Sedimentary evolution since Quaternary in the Shenhu hydrate drilling area, northern South China Sea

Abstract

The marine gas hydrate samples in China were for the first time acquired in the Shenhu area,northern continental slope of the South China Sea.However,due to erosion by the submarine canyons,the stratigraphic comparison and identification of sedimentary facies were difficult,leading to the lack of description of seismic-sedimentary facies,analysis of forming mechanisms and distribution of the sedimentary units.Moreover,few researches focused on the sedimentary evolution since Quaternary,and the crucial controlling factors on the heterogeneous distribution of gas hydrate are not clearly understood.Using high-resolution 2D/3Dseismic data,this study attempts to classify the seismic facies and discuss the sedimentary evolution in this area since Quaternary.This work is based on the high-resolution 2Dand 3Dseismic data obtained by Guangzhou Marine Geological Survey in 2006,with a trace interval of 12.5mand vertical resolution of 10~20 m.Firstly,we make an integrated analysis of seafloor topographic features,sequence stratigraphic framework and high-resolution seismic data,perform a classification of seismic facies have been classified,and describe morphological features and internal architectures accurately.Secondly,in terms of sediment supplies from the north and seafloor topography,deep-water sedimentary process is analyzed.Also,the sedimentary evolutions since Quaternary are re-built.Subsequently,the relationship between deep-water sediments since Quaternary and distribution of gas hydrates are discussed.Five types of seismic facies were identified,involving prograding slopes,small-scale turbidite channels in the Early Quaternary,sediment failures(including slides and slumps),submarine canyons and concomitant transformation of deposits,and the mixture of abysmal deposits and mass transport complex.The three phases of sedimentary evolution since Quaternary in the Shenhu area were re-built,which are turbidite channels erosion and re-deposited sediments,prograding slopes and sediments failures,and erosion and sedimentation of submarine canyons.Eroded by small-scale turbidites,re-deposited sediments would be near-source stacked with relatively good property conditions,which might be regarded as the potential suitable reservoirs for gas hydrates.The deposits transported from prograding slopes could lose the balance easily and result in sediments failures widespread in the Shenhu area.Because of the fine-grain particle and vertical continuity,sediment failures could be thought as the regional cap rock.Abundant submarine canyons and concomitant transformation of deposits could erode and destroy underlying strata,which modified suitable deposits as patch distribution.Hence,the sedimentary evolution since Quaternary was considered as one of the crucial controlling factors on the heterogeneous distribution of gas hydrate in the Shenhu hydrate drilling area,northern South China Sea.