Abstract:

The distributing features and change of sonic logging velocity and gas hydrate saturation in gas hydrate-bearing sediments were summarized with seismic and well logging data, furthermore, a comparative study was conducted between the theoretical calculation and in-situ measurement of gas hydrate saturation in site A, Shenhu area, northern slope of South China Sea. The relationship between sonic velocity and gas hydrate saturation in gas hydrate stability zone has been analyzed, too. The analysis results show that:(1)the thickness, sonic velocity, and gas hydrate saturation of gas hydrate-bearing zone are about 20 m, 1,873－2,226 m/s, 15.0%－47.3%, respectively;(2)the in-situ measurement of gas hydrate saturation which was derived from the field test of the chloride content in pore water is not direct proportion to sonic velocity for the influence of seafloor geological factors, but fluctuates according to sonic velocity with the range of 10%－20%, and generally increases with increasing sonic velocity;(3)the calculated value of gas hydrate aturation from velocity model of heat elasticity theory regularly rises with increasing velocity. The fit relationship between the theoretical calculation and in-situ value of gas hydrate saturation indicates that the established rock physics model is correct, and the selected parameters of model are reasonable. This method can be extended to the total study area to provide valuable data for resource evaluation of gas hydrate.

Key words:  gas hydrate, sonic logging velocity, saturation, Shenhu area, South China Sea