A study on natural gas hydrates and their forming model using marine controlled-source electromagnetic survey in the Qiongdongnan Basin

The Qiongdongnan Basin is one of important areas for gas hydrate prospecting in the South China Sea. However, because of the gentle terrain and the nearly horizontal seabeds, it is difficult to recognize the BSR (bottom simulating reflector) and to infer gas hydrates. To solve this problem, the first marine controlled-source electromagnetic detection experiment was carried out in this area, with the aim to image the gas hydrates and free gas below the GHSZ (gas hydrate stability zone) in the seabed. In total 10 receivers spacing 500 m were deployed on the seafloor at a depth of about 1360 m below water, and electromagnetic data were acquired along a 4. 5 km profile. By processing and two-dimensional (2D) inversion of the collected data, the resistivity profile of the study section of the seabed was obtained. It shows that there are several resistive anomalous bodies with resistivity between 2 Omega m and 10 Omega m characterized by discontinuous transverse distribution above the seabed of 330 mbsf (meter bottom of seafloor). Under the seafloor 330 mbsf, 3 high resistivity bodies with a resistivity of 2 similar to 4 Omega m are present laterally. According to the thermodynamic conditions of the study area, the thickness of GHSZ of biogenic gas and thermal genetic gas is estimated, and the location of BSR in the seismic section is deduced from the distribution characteristics of the high impedance bodies. On this basis, the resistivity section is interpreted, and the distribution of hydrates and the migration path of free gas are inferred. The thickness of GHSZ estimated by thermodynamic conditions and the BSR of the seismic profile have a good relationship with the bottom boundary of the high resistivity body. Furthermore the distribution of hydrates and the free gas migration pathway in the study area are explained by the inversion profile. The results show that the hydrate deposits in the study area belong to the type of lower generation and upper storage with natural gas migrating through faults and cracks, and the gas source of hydrates may be biogenic gas.