1. Exploration Geochemistry
Christopher W. Klein
GeothermEx, Inc.
5221 Central Ave. Suite 201
Richmond, CA 94804

2. GRC Exploration Workshop - GeothermEx, Inc.
Topics
Scope and Objectives of “Exploration”
The System Types: why Geochemistry?
Importance of an Integrated Approach
Choosing Tools: Strategy
Tactics: Data Basics
Water Tools
Gas Tools
Solids Tools
Chemical Equilibrium Thermodynamics
New Developments
Data Management
Further Information
Sept.8-9, 2006
GRC Exploration Workshop - GeothermEx, Inc.

3. 1. Scope and Objectives of Exploration
Given how poorly we understand so many geothermal systems, exploration encompasses almost all data gathering
At the least:
Reconnaissance
Pre-feasibility studies
Feasibility studies
Step-outs and field expansion during Development/Exploitation
The emphasis here
Sept.8-9, 2006
GRC Exploration Workshop - GeothermEx, Inc.

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Goals:
Commercial
Academic/Scientific
Blend
Depends a lot on who is paying.
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GRC Exploration Workshop - GeothermEx, Inc.

5. 2. The System Types: why Geochemistry?
Volcanic - magmatic
Andesitic / Island Arc
Basaltic / Oceanic Ridge - Hawaiian
Silicic / Continental (Calderas)
Deep Sedimentary Trough / Spreading Center
Continental Heat-Flow
Basin and Range (Extension/ high regional H-F)
‘Background’ H-F
Chemical/Phase Type
Liquid-dominated
Two-phase
Steam-dominated
Altered meteoric water
Altered seawater
2. The System Types: why Geochemistry?
Basic Manifestations:
Waters - springs, wells
Gases - fumaroles, springs, wells
Hydrothermal Alteration
Sept.8-9, 2006
GRC Exploration Workshop - GeothermEx, Inc.

6. 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
Sept.8-9, 2006
GRC Exploration Workshop - GeothermEx, Inc.

7. 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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GRC Exploration Workshop - GeothermEx, Inc.

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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
Sept.8-9, 2006
GRC Exploration Workshop - GeothermEx, Inc.

9. EXPLORATION TOOLS

10. GRC Exploration Workshop - GeothermEx, Inc.
6. WATER TOOLS
The H2O itself:
Isotopes
Phases (liquid / vapor)
What’s in it: solutes / gases
Chemistry
Sept.8-9, 2006
GRC Exploration Workshop - GeothermEx, Inc.

11. STABLE ISOTOPES OF WATER
Ratio
(R)
% Natural abundance
Reference Standard
Common
Precision of H2O Analysis 2H
Deuterium
2H/1H
0.015
VSMOW
δD ± 1.0 o/oo
18O
18O/16O
0.204
δ18O ± 0.1 o/oo
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
Sept.8-9, 2006
GRC Exploration Workshop - GeothermEx, Inc.

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GRC Exploration Workshop - GeothermEx, Inc.

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GRC Exploration Workshop - GeothermEx, Inc.

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GRC Exploration Workshop - GeothermEx, Inc.

16. Radioisotopes of Water
Half-life (yrs)
Decay mode
Principal Sources 3H
Tritium
12.43 β-
(yields 3He)
Cosmogenic
Weapons 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
Sept.8-9, 2006
GRC Exploration Workshop - GeothermEx, Inc.

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Solutes: Major Anions
Chloride
~50 to ~20,000 mg/kg
(to ~200,000 mg/kg in hypersaline brines)
Sources: traces of Na-K-Cl in volcanic rocks (seawater origins), connate seawater in sedimentary rocks, halite deposits
seawater Cl 19,350 mg/kg
Bicarbonate
<1 to several 1000 mg/kg
(for most purposes, effectively the same as “alkalinity”)
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)
Extremes of volcanic and steam heated are acidic (no HCO3)
Sept.8-9, 2006
GRC Exploration Workshop - GeothermEx, Inc.

18. Solutes: Major Anions and Cations3 1
3 component mixing 1 1 1 1 2
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Synthesis of Results: component origins on a map 2 2 1
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GRC Exploration Workshop - GeothermEx, Inc.

20. Tri-linear diagrams can be made using any three components
Schoeller (spider) diagrams can illustrate entire analyses
Log (concentration)
Species (Na, K, Ca, etc.)
Source: Giggenbach (1991)
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GRC Exploration Workshop - GeothermEx, Inc.

