8.3 Geochemical surveys
9 min read•august 21, 2024
are essential tools in geothermal systems engineering. They provide crucial data on subsurface conditions and resource potential by systematically collecting and analyzing chemical information from geological materials. This helps identify and characterize geothermal reservoirs.
These surveys have various applications, from identifying potential resources to assessing reservoir characteristics. They involve different types of surveys, including soil gas, water chemistry, and rock alteration analyses. Proper sampling techniques and methods are crucial for obtaining reliable data and making informed decisions in geothermal exploration and development projects.
Principles of geochemical surveys
- Geochemical surveys play a crucial role in geothermal systems engineering by providing valuable information about subsurface conditions and resource potential
- These surveys involve systematic collection and analysis of chemical data from various geological materials to identify and characterize geothermal reservoirs
Objectives and applications
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- Identify potential geothermal resources through chemical signatures in fluids and rocks
- Assess reservoir characteristics (temperature, pressure, fluid composition) to determine economic viability
- Monitor changes in geothermal systems over time for sustainable resource management
- Support decision-making processes for geothermal exploration and development projects
Types of geochemical surveys
- Soil gas surveys measure concentrations of gases (CO2, H2S, radon) emanating from geothermal systems
- Water chemistry surveys analyze chemical composition of surface and groundwater samples
- Rock alteration surveys examine mineralogical changes caused by geothermal fluid interactions
- Isotope surveys utilize stable and to trace fluid origins and movement
Sampling techniques and methods
- Systematic grid sampling ensures comprehensive coverage of the study area
- Depth-specific sampling targets different geological formations or aquifers
- Time-series sampling captures temporal variations in geochemical parameters
- Remote sensing techniques (hyperspectral imaging) complement ground-based sampling efforts
- Quality control measures include duplicate samples and field blanks to ensure data reliability
Geothermal fluid chemistry
- Understanding is essential for characterizing reservoir conditions and optimizing geothermal energy production
- Chemical composition of geothermal fluids provides insights into reservoir temperature, fluid origin, and potential scaling or corrosion issues
Major chemical constituents
- Chloride (Cl-) often dominates in high-temperature geothermal fluids, indicating deep circulation
- Bicarbonate (HCO3-) predominates in lower-temperature systems or near-surface waters
- Sodium (Na+) and potassium (K+) concentrations reflect water-rock interactions and temperature
- Silica (SiO2) content serves as a key geothermometer for estimating reservoir temperatures
- Sulfate (SO4 2-) levels can indicate oxidation processes or magmatic influence
Trace elements and isotopes
- Lithium (Li) concentrations often correlate with reservoir temperatures and fluid residence time
- Boron (B) serves as an indicator of fluid origin and potential magmatic input
- (δ18O and δD) help determine fluid sources (meteoric, magmatic, or marine)
- Radioactive isotopes (3H, 14C) provide information on fluid age and residence time
- Rare earth elements (REEs) can reveal insights into water-rock interactions and fluid pathways
pH and conductivity measurements
- pH values typically range from 5 to 9 in geothermal fluids, influencing mineral solubility
- Acidic fluids (pH < 7) may indicate presence of volcanic gases or steam-heated waters
- Neutral to alkaline fluids (pH ≥ 7) are common in many high-temperature geothermal systems
- Electrical conductivity correlates with total dissolved solids (TDS) content
- Conductivity measurements help identify zones of fluid upflow or mixing in geothermal areas
Geothermometers
- Geothermometers are chemical indicators used to estimate subsurface reservoir temperatures
- These tools rely on temperature-dependent chemical reactions and equilibria in geothermal systems
Silica geothermometers
- Based on temperature-dependent solubility of silica minerals (quartz, chalcedony, amorphous silica)
- Quartz geothermometer works well for high-temperature systems (>150°C)
- Chalcedony geothermometer more suitable for lower-temperature systems (<150°C)
- Amorphous silica geothermometer applicable in very low-temperature environments
