Shallow Hydrothermal Gas Sampling and Analysis as an Exploration Tool for Magmatic Geothermal Sources

Authors: Klingel, Eric.; Fischer, Tobias; Hanson, James
Keywords: Geothermometers; Lesser Antilles; Soil gas; Magmatic; Geothermal exploration; Gas analysis
Conference: Geothermal Resources Council Transactions Session: Exploration; Geochemical surveys; Geothermometers
Year: 2011 Language: English
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Abstract: Hydrothermal soil gas sampling and analysis procedure is a promising geothermal exploration tool for establishing magmatic sources, resource temperature estimates and siting locations for surface geophysics and slim holes. The procedure draws from established work from volcanic research, environmental soil vapor surveys and mineral exploration techniques. The hydrothermal soil gas survey takes gas samples in suspected areas of magmatic activity from an average depth of six feet by using driven soil gas probes and prepared glass gas sampling bottles. The collected gas is then analyzed for hydrothermal gases of magmatic origin, i.e., CO2, CH4, CO, He, H2S, and H2 and the air-derived gases N2, O2 and Ar. Recent hydrothermal gas sampling areas were located on an island of the lesser Antilles. The locations of the gas samples were based on areas of the most recent geothermal activity, accessibility in terms of drilling, plant construction and transmission line logistics and land control. Within these areas, nineteen gas sampling locations were identified, sampled and analyzed. Soil gas and headspace gas of water samples were analyzed for CO2, H2S, CH4, CO, N2, Ar, O2, He, and H2 contents. Detection limits vary depending on the gas species. The soil gas samples reported here represent a mixture of components: 1) air (N2, O2, Ar), 2) shallow organic volatiles (some of the N2, and some of the CH4), and 3) a deep component (CO2, H2, H2S, He). The deep component represents inputs from a) the mantle/magma and b) the hydrothermal fluid that moves from an area close to the magma towards the ocean. For the two samples that have the highest CO2 contents and CO2/Ar ratios the CO2/N2 and the CO2/Ar geothermometer give quite similar results; 141°C and 125°C for the crater and 136°C and 110°C for a flank sample. Notably, the H2S/Ar geothermometer and the widely-used CO2-CH4-H2S-H2 geothermometer give results within ± 10°C for most samples. The CO2-CH4 geothermometer, gives a temperature of 323 °C for the crater sample, which is realistic for deep temperatures in the magmatic hydrothermal systems of active volcanoes. For the flank samples that had detectable CH4 values ranged from 221 to 271 °C, again realistic for the deep temperatures of volcano-hosted geothermal systems.
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