Natural hydrogen systems: understanding accumulation and confronting subsurface challenges

Natural hydrogen is increasingly recognized as a potential low-emission energy resource, with exploration expanding across diverse geological settings. Unlike conventional hydrocarbons, natural hydrogen systems can involve ongoing generation, rapid migration, and strong interactions with rocks, fluids, and microbial communities. These features limit the direct transfer of established petroleum workflows and create uncertainty in resource estimation, producibility, and long-term performance. Here we synthesize current understanding of natural hydrogen from a subsurface and reservoir-engineering perspective, focusing on the processes that control generation, accumulation, preservation, and recoverability. We assess abiotic and biotic generation pathways, including water–rock reactions and microbial processes, and discuss the geological and kinetic controls that govern hydrogen supply at depth. We then examine exploration and appraisal challenges, highlighting why reserves are difficult to define in systems where charge and loss may occur simultaneously, and we use comparisons with petroleum and helium systems to clarify differences in trapping efficiency and depletion behavior. We discuss integrity requirements across the reservoir–well–caprock system, including material compatibility and leakage risks. We also outline the need for integrated modelling that couples flow, reactive transport and geomechanics, supported by data-driven methods where field datasets are sparse. Then, we review emerging production and enhancement strategies, including stimulation and controlled injection, and summarize how life-cycle assessment can support environmental evaluation across the natural hydrogen value chain. Finally, key research gaps are identified and proposed for consideration and future development.