Massive release of natural hydrogen from a geological seep (Chimaera, Turkey): Gas advection as a proxy of subsurface gas migration and pressurised accumulations

Subsurface geological reservoirs of natural hydrogen gas (H2), a clean fuel and energy vector, are currently a target for energy resource exploration. Such reservoirs can be revealed by the presence of H-2 within soil, analogous to hydrocarbon seepage in petroleum systems. Nevertheless, defining the level of soil H-2 that can indicate a potentially economic resource is currently impossible, and identifying geological H-2 within soil-gas is challenging because H-2 concentrations and the isotopic composition (delta H-2) may overlap with the in-situ biological signature. In spite of these limitations, analogies to conventional hydrocarbon systems suggest that the presence of surface advective gas flows can reveal (unlike diffusion) a subsoil source and even pressurised gas accumulations of H-2. Here, a massive release of H-2 is reported from a CH(4)eH(2) rich seep in Turkey, known as Chimaera, an emblematic example of H-2 advection. The site represents the first case where a closedchamber flux method was applied for H-2 seepage. H-2 advection at the site was clearly indicated by numerous gas vents and flames, and by the heterogeneous spatial distribution of pervasive, invisible exhalation (miniseepage), inducing rapid H2 concentration build-up within the chamber. H-2 emission (similar to 10 +/- 3 kg day(-1), with the highest H-2 emission factor reported, thus far, of similar to 5000 kg km(-2) day(-1)) is continuous and long lasting (flames have been documented for millennia) and, using an analogy for hydrocarbon seeps, may stem from pressurised accumulations. The Chimaera case is illustrative of how detecting soil H-2 advection may help unravel surface (biological) vs. subsoil (geological) gas origins in cases where, in the absence of significant gas seepage, soil H2 concentrations are within the range of biological production (10(0)-10(3) ppmv, e.g., as for fairy circles observed in several countries). Interpretations must, however, be supported by additional geochemical data and evaluations of potential biological H-2 production within the surface ecosystem. (c) 2022 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.

Automated summary

Subsurface geological reservoirs of natural hydrogen gas (H2) are currently being explored as clean fuel and energy sources. Detection and identification of the gas in soil remains challenging due to overlapping with biological signatures, but surface advective gas flows may reveal subsoil sources and pressurized gas accumulations. A case study in Turkey, known as Chimaera, demonstrates the massive release of H2 and the potential for using soil H2 advection to differentiate between biological and geological gas origins.