The Analysis Of Sedimentary Facies Controlled By Dual Slope Break Belts Of Wide And Low-gradient Shelf In Northern South China Sea

Pearl River Mouth Basin is the most typical Marine basin since Cenozoic.It is a marine-continental sedimentary basin with the evolution of the south China sea.In this paper,study arae has two typical slope-breaks---the shelf slope-break of Baiyun sag and the sedimentary slope break of Huizhou sag.The different main influencing factors which have tectogenesis,sea-levelchange,sediment supply and paleotopography will lead directly to the development of different types of slope-break zones.The types of slope-break zones are different,the corresponding sequence stratigraphic models are diffierent.For example,the shelf slope-break of Baiyun sag locates in the superimposed area of tectonic slope-break and sedimentary slope break,controlled by sea-level changing.So the sedimentary type of lowstand system tract is also more complex,mainly developed low wedge-shaped sand body and low fan sand body of gravity flow deposition.While in Huzhou area,sediment supply and carbonate platform are the mainly factors of sedimentary slope break developed.Slope break has a smaller scale,so the scale of tidal sand ridge and shoreland bar are smaller too,and the fan body is little developed.Based on theoretical guides of sequence stratigraphy,this paper takes the double slope belt as a research entry point.On the basis of a comprehensive analysis data including logging,drilling,earthquake,analysis and testing and etcetera,several conclusions are listeas below:1?By analyzing the lithofacies,combination characteristics,logging facies types,dividing up the sedimentary facies types and analyzing their characteristics of the study area,it can be concluded that the delta facies,the shallow water-bathyal region anddeep-water fan are developed in the study area.2?Based on the research results from a large number of research scholars,this paper uses the division method of the Vail sequence stratigraphy as a theoretical basis;and by comprehensively analyzing the development of the edge of double slope fold of the north of the South China Sea through data from logging,seismic,paleontological,drill core chemical data,and compared with previous data of Baiyun Depression and Huizhou Depression sequence,the Pearl River Formation and the Hanjiang Formation in the Pearl River Mouth Basin were divided into nine third-order sequences,named as SB23.8,SB21,SB18,SB17.5,SB16.5,SB15.5,SB13.8,SB12.5,SB10.5 by geochronology.The sequence stratigraphic patterns associated with the two slope belt zones were established respectively3?The sedimentary facies evolution characteristics of the study area are obtained:(1)During SQ1,the delta facies of Huizhou Depression were dominant,yet the sedi-mentary slope belt was not developed.The slope belt is located between Liwan North and Baiyunnan,and low-sandstone is developed at the bottom of the slope.(2)During SQ2 at ca.21 Ma,sea level decreased rapidly and slope belt with certain slope and tidal sand ridge developed in the Huizhou Depression.Delta was developed at the continental margin,slope fan was developed at the bottom area of continental margin slope,and the pelvic floor fan is developed at the foot of the slope;this was followed by slow sea level rise and low wedge development.During SQ2,the continental margin slope belt migrated from south to north in the Baiyun Depression.(3)During SQ3-SQ4,the sea level varied slightly,the sea level ascending as a whole comparing to SQ2.The delta narrowed,the slope of the slope belt in Huizhou Depression was reduced,the closed environment became open,and the sand body gradually turned into coastal sand bar deformed bywave.Delta was not developed along the Baiyun Depression continental margin,underwater channel – fan,lowstand wedges were smaller than that of SQ6 – SQ7.(4)During SQ5,the sea level continued to rise,and the carbonate platform in Huizhou Depression was submerged and the depositional slope belt disappeared.The muddy content of the sediments under the slope belt of Baiyun depression increased,and is dominated by muddy slumped block.(5)During SQ6-SQ7,the sea level decreased relative to the previous period,the delta extended,especially at the ca.13.8Ma ice age,global sea level significantly decreased,which led to a distinct development of continental margin slope belt in Baiyun Depression.During SQ7,the continental margin delta developed evidently,the sedimentary supply increased,and the development scale of the low-stand systems increased.However,the sediment muddy content increased and the size of the sand body was not larger than that of SQ2.(6)During SQ8,the sea level rose,and the slope belt was dominated by muddy slumped blocks.4?Under the Baiyun Depression slope belt,there mainly developed foreseet wedge-shaped sand body,moundy-shaped sand body formed by gravity,as well as water channel sand body deposited by gravity flow.Under the broad slope belt of the Huizhou Depression,the current lithologic oil and gas reservoirs are mostly tidal-deformed strip sand body.It is found through drilling that the shape of these sand bodies,reservoir properties have been significantly improved and are able to be favorable reservoir.