Mapping natural hydrogen occurrences in Romania

Natural hydrogen (H 2 ) produced in crustal and mantle rocks is a new target of geological exploration for its potential as an energy resource with low carbon emissions. H 2 occurrences are broadly listed and inventoried worldwide, but detailed mapping at a country scale is poorly documented. This study presents the first inventory and map of natural H 2 occurrences in Romania, one of the European countries with a large natural H 2 potential. The resource inventory includes 286 subsurface (deep wells) and 158 surface (gas seeps, mud volcanoes, and geothermal manifestations) data selected from studies published between 1970 and 2019, together with new analysis results reported in this study. The H 2 data are distinguished between concentrations in the gas phase (vol%) and those dissolved in water (μM), and include sites where H 2 was not detected. Normal Probability Plot analysis shows 14 concentration outliers and that more than 90% of the data refer to trace H 2 levels (up to ∼0.1 vol%). Although H 2 concentrations are generally low, Romania has an exceptional case of natural H 2 occurrence (28.7 vol%) related to serpentinisation in the Tişovița-Iuți ophiolite. The outliers occur in eight major tectonic units, characterised by the presence of potential H 2 source rocks, including serpentinised peridotites in ophiolites, radiolytic basement (crystalline rocks), high-maturity sedimentary rocks in hydrocarbon-bearing fields, and geothermal systems with high-temperature, deep crustal or mantle-derived fluids. The natural H 2 map of Romania represents a European example serving as an experimental reference for scientific research and exploration, supporting policy formulation on natural H 2 exploitation.