Frery, and 3 co-authors, reported in 2021 that natural hydrogen seeps called fairy circles have been identified in Mali, Brazil, Russia, and United States. Natural hydrogen is produced from a field in Mali for 7 years. The natural hydrogen system is still poorly understood and needs to be studied in new geological contexts as to streamline hypotheses on the system dynamics and elements into concepts. In Australia, numerous circular surface features, commonly called salt lakes or swaps, are visible from the sky but no existing work shows nor quantifies any hydrogen content in those features.
In this study, the authors reviewed the existing literature on fairy circles as to determine and directly test with soil-gas measurements if hydrogen surface-emitting features are present in Australia. They determined the best candidates to test were with a multi-disciplinary approach linking geology, multi-physical imaging, and seismic interpretation. Soil-gas measurements showed persistent hydrogen concentration localized in the external ring of circular depressions aligned along the Darling Fault, a major crustal boundary between the granitic, mafic and ultramafic rocks of the Yilgarn Craton from the sedimentary rocks of the Perth Basin.
This work proved the existence of so-called fairy circles, in the meaning of H2 emitting structures, and they are present in Australia and opens the door to new prospectivity pathways by evaluating original hypotheses on natural hydrogen generation, migration pathways and entrapment.
This geological setting promotes deep serpentinization of ultramafic rocks as well as oxidation of iron-rich Archean rocks and mafic dikes as potential hydrogen sources that are both of massive potential economic value. This hydrogen can circulate and be entrapped as aqueous hydrogen in low-salinity aquifers or migrate in gaseous phase in fault zones up to intermediate structural reservoir or to the surface.
Conclusive remarks from this study include:
- They determined best surface depression targets to analysis by cross-reference of size, geometry, distribution and vegetation with known fairy circles in Brazil, Mali, Russia and the United States and a multi-disciplinary approach integrating interpretation of geophysical data and field geology.
- Soil-gas measurements in the Moora-Pingarrega region, in the North Perth Basin (Western Australia) reveal the present of hydrogen in the external ring of circular surface depressions. We are improving our measurement method to mitigate the proportions of the soil-gas leaking between the drilling and measurement operations.
- Interpretation of seismic data underneath studied fairy circles strongly suggests that faults can act as vertical migration pathways for hydrogen.
- The hydrogen soil-gas measured values are not function of the depression slope or vegetation. However, a correlation between the near-surface geology and the intensity of the signal is noticeable.
Our region generation model relies on the hydrogen being generated by two main sources:
- A deep source linked with the serpentinization of the ultramafic rocks with a major source located below the Yilgarn Craton and a potential secondary source located in the Pinjarra Orogen rocks located below the North Perth Basin;
- A shallower source located in the first kilometer below the surface that is linked with the oxidation of the mafic dikes, iron-rich granites and the iron rich rocks of the Archean Great oxide event massively presented in the proximal Yilgarn Craton and the Pinjarra Orogen.
- The authors' regional migration model suggests that the hydrogen can migrate in gaseous phase via major faults embedded in the basement as well as in aqueous phase in low salinity aquifers that could be shallowed than a kilometer depth.
- The authors' regional migration model also suggests that the hydrogen can be entrapped in low-salinity aquifers as mentioned in the previous points or below very-low permeability formations (e.g. salt or shale).
- The investigation of fairy circles and the deployment of light weight gas analyzer could present a time- and cost-effective way to assess the natural hydrogen system of different regions and demonstrate the presence of economic natural hydrogen resources.
Further investigations on seep monitoring, total gas geochemical signature, isotopy, and link with the biostratum is necessary to determine between the hypotheses on this hydrogen system.
Read original URL: https://www.sciencedirect.com/science/article/abs/pii/S0360319921026045
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