**9. Conclusions**

Hydrogen exploration requires a combination of the techniques and data used for both conventional petroleum and mining exploration. The first elementary bricks we provide here to evaluate the sources, migration and trapping are certainly not enough, but the following general guidelines will be extremely valuable in targeting the fertile H2 area in intra-cratonic areas.

Explore in old provinces where basement rocks are Archean to Paleoproterozoic. Use lithologies (ultramafic rocks, U-, Th-, K-rich rocks), and He (R/Ra) as pathfinders for the H2 generation potential. Carefully consider the local geothermal gradient as it may be of use to infer fertile zones for active serpentinization.

Identify the location of faults deeply rooted in the basement, such as horst and graben structures. Pay attention to the topography of the unconformity, which represents both a major drainage and trapping area.

Target relatively shallow traps. As for He, H2 partitions into gas are better at a shallow depth. Surface rounded depressions, karsts and sinkholes seem to represent favorable collecting zones prior to H2 escaping into the atmosphere. A field investigation based on electromagnetic and gravimetric prospections could help to characterize the structure of the sinkhole at depth in order to set up a geotechnical drilling at the right location. Such boreholes, carried out in the first hundreds of meters, could provide valuable information on the H2 concentration gradient down to the upper karstic formation, which is often not possible from classical oil and gas exploration well drilling.

Dedicated exploration of boreholes is definitely required to improve this preliminary exploration guide and to strengthen the accuracy for H2 flux measurements. Additional constraints on the

H2–accompanying gases (He, N2, Ne, Hydrocarbons, Rn) and on the role of H2-consuming microbial communities at the subsurface within the emitting structure (Myagkiy et al., 2020) [63] will be extremely valuable.

**Author Contributions:** All authors contributed to the writing of the paper. All authors have read and agreed to the published version of the manuscript.

**Funding:** The work reported here was not supported by any gran<sup>t</sup> by any agency.

**Acknowledgments:** Grateful thanks to Guest Editor Lawrence Cathles and two anonymous reviewers for their constructive reviews. The first author would like to thank Sophie-Adélaïde Magnier for helpful discussions. Laurent Truche gratefully acknowledges the support from the Institut Universitaire de France.

**Conflicts of Interest:** The authors declare no conflict of interest.
