Characteristics of Pockmarks and Their Genesis Zhongjian Offshore Area, South China Sea

Pockmarks are very important in delineating the prospective areas of gas hydrate and indicating geological hazards. Based on the high-density 3D seismic data of Zhongjian offshore area of South China Sea, the submarine geomorphic characteristics of the study area are described by automatic tracking technology. Many pockmarks with different shapes and sizes are found, which can be divided into five categories: round pockmarks, oval pockmarks, elongated pockmarks, crescent pockmarks and composite pockmarks. The pockmarks in the Zhongjian offshore area are mainly developed in groups and/or in the belt around the seabed of slope break, in groups and belts, but rarely found in the flat topographic regions. Our survey suggest that these pockmarks are formed by volcanic activities, fault activities, hydrate decomposition, sea bottom currents etc. Thus there are four kinds of fluid-formed pockmark on the seabed, i.e. volcanic hydrothermal fluid, gas hydrate decomposition, oil and gas migrating upward along faults, and mixture of the above.