H2, being a small and highly mobile molecule, can easily escape through even tiny cracks or faults in rock formations. Therefore, an effective seal is crucial for trapping H2. The most effective caprocks are those with very low permeability, such as salt, anhydrite, and tightly compacted shale, which prevent H2 from diffusing out of the reservoir. The overlying caprock works effectively due to the extremely small molecular size, high diffusivity, and low density of H2, it exhibits enhanced permeability through geological formations and presents challenges for effective containment by conventional caprocks.