EG_Isotope Geochemistry for Geothermal

Isotopic Chemistry for Geothermal
Exploration

Eksplorasi Geothermal
Geoscience, Faculty of Mathematics & Natural Sciences

Universitas Indonesia
2019

E-mail: [email protected]

DNS - 2019 Eksplorasi Geothermal

The application of isotopic geochemistry in geoscience

Unstable Isotope Stable Isotope

• KEYWORD: Geochronology • KEYWORD: Fractionations
• Determine the age of the Earth • can help understand the change in
• Determining the age of the rocks, thus
biosphere / atmosphere
understanding the age and and the • Geothermometry
extent of geological processes • Tracers
• Microbial decomposition, fossil

organisms
• Understanding the composition of the

mantle

As an aid to investigate mantle dynamics, ore genesis, hydrology, hydrocarbon
migration, crustal evolution, volcanology, oceanic circulation, paleontology and

environmental monitoring

Radioactivity & Geochronology

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Isotope Chemistry 2 types of isotopes

• SMOW (18O and D): the composition of Stable
standard mean ocean water, used as a isotopes
standard for oxygen and hydrogen
determinations. Radioactive
isotopes
• Craig (1963) demonstrated that the δD
content of geothermal waters is similar to
that of local meteoric water and δ18O value
of the geothermal water is more positive
than meteoric water

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Stable Isotope Geochemistry

• Concerned with variations in the isotopic
compositions of elements arising from
physiochemical processes rather than nuclear
processes.

• Fractionation: change in an isotope ratio that
arises as a result of some chemical or physical
process.

• Small differences of different isotopes of an
element can provide a very large amount of
useful information about chemical processes.

• Usually expressed in parts per thousands to
parts per hundred or parts per mil deviations (δ)

• The property of mass fractionation is employed
in stable isotope to hydrological studies

DNS - 2019 Eksplorasi Geothermal

Stable Isotope Geochemistry

• Fractionation: change in
an isotope ratio that
arises as a result of some
chemical or physical
process.

• Small differences in
MASS of different
isotopes of an element
can provide a very large
amount of useful
information about
chemical processes.

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Portioning isotopes in vapor and precipitation

Light

Heavy Higher topography
Lower topography
Heavy

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Stable Isotope Geochemistry

• Some elements are reported relative to its standard
• Hydrogen: SMOW – Standard Mean Oceanic Water
• Carbon: PDB – Pee Dee Belemnite
• Nitrogen: atmosphere

• Ratio depends on the abundances of the isotopes

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Common Isotope in Geothermal System

16O 18O 1H 2D 3T

12C 13C 14C

• Remarks:

• H = Hydrogen, D= Deuterium, T = Tritium
• O = Oxygen
• C = Carbon

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Isotope Chemistry for Geothermal

• Stable isotopes:

• Origin of geothermal water (magmatic or meteoric)

• Geothermometers (Oxygen isotope, hydrogen isotope)
• Reservoir monitoring

• Variation in 18O and D can indicate dilution / recharge / mixing

• Creating hydrogeochemical model

SW Gunung Panjang NE Not to be scaled vertically Limestone
Legend Volcanic breccia
Travertine terrace 100 Weathered marine sedimentary rock
Arbitrary Fault Siltstone, claystone, sandstone
60 ºC Warm Springs
Tuff sandstone
80 ºC Cold Water Flow

Warm Water Circulation

Thermal Water Convection Flow

Isotherm Line

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Hydrogeochemical Conceptual Model

• as

Alam, 2019

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How do you retrieve isotopic data?

• Water and or gas sample from geothermal manifestations or
weirbox.

water / gas
sampling

laboratory
analysis

results

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Radioactive Isotope for Geothermal

• Dating water age / residence time

• 3T and 14C to date residence time of water
in cold water aquifers

• Tracers

• Introduced to geothermal fluid production
• To determine flow direction
• Geological controls on subsurface flow

and flow rates
• Determine reinjection waste water
• Reservoir modelling

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Process influencing isotopic composition

Rock water reactions Boiling & Conduction Mixing Steam heating

• enriching 18O in • fluid concentration • with meteoric water • produces acidic fluid
geothermal water changes as the system • most meteoric water (near surface oxidation)
relative to local meteoric boils
water; causing the shift have lower 18O content • shows an enrichment of
in SMOW graph • Depends on the 18O and D relative to
temperature meteoric water
• Exchange of oxygen
with heavier isotope in • Allowing the change in • sulphate water + isotopic
liquid phase 18O composition composition of
fumaroles and help
• Not much 2D shift in deduce reservoir fluid
water
• can indicate magmatic
• Depends on the steam
permeability, porosity,
age of the system, rock-
water ratio

• The older the age the
smaller the shift in 18O

" → "
Mass fractionation occurs

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Isotope Chemistry • Process influencing fluid isotopic
composition:
Higher altitude
• Rock-water interaction at depth –
More exchange of oxygen and heavier
fumaroles isotope preferentially concentrating
appear in the liquid phase (18O)

• Boiling: depends on the temperature
of boiling and water-steam
separation.

• Conduction: occurs at near-surface
rock-water reactions

• Mixing with meteoric water
• Steam heating: acidic water enriches

18O and D relative to meteoric water

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Isotope Geothermometers

• An exchange of isotope must
occurs; meaning that different
isotopes (1H, 2D, 3T) must co
exists in the fluid / rock in the
reservoir

• Water – rock reaction must be
at equilibrium

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Isotope geothermometer

Hydrogen Carbon Sulphur
• Exchange of D and • Chemical
• Sulphur exchange
H within the water transformation from between SO4 and
molecule CO2 to CH4 H2S
• Exchange of 13C and
12C in CO2 and CH4 • Depends on sulphur
gas concentrations,
temperature and pH

• Occur under acidic
condition

" + ⇌ + " " + 4 " ⇌ / + 2 "