Hydrogen generation potential from Glauconitic sandstone during water-rock reactions: insights from hydrothermal experiments

Natural hydrogen exploration has focused largely on ultramafic systems, yet Fe-bearing sedimentary minerals may also generate H 2 during water-rock interactions if reactive iron is available. Herein, batch experiments with variable temperature and solid-to-liquid ratios (S/L) were performed for an exploratory assessment of H 2 -rich gas generation from glauconitic sandstone of the Arnager Greensand Formation. Across the investigated conditions, the generated gas was commonly H 2 -dominant in relative composition, with impurities such as H 2 S and CO gases whose proportions vary with temperature and S/L. Post-reaction XRD and XRF data indicate that quartz remained the dominant framework phase, whereas glauconite abundance and bulk Fe decreased relative to the starting material. These changes are consistent with selective alteration of the Fe-bearing fraction during H 2 -generating reactions. These results indicate that glauconitic sandstones can generate H 2 -dominant gas under shallow subsurface conditions and may represent an underexplored sedimentary host for geologic hydrogen generation worthy of further investigation, while also highlighting the importance of sulfur-related controls, carbon speciation, and fluid-rock ratio in determining gas quality. Absolute quantification of H 2 yield, together with broader experimental conditions, is identified as a key priority for future studies.