We monitored the stability of CO2 hydrate by investigating its three-phase (L-W-H-V) equilibrium at different marine environmental conditions in the range of 273.3-280 K and 17-30 bar. The phase equilibrium of CO2 hydrate in synthetic seawater (aqueous solution with 3.5% NaCl and 0.1 M NaHCO3) was observed in higher pressure and lower temperature region of phase diagram (Delta P: 5.063 +/- 0.019 bar, Delta T: -1.950 +/- 0.019K), compared to that in deionized water (DIW) control. Effect of soil minerals (illite and pyrite) and organic matters (glycine and humic acid) on CO2 hydrate phase equilibrium was not significant in both DIW and synthetic seawater. The phase equilibrium of CO2 hydrate in Ulleung basin (UB) sediment suspension was very similar to that of the synthetic seawater control (Delta P: 0.115 +/- 7.969 x 10(-4) bar, Delta T: 0.038 +/- 8.466 x 10(-5) K); however, in the absence of organic fraction in UB sample by baking the sediment, its phase equilibrium shifted to the unstable upper left region (Delta P: 0.360 +/- 0.043 bar, Delta T: -0.215 +/- 8.442 x 10(-4) K). The results can provide fundamental knowledge to predict the stability of sequestered CO2 and estimate CO2 leakage from the sites in seabed sediment environments as a part of significant criteria to implement successful offshore CO2 storage. (C) 2013 Elsevier Ltd. All rights reserved.