Cation Geothermometers

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Exploration Technique: Cation Geothermometers

Exploration Technique Information
Exploration Group: Geochemical Techniques
Exploration Sub Group: Geochemical Data Analysis
Parent Exploration Technique: Liquid Geothermometry
Information Provided by Technique
Lithology:
Stratigraphic/Structural:
Hydrological:
Thermal: Used to estimate reservoir temperatures.
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Cation Geothermometers:
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Introduction
Some experts have stated that the factor that changes the risk assessment of a geothermal prospect the fastest is obtaining attractive chemical confirmation (geothermometry, gas analyses) that a thermal resource exists in that location. As with all geothermal exploration methods for both hidden and conventional systems, geochemical interpretations are more reliable when integrated into a conceptual model that is consistent with other data.

Silica and cation geothermometers have been used to estimate reservoir rock-water equilibration temperatures in geothermal resource evaluation since the mid-1980s.[1] Cation geothermometers that use Na, Ca, and K were well established by that time, and were followed by the addition of geothermometers that incorporate the Mg and Li contents of thermal waters by about 1990.[2] Variations of the original defining equations are occasionally published to date, based on data sets from selected locations and/or refined mathematical approaches (e.g. Can, 2002).[3]

 
Use in Geothermal Exploration
Cation geothermometers are based on steady-state ion exchange reactions between hydrothermal fluids and different reservoir rock mineral groups (such as feldspars, micas, zeolites, or clays) at equilibrium conditions.[4] [5] Because the reaction rates for mineral solubility in a hydrothermal system are strongly dependent on temperature, concentrations of certain dissolved species in thermal fluid samples relate to the maximum reservoir temperature and fluid residence times (as well as several other variables).[6] Most geothermometers can now be applied automatically for each new chemical analysis of sampled fluid by inputting the results into a spreadsheet. However, reliable interpretation of the calculated temperatures obtained from different geothermometers requires knowledge of the most likely reactions that occur between thermal fluids and reservoir rocks with varying mineral compositions.[5] Also, rapid upwelling of thermal fluids to the surface is an assumed condition when applying most geothermometers, which require minimal re-equilibration of fluids with wall rocks during ascent for temperature estimates to be considered representative of reservoir conditions. Geothermometers commonly used in the evaluation of geothermal systems include:

  • Na-K;[7] based on the exchange of Na+ and K+ ions between reservoir fluids and the alkali feldspars. Numerous iterations of the Na-K geothermometer have been introduced since the original version was published in 1975, and show good agreement in estimated temperatures in the vicinity of 300°C.[8][9][10][11] Refined equations also continue to be published.[3] Na+ and K+ are generally thought to take longer to re-equilibrate than the components used by other geothermometers, and so the method may be used to estimate the possible highest temperatures in deeper parts of a geothermal system, where waters reside for relatively long periods of time. At lower temperatures, ion exchange between the thermal fluids, different metastable structural states of alkali feldspar, and Na- and K-bearing montmorillonite clays complicates application of the Na-K geothermometers, leading to scatter in temperature estimates calculated using the different formulations.[4] Consequently, variations of the Na-K geothermometer may be applied to some well water samples, but may only be tenuously applied to hot spring waters.
  • Na-K-Ca;[12] assumes that Na+, K+, and Ca+ cations are in equilibrium with feldspars at depth. Calculated times to reach equilibrium between thermal fluids and feldspar minerals for the Na-K-Ca geothermometer varies from tens of hours at 500°C to approximately 100 years at 150°C.[6] There are two formulations of the Na-K-Ca geothermometer that account for these strong temperature dependencies, one for lower (<100°C) temperature waters and one for higher (>100°C) temperature waters.
  • Na-K-Ca-Mg;[13] probably the most commonly applied cation geothermometer, used extensively in the 1978 U.S. Geological Survey geothermal resource assessment.[14] It is based on a correction to the Na-K-Ca geothermometer that considers the reaction of the higher temperature fluids (>70°C) with Mg-bearing mineral phases (e.g. chlorite, Mg-rich calcite) in the reservoir.
  • K-Mg[11] and Li-Mg[15]; These geothermometers are based on rapid Mg cation exchange reactions at lower temperatures, and provide reliable temperature estimates for the last temperature of water-rock equilibration.[5] Good agreement between results calculated from the K-Mg, Li-Mg, and other geothermometers indicates that the estimated temperature is fairly close to the real temperature of water-rock equilibration, and that minimal water-rock reaction occurrs during upflow. The K-Mg geothermometer also appears to provide estimated temperatures reasonably close to measured temperatures ranging from 90° to 130°C in drilled geothermal systems, regardless of the type of water chemistry.[5]

