exploration geochemistry christopher w. klein geothermex, inc. 5221 central ave. suite 201 richmond,...

Exploration Geochemistry

Christopher W. Klein

GeothermEx, Inc.5221 Central Ave. Suite 201

Richmond, CA 94804

Sept.8-9, 2006 GRC Exploration Workshop - GeothermEx, Inc. 2

Topics1. Scope and Objectives of “Exploration”2. The System Types: why Geochemistry?3. Importance of an Integrated Approach4. Choosing Tools: Strategy5. Tactics: Data Basics6. Water Tools7. Gas Tools 8. Solids Tools9. Chemical Equilibrium Thermodynamics10. New Developments11. Data Management12. Further Information


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


• Goals:– Commercial– Academic/Scientific– Blend– Depends a lot on who is paying.


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

Basic Manifestations:

Waters - springs, wells

Gases - fumaroles, springs, wells

Hydrothermal Alteration


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


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?


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

EXPLORATION TOOLS


6. WATER TOOLS

• The H2O itself: – Isotopes – Phases (liquid / vapor)

• What’s in it: solutes / gases– Chemistry– Isotopes


Isotope Ratio

(R)

% Natural abundance

Reference Standard

Common

Precision of H2O Analysis

2HDeuterium

2H/1H 0.015 VSMOW δD ± 1.0 o/oo

18O 18O/16O 0.204 VSMOW 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”H2

16O is about 10% lighter than H218O, and chemically more reactive

STABLE ISOTOPES OF WATER


Radioisotopes of Water

Isotope 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


Solutes: Major AnionsChloride

~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

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)

Extremes of volcanic and steam heated are acidic (no HCO3)

Approximate range among non-volcanic geothermal systems (higher SO4 exist)

seawater Cl 19,350 mg/kg


Solutes: Major Anions and Cations

3 component mixing

1

1

111

2

2

3


221

3

Synthesis of Results: component origins on a map


Tri-linear diagrams can be made using any three components

Source: Giggenbach (1991)

Species (Na, K, Ca, etc.)

Schoeller (spider) diagrams can illustrate entire analyses

Log

(con

cent

ratio

n)


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.


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 ofreaction times(Henley and others, 1984)

e.g. reactions that control pH, Carbonate deposition


Silica Geothermometers


Silica: The

Chalcedony – Quartz Problem

Data from geothermalwells in Nevada


Silica: Salinity Effects - 1


Silica: Salinity Effects - 2


Cation Geothermometers - 1

• 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)

• Na/K - Ion exchange in alkali feldspars (common in volcanic rocks) causes Na/K to decrease as T increases.



• 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)



• 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


Effects of Reservoir Cooling:

Silica, Na/K and Na-K-Ca geothermometers

All wells are within a single geothermal field in Nevada, USA


Effects of Reservoir Cooling:

K-Mg and Na/K geo-thermometersCalibrations by Giggenbach, Fournier, Arnorsson


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.


Mathematical Mixing Models

Fraction seawater in sample

Che

mic

al T

empe

ratu

re (

°C)

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


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


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


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.


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


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.


Gas Geothermometry - 2

Giggenbach Gas Ratio Grids: thermodynamic basis, with simplifying assumptions

Example: H2/Ar vs. CO2/Ar Others: H2/Ar vs. TCH4/CO2 vs. CO/CO2

CO/CO2 vs H2/Ar


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


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


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


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


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


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)

http://www.wileywater.com/

exploration geochemistry christopher w. klein geothermex, inc. 5221 central ave. suite 201 richmond,...

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