Exploration Geochemistry
Christopher W. Klein
GeothermEx, Inc.
5221 Central Ave. Suite 201
Richmond, CA 94804
Topics
1. Scope and Objectives of “Exploration”
2. The System Types: why Geochemistry?
3. Importance of an Integrated Approach
4. Choosing Tools: Strategy
5. Tactics: Data Basics
6. Water Tools
7. Gas Tools
8. Solids Tools
9. Chemical Equilibrium Thermodynamics
10. New Developments
11. Data Management
12. Further Information
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1. Scope and Objectives
of Exploration
• Given how poorly we understand so many
geothermal systems, exploration
encompasses almost all data gathering
• At the least:
The
emphasis
here
– Reconnaissance
– Pre-feasibility studies
– Feasibility studies
– Step-outs and field expansion during
Development/Exploitation
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• Goals:
– Commercial
– Academic/Scientific
– Blend
– Depends a lot on who is paying.
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• Volcanic - magmatic
– Andesitic / Island Arc
– Basaltic / Oceanic Ridge Hawaiian
– Silicic / Continental (Calderas)
– Deep Sedimentary Trough /
Spreading Center
• Continental Heat-Flow
– Basin and Range (Extension/
2. The System
Types: why
Geochemistry?
Basic Manifestations:
high regional H-F)
– ‘Background’ H-F
• Chemical/Phase Type
–
–
–
–
–
Liquid-dominated
Two-phase
Steam-dominated
Altered meteoric water
Altered seawater
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Waters - springs, wells
Gases - fumaroles, springs, wells
Hydrothermal Alteration
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3. Importance of
an Integrated Approach
• Don’t limit the geochemical point-of-view
to one discipline if others may be relevant
• Conclusions must be reasonable in light of
other data and information:
– Geology
– Temperature
– Well data
– Geophysics
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4. Choosing Tools: Strategy
• Commercial viewpoint:
– Try to avoid discovering what you already
know, or more than you need to know.
– Does the proposed study have a reasonable
chance of assisting a project decision
(resource assessment / drilling / finance / etc.)
in a way that other information could not?
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5. Tactics: Data Basics
•
•
•
•
Too much data rarely the problem
Wrong data can be a problem
Thorough and disciplined record-keeping
Location, location, location
– GPS
– Maps of results and synthesis of data at common scale
– Contours drawn by hand (not by computer)
• Quality control
– During data gathering/generation
– During data analysis
• Data management
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EXPLORATION TOOLS
6. WATER TOOLS
• The H2O itself:
– Isotopes
– Phases (liquid / vapor)
• What’s in it: solutes / gases
– Chemistry
– Isotopes
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STABLE ISOTOPES OF WATER
Isotope
Ratio
(R)
% Natural
abundance
Reference
Standard
Common
Precision of
H2O Analysis
2H
2H/1H
0.015
VSMOW δD ± 1.0 o/oo
0.204
δ18O ± 0.1 o/oo
Deuterium
18O
18O/16O
VSMOW
δD or δ18O = 1000 * (Rsample – Rstd)/Rstd (permil or o/oo)
So:
Seawater δD = 0 o/oo and δ18O = 0 o/oo
δD or δ18O < 0 = “lighter”
δD or δ18O < 0 = “heavier”
H216O is about 10% lighter than H218O, and chemically more reactive
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Radioisotopes of Water
Isotope
Half-life
(yrs)
Decay
mode
3H
12.43
Cosmogenic
β(yields 3He) Weapons
Tritium
Principal
Sources
Tests
1 Tritium Unit (TU) = 1 atom 3H per 1018 atoms 1H
Before 1953: atmospheric TU ~3-5
By 1963: atmosphere at several 1000 TU
European atmosphere now <10 TU
Groundwater: >30 TU implies recharge in 1960s; <1 TU implies older
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Solutes: Major Anions
Chloride
~50 to ~20,000 mg/kg
seawater Cl
19,350 mg/kg
Bicarbonate
<1 to several 1000
mg/kg
(to ~200,000 mg/kg in
hypersaline brines)
(for most purposes,
effectively the same as
“alkalinity”)
Sources: traces of Na-K-Cl in
volcanic rocks (seawater origins),
connate seawater in sedimentary
rocks, halite deposits
Sources: reactions of
dissolved CO2 from
atmosphere and/or in
geothermal/volcanic
steam, with silicate
minerals in rocks, with
carbonate minerals
(limestone)
Sulfate
~10 to ~1500 mg/kg
(to ~100,000 mg/kg in acid
volcanic steam condensates
Sources: oxidized sulfide
minerals and H2S, sulfate
mineral deposits (gypsum,
anhydrite)
Approximate range among non-volcanic
geothermal systems (higher SO4 exist)
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Extremes of volcanic and steam
heated are acidic (no HCO3)
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Solutes: Major Anions and Cations
3
1
3 component
mixing
111
1
2
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3
Synthesis of
Results:
component
origins on a map
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2
1
19
Tri-linear diagrams can be made
using any three components
Log (concentration)
Schoeller (spider) diagrams
can illustrate entire analyses
Species (Na, K, Ca, etc.)