- assume equilibrium between fluid and host rock, and no mixing or precipitation
Cation geothermometers
- Na-K geothermometer based on temperature-dependent ion exchange between feldspars
- Na-K-Ca geothermometer incorporates calcium to account for water-rock interactions
- K-Mg geothermometer useful for estimating temperatures in rapidly equilibrating systems
- Li-Mg geothermometer applicable in chloride-rich geothermal fluids
- generally more resistant to dilution effects than silica geothermometers
Gas geothermometers
- CO2-CH4-H2 geothermometer based on equilibrium between carbon and hydrogen species
- H2S-H2-CH4 geothermometer utilizes sulfur-carbon-hydrogen equilibria
- CO-CO2 geothermometer relies on carbon monoxide-carbon dioxide equilibrium
- particularly useful in vapor-dominated geothermal systems
- Often applied in conjunction with liquid-phase geothermometers for comprehensive temperature estimation
Geochemical indicators
- Geochemical indicators provide valuable information about reservoir conditions, fluid origins, and subsurface processes
- Interpretation of these indicators requires consideration of multiple factors and cross-validation with other data sources
Reservoir temperature estimation
- Combination of multiple geothermometers yields more reliable temperature estimates
- Silica-enthalpy mixing models account for dilution effects on silica concentrations
- Isotope (e.g., 18O-SO4-H2O) provides independent temperature estimates
- Fluid inclusion studies in hydrothermal minerals offer direct evidence of past temperatures
- Consideration of reservoir processes (boiling, mixing) essential for accurate temperature estimation
Fluid origin and evolution
- Cl/B ratios help distinguish between magmatic, marine, and meteoric fluid sources
- δ18O and δD values plotted against global meteoric water line reveal fluid source and evolution
- 87Sr/86Sr ratios indicate extent of water-rock interaction and lithological influences
- Noble gas isotopes (3He/4He) provide insights into mantle contribution to geothermal fluids
- Trace element patterns reflect fluid-rock interactions and potential magmatic input
Mixing and dilution processes
- Conservative element ratios (e.g., Cl/B) remain constant during mixing, aiding in identification
- Silica-enthalpy diagrams reveal mixing trends between hot and cold end-members
- Isotope mixing lines on δ18O vs δD plots indicate mixing between different water types
- Cl-HCO3-SO4 ternary diagrams illustrate mixing between different fluid types
- Recognition of mixing and dilution crucial for accurate interpretation of geothermometer results
Sampling and analysis techniques
- Proper sampling and analysis techniques are critical for obtaining reliable geochemical data
- Adherence to standardized protocols ensures data quality and comparability across different studies
Field sampling procedures
- Use of appropriate sampling containers (plastic, glass) based on target analytes
- Field filtration (0.45 μm filters) for dissolved constituents analysis
- Acidification of samples for cation preservation (pH < 2)
- On-site measurements of unstable parameters (pH, temperature, conductivity)
- Proper labeling, storage, and transportation of samples to maintain integrity
Laboratory analysis methods
- Inductively Coupled Plasma (ICP-MS) for trace element analysis
- Ion Chromatography (IC) for major anion determination
- Atomic Absorption Spectroscopy (AAS) for major cation analysis
- Gas Chromatography (GC) for dissolved gas composition
- Isotope Ratio Mass Spectrometry (IRMS) for stable isotope measurements
Quality control and assurance
- Implementation of chain of custody procedures for sample tracking
- Analysis of field and laboratory blanks to detect contamination
- Use of certified reference materials to ensure analytical accuracy
- Duplicate sample analysis to assess precision and reproducibility
- Inter-laboratory comparisons to validate results and methodologies
Data interpretation and modeling
- Geochemical data interpretation requires integration of multiple techniques and consideration of geological context
- Advanced modeling tools aid in understanding complex geochemical processes and predicting system behavior
Geochemical plots and diagrams
- classify water types based on major ion compositions
- assess fluid-mineral equilibrium states
- compare relative concentrations of major ions across samples
- provide visual representation of major ion concentrations
- illustrate relationships between major cations and anions
Multivariate statistical analysis
- Principal Component Analysis (PCA) identifies main factors controlling geochemical variations
- Cluster Analysis groups samples with similar geochemical characteristics
