The Study On Carbonate Distribution Of Baiyun Deep-water Area In Pearl River Mouth Basin

The study area is located in Baiyun Sag of Pearl River Mouth Basin and its periphery,the key strata are Oligocene Zhuhai Formation and Miocene Zhujiang Formation.Based on the sufficient research status in deep-water area of carbonate rocks and the tectonics,deposits,reservoir forming conditions and other aspects in Baiyun Sag,combined with drilling,logging,thin sections and seismic data,the tectonic background and sedimentary background of carbonate rocks in the study area are deeply studied.With the investigation of previous research and combination with well-log and seismic data analysis,we established carbonate rock identification marks in deep water,recognized suspected reefs and distinguished them from similar mounds,analyzed carbonate rocks and reefs distribution regularity and summarized the development model of carbonate rocks in this article.Pearl River Mouth Basin was influenced by 2 east-west and 3 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 ShenHu Movement,ZhuQiong Movement episode 1,ZhuQiong Movement episode 2,the South China Sea Movement,Baiyun Movement and DongSha Movement,which developed three-layer structures including fault depression,fault-sag and depression.The South China Sea Movement and Baiyun Movement had a great influence on the sedimentary pattern and the carbonate rocks development of Baiyun Sag.The South China Sea Movement was a “Thalassogenic Movement” for Pearl River Mouth Basin,Baiyun Sag transformed continental facies into marine facies,Baiyun Movement caused the shelf break of Baiyun Sag move from south margin to north.Baiyun Sag transformed shallow shelf into deep-water slope sediments.Through the research of tectonic characteristics of carbonate development in the South China Sea deep-water area,and compared the tectonic background of Baiyun Sag and its peripheral with Qiongdongnan Basin,Nankang carbonate platform(Mega Platform)and other similar deep-water carbonate deposits,we could confirm that the study area have the tectonic background to develop carbonate rocks and reefs.Compared relative sea-level change in Pearl River Mouth Basin with global sea level change,analyzed the sedimentary background of Baiyun Sag and used the well lithology,well logging,thin sections and other related information,combined with the cross-well seismic profile in the study area,Dongsha Uplift in the northeast of Baiyun Sag and on Southern Uplift Zone in the southern of Baiyun Sag,Zhuhai Formation-Zhujiang Formation had the sedimentary background to develop carbonate rocks and reefs.Based on the research of seismic response characteristics on Dongsha Uplift carbonate reefs and carried out well-log and seismic data contrast among deep-water carbonate layers,seismic facies identification marks of deep-water reefs had been established.By precisely searching of 2D and 3D seismic profiles,11 suspected reefs on 3D profiles and 23 suspected reefs on 2D profiles are found,the difference between reefs and volcano mounds,contourite mounds,basin floor fans had been listed.Based on the seismic facies recognition of reefs,synthesize analysis of drilling,logging,thin sections,seismic data,the vertical and horizontal distribution regulation of carbonate rocks were summarized.The carbonate rocks of Baiyun sag and its peripheral mainly developed in Zhuhai-Zhujiang Formation,two platform types are identified on seismic section,one is volcanic reef of carbonate platform which is characterized by small area distribution,large thickness;another is ancient uplift rimmed carbonate platform which is characterized by large area distribution,smaller thickness.Combined with the analysis of well facies,two major carbonate types were summarized on the plane distribution of the study area,including volcanic reef of carbonate platform: volcanic reef of carbonate platform which included the area of Well B6-1-southeast of Yunli Low Uplift-northwest of Liwan Sag;rimmed carbonate platform which can be subdivided into 3 types:(1)southwest of Dongsha Uplift,southeast of Yunli Low Uplift and northern Shunhe Uplift rimmed carbonate platform;(2)rimmed carbonate platform of southern Dongsha Uplift;(3)rimmed carbonate platform of northern Yunli Low Uplift.Summarize the carbonate rocks development mode,the important geological events from 23.8Ma(Baiyun Movement)and extensive transgression events from 18.5Ma have a great influence on the distribution of carbonate rocks of the study area,During Zhuhai Formation sedimentary period,sea water intruded into Baiyun Sag from low tectonic parts between Liwan Sag,Yunli Low Uplift and Yunkai Low Uplift,most of Shunhe Uplift?southeastern Yunli Low Uplift and Dongsha Uplit exposed on the surface and suffered from exposure and erosion,Baiyun Sag high topography developed rimmed carbonate platform deposits(Mixed deposits)which was less affected by sources,Baiyun Sag-Liwan Sag deep-water area volcanic island was less affected by sources,developed volcano island reef isolated platform.After Baiyun Movement,lower Zhujiang Formation of Dongsha Uplift,Yun Li Low Uplift and 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 18.5Ma,most of the platform disappeared,only local high points on the platform developed limestone.