Source:
Giggenbach (1991)
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Mixing diagrams can
be constructed
comparing dissolved
species to enthalpy
(temperature)
Chloride ion is best for
this, because it does not
participate in chemical
reactions.
Other ‘conservative’ or
nearly ‘conservative’
species (aqueous
tracers):
B, Li, Rb, Cs, Br, the
stable isotopes of water.
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Chemical
Geothermometers
Qualitative
comparison of
reaction times
(Henley and others, 1984)
Rely upon chemical
species (solutes, gases,
isotopes) reaching a state
of reaction equilibrium in
the reservoir, then leaving
the reservoir and
appearing at
wells/springs/fumaroles
before significant reequilibration can occur.
e.g. reactions that control
pH, Carbonate deposition
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Silica Geothermometers
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Data from geothermal
wells in Nevada
Silica:
The
Chalcedony –
Quartz Problem
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Silica: Salinity Effects - 1
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Silica: Salinity Effects - 2
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Cation Geothermometers - 1
• Na/K - Ion exchange in alkali feldspars
(common in volcanic rocks) causes Na/K to
decrease as T increases.
• Simple plots of K vs Na can be
a guide to relative source
temperatures.
• Considered applicable only at
>150°C.
• Clay mineral interference at
<200°C can yield temperatures
that are too high.
• Various calibrations available
(Fournier, Giggenbach,
Truesdell, Arnorsson)
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Cation Geothermometers - 2
• Na-K-Ca – Developed and calibrated by Fournier
•
•
•
•
•
and Truesdell (1973).
Empirical, but based on a theoretical consideration of
likely silicate reactions, to incorporate the influence of
calcium-bearing minerals (feldspars, epidote, calcite)
Considered more acceptable than Na/K over 100-300°C
High Pco2 at low temperature yields poor results due to
high Ca. Pco2 correction can be applied.
Eqn has two forms: the correct one needs to be applied
(depends on T°C, Ca, Na)
Other versions available: Benjamin and others, 1983; illite form
of Ballantyne and Moore, 1990)
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Cation Geothermometers - 3
• Lower-T waters and shallow-cooled
reservoir zones: if Mg >~1 ppm.
–Na-K-Ca-Mg : Applies correction to Na-KCa. Developed and calibrated by Fournier and Potter
(1978)
–K-Mg : Developed by Giggenbach, alternate
calibrations by Fournier, Arnorsson
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Effects of
Reservoir Cooling:
Silica, Na/K and
Na-K-Ca
geothermometers
All wells are within a
single geothermal
field in Nevada, USA
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Effects of
Reservoir
Cooling:
K-Mg and
Na/K geothermometers
Calibrations by
Giggenbach,
Fournier, Arnorsson
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Other Aqueous Geothermometers
• Sulfate-Water Oxygen Isotope: re-equilibrates
very slowly with cooling, may be very accurate if
SO4 not added/removed (mixing,
anhydrite/gypsum)
• Anhydrite equilibrium (CaSO4): Accuracy
depends upon thermodynamic data for the
equilibrium reaction.
• K-Mg-Ca (Giggenbach): simultaneous T
dependence of K2/Ca and K2/Mg (reactions involving
feldspars, mica, Ca-Al-silicate, calcite, CO2, chlorite)
• Na/Li and other ion ratios: rarely used.
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Mathematical Mixing Models
Process: remove
seawater to the
point where the
thermal
component
contains 1 mg/kg
of Mg.
Result: thermal Cl
at ~11,000 mg/kg,
geothermometers
converging at
Chemical Temperature (°C)
Example: Nevis, W.I., 55°C submarine spring: Cl at
16,400 mg/kg (thermal water contaminated by seawater).
175°C
~175°C
Fraction seawater in sample
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Other Water Parameters
(Less Widely Used)
• To distinguish provenance
– Isotopes of C, S, B, Cl
– Rare earth elements, Y
• Isotope geothermometers (gas–water,
gas-gas)
–
–
–
–
: H2O – CO2
2H : H2 – H2O, H2 – CH4, H2O – CH4
13C : CO2 – CH4, CO2 – HCO3
34S : SO4 – H2S
18O
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7. Gas Tools
• Advantages at volcanic systems:
– more fumaroles/seeps than springs
– fumaroles usually above reservoir (short
pathway to surface)
• Limitations:
– minor to insignificant in outflow zones and in
non-volcanic settings
– chemistry more complex than water
– greater difficulty and expense to sample
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Significant Gas Components
• Relatively more soluble in water:
– NH3, H2S, CO2
• Relatively less soluble:
– CH4, H2, N2, Ar, He, (other noble gases)
•
•
•
•
Higher T systems: significant CO2, CH4, H2
Lower T systems: dominated by N2
Volcanic/magmatic: SO2, HCl, HF
Measurable O2 indicates contamination by air
from shallow source or during sampling.