- Discriminant Analysis classifies samples based on predefined geochemical criteria
- Factor Analysis reveals underlying relationships between geochemical variables
- Time series analysis detects temporal trends in geochemical parameters
Geochemical modeling software
- PHREEQC simulates geochemical reactions and calculates mineral saturation indices
- SOLVEQ-XPT models fluid-mineral equilibria at high temperatures and pressures
- The Geochemist's Workbench provides tools for reaction path modeling and phase diagrams
- EQ3/6 performs speciation calculations and reaction path simulations
- TOUGHREACT couples geochemical reactions with heat and mass transport simulations
Environmental considerations
- Geothermal fluid chemistry can have significant environmental impacts that must be addressed
- Proper management and mitigation strategies are essential for sustainable geothermal development
Fluid discharge impacts
- Thermal pollution in surface waters can affect aquatic ecosystems
- High concentrations of dissolved solids may contaminate soil and groundwater
- Release of toxic elements (As, B, Hg) poses risks to human health and environment
- Gaseous emissions (H2S, CO2) contribute to air quality concerns
- Proper reinjection practices minimize environmental impacts of geothermal fluid disposal
Scaling and corrosion issues
- Silica scaling occurs when supersaturated fluids cool and precipitate amorphous silica
- Carbonate scaling (calcite, aragonite) forms due to pressure drops and CO2 degassing
- Sulfide scaling (pyrite, chalcopyrite) results from changes in fluid chemistry or temperature
- Chloride-rich fluids can cause severe corrosion of well casings and surface equipment
- Scale inhibitors and corrosion-resistant materials mitigate these operational challenges
Mitigation strategies
- Reinjection of spent geothermal fluids maintains reservoir pressure and reduces surface disposal
- Chemical treatment of fluids to prevent scaling and corrosion in power plant components
- Use of binary cycle power plants to isolate geothermal fluids from the environment
- Implementation of air-cooled systems to reduce water consumption and thermal pollution
- Continuous monitoring and adaptive management to address evolving environmental concerns
Integration with other surveys
- Integration of geochemical data with other survey types provides a comprehensive understanding of geothermal systems
- Multidisciplinary approach enhances exploration success and accuracy
Geophysical survey correlation
- Magnetotelluric (MT) surveys detect conductive zones associated with geothermal fluids
- Gravity surveys identify density contrasts related to hydrothermal alteration or fluid circulation
- Seismic surveys reveal structural features controlling fluid flow and reservoir geometry
- Heat flow measurements corroborate geochemical temperature estimates
- Integration of geochemical and geophysical data improves conceptual models of geothermal systems
Geological survey integration
- Surface identifies potential fluid upflow zones and structural controls
- Stratigraphic analysis provides context for interpreting fluid chemistry and circulation patterns
- Petrological studies reveal water-rock interactions and alteration mineralogy
- Structural geology informs fluid flow pathways and reservoir compartmentalization
- Integration of geochemical and geological data refines conceptual models of geothermal systems
Reservoir characterization synthesis
- Combines geochemical, geophysical, and geological data to develop comprehensive reservoir models
- Identifies key reservoir parameters (temperature, permeability, fluid chemistry) for
- Guides well targeting and development strategies based on integrated data interpretation
- Supports numerical simulation of reservoir behavior and production scenarios
- Facilitates risk assessment and decision-making in geothermal exploration and development projects
Case studies
- Case studies provide valuable insights into the application of geochemical surveys in real-world geothermal projects
- Lessons learned from these studies inform best practices and improve future exploration and development efforts
Successful geochemical survey examples
- Geysers geothermal field (California, USA) utilized gas geochemistry to optimize steam production
- Olkaria geothermal field (Kenya) employed fluid geochemistry to guide reservoir management
- Wairakei geothermal field (New Zealand) used geochemical monitoring to assess reservoir sustainability
- Larderello geothermal field (Italy) applied to understand fluid origins and recharge
- Salton Sea geothermal field (California, USA) utilized geochemistry to characterize high-salinity brines
