The Seismic Facies Study Of Xing Ning Sag Eocene In Northeast Of South China Sea

The Pearl River Mouth Basin is located in northeastern of South China Sea,the conversion part of Pacific tectonic tracts and Tethyan tectonic tracts,and it is controlled by Eurasian plate,Pacific plate and India-Australian plate.The study area,Xing Ning Sag,is a secondary tectonic,which is located in east edge of The Pearl River Mouth Basin,and it is a concave between Dong Sha Uplift and Li Wan Sag.CNOOC(China)Ltd.Shenzhen branch aims at Xing Ning Sag in the early stage of the oil and gas exploration,collected high-quality 2d seismic data covering all concave,Xing Ning Sag is the ideal region where carries out the study of seismic facies and seismic sedimentology.The paper is guided by the seismic sedimentology and sequence stratigraphy theory,absorbing the sedimentary sequence research results from the adjacent region fully,using the seismic facies analysis technique,carrying out earthquake sequence stratigraphy,seismic facies,sedimentary system types and distribution research in different periods of Eocene,reveals the sedimentary evolution of Xing Ning Sag Eocene profoundly.Under the background of regional geological structure,this paper identify three structure interface based on 80 seismic line with covering the entire area: Tg is a Cenozoic basement interface,is a dehiscent unconformity surface of passive continental margin basin;T80 interface affected by Zhuqiong ? act tectonic movement,the uplift accords to denude,the locality takes angular unconformity,has clear truncation and onlap on the edge of the basin;T70 interface affected by The south China sea movement to occur intensive stretching and the uplift according to denude,takes angular unconformity,has down truncation and top onlap feature.Between Tg and T80 is Wen Chang coset,Between T80 and T70 is En Ping coset.Wen Chang coset recognizes three three-grade sequence interfaces,it divides into four cosets;En Ping coset recognizes two three-grade sequence interfaces,it divides into three cosets.The classification scheme is based on an interface on the overlap and a foreset or truncation under the interface.That sets up seismic sequence stratigraphic of seismic-sedimentary evolution,which controls the whole region.In the seismic sequence stratigraphic,based on the basis of seismic facies analysis,which means seismic reflection structure,seismic reflection configurations and unit shape of seismic reflection,the study area divides into various kinds ofseismic facies types,which the progradation seismic facies is located in gentle slope mainly.It is visible in the sequence except WCSQ2.It reflects the delta front depositional environment;Parallel continuous reflection seismic facies locating in gentle slope is speculated a floodplain and which in the center of the sag reflexes lacustrine deposit.It is visible in all sequences;Wedge chaotic seismic facies is located in the steep slope.It is visible in all sequences.It reflexes fan delta depositional;Erosion filling strong clutter amplitude seismic facies is located in gentle slope mainly.It reflexes deltaic plain distributary channel.Beside,mounds seismic facies reflexes delta front;lens seismic facies reflexes neritic subaqueous fan.The distribution range of seismic facies has experienced process gradually expanded to shrink and then expand from the longitudinal,which means it has a good inheritance.The distribution in the northeast,southwest and center of the sag has an asymmetry on horizontal.According to the seismic facies on behalf of the significance of sedimentary facies,in combination with characteristics of sedimentary bodies seismic reflection,the study area is divided into seven sedimentary facies: flood plain,shore lake,delta plain watercourse,delta front,fan delta,shallow lake,half deep lake-deep lake.On this basis,according to the residual thickness and provenance analysis,plane distribution of sedimentary facies is compiled systematically.It has delta-lake sedimentary mode.Xing Ning Sag of Eocene is faulting stage.Beside,Wen Chang coset is early faulting stage,the range of lake basin is small and it has limitation in WCSQ2 and WCSQ3.Source mainly comes from the west and northeast.It mainly deposits a shallow lake facies and develops a few delta and fan delta deposits.Until WCSQ4 and WCSQ5,the tectonic extension is enhancing and the lake basin is expanding quickly.Then it takes two almost parallel half deep lake-deep lake facies nearby control sag fault zone.The provenance in the east and west is stable,enhancing in the northern.It is a characteristic that develop delta in gentle slope,strand shallow lake in shoal water of gentle slope,fan-delta in steep slope,and half deep lake-deep lake facies in sag centers.Sedimentary evolution pattern of En Ping coset has obvious similarities with Wen Chang coset.The pattern of sag manifests as characteristics of NW – SE.Deposition is still basis on fan dalta or neritic subaqueous fan in steep slope belt of southwestern,while delta in gentle slope belt of east and north.And the distribution range of shore shallow lake in gentle slope belt of east and north has gradually narrowed from north to southeast.At the same time,the provenance in northern is gradually apparent with the expansion of water bodiesin the basin.