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As with solutes
in water, any
three gas
components can
be combined in
a tri-linear
diagram
An alternate view
puts He (which
comes from
radioactive decay
in the earth’s crust)
at this apex.
CH4 – H2S – CO2
can be useful to show
boiling trends
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Gas Geothermometry - 1
• Empirical
– determined for studied areas (e.g. Iceland)
– best fits of data to source temperature
• Theoretical / thermodynamic
– based on chemical reactions, some with
minerals, assuming equilibrium
• Major ambiguity - whether gases sampled
originate from reservoir steam, boiling of liquid,
or both.
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Gas Geothermometry - 2
Giggenbach Gas Ratio
Grids: thermodynamic
basis, with simplifying
assumptions
Example: H2/Ar vs. CO2/Ar
Others:
H2/Ar vs. T
CH4/CO2 vs. CO/CO2
CO/CO2 vs H2/Ar
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Other Gas Parameters
3He/4He
– magmatic (high) vs. crustal (low)
(3He = mantle source; 4He = decay of U, Th, Ar)
40Ar/36Ar
– atmospheric (low) vs. magmatic
(high)
Noble gas ratios (various)
Stable isotopes of steam condensate
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8. Solids Tools
• Hydrothermal Alteration Maps
–
–
–
–
Can outline extent of reservoir
Fluid type(s) responsible
Temperature(s) of alteration
Limitation: may indicate paleo-conditions only
• Fluid Inclusion Analysis
• Leakage Detection Surveys (faults/fractures)
– Soil gas: Hg, Rn, CO2
– Soil: ammonia, Sb, As, B, Hg, Gamma
• Evidence of reservoir cap rock (clay minerals)
– May assist resistivity survey interpretation
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9. Chemical Equilibrium
Thermodynamics
• Calculate simultaneous chemical reaction
states in a large suite of dissolved and
solid species
• Requires good data (esp. pH, alkalinity /
bicarbonate, Al)
• Useful for geothermometry, mixing,
precipitation and dissolution of solids
• Some thermodynamic data are uncertain
• Available codes differ in capabilities
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10. New Developments
• Software and Equipment
–
–
–
–
Database software
Graphing software
In the field: GPS
High Performance/Pressure Liquid Chromatography:
better anion data, esp. SO4
• Methods
– More common/refined use of AA for SiO2
• Biggest Downer: increased difficulty of shipping
samples, esp. gases
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11. Data Management
• Spreadsheets
–
–
–
–
–
Convenient for smaller amounts of data
Lead to sloppy/inconsistent formatting
Limited input/edit forms screen capability
Calculations may contain hidden errors
Graphing can suffer from inadequate format control
• Databases
–
–
–
–
–
Better for data sets with >25~40 analyses
Enforce discipline in formatting
Unlimited input/edit forms screen capability
Calculations are external to the data
Use separate graphing package
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12. Further Information
•
•
•
•
•
•
•
•
Arnórsson, S., 2000. Isotopic and Chemical Techniques in Geothermal Exploration, Development
and Use: Sampling Methods, Data Handling, Interpretation. International Atomic Energy Agency,
Vienna
Bethke, C.M., 1996. Geochemical Reaction Modeling, Concepts and Applications. Oxford
University Press, New York, Oxford.
D’Amore, F. (Co-ordinator), 1991. Applications of Geochemistry in Geothermal Reservoir
Development. Series of Technical Guides on the Use of Geothermal Energy. UNITAR/UNDP
Centre on Small Energy Resources, Rome – Italy.
Ellis, A.J. and W.A.J. Mahon, 1977. Chemistry and Geothermal Systems. Academic Press.
Henley, R.W., Truesdell, A.H. and Barton, P.B., 1984. Fluid-Mineral Equilibria in Hydrothermal
Systems; Reviews in Economic Geology, Vol. 1, Society of Economic Geologists, Univ. Texas, El
Paso, TX
Hem, J.D., 1989. Study and Interpretation of the Chemical Characteristics of Natural Water.
United States Geological Survey Water-Supply Paper 2254.
Nicholson, K., 1993. Geothermal Fluids: Chemistry and Exploration Techniques. Springer-Verlag.
The Encyclopedia of Water: Environmental Isotopes in Hydrology (at www.wileywater.com)
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