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Understanding of gas hydrates, for example, phase equilibria, in reservoir sediments is one of the important issues in the fields of energy production and CO₂ sequestration via CH₄ hydrate exploitation and CO₂ hydrate formation. The composition of water in clay can change their physical properties, which influences the state in which gas hydrates exist in clays. We investigated gas hydrates in crystalline-swelled Na-montmorillonite (CS-mon) having water content of <27 wt % and maintaining crystallinity of clay particles. Instead of a gas hydrate forming in the interlayer, gas is dissolved in the swelled interlayers of clay, acting as a favorable storage site for hydrophilic CO₂. In addition, gas hydrates formed in CS-mon showed both inhibited and promoted phase behavior. This revealed that the clay surface can promote the formation of a hydrate structure, but confinement in nanosized space dominates the promotion …