Characterization of coexistence of gas hydrate and free gas using sonic logging data in the Shenhu Area, South China Sea

Recent studies have confirmed the coexistence of gas hydrate (GH) and free gas (FG) in gas hydrate stability zone (GHSZ). However, the acoustic properties of the coexisting sediment, GH-bearing sediment, and FG-bearing sediment are needed to be specified, for discriminating these three coexisting phases using sonic logs. In this study, the compressional-(P-) and shear (S-) wave attenuations of the sediment with coexisting GH and FG are calculated from the sonic logs in the South China Sea (SCS). The obtained results reveal that the P-wave and S wave velocities of the sediment with coexisting GH and FG are decreased by 14% and 11%, P-wave attenuation enhanced by 104%, and S-wave attenuation decreased by 44%, respectively, as compared with the sediment only containing GH at well SHSC-4J1. A similar result can be obtained for well GMGS6-SH02. The highest P-wave attenuation appears in the sediment with coexisting GH and FG,-0.66 at well SHSC-4J1 and-0.60 at well GMGS6-SH02, respectively. Moreover, cross plots of the measured P-and S-wave velocities and attenuations suggest that attenuation is more sensitive to the coexistence of GH and FG than velocity. The saturation of coexisting GH and FG in sediment can be calculated from the sonic velocities using rock physics modeling, a maximum value of-26% (av. 13%) at well SHSC-4J1 and a maximum value of-23% (av. 7%) at well GMGS6SH02.