To accurately use geothermometry for data interpretation, it is important to consider the reaction kinetics, thermodynamics, mineral suites, and reaction or fluid paths that are involved and to keep in mind that the components of the liquid geothermometers are not sufficiently soluble in steam, and are, therefore, applicable only to uncondensed water samples.[16] Adherence to these principles is exemplified in the discussion of a study published in 1991, which integrated information from previous scientific and private industry investigations with new data from fluid sampling, test drilling, and geological and geophysical studies conducted between 1985-1988 into a comprehensive conceptual model of the present-day hydrothermal flow system at Long Valley Caldera, CA.[17] Temperature gradient data from many of the wells are available online through the U.S. Geological Survey.[18] Relevant data from chemical and isotopic studies published during the same year are also considered in the model.[19][20][21][17] Estimated temperatures for the Long Valley geothermal reservoir were calculated for fluid samples from the MBP-1 and MBP-3 wells at Casa Diablo and from the RDO-8 well using five different geothermometers, which produced a range of temperatures from 181-248°C. The Na-K and Na-K-Ca cation geothermometers yielded an average temperature of 218°C, in good agreement with the maximum temperature of 214°C measured in the reservoir at that time. Silica geothermometer temperature estimates for the well samples ranged from 196-202°C. This temperature range is lower than the cation geothermometer temperature estimates for the same samples, indicating loss of silica in association with declining reservoir temperatures or with dilution by waters of comparatively lower silica content. Sulfate-water isotope geothermometer temperature estimates for the two Casa Diablo wells were 222 and 232°C, whereas the anhydrite solubility geothermometer temperature estimates for the RDO-8 and Casa Diablo MBP-3 wells were 231 and 248°C, respectively. Given the lack of information on the existence of anhydrite in the reservoir rock at the time, the sulfate-water isotope temperature estimate was considered the more reliable of the estimates provided by these two sulfate-dependent geothermometers.




 
Data Access and Acquisition
In the United States, geochemical analyses have been performed and cataloged for most places that have accessible surface waters (e.g. the U.S. Geological Survey GEOTHERM geochemistry database,[22] University of Nevada, Reno’s geochemistry database for the Great Basin[23]). As the resource areas with the most easily interpreted chemistry are developed, the uncertainty in the interpretation of the data from the surface manifestations of the remaining prospects becomes a greater challenge. For hidden systems, a strategy is needed to get a suitable water sample at low enough cost from a slim hole, a water well, or a core hole drilled for mineral exploration.
 
Best Practices

  • The best practice in using geothermometers is to apply them with caution. Understand what is being asked. Understand the system in which they are being applied. Understand the assumptions. Use correction factors, if available, or develop new correction factors, if possible. Use geothermometers that are not solute dependent, if available.
  • Geothermometer systems (e.g. K-Mg), have been developed through the collection, cataloging, and mapping of chemical data in known geothermal systems. In the Basin and Range Region, geothermal fluids often flow through Na-K-Ca rich rocks and alluvium on their way to the surface, which can alter the fluid chemistry and complicate interpretations when attempting to apply the Na-K-Ca geothermometer. Without additional information about a hydrothermal system (e.g., pH, TDS, and dissolved gases), it is unclear how widespread these geothermometers can be applied, though they appear to be less affected by mixing and boiling than silica geothermometers.[5]

