Understanding natural hydrogen systems: From generation to surface emissions

Natural hydrogen (H₂), often referred to as white hydrogen, is attracting increasing attention as a potential subsurface energy resource. Its occurrence, migration, and preservation are strongly controlled by faults and fracture networks, which regulate fluid flow, fluid–rock interactions, and overall reservoir integrity. This contribution provides a state-of-the-art review of current research on natural hydrogen systems, with particular focus on the role of fault and fracture zones and on recent advances from Italy as an emerging natural laboratory.

At the global scale, natural hydrogen has been reported in a wide range of structurally complex geological settings, including rift zones, ophiolitic complexes, mid-ocean ridges, sedimentary basins, and fractured crystalline basement (e.g., Zgonnik, 2020; Wang et al., 2023; Sequeira et al., 2025; Gorain, 2025). Hydrogen can be generated through multiple processes—such as serpentinization, radiolysis, organic matter pyrolysis, and mantle degassing—that commonly operate in tectonically active and faulted environments. Owing to its small molecular size and high diffusion coefficient, hydrogen migration is particularly sensitive to fracture connectivity, fault permeability, and fault (re-) activation, making structural architecture a primary control on both accumulation and leakage.

Field observations, well data, and monitoring studies indicate that hydrogen frequently migrates along fault and fracture networks, may accumulate transiently within structurally controlled traps, or is released at the surface through focused seepage (Prinzhofer et al., 2019; Baciu and Etiope, 2024). Recent studies emphasize that circulation of hydrogen-rich fluids within fault zones can significantly modify the mechanical and transport properties of host rocks through fluid–rock interactions, potentially leading to either enhanced or reduced permeability and sealing capacity (Sequeira et al., 2025; Gorain, 2025). These coupled processes have important implications for fault stability, leakage risk, and the long-term viability of subsurface energy systems.

In this context, Italy is a particularly favourable setting for research on natural hydrogen. The country hosts a broad spectrum of geological environments conducive to hydrogen generation and migration, including ophiolites, such as those exposed in the Tuscan–Emilian Apennines, active fault systems, geothermal areas, and sedimentary basins sealed by evaporites. Recent structural, geochemical, and geophysical studies suggest that the occurrence of hydrogen in Italy is closely linked to fault architecture, deformation processes, and multiscale fluid circulation (Azor de Freitas et al., 2025).

By integrating global observations with insights from Italian case studies, this review outlines current research trends, identifies key knowledge gaps, and highlights the need for multidisciplinary approaches combining field investigations, monitoring of potential gas emissions from active fault systems, interpretation of subsurface data and conceptual modelling of potential reservoirs and hydrogen emission areas. These insights are directly relevant to low-carbon energy exploration and to the assessment of fault-controlled leakage, reservoir performance, and system stability in subsurface energy applications.