Natural hydrogen migration and trapping in the Bulqizë ophiolitic massif (Albania)

The Bulqizë ophiolitic massif (Albania) hosts a significant natural hydrogen system, with underground emissions estimated at several hundred tons of H 2 per year. Despite these high fluxes in mine galleries, no surface seepage has been detected, indicating deep storage and effective sealing. The processes controlling hydrogen accumulation, migration, and structural trapping, however, remain poorly constrained. This study presents the first integrated reservoir-scale characterization of a natural H 2 system in an ophiolite, combining mineralogical, petrophysical, and structural analyses across surface outcrops and underground workings. The aim is to evaluate whether the Bulqizë massif can function as a geological reservoir capable of storing substantial volumes of natural H 2 . Petrophysical analyses reveal that fresh ultramafic rocks have very low primary porosity (12.7 m⁻ 1 near major faults at −110 m bmsl, where the highest H 2 fluxes are observed. Gas emissions are concentrated along a major fault intersecting the chromitite ore body. The absence of surface seepage, combined with decreasing fracture density toward the top of the massif, highlights the presence of an efficient structural caprock. These results demonstrate that natural hydrogen can accumulate in structurally sealed ultramafic reservoirs. The Bulqizë system behaves as a hybrid reservoir, where serpentinization generates both hydrogen and porosity, and post-Miocene tectonics controls migration and accumulation. A conceptual model identifies the most favorable zones at the intersection of serpentinization fronts and late-stage faults. This study provides a critical geological framework for natural hydrogen exploration in ophiolitic environments.