Hydrogen Adsorption Potential in Unconventional Carbonate Source Rocks: Challenges and Opportunities

Abstract The global push for decarbonization has renewed interest in underground hydrogen storage (UHS), with unconventional reservoirs emerging as potential candidates. However, the effects of source rock composition and thermal maturity on hydrogen adsorption remain poorly understood. This study utilizes the Upper Cretaceous (Maastrichtian) Jordan Source Rock (JSR) interval from the Al-Lajjun area of Western Jordan to investigate the multifaceted relationship between source rock composition, thermal maturation, and hydrogen adsorption. The JSR is a carbonate-rich, thermally immature, Type IIS source rock that shares strong compositional similarities with key regional unconventional carbonate source rocks globally. Initial sample characterization revealed significant heterogeneity in total organic carbon (TOC) and mineralogy, including variations in carbonate and silica content. Hydrogen adsorption experiments revealed a Type I isotherm behavior, indicative of micropore-dominated adsorption mechanisms. The organic-rich, carbonate-dominated samples (i.e. JSR-1) exhibited the highest hydrogen adsorption, whereas samples with either lower TOC (i.e. JSR-3) or higher quartz content (i.e. JSR-2) showed reduced adsorption capacity. This indicates that organic matter contributes significantly to nanopore development, while mineralogical variations, particularly carbonate versus quartz content, influence the surface reactivity and available adsorption sites. Post-maturation, the hydrogen adsorption experiments revealed no detectable adsorption, likely due to the presence of in-situ generated hydrocarbons occupying pore spaces and preventing hydrogen interaction with the rock matrix. This observation highlights that thermal evolution alters not only the pore structure but also the accessibility of adsorption sites. Collectively, the findings underscore the importance of both organic richness and mineral matrix in controlling hydrogen adsorption in immature source rocks. However, as maturity progresses and hydrocarbons are generated, hydrogen adsorption potential diminishes regardless of initial composition. This suggests that thermally mature, depleted source rocks with limited residual adsorption potential may offer favorable conditions for cyclic or seasonal hydrogen storage, especially where retrieval efficiency is critical.