Field Sampling

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Exploration Technique: Field Sampling

Exploration Technique Information
Exploration Group: Field Techniques
Exploration Sub Group: Field Sampling
Parent Exploration Technique: Field Techniques
Information Provided by Technique
Lithology: Rock samples are used to define lithology. Field and lab analyses can be used to measure the chemical and isotopic constituents of rock samples.
Stratigraphic/Structural: Can reveal relatively high permeability zones. Provides information about the time and environment which formed a particular geologic unit. Microscopic rock textures can be used to estimate the history of stress and strain, and/or faulting.
Hydrological: Water composition and source of fluids. Isotope geochemistry can reveal fluid circulation of a geothermal system.
Thermal: Water temperature. Used to locate active hydrothermal systems. Thermal conductivity of a rock sample can provide information to calculate heat flow. Hydrothermal alteration of a rock sample can indicate certain temperature or fluid compositions.
Field Sampling:
Systematic field sampling is critical for reliable characterize a geothermal resource. Some of the physical and chemical properties of rock samples can be estimated by visual inspection, but accurate determination of these properties requires detailed laboratory analysis. Surface or subsurface fluid sampling is also routinely performed to characterize the chemical, thermal, or hydrological properties of a hydrothermal system. Combinations of these sampling techniques have traditionally been used to obtain important information used to determine whether or not a viable power generation or heat utilization facility can be developed at a prospect. Soil sampling is a less commonly used method for exploration of geothermal resources that lack obvious surface manifestations. Soils that are above or adjacent to a “hidden” hydrothermal system will have a unique chemistry that can be indicative of a hydrothermal system at depth or of zones of relatively high subsurface permeability.
Other definitions:Wikipedia Reegle

Field sampling is done to characterize important properties of the geothermal system under investigation. Reservoir properties determined through analysis of rock samples can assist in defining a geothermal resource by providing information regarding the temperature history, volume, heat source, degree of alteration/water-rock interaction, and rate of fluid recharge that occur in the hydrothermal system. There are dozens of techniques that can be applied to measure physical and chemical properties of a single rock sample, as well as the hydrological and thermal environment that the rock came from. Reservoir rock properties strongly influence the unique chemical signature of geothermal fluids, which evolve through specific processes within the dynamic hydrothermal system. Water and gas samples can be collected at the surface from springs, geysers, fumaroles, or drilled wells, or from a specific depth interval in geothermal wells. For geothermal resource areas that lack surface manifestations (hot springs, fumaroles, mud volcanoes, etc.), soil sampling can be a useful tool for identifying blind geothermal systems at depth. Soils act similar to a sponge and absorb certain elements or mineral alterations that are associated with hydrothermal activity.

Geochemist sampling a cold seep near the Poncha Hot Springs, CO.

Use in Geothermal Exploration
A single rock sample is used to characterize a section of a well or formation, while a collection of rock samples can be used to determine if whether reservoir properties are sufficient to sustain a power generation or heat utilization facility. Drilling wells and collecting drill cores is a common practice for determining the location of a potential geothermal reservoir. Comprehensive characterization of a reservoir requires detailed knowledge and analysis of the rock samples collected from a well. Typically the most critical information gathered are physical properties such as porosity, permeability, and thermal conductivity. However, there are other more detailed techniques that can reveal chemical composition, alteration mineralogy, the peak temperature experienced by the hydrothermal system, etc.

Water and gas sampling are routinely used in geothermal exploration and monitoring to characterize the chemical composition of the fluid, measure the temperature, or conduct isotope studies to derive the provenance of thermal fluids. Fluid sampling is a critical aspect of characterizing a geothermal system because the water chemistry, temperature and source can reveal the quality of the resource. Water chemistry is largely controlled by temperature, water-rock interactions, volume of water vs rock, residence time, and contributions from other fluids (mixing), such as cold groundwater, seawater, magmatic fluids, etc.[1] Waters that discharge at the surface are commonly over saturated with silica or carbonate at surface conditions and precipitate sinter or travertine, respectively.[2][1] Some geothermal fluids that reach the surface form acid-sulfate springs, generated from rising steam and volatile compounds that condense and mix with an overlying freshwater aquifer, whereupon the H2S in the steam oxidizes to form sulfuric acid.[2][1][3]

Soil sampling is typically performed in a methodical and structured way, so that the results of the geochemical analysis can be plotted spatially on a map. An active hydrothermal system releases fluids with chemical signatures that are anomalous in typical surface environments. The most volatile gases are able to escape the heat source and permeate through overlying formations and structures, casting an imprint either on the soil or into the atmosphere that is used to locate a potential geothermal resource. Analysis of soil samples is particularly useful for locating blind or covered structures in geothermal areas, which act as conduits for the ascending fluids. The successful identification of these soils can reveal relatively high permeability zones associated with otherwise unknown geothermal resources.

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