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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.
Sept.8-9, 2006
GRC Exploration Workshop - GeothermEx, Inc.

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Chemical Geothermometers 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 re-equilibration can occur.
Qualitative
comparison of
reaction times
(Henley and others, 1984)
e.g. reactions that control pH, Carbonate deposition
Sept.8-9, 2006
GRC Exploration Workshop - GeothermEx, Inc.

23. Silica Geothermometers
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GRC Exploration Workshop - GeothermEx, Inc.

24. Silica: The Chalcedony – Quartz Problem
Data from geothermal
wells in Nevada
Silica: The Chalcedony – Quartz Problem
Sept.8-9, 2006
GRC Exploration Workshop - GeothermEx, Inc.

25. Silica: Salinity Effects - 1
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GRC Exploration Workshop - GeothermEx, Inc.

26. Silica: Salinity Effects - 2
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GRC Exploration Workshop - GeothermEx, Inc.

27. 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)
Sept.8-9, 2006
GRC Exploration Workshop - GeothermEx, Inc.

28. 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 °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)
Sept.8-9, 2006
GRC Exploration Workshop - GeothermEx, Inc.

29. Cation Geothermometers - 3
Lower-T waters and shallow-cooled reservoir zones: if Mg >~1 ppm.
Na-K-Ca-Mg : Applies correction to Na-K-Ca. Developed and calibrated by Fournier and Potter (1978)
K-Mg : Developed by Giggenbach, alternate calibrations by Fournier, Arnorsson
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GRC Exploration Workshop - GeothermEx, Inc.

30. GRC Exploration Workshop - GeothermEx, Inc.
Effects of Reservoir Cooling:
Silica, Na/K and Na-K-Ca geothermometers
All wells are within a single geothermal field in Nevada, USA
Sept.8-9, 2006
GRC Exploration Workshop - GeothermEx, Inc.

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Effects of Reservoir Cooling:
K-Mg and Na/K geo-thermometers
Calibrations by Giggenbach, Fournier, Arnorsson
Sept.8-9, 2006
GRC Exploration Workshop - GeothermEx, Inc.

32. 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.
Sept.8-9, 2006
GRC Exploration Workshop - GeothermEx, Inc.

33. Mathematical Mixing Models
Example: Nevis, W.I., 55°C submarine spring: Cl at 16,400 mg/kg (thermal water contaminated by seawater).
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 ~175°C
175°C
Chemical Temperature (°C)
Fraction seawater in sample
Sept.8-9, 2006
GRC Exploration Workshop - GeothermEx, Inc.

34. 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)
18O : H2O – CO2
2H : H2 – H2O, H2 – CH4, H2O – CH4
13C : CO2 – CH4, CO2 – HCO3
34S : SO4 – H2S
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GRC Exploration Workshop - GeothermEx, Inc.

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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
Sept.8-9, 2006
GRC Exploration Workshop - GeothermEx, Inc.

36. 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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GRC Exploration Workshop - GeothermEx, Inc.

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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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GRC Exploration Workshop - GeothermEx, Inc.

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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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GRC Exploration Workshop - GeothermEx, Inc.

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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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GRC Exploration Workshop - GeothermEx, Inc.

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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
Sept.8-9, 2006
GRC Exploration Workshop - GeothermEx, Inc.

42. 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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GRC Exploration Workshop - GeothermEx, Inc.

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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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GRC Exploration Workshop - GeothermEx, Inc.

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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., Isotopic and Chemical Techniques in Geothermal Exploration, Development and Use: Sampling Methods, Data Handling, Interpretation. International Atomic Energy Agency, Vienna
Bethke, C.M., Geochemical Reaction Modeling, Concepts and Applications. Oxford University Press, New York, Oxford.
D’Amore, F. (Co-ordinator), 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, Chemistry and Geothermal Systems. Academic Press.
Henley, R.W., Truesdell, A.H. and Barton, P.B., Fluid-Mineral Equilibria in Hydrothermal Systems; Reviews in Economic Geology, Vol. 1, Society of Economic Geologists, Univ. Texas, El Paso, TX
Hem, J.D., Study and Interpretation of the Chemical Characteristics of Natural Water. United States Geological Survey Water-Supply Paper 2254.
Nicholson, K., Geothermal Fluids: Chemistry and Exploration Techniques. Springer-Verlag.
The Encyclopedia of Water: Environmental Isotopes in Hydrology (at
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