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• The phase equilibria of natural gas hydrate in real marine sediments were determined. • The endothermic peak bottom was proved corresponding to the phase equilibrium point. • Moisture difference affected decomposition pressure via changing sediment salinity. • The sediment salinity and gas components should be considered in spot production. Marine natural gas hydrate production has become a hot topic in recent years. There is still a long way to achieve the commercial production of marine hydrate reservoirs. The purpose of this work is to investigate the natural gas hydrate decomposition characteristics in the real marine sediments obtained from the South China Sea. A high-pressure micro-differential scanning calorimeter was employed to study the thermodynamic properties of natural gas hydrates. The results indicated that the increase in gas pressure or sediment salinity decreased the ice melting temperature by changing the vapor pressure of water. In addition, the decrease in sediment moisture from 40% to 30% increased the decomposition pressure of hydrates by 4.4 MPa at 278 K of sea bottom temperature, due to the water absorption capacity of the sediments increases the actual salinity furtherly. The influence of mixed gases on the decomposition pressure of hydrates was also investigated. The findings of this work on the effects of salinity and gas components on the decomposition pressure are significant to the spot safety production of marine natural gas hydrate reservoirs. [ABSTRACT FROM AUTHOR]