Haima Cold Seeps: Direct Evidence for the Occurrence of Gas Hydrates—A Case Study from the Qiongdongnan Sea Area, South China Sea

Cold seeps are natural “outcrop” evidence of marine gas hydrates and represent one of the most direct indicators of gas hydrate occurrence. This study used high-resolution 3D seismic data covering approximately 400 km2 in the deep-water southern Qiongdongnan Basin, northern South China Sea. Sub-bottom profiles and multibeam bathymetric data were also incorporated. Several seismic features indicative of fluid migration and accumulation were recognized, including gas chimneys, bottom-simulating reflectors (BSRs), seafloor pockmarks, seismic pull-up events, and bright spots. The results confirm that gas chimneys act as the main pathways for the upward migration of deep high-temperature, high-pressure fluids. The analysis shows that the sustained activity of gas chimneys not only supplies hydrocarbon gases for hydrate accumulation but also, through associated thermal fluids, can cause the dissociation of pre-existing hydrates, resulting in high-flux methane seepage and consequently nurturing cold seep systems. Through in situ observations and sampling by the remotely operated vehicle (ROV) Haima, a “Haima Cold Seep Area” associated with gas hydrates was discovered in the Qiongdongnan Basin. The study indicates that the identification of fluid seepage conduits on 3D seismic profiles is the most critical seismic characteristic for cold seep recognition. As direct outcrop evidence of hydrate occurrence, cold seeps can effectively compensate for the ambiguity of indirect indicators such as BSR, thus providing a reliable basis for fine-scale assessment of marine gas hydrate resources and investigation of accumulation mechanisms.