Challenges and limitations
- Fluid mixing and dilution can complicate geothermometer interpretations
- Non-equilibrium conditions may lead to inaccurate temperature estimates
- Limited surface manifestations in blind geothermal systems pose sampling challenges
- Temporal variations in fluid chemistry require long-term monitoring for accurate characterization
- Complex geological settings can result in ambiguous or conflicting geochemical signals
Lessons learned and best practices
- Integration of multiple geochemical techniques improves reliability of reservoir assessments
- Regular sampling and monitoring programs essential for detecting temporal changes
- Consideration of geological context crucial for accurate interpretation of geochemical data
- Collaboration between geochemists, geologists, and engineers enhances project outcomes
- Adaptive exploration and development strategies based on evolving geochemical understanding
- Investment in high-quality sampling and analysis pays dividends in improved decision-making
Key Terms to Review (31)
Baseline geochemistry: Baseline geochemistry refers to the natural concentrations and variations of chemical elements and compounds found in geological materials, such as rocks, soils, and groundwater, before any anthropogenic or external influences. Understanding baseline geochemistry is crucial for assessing environmental changes, particularly in the context of resource exploration and environmental monitoring.
Carbon dioxide concentration: Carbon dioxide concentration refers to the amount of carbon dioxide (CO2) present in a given volume of air or water, usually expressed in parts per million (ppm) or as a percentage. This measurement is critical in understanding the geochemical processes occurring in geothermal systems, as higher concentrations can indicate volcanic activity or hydrothermal systems, and influence the potential for energy production.
Cation Geothermometers: Cation geothermometers are tools used to estimate the temperature of geothermal fluids based on the concentrations of specific cations (positively charged ions) present in the fluid. These geothermometers rely on the principle that the equilibrium distribution of these cations varies with temperature, allowing scientists to infer thermal conditions from their relative abundances. This makes cation geothermometers particularly useful for understanding geothermal systems and assessing the potential energy resources within them.
Contamination risk: Contamination risk refers to the potential for harmful substances or pollutants to negatively impact environmental and human health. This concept is critical in various industries, as it influences decision-making processes related to resource management, safety protocols, and regulatory compliance.
David H. Christensen: David H. Christensen is a notable figure in the field of geothermal energy, particularly recognized for his contributions to the understanding and implementation of geochemical surveys in geothermal systems. His work has helped advance methodologies for assessing geothermal resources, emphasizing the importance of geochemical data in identifying potential sites for development and enhancing the efficiency of geothermal energy extraction.
Gas geothermometers: Gas geothermometers are tools used to estimate the temperature of geothermal fluids by analyzing the concentrations of certain gases, such as CO$_2$, H$_2$S, and CH$_4$, dissolved in those fluids. These geothermometers rely on the known relationships between gas solubility and temperature, allowing researchers to infer thermal conditions of geothermal reservoirs. By measuring the amounts of these gases, insights into the geochemistry of geothermal fluids can be gained, which is crucial for understanding geothermal systems and conducting effective geochemical surveys.
Gas Sampling: Gas sampling is the process of collecting gas samples from the environment, typically to analyze their composition and concentrations. This technique is crucial for understanding geochemical processes and assessing the presence of geothermal resources, as different gases can indicate various subsurface conditions and geothermal activities.
Geochemical surveys: Geochemical surveys are systematic investigations of the chemical composition of geological materials, primarily soil, rock, and water, to identify variations in geochemical properties that can indicate the presence of geothermal resources. These surveys are essential in locating and evaluating potential geothermal sites by analyzing elements, minerals, and compounds that may suggest subsurface heat flow and reservoir characteristics.
