Gas Sampling

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Exploration Technique: Gas Sampling

Exploration Technique Information
Exploration Group: Field Techniques
Exploration Sub Group: Field Sampling
Parent Exploration Technique: Field Sampling
Information Provided by Technique
Stratigraphic/Structural: High flux can be indicative of conduits for fluid flow.
Hydrological: Gas composition and source of fluids.
Thermal: Anomalous flux is associated with active hydrothermal activity. Distinguish magmatic/mantle heat inputs. Can be used to estimate reservoir fluid temperatures.
Gas Sampling:
Gas sampling is done to characterize the chemical, thermal, and hydrological properties of a surface or subsurface hydrothermal system. Various methods are applied to obtain samples used for determination of the composition of gases present in soils or hydrothermal discharges. The flux of volatile gases emitted from a hydrothermal system can also be determined by measuring the flow of gases at specific locations and comparing it to average background emissions. Anomalously high gas flux can provide an indication of hydrothermal activity at depth that is otherwise not apparent.
Other definitions:Wikipedia Reegle

A variety of gas sampling techniques are used to evaluate and monitor geothermal systems. Surface gas sampling and soil gas sampling may be used to analyze the volatile gas contents emanating from a hydrothermal system. Typical geothermal resources expel anomalously high concentrations of gases (CO2, CH4, N2O, He, etc.) indicating the presence of enhanced permeability and of high temperature resources that may be present at depth. Gas flux sampling is used to study the rate of discharge of volatile gases from the geothermal system. These techniques may also be used for ongoing monitoring of the geothermal system during resource development and production.

U.S. Geological Survey scientist sampling a fumarole at Mount Baker, WA. Photo taken by W. Chadwick in 1981, featured in the Wikipedia public domain image cache.

Use in Geothermal Exploration
Gas sampling of springs, fumaroles, and wellheads is sometimes used in geothermal exploration and monitoring to better characterize the bulk chemical composition of the thermal fluids, measure the temperature and pressure of surface gas discharges, or conduct isotope studies to determine the source of heat sustaining the system. Similar to water sampling, gas sampling is an important tool for characterizing a geothermal system because the unique chemical and isotopic signature of gas discharges can assist in determining the quality of the resource. The gas chemistry relates to reservoir temperature, pressure, water-rock interactions at depth, and mixing with other fluids (such as cold groundwater, seawater, magmatic fluids, etc) prior to phase separation from the thermal fluid.[1] Analysis of gas samples can assist in determining the source of heat in the geothermal system, and may also help to constrain the residence time of thermal fluids within the reservoir. Some gases that reach the surface in geothermal systems 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] These acidic discharges form distinct alteration footprints that may consist of a silica leach capping (consisting predominantly of remnant silica) and/or advanced argillic alteration, both of which may be recognized at the surface using remote sensing or basic field mapping techniques.

Gas flux sampling is considered to be an effective exploration technique for “hidden” geothermal resources that lack obvious surface manifestations, such as fumaroles, geysers, hot springs, etc.[4][5] Hydrothermal activity brings volatile gases to the surface that are chemically and isotopically distinct from meteoric fluids, resulting in an anomalously high flux of specific gas species compared to what is typical of average background gas concentrations. This anomalous gas flux can be measured, and can be used to identify permeable conduits that allow thermal fluids to escape the hydrothermal system.[6][7][4][5]

Soil gas sampling has similar applications, derived from the use of the technique in ongoing measurement of the gas contents of soils in volcanic hazard monitoring. For example, gas flux and soil gas sampling techniques have been used to monitor potentially deadly CO2 emissions at the Long Valley Caldera, CA since the early 90s, when a period of intense seismic activity associated with dike intrusion and/or magmatic fluid migration caused an increase in CO2 flux to the surface, resulting in localized tree kills around Horseshoe Lake at the base of Mammoth Mountain.[8][9]

Best Practices
Gas sampling is best carried out by a qualified hydrologist, geologist, or geochemist familiar with current sampling standards. A practical understanding of how different surface features relate to hydrothermal processes within the geothermal system is also ideal for the purposes of data interpretation, application of various chemical and isotopic geothermometers, and geochemical modeling of the reservoir.
Potential Pitfalls
The various methods for sampling gases in geothermal systems come with limitations that are specific to the technique to be applied. In the case of surface gas sampling, methods are designed to prevent dilution and re-equilibration of samples through exposure to the atmosphere.[10] Failure to adhere to proper sampling procedures and treatment practices can result in re-equilibration of the sample at surface conditions, which disturb the chemical composition of the fluid prior to analysis. These processes shift the fluid chemistry of the sample away from that of fluids present in the geothermal reservoir at depth, which impacts the results of the fluid analyses and will ultimately affect the results of geothermometric calculations and geochemical modeling of the data. In the case of gas flux sampling, different measurement techniques and devices may disrupt or alter the natural flow of gases to the surface, or are unable to measure the flux over a wide area. These limitations can result in data that are not representative of the real flux of gases from the geothermal system.

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