Trapping processes of large volumes of natural hydrogen in the subsurface: The emblematic case of the Bourakebougou H2 field in Mali

Most of the ongoing discussions about natural hydrogen (H2) exploration focus on surface hydrogen occurrences and generation processes. However, the trapping process is the most critical issue with regard to both discovering significant accumulations of hydrogen in the subsurface and providing profitable clean energy production for humankind. The extremely high mobility of hydrogen molecules is due to their very small size that confers very diffusive properties, and this results in frequent surface seepages that have recently been recorded worldwide. During this study, we performed a detailed characterization of the caprocks (dolerites) that retain hydrogen in the Bourakebougou natural H2 field. Our investigation on the sealing capacity of these dolerites revealed that their thickness is important in addition to the density and size of their fractures. The role of aquifers has also been highlighted as contributing to the retention of H2 in subsurface. Indeed, hydrogen being poorly soluble in water at low pressure and temperature conditions, it is less likely to diffuse easily to the surface at shallow depth. This study highlights that the exploration of H2 fields should not be based only on the presence of an H2 generation process, but also based on the presence of a very efficient trapping system.