Multi-beam and seismic investigations of the active Haima cold seeps,northwestern South China Sea

To confirm the seabed fluid flow at the Haima cold seeps,an integrated study of multi-beam and seismic data reveals the morphology and fate of four bubble plumes and investigates the detailed subsurface structure of the active seepage area.The shapes of bubble plumes are not constant and influenced by the northeastward bottom currents,but the water depth where these bubble plumes disappear(630–650 m below the sea level)(mbsl)is very close to the upper limit of the gas hydrate stability zone in the water column(620 m below the sea level),as calculated from the CTD data within the study area,supporting the“hydrate skin”hypothesis.Gas chimneys directly below the bottom simulating reflectors,found at most sites,are speculated as essential pathways for both thermogenic gas and biogenic gas migrating from deep formations to the gas hydrate stability zone.The fracture network on the top of the basement uplift may be heavily gas-charged,which accounts for the chimney with several kilometers in diameter(beneath Plumes B and C).The much smaller gas chimney(beneath Plume D)may stem from gas saturated localized strong permeability zone.High-resolution seismic profiles reveal pipe-like structures,characterized by stacked localized amplitude anomalies,just beneath all the plumes,which act as the fluid conduits conveying gas from the gas hydrate-bearing sediments to the seafloor,feeding the gas plumes.The differences between these pipe-like structures indicate the dynamic process of gas seepage,which may be controlled by the build-up and dissipation of pore pressure.The 3D seismic data show high saturated gas hydrates with high RMS amplitude tend to cluster on the periphery of the gas chimney.Understanding the fluid migration and hydrate accumulation pattern of the Haima cold seeps can aid in the further exploration and study on the dynamic gas hydrate system in the South China Sea.

Distinct BSRs and their implications for natural gas hydrate formation and distribution at the submarine Makran accretionary zone

To investigate the nature of gas hydrates in the Makran area,new high-resolution geophysical data were acquired between 2018-2019.The data collected comprise multibeam and two-dimensional multi-channel seismic reflection data.The multibeam bathymetry data show East-North-East(ENE)ridges,piggy-back basins,canyon and channel systems,and the morphology of the abyssal plain.Continuous and discontinuous bottom simulating reflectors(BSRs)occur in the piggy-back basins on most of the seismic profiles available.The BSRs cut the dipping layers with strong amplitude and reversed polarity.Discontinuous BSRs indicate a transition along a dipping high-permeable sand layers from gas-rich segment to the gas hydrate-bearing segment and sugge st alternating sediments of fine and relatively coarse grain size.Double BSRs are highly dynamic and attributed to slumps occurring in the study area.The BSRs induced by slumps are located both at deep and shallow depths,responding to the temperature or pressure variation.For the first time,BSRs are observed in the abyssal plain of the Makran area,being associated with anticline structures,which do not show large spatial continuity and are strongly conditioned by structural conditions such as anticlines and fluid migration pathways,including deep fault,gas chimney,and high-permeable sedimentary layer.Our results may help to assess the gas hydrate potential within the piggy-back basins and to determine the most promising target areas.Moreover,results about the abyssal plain BSR may help to locate hydrocarbon reservoirs in the deep ocean.