Transcript Slides

Exploration of potential geothermal
reservoirs: use of the chemical Na/Li
geothermometer and lithium isotopes
Bernard Sanjuan, Romain Millot, Michel Brach
Division Metrology, Monitoring, Analysis (MMA)
ENGINE Workshop, Postdam (Germany), November 6-8, 2006
Main objectives of this study
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Since 1965, several chemical and isotopic geothermometers,
based on empirical or semi-empirical laws, are commonly used
in geothermal exploration but unfortunately, the estimates of
reservoir temperatures using these classical tools are not
always concordant
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Since 1981 and 1989, the use of the Na/Li thermometric
relationships is a relevant tool which often yields more reliable
estimates of deep temperature than that of the other
geothermometers
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By integrating more than 120 additional data relative to
hydrothermal, geothermal, oil-field and sedimentary fluids, this
study re-examines the three previously determined Na/Li
relationships and new possible equations
Division Metrology, Monitoring, Analysis (MMA)
ENGINE Workshop, Postdam (Germany), November 6-8, 2006
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Na/Li thermometric relationships
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log (Na/Li) (molar ratio)
Conditions of use of the chemical Na/Li geothermometer
Basalt - seawater
interactions
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Sedimentary
basins
y = 0.8553x + 1.2752
2
R = 0.9670
y = 1.5883x - 1.286
Very altered
EPR, MAR
3
2
R = 0.9809
Dilute fluids
EPR
MAR
y = 1.0744x - 0.5992
2
R = 0.9752
2
Saline fluids
y = 1.2216x + 0.0306
2
R = 0.979
1
1.0
400 350 300
1.5
250
200
150
2.0
2.5
100
3.0
50
25
3.5
0°C
4.0
3
10 /T (°K)
Division Metrology, Monitoring, Analysis (MMA)
ENGINE Workshop, Postdam (Germany), November 6-8, 2006
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Examples of Li isotope application
d7Li values versus mass
lithium/chloride ratios
for seawater and fluids
collected from thermal
springs located in the
Bouillante geothermal
field (Guadeloupe) and
in the Lamentin plain
(Martinique, French
West Indies)
Note the existence of
hyperbolic functions
which traduces mixing
processes between
seawater and a hot fluid
end-member (reservoir
fluid)
Division Metrology, Monitoring, Analysis (MMA)
ENGINE Workshop, Postdam (Germany), November 6-8, 2006
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Main conclusions
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This study has allowed to confirm and refine the three existing
Na/Li thermometric relationships for geothermal exploration. A
new Na/Li thermometric relationship for fluids discharged from
the oceanic ridges and emerged rifts is proposed
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The use of the Na/Li geothermometer must be combined with
that of the other classical geothermometers in geothermal
exploration
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Given the scarcity of the d7Li values analyzed in the geothermal
and oil-field fluids and the available data, it is necessary to
acquire additional data in both fluids and rocks to better
understand the Li behavior
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It is recommended to study the Li behavior at different
temperatures for a same type of process of water-rock
interactions in order to obtain a thermometric relationship
involving the d7Li values
Division Metrology, Monitoring, Analysis (MMA)
ENGINE Workshop, Postdam (Germany), November 6-8, 2006
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