 
Potential Pitfalls

  • Some commonly used geothermometers (e.g. silica, Na-K) were developed in high-temperature (>90°C) magmatic-volcanic geothermal systems, so trying to apply them to other regions—like the Basin and Range—can give misleading and discordant results.[24]
  • Data from laboratory experiments and field studies of sodium-chloride solutions were used to develop the mathematical equations for several cation geothermometers. In geothermal prospect areas of the Basin and Range region, sodium-bicarbonate waters are the predominate type of thermal fluid encountered. As a result, extra caution should be used when applying geothermometers calibrated for analyses of dilute, sodium-chloride type waters with near-neutral pH.[5]
  • Several geothermometers, particularly those that incorporate Ca2+, are sensitive to the effects of the partial pressure of CO2 in the solution and to the effects of calcite precipitation on fluid chemistry during cooling and/or ascent.[4]
  • Cation geothermometers are still incorrectly applied to low-pH fumarole condensates where the cation equilibration is dominated by leaching under acid conditions yielding anomalously and unreasonably high geothermometer estimates.







 
References
  1. Advances in the Past 20 Years: Geochemistry in Geothermal Exploration, Resource Evaluation and Reservoir Management
  2. Isotopic and Chemical Techniques in Geothermal Exploration, Development and Use: Sampling Methods, Data Handling, Interpretation
  3. 3.0 3.1 A New Improved Na/K Geothermometer by Artificial Neural Networks
  4. 4.0 4.1 4.2 Lectures on Geochemical Interpretation of Hydrothermal Waters
  5. 5.0 5.1 5.2 5.3 5.4 5.5 Geothermometer Calculations for Geothermal Assessment
  6. 6.0 6.1 Chapter 14: High Temperature Calculations Applied to Ore Deposits
  7. Summary of Section III: Geochemical Techniques in Exploration
  8. A Revised Equation for the NA/K Geothermometer
  9. Some Remarks on the Application of Geochemical Techniques in Geothermal Exploration
  10. Chemical Equilibria in Icelandic Geothermal Systems—Implications for Chemical Geothermometry Investigations
  11. 11.0 11.1 Geothermal Solute Equilibria: Derivation of Na-K-Mg-Ca Geoindicators
  12. An Empirical Na-K-Ca Geothermometer for Natural Waters
  13. A Magnesium Correction for the Na-K-Ca Chemical Geothermometer
  14. Hydrothermal Convection Systems with Reservoir Temperatures ≥ 90°C
  15. Chapter 6: Chemical Geothermometers and Their Application to Formation Waters from Sedimentary Basins
  16. Thermal and Mineral Waters of Nonmeteoric Origin, California Coast Ranges
  17. 17.0 17.1 Michael L. Sorey,Gene A. Suemnicht,Neil C. Sturchio,Gregg A. Nordquist. 12/1991. New Evidence On The Hydrothermal System In Long Valley Caldera, California, From Wells, Fluid Sampling, Electrical Geophysics, And Age Determinations Of Hot-Spring Deposits. Journal of Volcanology and Geothermal Research. 48(3-4):229-263.
  18. Christopher Farrar,Jacob DeAngelo,Colin Williams,Frederick Grubb,Shaul Hurwitz. 2010. Temperature Data From Wells in Long Valley Caldera, California. (!) : U.S. Geological Survey. Report No.: Data Series 523.
  19. Fraser Goff,Harold A. Wollenberg,D. C. Brookins,Ronald W. Kistler. 12/1991. A Sr-Isotopic Comparison Between Thermal Waters, Rocks, And Hydrothermal Calcites, Long Valley Caldera, California. Journal of Volcanology and Geothermal Research. 48(3-4):265-281.
  20. Steven Flexser. 1991. Hydrothermal Alteration and Past and Present Thermal Regimes in the Western Moat of Long Valley Caldera. Journal of Volcanology and Geothermal Research. 48(3-4):303-318.
  21. Brian M. Smith,Gene A. Suemnicht. 12/1991. Oxygen Isotope Evidence For Past And Present Hydrothermal Regimes Of Long Valley Caldera, California. Journal of Volcanology and Geothermal Research. 48(3-4):319-339.
  22. GEOTHERM Database
  23. Great Basin Groundwater Geochemical Database
  24. Evaluation of Chemical Geothermometers for Calculating Reservoir Temperatures at Nevada Geothermal Power Plants




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