - tMgoC
R = (Mg / Mg + 0.61Ca + 0.31K) x 100
if R from 1.5 to 5
tMgoC = -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
tMgoC=10.66-4.7415 log R+325.87(log R)21.032x105(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
Reference
Fournier
and Potter
(1979)
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
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 waterrock 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).
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).
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.
SILICA-ENTHALPY MIXING
MODEL
Fig. 5. Dissolved silicaenthalpy 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)
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
(1000C)
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
(1000C)
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 hotwater component.
From the steam tables, the
reservoir temperature of the
hot-water component is
determined.
419 J/g
(1000C)
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)
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)
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).
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)
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.
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
B
D
E
B
steam
G
steam
F
C
N
H
hot water
steam
mixed water
residual liquid from boiling
cold water reservoir
hot water reservoir
H
hot water undergoing
conductive cooling
mixed water undergoing
conductive cooling
residual liquid undergoing
conductive cooling
ISOTOPES
IN
GEOTHERMAL
EXPLORATION
& DEVELOPMENT
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)
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)
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)
Sources of Geothermal Fluids and
Physico-Chemical Processes
STABLE
H- & O-ISOTOPES
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
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
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
Sources of Geothermal Fluids
Stable H- & O-Isotopes
Sources of Natural Waters:
1.
2.
3.
4.
5.
Meteoric Water (rain, snow)
Sea Water
Fossil Waters (trapped in sediments in sedimanary basins)
Magmatic Waters
Metamorphic Waters
Sources of Geothermal Fluids
Stable H- & O-Isotopes
SMOW
+
0
Metamorphic
Waters
-40
D (per mil)
Field of
Formation
Waters
-80
Magmatic
Waters
Most igneous
biotites &
hornblendes
-120
-20
-10
0
10
18O (per mil)
20
30
Sources of Geothermal Fluids
Stable H- & O-Isotopes
precipitation
1
H, 16O
D, 18O
precipitation
1
H, 16O
1
D, 18O
H, 16O
D, 18O
evaporation
D, 18O
Ocean
D, 18O
Seepage
(D/H)
vapor
(18O /
< (D/H) water
18
O) vapor< ( O /
16
River
16
O)water
1
H, 16O
Sources of Geothermal Fluids
Stable H- & O-Isotopes
+
0
SMOW
Evaporation
-40
Condensation
Water-Rock
Interaction
-80
-120
-12
-8
-4
del-18 O (per mil)
0
Sources of Geothermal Fluids
Stable H- & O-Isotopes
0
Magmatik
Sular
Larderello
The Geysers
Iceland
-50
D (per mil)
Niland
Lassen Park
-100
Steamboat Kaynakları
-150
-15
-10
-5
18O (per mil)
0
+5
+10
Physico-Chemical Processes:
Stable H- & O-Isotopes
Latitute 
D
18O
Altitute from Sea level 
D
18O
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
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)
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
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
(Scaling)
Vapour Separation
Volume of (residual) liquid 
Concentration of dissolved components
in liquid 
Liquid will become oversaturated
Component (calcite, silica, etc.) will
precipitate
Scaling
Dating of Geothermal Fluids
3H-
& 3He-ISOTOPES
Dating of Geothermal Fluids
Time elapsed between RechargeDischarge or Recharge-Sampling
points (subsurface residence residence
time)
– 3H method
– 3H-3He method
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  31H +
12
6C
 Nuclear testing
3H
concentration
 Tritium Unit (TU)
 1 TU = 1 atom 3H / 1018 atom H
 3He + 
– Half-life = 12.26 year
– Decay constant () = 0.056 y-1
3H
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
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
3H= 3H
0e
3H
-t
0 (before
1963)  10 TU
 = 0.056 y-1
t = 2003-1963 = 40 years
 3H  1.1 TU
3H
– Dating Method
APPARENT AGE
3H= 3H e-t
0
3H
= measured at sampling point
3H
0
= measured at recharge point
(assumed to be the initial tritium concentration)
 = 0.056 y-1
t = apparent age
–
Dating Method
3H
3He
= 3H0 – 3H
(D = N0-N)
3H= 3H e-t
(N = N0e -t)
0
3H = 3H et
0
3He = 3H et - 3H = 3H (et – 1)
3He
t = 1/ * ln (3He/3H + 1)
3He
& 3H – present-day concentrations measured in water sample
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
Isotope Geothermometers
12CO
13CH = 13CO + 12CH (CO gas - methane gas)
+
2
2
4
2
4
CH3D + H2O = HDO + CH4 (methane gas – water vapor)
HD + H2O = H2 + HDO (H2 gas – water vapor)
S16O4 + H218O = S18O4 + H216O (dissolved sulphate-water)

1000 ln  (SO4 – H2O) = 2.88 x 106/T2 – 4.1
(T = degree Kelvin = K )
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
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
S-Isotope Geothermometer
34S = 34S(mineral 1) - 34S(mineral 2)
34S = 34S= A (106/T2) + B
Temperature 0C
800
12
100
200 150
400
50
Pyrite-Galena
8
4
0
Sphalerite-Galena
8
4
0
4
Pyrite-Sphalerite
2
0
0
2
4
6
10 / T
6
2
( 0 K-2 )
8
10
12