Geological mapping: Geological mapping is the process of creating detailed representations of the Earth's surface and subsurface geology, highlighting the distribution, nature, and age of rock formations and geological features. This practice is essential for understanding geological structures, identifying natural resources, and assessing geological hazards. It plays a critical role in resource exploration and management, as well as informing environmental assessments and land-use planning.
Geothermal fluid chemistry: Geothermal fluid chemistry refers to the study of the chemical composition and properties of fluids found in geothermal systems, which can include water, steam, and dissolved minerals. Understanding this chemistry is crucial for evaluating the potential of geothermal resources, assessing environmental impacts, and optimizing energy production from these systems.
Geothermometry: Geothermometry is a method used to estimate the temperature of geothermal systems by analyzing the chemical composition of geothermal fluids. This technique is crucial for understanding the thermal conditions and processes occurring in geothermal reservoirs, helping to identify potential resources and optimize their utilization.
Giggenbach Ternary Diagrams: Giggenbach Ternary Diagrams are graphical representations used in geochemistry to analyze the relationships between three different chemical species in geothermal fluids. These diagrams help identify the chemical composition and origin of geothermal fluids, as well as their temperature and pressure conditions, which are crucial for understanding geothermal systems.
Hydrogen sulfide: Hydrogen sulfide is a colorless gas known for its distinct rotten egg smell, produced naturally through the breakdown of organic matter and is often found in geothermal systems. In the context of geochemical surveys, it serves as an important indicator of geothermal activity, as high concentrations can indicate the presence of geothermal fluids that originate from deep within the Earth.
Isotope geochemistry: Isotope geochemistry is the study of the distribution and abundance of isotopes in geological materials, which provides valuable information about the processes that shape the Earth's composition and history. By analyzing isotopic ratios, scientists can gain insights into various geological phenomena, such as temperature changes, fluid interactions, and the age of rocks and minerals. This field plays a crucial role in understanding the Earth’s formation, evolution, and the mechanisms driving geochemical cycles.
Langelier-Ludwig Plots: Langelier-Ludwig plots are graphical representations used to assess the chemical balance of water, particularly in relation to its scaling potential and corrosivity. These plots help in understanding the carbonate equilibrium in water samples and can indicate the stability of water with respect to calcium carbonate precipitation, which is crucial for evaluating water quality in geothermal systems.
Mass spectrometry: Mass spectrometry is an analytical technique used to measure the mass-to-charge ratio of ions, allowing for the identification and quantification of molecules in a sample. This technique plays a crucial role in geochemical surveys by providing detailed information about the elemental and isotopic composition of materials, which helps in understanding the processes occurring within geothermal systems.
Mineral dissolution: Mineral dissolution is the process by which minerals break down and dissolve into their constituent ions in a solution, usually due to the influence of water or other solvents. This process is crucial for understanding the geochemical processes that affect groundwater quality, mineral stability, and nutrient availability in geothermal systems. The rate of dissolution can be influenced by factors such as temperature, pressure, and the presence of other dissolved substances.
Petrography: Petrography is the branch of geology that focuses on the detailed description and classification of rocks, particularly through thin section analysis under a microscope. This method allows geologists to observe the mineralogical composition, texture, and structure of rocks, providing essential insights into their origin and formation processes. Understanding petrography is crucial for geochemical surveys as it aids in linking rock characteristics to geothermal systems and potential resource locations.
Piper Diagrams: Piper diagrams are graphical representations used to visualize the chemical composition of water samples, particularly in hydrogeology and geochemistry. They help in interpreting water quality data by plotting the relative proportions of major ions, making it easier to identify water types and assess their chemical relationships. By providing a visual format, these diagrams facilitate the comparison of different water sources and support geochemical surveys in understanding groundwater and surface water chemistry.
