Exploration of potential geothermal reservoirs: use of the chemical Na/Li

geothermometer and lithium isotopesBernard Sanjuan, Romain Millot, Michel Brach

Division Metrology, Monitoring, Analysis (MMA)

ENGINE Workshop, Postdam (Germany), November 6-8, 2006

ENGINE Workshop, Postdam (Germany), November 6-8, 2006

Division Metrology, Monitoring, Analysis (MMA)

> 2

Main objectives of this study

> 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

> 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

> 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

ENGINE Workshop, Postdam (Germany), November 6-8, 2006

Division Metrology, Monitoring, Analysis (MMA)

> 3

Na/Li thermometric relationships

y = 1.0744x - 0.5992

R2 = 0.9752

y = 1.2216x + 0.0306

R2 = 0.979

y = 0.8553x + 1.2752

R2 = 0.9670 y = 1.5883x - 1.286

R2 = 0.9809

1

2

3

4

5

1.0 1.5 2.0 2.5 3.0 3.5 4.0

103/T (°K)

log

(N

a/L

i) (

mo

lar

rati

o)

250 200 150 25100 50300350

Basalt - seawater interactions

Saline fluids

EPRMAR

Very altered EPR, MAR

400 0°C

Sedimentary basins

Dilute fluids

Conditions of use of the chemical Na/Li geothermometer

ENGINE Workshop, Postdam (Germany), November 6-8, 2006

Division Metrology, Monitoring, Analysis (MMA)

> 4

Examples of Li isotope application

7Li 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)

ENGINE Workshop, Postdam (Germany), November 6-8, 2006

Division Metrology, Monitoring, Analysis (MMA)

> 5

Main conclusions

> 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

> The use of the Na/Li geothermometer must be combined with that of the other classical geothermometers in geothermal exploration

> Given the scarcity of the 7Li 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

> 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 7Li values