Abstract:

With analysis of the distributing feature and the changing law of the sonic velocity and density of sediments with gas hydrate in Shenhu Area of the South China Sea, by comparing with logging data of gas hydrate formation at Site 570 of DSDP 84，the logging response characteristics of gas hydrate formation in Shenhu Area were conducted. The results show that:(1)Sediments with gas hydrate in Shenhu Area have the clear features that the sonic velocity is high and density is low, and the density is generally decreased with the increasing of the sonic velocity.(2)The high sonic velocity section in sediments with gas hydrate is in the depth of 197 to 220 m, and the low density section is in the depth of 200 to 212 m; the value of saturation is in the range of 15％to 47％.(3)The average sonic velocities of sediments with gas hydrates and the overlying strata and underlying strata are 2,076 m/s, 1,903 m/s and 1,892 m/s, respectively, and the corresponding densities are 1.89 g/cm3, 1.98 g/cm3 and 2.03 g/cm3, respectively; sonic velocity is effected by porosity and saturation, and density is greatly influenced by saturation; (4) The sonic velocity increases by 9.1％ from the overlying strata of hydrates to the maximumvalue section, and the corresponding density decreased by 4.55％; the sonic velocity decreases by 8.86％ from the maximumvalue section to the underlying strata of hydrates, and the corresponding density increases by 7.41％. The logging response characteristics can be used to identify the existence of gas hydrates and to calculate the saturation of gas hydrate, furthermore, to provide accurate area and thickness of gas hydrate zone for calculating gas hydrates reserves with combination of other geological and geophysical data.

Key words: gas hydrate, logging response, velocity, density, porosity, saturation