Radioactive isotopes: Radioactive isotopes are variants of chemical elements that have unstable nuclei and emit radiation during their decay process. This characteristic makes them useful in various applications, including the analysis of geothermal fluids and the execution of geochemical surveys, as they can provide insights into the origin and evolution of geothermal systems and help in tracing fluid movement and interactions within the Earth's crust.
Reservoir characterization: Reservoir characterization is the process of defining and understanding the properties and behavior of a geothermal reservoir, including its physical, thermal, and hydraulic characteristics. This understanding is essential for effective resource management, development planning, and optimizing energy extraction. It involves various techniques to assess how fluids move within the reservoir, the heat content available, and how these factors influence well performance and sustainability.
Resource Assessment: Resource assessment is the systematic evaluation of geothermal resources to determine their availability, potential energy output, and economic viability. This process helps identify suitable sites for geothermal development by analyzing geological, geochemical, and engineering data to estimate the capacity and characteristics of geothermal reservoirs.
Robert E. McNitt: Robert E. McNitt is a prominent figure in the field of geothermal energy, known for his contributions to geochemical surveys and research methodologies that enhance the exploration and development of geothermal systems. His work emphasizes the significance of geochemical analysis in identifying geothermal resources, helping to improve the understanding of subsurface conditions necessary for effective geothermal energy production.
Schoeller Diagrams: Schoeller diagrams are graphical representations used in geochemistry to display the chemical composition of water samples, highlighting variations in ion concentrations. These diagrams help visualize geochemical data, making it easier to interpret the relationships between different elements and compounds present in geothermal systems.
Silica geothermometers: Silica geothermometers are tools used to estimate the temperature of geothermal systems based on the concentration of dissolved silica in hydrothermal fluids. They work on the principle that the solubility of silica in water changes with temperature, allowing scientists to infer subsurface temperatures by analyzing surface water samples. Understanding these temperature estimates is crucial for assessing geothermal resources and conducting effective geochemical surveys.
Soil gas analysis: Soil gas analysis is a geochemical method used to sample and analyze gases present in the soil, typically to assess subsurface conditions and detect potential geothermal resources. This technique is valuable for identifying the concentration of gases such as carbon dioxide, methane, and radon, which can indicate geological processes or the presence of geothermal activity. By understanding the composition of soil gases, researchers can infer information about the underlying geology and potential geothermal reservoirs.
Stable isotopes: Stable isotopes are non-radioactive variants of elements that have the same number of protons but different numbers of neutrons, resulting in different atomic masses. These isotopes do not undergo radioactive decay, making them useful for a variety of scientific applications, particularly in understanding geochemical processes and tracing the origins and movements of fluids in geothermal systems.
Stiff diagrams: Stiff diagrams are graphical representations used to illustrate the chemical composition of water samples, specifically in geochemical surveys. These diagrams help visualize the relationships between different ions and can reveal important insights about the water’s source, its geochemical processes, and any potential contamination. By displaying the concentration of major cations and anions, stiff diagrams provide a quick way to assess water quality and characterize geothermal systems.
Thermal convection: Thermal convection is the process of heat transfer through the movement of fluids, driven by temperature differences within the fluid. As a fluid is heated, it becomes less dense and rises, while cooler fluid descends to take its place, creating a continuous circulation pattern. This process plays a crucial role in geothermal systems as it affects heat distribution and fluid movement, and is also significant in understanding the geochemical properties of fluids.
Trace elements: Trace elements are chemical elements present in minute quantities in a substance but are essential for various biological and geochemical processes. Their presence in geothermal systems can provide important insights into the chemical composition of fluids and rocks, and they play a critical role in understanding geothermal energy potential and environmental impacts.
X-ray fluorescence: X-ray fluorescence (XRF) is a non-destructive analytical technique used to determine the elemental composition of materials by measuring the fluorescent X-rays emitted from a sample when it is excited by high-energy X-rays. This method allows scientists to analyze the chemical makeup of rocks, minerals, and other geological samples, making it a vital tool in geochemical surveys for understanding subsurface conditions.