Display omitted •CH4/CO2 replacement behaviors in natural marine sediments are investigated.•CO2 diffusion is weakened by clayey sediment during replacement process.•NGH reservoir with moderate water content has excellent potential for replacement.•Optimized P-T conditions are crucial to improve CH4 recovery from hydrates. The use of CH4/CO2 replacement from hydrate bearing sediments for CH4 recovery and CO2 storage is an alternative option to mitigate energy shortage and global warming. Fine marine sediments are highly attractive for abundant gas hydrate reserves and tremendous CO2 sequestration potential. However, the CH4/CO2 replacement regularity previously obtained from coarse sands may not be suitable for fine marine sediments because of their distinct differences in physical properties. In this study, fine natural marine sediments obtained from the South China Sea were used as porous media to investigate the CH4/CO2 replacement characteristics. The results indicated that the pressure and the temperature were the main controlling factors affecting the replacement efficiency. The content of initial methane hydrate and water in reservoir had a more significant effect on CO2 storage than CH4 recovery. The gas exchange kinetics in hydrates presented in fine marine sediments were significantly different from those in coarse grained sediments. CH4/CO2 replacement in fine marine sediment seemed to be inhibited by weak CO2 diffusion as peculiarities of the sediment including fine grain size, clay swelling and high proportion of bound water. According to the observed experimental results, the pressure and temperature conditions should be comprehensively optimized to enhance the mass transfer effect and improve economic benefits. This work provided greater insights into future marine NGH exploitation and contributed to carbon sequestration with applying CH4/CO2 replacement method.