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Exploration Geochemistry

Exploration Geochemistry. Christopher W. Klein GeothermEx, Inc. 5221 Central Ave. Suite 201 Richmond, CA 94804. Topics. Scope and Objectives of “Exploration” The System Types: why Geochemistry? Importance of an Integrated Approach Choosing Tools: Strategy Tactics: Data Basics

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Exploration Geochemistry

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

  2. 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 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 GRC Exploration Workshop - GeothermEx, Inc.

  4. Goals: • Commercial • Academic/Scientific • Blend • Depends a lot on who is paying. 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 Basic Manifestations: Waters - springs, wells Gases - fumaroles, springs, wells Hydrothermal Alteration 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 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? GRC Exploration Workshop - GeothermEx, Inc.

  8. 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 GRC Exploration Workshop - GeothermEx, Inc.

  9. EXPLORATION TOOLS

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

  11. STABLE ISOTOPES OF WATER 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 GRC Exploration Workshop - GeothermEx, Inc.

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  16. Radioisotopes of Water 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 GRC Exploration Workshop - GeothermEx, Inc.

  17. 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) GRC Exploration Workshop - GeothermEx, Inc.

  18. Solutes: Major Anions and Cations 3 1 3 component mixing 1 1 1 1 2 GRC Exploration Workshop - GeothermEx, Inc. 2

  19. 3 Synthesis of Results: component origins on a map 2 2 1 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) GRC Exploration Workshop - GeothermEx, Inc.

  21. 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. GRC Exploration Workshop - GeothermEx, Inc.

  22. 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 GRC Exploration Workshop - GeothermEx, Inc.

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

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

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

  26. Silica: Salinity Effects - 2 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) 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 100-300°C • High Pco2 at low temperature yields poor results due to high Ca. Pco2correction 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) 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 GRC Exploration Workshop - GeothermEx, Inc.

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

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

  35. 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 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. GRC Exploration Workshop - GeothermEx, Inc.

  37. 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 GRC Exploration Workshop - GeothermEx, Inc.

  38. 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. GRC Exploration Workshop - GeothermEx, Inc.

  39. 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 GRC Exploration Workshop - GeothermEx, Inc.

  40. 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 GRC Exploration Workshop - GeothermEx, Inc.

  41. 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 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 GRC Exploration Workshop - GeothermEx, Inc.

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  44. 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 GRC Exploration Workshop - GeothermEx, Inc.

  45. 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 GRC Exploration Workshop - GeothermEx, Inc.

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  47. 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) GRC Exploration Workshop - GeothermEx, Inc.

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