Surveying Natural Gas Emissions in Geothermal Exploration

The spatial variability of gas emissions at Earth's surface is a proxy for structural discontinuities in the subsurface of geothermal systems. Increased gas emissions, e.g. carbon dioxide or methane, indicate permeable segments in fracture networks, which are connected to the deep hydrothermal system. Thus, they provide reliable information on reservoir-scale to reduce the high exploration risk in geothermal projects and contribute to the site selection of costly production wells. In addition, permanent monitoring of volcanic gases at surface allows to investigate temporal changes in gas emissions related to changing reservoir conditions. This represents an additional application of soil gas studies for a successful reservoir management. Here, we show a summary of selected case studies in geothermal systems worldwide highlighting the successful performance of comprehensive and systematic soil gas studies as a reliable geothermal exploration tool. In our studies we use integrated approaches by analyzing emission rates, concentration, and isotopes of the different volcanic gases and linking them to the deep reservoir. We consider soil gas studies as a complementary technique to well-established geophysical or geochemical exploration methods, which will help to improve conceptual models of geothermal systems. For this purpose, we have developed a mobile soil gas lab for flexible in-situ applications. New projects, like fLUXtec./, will focus on the optimization and promotion of this type of survey, which is not yet commonly used in geothermal exploration.