A marine controlled-source electromagnetic survey to detect gas hydrates in the Qiongdongnan Basin, South China Sea

The Qiongdongnan Basin (QB) is an important prospecting area for gas hydrates in the South China Sea (SCS). However, it is difficult to infer gas hydrates using a bottom simulating reflector (BSR) under the influence of flat seabed, multiples and bubble effects. In the QB, the upper limit of the gas hydrate stability zone (GHSZ) and the distribution of gas hydrates in the shallow seabed are still not well understood. A marine controlled-source electromagnetic (MCSEM) survey was completed in order to collect electric resistivity information along a 4.5-km profile. Electromagnetic data were acquired and processed at nine sites. The seafloor receiver orientations were evaluated using the orthogonal procrust rotation analysis (OPRA) method. As a result of isotropic, vertical transverse-isotropic (VTI), and triaxial anisotropic inversion of electric field Pmax data, a 2-D resistivity image of the seabed was presented. Preliminary results show that the shallow seabed can be divided into four electric layers. According to thermodynamic conditions in the QB, the estimated thickness of GHSZ is approximately 320 m for biogenic methane and 272 m for thermogenic gas in the study area. The BSR and hydrates were interpreted by integrating the resistivity model along with thermodynamic calculation and seismic reflection data. The MCSEM result indicates there are abundant natural gas originating or partly originating from a microbial source beneath the GHSZ.