An Insight into Shallow Gas Hydrates in the Dongsha Area, South China Sea

Previous studies of gas hydrates in the Dongsha area, mainly focused on the deep-seated gas hydrates that have a high energy potential, while little attention has been paid to the shallow gas hydrates occurrences. Shallow gas hydrates have been confirmed by drill cores at three sites (GMGS2-08, 09 and 16) during the GMGS2 Cruise, which occur as veins, blocky nodules or massive layers, at 8-30 m below the seafloor. Gas chimneys and faults observed on the seismic sections are the two main fluid migration pathways supporting shallow gas hydrate formation and sustainability. The gas hydrate, marked by Bottom Simulating Reflectors, and the shallow hydrate-bearing sediments, are two main seals for the migrating gas. The occurrences of shallow gas hydrates are controlled by fluid migration along shallow faults and the presence of deep-seated gas hydrates. Active gas leakage is taking place at a relatively high-flux state through the vent structures identified on the geophysical data at the seafloor, although without resulting in gas plumes easily detectable by acoustic methods. Shallow gas hydrate accumulations are not easy to recognize, but the presence of strong reflections in the high-resolution seismic profiles and dim or chaotic layers in the sub-bottom profiles are most likely good indicators of shallow gas hydrates in the Dongsha area. Active cold seeps, either indicated by gas plumes or seepage vents, can also be used as indicators for neighboring shallow gas hydrates, which confirm a highly dynamic gas hydrate system in the Dongsha area.