Research Of Sequence Stratigraphy And Sedimentary System Of Zhuhai Formation On The North Slope Of Baiyun Depression

The Pearl River Mouth Basin is the most typical marine basin of the Cenozoic in China,and it was formed along with the evolution of the formation of the South China Sea.From the beginning of the expansion of the South China Sea at the end of the Oligocene to the present day,a variety of types of favourable reservoir sands have formed within the basin,which are rich in hydrocarbon resources and have maintained crude oil production of over 10 million cubic metres since 1996.With the advancement of the exploration and development process,there has been a shift in the direction of exploration from shallow water to deep water and from shallow to deep layers in recent years.In this context,the Zhuhai Formation of the Baiyun Depression has gradually become a hot spot for exploration and development.At present,the exploration of large dominant structure oil and gas reservoirs in Baiyun sag has basically finished,and the exploration target has gradually turned to small stratigraphic lithologic oil and gas reservoirs.The key problem of lithologic reservoir exploration is to determine the type and distribution of sedimentary facies.However,the type and distribution of sedimentary facies in the Zhuhai Formation of Baiyun Depression are not clear yet.This paper takes the study of the sedimentary characteristics of the Zhuhai Formation on the north slope of the Baiyun Depression as an entry point,and uses classical stratigraphic theory as a guide to establish a stratigraphic stratigraphic framework on the north slope of Baiyun,and on this basis discusses the spreading and evolution of the sedimentary system within the isochronous stratigraphic framework.A comparison of the trace element characteristics of 35 bedrock samples from the modern South China coastal Pearl River Basin and 14 samples from the Zhuhai Formation on the northern slope of the Baiyun Depression reveals that the sediments of the Zhuhai Formation on the northern slope of the Baiyun Depression are mainly from the ancient Pearl River Basin,with a mixed source of granite,alkaline basalt and ancient sedimentary rocks,and a passive continental margin as the tectonic background of the source area.In view of the limited analysis and testing of light and heavy minerals in the study area,this paper further verifies the provenance supply direction of the Zhuhai Formation on the north slope of Baiyun Depression through the orientation interpretation of a large number of seismic profiles of the forebody direction.The results show that the provenance of the Zhuhai Formation on the north slope of Baiyun Depression comes from the ancient Pearl River and its branches in the northwest of the study area,and is supplied to the interior of the depression in a multi-point radial pattern.Based on the detailed observation of the 44.3-meter cores from three Wells,and by integrating the sedimentary characteristics of lithology,sedimentary structure and genetic mechanism of rock structure,three types of conglomerate facies,sandstone facies and fine grained rocks are identified in the Zhuhai Formation in the study area.It can be further divided into 12 Kind of rock facies,include scour and fill structural fine conglomerate facies,cross bedded pebbled sandstone facies,massive bedded pebbled sandstone facies,biodisturbed structural sandstone facies,wavy bedded sandstone facies,argillaceous laminated sandstone facies and parallel bedded sandstone facies,biodisturbed structural fine grained lithofacies,glaucone-bearing fine grained lithofacies,horizontal bedded fine grained lithofacies,drainage structural fine grained lithofacies and sand bedded fine grained lithofacies.Based on the vertical combination of rock facies,4 rock facies combinations were identified,corresponding to 4 sedimentary microfacies,namely,underwater distributary channel,estuary bar,interdistributary bay and distal bar.With seismic sequence stratigraphy as the main line,combined with logging sequence stratigraphy division,and bridged by precise time-depth relationships,two second-order sequence interfaces and five third-order sequence interfaces are identified in Zhuhai Formation on the north slope of Baiyun Depression,and the target layer is divided into one second-order sequence and six third-order sequence.The second-order sequence interfaces of ZHSB1 and ZHSB7 are tectonic unconformities corresponding to the South China Sea movement and the Baiyun movement respectively,which are shown as truncated reflection characteristics on seismic profiles.ZHSB2-ZHSB6 is a third-order sequence interface,which mainly corresponds to the rise and fall of sea level,and its interface features are two characteristics.The seismic facies of Zhuhai Formation in the study area are divided according to seismic physical parameters,internal reflection structure of seismic reflection and external morphology of seismic facies.Combined with wells sedimentary facies analysis,sedimentary relative ratio of connected well,seismic facies analysis of section,seismic attribute optimization and planar distribution of attribute value,the analysis of sedimentary facies types and distribution of each three-level sequence is completed.The results show that during the deposition period of Zhuhai Formation on the north slope of Baiyun Depression,the sea level rises on the whole,and there is a small decrease in the sea level at ZHSQ4 and ZHSQ6.The Zhuhai Formation on the northern slope of the Baiyun Depression is mainly developed as a deltaic plain,deltaic foreland and shallow marine deposits.When sea level rise and fall is high,the study area has multiple phases of deltaic doropods accretion and decretion,with a clear degree of divergence along the source direction.When sea level rise and fall is small,the deltaic doropods tend to oscillate in the vertical direction of the source,forming a continuous deltaic doropods complex.