Distribution and characteristics of gas chimneys in the passive margin offshore SW Taiwan

We analyzed original and published seismic data to investigate the distribution and characteristics of subsurface fluid flow features in the gas hydrate provinces along the northeastern South China Sea (SCS) continental slope, including Jiulong Ridge, Horseshoe Ridge, Pointer Ridge, and Formosa Ridge. Numerous features indicating the presence/migration of hydrocarbons, such as bottom simulating reflections (BSRs), bright spots, gas chimneys, and fluid flow through faults, are identified. The results reveal that the hydrofracturing-induced gas chimneys act as the primary conduits for the overall focused fluid flow in the study area, though fluid flow along permeable faults is also observed at Pointer Ridge. Thirty-three gas chimneys identified are categorized into two types depending on their capability for focused fluid flow migrating into the gas hydrate stability zone (GHSZ). The type-I chimneys that transport fluids into the GHSZ contribute to gas hydrate formation and may even lead to seafloor seepage. Buried by a considerable thickness of sediments, the type-II chimneys cannot directly contribute to generating gas hydrate and surface seepage. Our results suggest that the sedimentary processes and fluid accumulation significantly control the development of gas chimneys in the study area. Since the focused fluid flow conduits that indicate overpressured fluids are critical pathways that feed gas into the GHSZ and form gas hydrates, utmost attention should be paid to them during hydrocarbon exploration.