ESTIMATION OF GAS HYDRATE SATURATION WITH MODIFIED BIOT-GASSMANN THEORY: A CASE FROM NORTHERN SOUTH CHINA SEA

The modified Biot-Gassmann theory (BGTL) proposed by Lee (2002) is applied in this study to estimation of the saturation of gas hydroate in the deep-water unconsolidated clayey sediments from the Well A, Shenhu area, northern South China Sea. The BGTL theory assumes that the ratio of the shear to compressional velocities of an unconsolidated sediment is related to the ratio of the shear to compressional velocities of the solid matrix in the sediment and its porosity. Parameters involved in the model are related to the occurrence of gas hydrate, mineral components,pressure differentiation,porosity,and pore structure of the sediments. The cross plot of velocity from sonic logging vs. concentration of gas hydrate from core measurements suggest that the occurrence of gas hydrate in the sediments of Well A is more or less close to the matrix model. Statistics from core smear slide data suggest that the sediment matrix in well A can be simplified as three major mineral components, i.e clay, carbonate, and terrigenous clastic minerals. The elastic modulus and density values of the matrix are calculated by the elastic modulus and density values of the individual mineral components and their volume percentage. As estimated, the gas hydrate in Well A is mainly distributed in the depth interval of 195 to 220 meters below sea floor with a highest concentration of gas hydrate up to 47%, which matches well with the results from core measurements.