Soil degassing at the Los Humeros geothermal field (Mexico)

Abstract The Los Humeros geothermal field is the third most important producer of geothermal electricity (70 MW) in Mexico. Geothermal fluids are hosted in fractured andesitic lavas and mostly consist of high enthalpy steam with limited water content (vapor fraction > 0.9). Despite the high reservoir temperature (~300–400 °C), thermal manifestations at the surface are scarce and locally appear as steaming grounds, weak steam vents and advanced argillic alteration. Geothermal fluid upflow from the reservoir towards the surface is limited by welded ignimbrite deposits that act as a low-permeability barrier. In this study, we present the first measurements of CO 2 , CH 4 and H 2 S degassing rates from the soil performed at Los Humeros. Flux measurements were complemented with δ 13 C composition of degassing CO 2 and soil temperatures to discuss gas origin and thermal anomalies. We measured high soil degassing rates (up to 7530 g m −2  d −1 CO 2 , 33 g m −2  d −1 CH 4 and 22 g m −2  d −1 H 2 S) in three localized areas (Humeros North – HN, Humeros South – HS and Xalapazco – XA) as well as high soil temperatures reaching the boiling temperature at the local altitude (90.6 °C). The particular location of these three areas suggests that the steam-dominated reservoir degases to the surface through permeable faults crossing the ignimbritic deposits. The remaining surveyed areas are characterized by weak CO 2 fluxes (≤44 g m −2  d −1 ), non-detectable CH 4 and H 2 S fluxes, and lower soil temperatures (5–21 °C). The compositions in δ 13 C CO2 from HN-HS-XA areas (δ 13 C CO2  = −7.94 to −2.73‰) reflect a magmatic source with some possible contribution from the sedimentary basement, as well as fractionation induced by boiling and CO 2 dissolution in shallow water bodies. We also discuss the processes causing the spread in CO 2 /CH 4 flux ratios. Finally, we estimate the heat output from the three high degassing areas to a value of 16.4 MWt.