| Title | Volcano-Tectonic Structures and Their Influence on Soil Gas Emissions in a Low Permeable Geothermal Reservoir – A Case Study from Los Humeros Volcanic Complex, Mexico |
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| Authors | Anna JENTSCH, Egbert JOLIE, Dave G. JONES, Helen-Taylor CURRAN, Loic PEIFFER, and Martin ZIMMER |
| Year | 2020 |
| Conference | World Geothermal Congress |
| Keywords | Scouting survey, soil degassing, CO2 efflux, carbon and helium isotopes, radon, fault zone analysis, systematic sampling approach |
| Abstract | The Mexican-European research cooperation GEMex enables to study one of the largest geothermal fields in Mexico, the Los Humeros Volcanic Complex (LHVC). By means of different soil gas techniques, we investigated their applicability to understand the structural pattern and characteristics of the volcanic-geothermal system. The geothermal field is characterized by a low permeable caprock where fluid upflow is mainly controlled by volcano-tectonic structures. We developed an optimal sampling approach and network covering the main production zone of the geothermal field measuring different soil gases in order to understand their spatial variability. CO2 efflux measurements were performed by the accumulation chamber technique and elevated emissions were measured along known faults as well as in areas of unknown fault related permeability. This was complemented by alpha spectroscopy for 222Rn and 220Rn emissions as well as soil temperature measurements at 50 cm depth. The results show a positive correlation to CO2 efflux although in a few areas different behavior was observed for these parameters. Additional work focused on the origin of geothermal gases. δ13C analyses of degassing CO2 and 3He/4He ratios can be a useful tool for the differentiation of the source of gas. δ13C results give evidence for a magmatic system with some contribution from the meta carbonates composing the basement. Elevated 3He/4He ratios indicate a mantle contribution in areas of anomalous CO2 and 222Rn degassing. Our results suggest that especially the combination of different soil gas measurements is a useful approach to indicate structural discontinuities in the subsurface that act as migration pathways of hydrothermal fluids. Some of the highest values from the investigated parameters were measured in areas of unknown structural permeability. Furthermore, we would like to point out that a strategic sampling network plays a major role not only to identify but also to determine the geometry and distribution of the most permeable parts along volcano-tectonic structures. |