Gas hydrate accumulation associated with fluid escape structure in the western margin of South China Sea

Gas hydrates have been found in the western continental margin of South China Sea,which are revealed by widespread bottom simulating reflectors(BSRs)imaged from a three-dimensional(3D)seismic volume near the Guangle carbonate platform in the western South China Sea.Fluid-escape structures(faults and gas chimneys)are originated below BSR were distinguished.A comprehensive model in three-level structure was proposed to depict the gas hydrate accumulation in the study area.In Level 1,regional major faults and gas chimneys provide the first pathways of upward migration of gas near basement.In Level 2,pervasive polygonal faults in carbonate layer promote the migration of gas.In Level 3,gases sourced from near-basement accumulate within shallow sediment layers and form gas hydrate above the unit with faults once appropriate temperature and pressure occur.The gas hydrates in the study area are mainly in microbial origin,and their accumulation occurs only when fluid-escape structures align in all the three levels.The proposed model of the gas hydrate accumulation in western SCS margin provides new insights for further studies in this poorly studied area.

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.

Seismic Data Processing Approaches for the Study of Gas Hydrates in the East China Sea

A comprehensive study of the data profiles, including the 2D seismic data, single channel seismic data, shallow sections, etc., reveals that gas hydrates occur in the East China Sea. A series of special techniques are used in the processing of seismic data, which include enhancing the accuracy of velocity analysis and resolution, estimating the wavelet, suppressing the multiple, preserving the relative amplitude, using the DMO and AVO techniques and some special techniques in dealing with the wave impedance. The existence of gas hydrates is reflected in the subbottom profiles in the appearance of BSRs, amplitude anomalies, velocity anomalies and AVO anomalies, etc. Hence the gas hydrates can be identified and predicted. It is pointed out that the East China Sea is a favorable area of the gas hydrates resource, and the Okinawa Trough is a target area of gas hydrates reservoir.

Comment on "Estimation of potential distribution of gas hydrate in the northern South China Sea" by Chunjuan Wang et al.

Herein we would like to comment on the paper ＂Estimation of potential distribution of gas hydrate in the northern South China Sea＂ by Wang et al. 2010 in Chinese Journal of Oceanology and Lirnnology, 28（3）： 693-699. The purpose of this comment is to point out that the given probabilities of gas hydrate occtwrence in the northern Zhujiang Mouth Basin and the Yinggehai Basin in the figure of Wang et al. （2010） are improper. After introducing our work of estimation of gas hydrate stability distribution in the northern South China Sea, we suggest that Wang et al. （2010） dismissed the basic P-T rule for the existence of gas hydrate. They should consider more the variables of water depth, seabed temperature and geothermal gradient in their gas hydrate distribution model in future studies.

Geophysical evidence of a large occurrence of mud volcanoes associated with gas plumbing system in the Ross Sea(Antarctica)

Seafloor and buried reliefs occur along continental margin of the Ross Sea(Antarctica).These features are several kilometres wide and tens of metres high,exhibiting cone or flat-top dome shapes.Previous studies have proposed a volcanic or glacial origin for these formations,but these hypotheses do not account for all the available evidence.In this study,we use morpho-bathymetric data,intermediate resolution multichannel seismic and high resolution chirp profiles,as well as magnetic lines to investigate these clusters of mounds.By employing targeted processing techniques to enhance the geophysical characterization of the seafloor and buried reliefs,and to understand the underlying geological features,we propose that the reliefs are mud volcanoes.Some of these formations appear to be associated with a plumbing system,as indicated by acoustic anomalies linked to sediment containing gas.These formations are likely fed by clayey source rocks of Miocene age.Additionally,other reliefs might be the result of mud mobilisation caused by gravity instability and fluid overpressure.