Silica Geothermometers

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Exploration Technique: Silica Geothermometers

Exploration Technique Information
Exploration Group: Geochemical Techniques
Exploration Sub Group: Geochemical Data Analysis
Parent Exploration Technique: Liquid Geothermometry
Information Provided by Technique
Thermal: Used to estimate reservoir temperatures.
Silica Geothermometers:
No definition has been provided for this term.

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.
Use in Geothermal Exploration
Silica and cation geothermometers have been used to estimate reservoir rock-water equilibration temperatures in geothermal resource evaluation since the mid-1980s.[1] Silica geothermometers are based on steady-state ion exchange reactions between hydrothermal fluids and different silica phases in reservoir rocks (i.e., quartz, amorphous silica, chalcedony) at equilibrium conditions.[2] [3] The reaction rates for different silica phases in a hydrothermal system are strongly dependent on temperature, pressure, and fluid acidity.[3] Consequently, the concentrations of dissolved SiO2 in thermal fluid samples with near-neutral pH (between ~5 and 7)[4] relates to equilibration with the various silica phases stable at reservoir temperatures, assuming that sampling was conducted following rapid ascent of the fluid to the surface.[5] 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 silica geothermometers requires knowledge of the most likely reactions that occur between thermal fluids and reservoir rocks at different temperatures.[3] Fournier (1989, 1992) discusses a series of silica geothermometers based on the solubility of amorphous silica, opal, α−cristobalite, chalcedony, and quartz between temperatures of 20° and 250°C:[2] [4]

  • Below 180°C, the chalcedony and quartz geothermometers may be used to estimate reservoir temperatures, as both of these mineral species affect the dissolved silica content of the fluid, depending on the reservoir rock composition. A transitional silica geothermometer has been developed that approximates the calculated temperature between the low temperature chalcedony solubility control and the quartz solubility control at high temperature.[6] The transitional geothermometer eliminates some of the ambiguity in silica-based temperature estimates between 20° and 200°C, and is the preferred method used in the 2008 U.S. Geological Survey Geothermal Resource Assessment.
  • Above 200°C, the quartz geothermometer is considered reliable for estimating reservoir temperatures. Equilibration between thermal fluids and silica species in wall rocks used in the quartz geothermometer takes from tens of hours at 250°C to tens of years at 100°C.[5]
  • At temperatures in excess of 300°C, high concentrations of other dissolved ions increasingly affect silica solubility, leading to erroneous temperature estimates using the silica geothermometers.

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.[7] 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.[8] Temperature gradient data from many of the wells are available online through the U.S. Geological Survey.[9] Relevant data from chemical and isotopic studies published during the same year are also considered in the model.[10][11][12][8] 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,[13] University of Nevada, Reno’s geochemistry database for the Great Basin[14]). 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.
  • In many low-temperature Great Basin geothermal prospects, the silica geothermometer has been considered the most applicable geothermometer, because its thermodynamics have been studied in greater detail than other geothermometers. The fact that there are multiple phases of silica, however, impacts the ability to interpret geothermometry data. Silica geothermometry data can give erroneous results due to the: [15]
    • Presence of high salinity fluids, which alter quartz solubility.
    • Effects of steam separation, which can concentrate the fluid causing early precipitation of silica.
    • Effects of precipitation after sampling, since the rate of quartz precipitation increases drastically as temperature drops.
    • Effect of pH on quartz solubility.
    • Effects of dilution due to cold water mixing.

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.[16]

  1. Advances in the Past 20 Years: Geochemistry in Geothermal Exploration, Resource Evaluation and Reservoir Management
  2. 2.0 2.1 Lectures on Geochemical Interpretation of Hydrothermal Waters
  3. 3.0 3.1 3.2 Geothermometer Calculations for Geothermal Assessment
  4. 4.0 4.1 Chapter 2: Water Geothermometers Applied to Geothermal Energy
  5. 5.0 5.1 Chapter 14: High Temperature Calculations Applied to Ore Deposits
  6. Chapter 5: Chemical Techniques in Geothermal Exploration
  7. Thermal and Mineral Waters of Nonmeteoric Origin, California Coast Ranges
  8. 8.0 8.1 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.
  9. 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.
  10. 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.
  11. 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.
  12. 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.
  13. GEOTHERM Database
  14. Great Basin Groundwater Geochemical Database
  15. Application of Water Geochemistry to Geothermal Exploration and Reservoir Engineering
  16. Evaluation of Chemical Geothermometers for Calculating Reservoir Temperatures at Nevada Geothermal Power Plants

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