The Study On Sequence Stratigraphy And Sedimentary Facies Of Baiyun Deep-water Area

The study area is located in the Baiyun Sag of Pearl River Mouth Basin and its periphery area,the key strata are the Oligocene Zhuhai Formation and the Miocene Zhujiang Formation.Based on the overseas and domestic research status of carbonate sedimentary facies and sequence,combine with the material of drilling,logging,debris thin section,geochemistry and seismic data in the study area,the petrological and sedimentological features of this area has been further studied.With combination of well-log and seismic data analysis,we establish carbonate rocks' identification marks in deep-water area,to recognize the sequence boundary,and then compair each single well's sequence to divide and set up the sequence stratigraphic framework of the region.On this basis,we make a reseach in the distrbution and evolution of the sedimentary facies within the sequence,and then analyze carbonate rock and organic reef's distribution characteristics,ultimately,summarize the development model of carbonate rocks.Pearl River Mouth Basin was influenced by the split of two east-west and three north-south fault segmentations and totally showed frameworks of “segmentation in E-W direction”,“zonation in N-S direction” and “three uplifts and two sags”.Cenozoic experienced six big tectonic movements,namely the ShenHu Movement,the ZhuQiong Movement episode one,the ZhuQiong Movement episode two,the South China Sea Movement,the Baiyun Movement and the DongSha Movement,which developed three-layer structures including fault depression,fault-sag and depression.The South China Sea Movement and the Baiyun Movement had a great influence on the sedimentary pattern and the carbonate rocks development of the Baiyun Sag.The South China Sea Movement was a “Thalassogenic Movement” for the Pearl River Mouth Basin,the Baiyun Sag transformed continental facies into marine facies,the Baiyun Movement caused the shelf break of the Baiyun Sag moving from the south margin to the north margin.Thus,the Baiyun Sag transformed shallow shelf into deep-water slope sediments.Through identification marks,such as,lithological association,geochemical elements,log response and seismic facies,the study area is divided into carbonate sedimentary system and clastic depositional system.Carbonate sedimentary system include shallow sea shelf,rimmed platform,isolated platform and platform margin of four sedimentary facies,and then with 6 sedimentary subfacies and 18 sedimentary microfacies.Based on the Vail sequence stratigraphy theory,combined with the tectonic and sedimentary background in the area,making comprehensive study of the sequence boundary,we have subdivided 6 third sequences(SQ4~SQ9)in the Zhuhai-Zhujiang Formation,SQ4,SQ5,SQ6 for the Zhuhai Formation and SQ7,SQ8,SQ9 for the Zhujiang Formation.Through drilling and seismic studies,establish third order sequence framework.On the basis of analyzing point(thin section of each sing well)to line(well section)and to plane(seism),in order to study the sedimentary facies distribution and the main dominate factors within the sequence framework.The major controlling factors of the third order sequence development in the study area include tectonic causes,sea level eustacy and the ancient Pearl River Delta.Above all,the sea level eustacy plays an important role in the development of third sequence.Based on the seismic facies recognition of carbonate rocks,synthesize analysis of drilling,logging,thin section,summarize the plane distribution characteristics of carbonate rocks.The carbonate rocks of Baiyun deep-water area is mainly developed in SQ4,SQ5 of the Zhuhai Formation,and SQ6,SQ7,SQ8 of the Zhujiang Formation,distributing from southwestern Dongsha Uplift,the Yunli Low Uplift,northeastern Shunhe Uplift,Well B7-1 to Well B6-1.There are two development models which are identified on seismic section,one is ancient uplift rimmed carbonate platform which is characterized by large area distribution,smaller thickness;another is volcanic mound isolated carbonate platform which is characterized by small area distribution,large thickness.According to tectonic units and developmental strata,the rimmed carbonate platform can be subdivided into 3 types:(1)the Enping Formation rimmed carbonate platform in the south rim of the Baiyun Sag(the southeast of Yunli Low Uplift);(2)the Zhuhai Formation rimmed carbonate platform in the southwest of the Baiyun Sag(the north of Shunhe Uplift);(3)the Zhujiang Formation rimmed carbonate platform in the northeast of the Baiyun Sag(the south of Dongsha Uplift).The isolated carbonate platform distributes in the area of Well B7-1 to Well B6-1 and to southeast of Yunli Low Uplift.Discuss the development model of rimmed carbonate platform and isolated carbonate platform.Until now,the important geological event from 23.03Ma(the Baiyun Movement)and extensive transgression event from 19.1Ma have a great influence on the distribution of carbonate rocks in the study area.During the Zhuhai Formation sedimentary period,sea water intruded into the Baiyun Sag from low tectonic parts between the Liwan Sag,the Yunli Low Uplift and the Yunkai Low Uplift,most of the Shunhe Uplift?southeastern Yunli Low Uplift and the Dongsha Uplift exposed on the surface and suffered from exposure and erosion,high topography of the Baiyun Sag developed rimmed carbonate platform deposits(Mixed deposits)which was less affected by sources,the Baiyun Sag-Liwan Sag deep-water area volcanic mound was less affected by sources,developed into volcanic mound isolated platform.After the Baiyun Movement,lower Zhujiang Formation of the Dongsha Uplift,the Yunli Low Uplift and the Shunhe Uplift were all flooded by seawater,carbonate rocks developed in good condition,thin limestone was widely developed in the platform and rimmed shelf,reef developed widely;after extensive transgression events from 19.1Ma,most of the platform disappeared,only local high points on the platform developed limestone.