Accumulation Conditions And Distribution Controlling Factors Of Natural Gas Hydrate In Dongsha Prospect

As the resources of oil and gas are declining,natural gas hydrate is generally considered to be one of the most promising new type of clean energies to replace oil and gas,and it has enormous reserves in ocean and permafrost zones which is still untapped.Both seismic and well logging data showed plendid gas hydrate resources in Dongsha prospect,northern South China Sea.In 2013,Guangzhou Marine Geological Survey drilled 13 wells in Dongsha prospect,and got different occurrences of gas hydrate with high purity.However,research is not deep enough on gas hydrate reservoir geological rules of Dongsha prospect.Based on the geological,seismic,well logging,geochemical and in situ test data of Dongsha prospect,as well as comparing with abroad typical hydrate prospect,temperature and pressure conditions,gas sources,fluid migration paths and reservoir conditions are analyzed in Dongsha prospect.Combining with the distribution characteristics of gas hydrate,the main controlling factors of gas hydrate accumulation is studied and an accumulation model is set up.The results are showed as following: Dongsha prospect has good accumulation conditions for gas hydrate.The gas source is mainly biological gas via microbial carbon dioxide reduction,followed by thermogenic gas.The abyssal mudstones of Miocene to Quaternary and another three sets of deep source rocks provided tremendous natural gas for gas hydrate.Faults,chimney structures and fractures are the mainly vertical migration paths.After migrating to the shallow layers,natural gas will migrate horizontally through slumping surface and sandstones to gas hydrate stability zone to form gas hydrate.Temperature and pressure are well-qualified for Dongsha prospect,and the thickness of gas hydrate stability zone is about 110~320m,which is thick in south and thin in north.Sediments of Dongsha prospect are relatively fine,mainly mudstones or silly mudstones,and only a few sandstones are found in two wells.Gas hydrate are distributed mostly in disperse and laminated states in mudstone strata,which has a relatively low saturation.But the occurrence of gas hydrate in sandstones are nodular or lumpy which have a relatively high saturation.The gas hydrate distributes in one or two layers in Dongsha prospect,and the controlling factors for these two layers are different.After comprehensive analysis,temperature and pressure conditions and sedimental sources are thought to be the main distribution controlling factors for the relatively deep gas hydrate layer;while depth of sulfate-methane interface,submarine topography and distribution of sand and clay are the main distribution controlling factors for the relatively shallow gas hydrate layer.