Seismic Evidence for Gas Hydrate in Dongsha Area, Northern South China Sea

A large cold vent, named Jiulong Methane Seepage, was discovered in the Dongsha Area of northern South China Sea (SCS) in 2004. We present seismic evidence of the existence of gas hydrates and free gas beneath Jiulong Methane Seepage. Bottom Simulating Reflectors (BSRs) are identified in the seismic profiles, inferring the presence of gas hydrate. Seismic attributes, interval velocity model, relative acoustic P-impedance inversion were conducted to better image the distribution of gas hydrate. Hydrate-bearing sediments are characterized by weak reflection (blanking zone) above BSRs, high velocities and relatively small impedance differences, while the enhanced reflection, low frequency, and low velocities suggest the presence of associated free gas below solid hydrate. The formation of gas hydrate is related to widespread faults and mud diapirs (gas chimneys) in Dongsha Area. The presence of faults and mud diapirs provide useful conduits to gas migration upward, linking the free gas zone below BSRs and hydrate zone and seafloor, and hence causing methane leakage on the seafloor.