Source-to-sink(S2S)system Analysis Of The Paleogene In The Pearl River Mouth Basin,Northern South China Sea

The Pearl River Mouth Basin is the largest Cenozoic sedimentary basin in the northern margin of the South China Sea.Its structural evolution process is complex.Its rifting period has experienced multiple rifting cycles,forming a structural sedimentary pattern of three uplifts and two depressions belts.At present,with the development of drilling technology and exploration process,the key strata of oil and gas exploration in the Pearl River Mouth Basin have gradually shifted from the shallow(Neogene)to the deep(Paleogene),and the exploration target has gradually shifted from the shallow water area to the deep water area.However,restricted by the structural sedimentary pattern of "uplift depression alternation",the development characteristics and differences of the Paleogene provenance sedimentary system in the South and North depressions of the Pearl River Mouth Basin,as well as the distribution law of the high-quality reservoirs in the middle and deep under the influence of magma(or volcaniclastic)are not systematically studied,which will seriously restrict the oil and gas exploration deployment in the next stage.In this paper,the Paleogene Wenchang Formation(rifting scene I: 49-38ma)and Enping formation(rifting scene II: 38-33.9ma)in the Pearl River Mouth Basin are taken as the main research objects.Based on the data analysis of high-resolution threedimensional earthquake,drilling core and zircon U-Pb dating which are widely covered in the whole area,and guided by sequence stratigraphy and seismic sedimentology theory,from the perspective of "source channel sink" system(S2S)research In this paper,the source sink system development and evolution characteristics of the Paleogene in different depressions(Zhu-I depression and Baiyun Sag)in the South and north of the Pearl River Mouth Basin are comprehensively discussed,the source supply mode,main types of sediment transport channels,sedimentary filling characteristics and sand body distribution rules in different rifting stages are clarified,and the provenance and sedimentary system development differences and provenance-transformation characteristics of the South and North depressions are summarized.Finally,a set of application guide of source-to-sink system in Paleogene rifting period of Pearl River Mouth Basin is constructed,which provides a reference for searching for favorable reservoir sand bodies in the next stage of the study area.Based on the theory of sequence stratigraphy,the Paleogene rifting period in the Pearl River Mouth Basin is divided into four semi-second-order sequences,i.e.the lower Wenchang Formation,the upper Wenchang Formation,the lower Enping formation and the upper Enping formation from the bottom to the top.Three main sequence stratigraphic patterns,i.e.the fault steep slope type,the fault gentle slope type and the slope type,are established,and the plane development position and vertical evolution characteristics of different sequence stratigraphic patterns are defined.Based on the U-Pb dating trace of detrital zircon and the comprehensive analysis of the source system based on 3D seismic data,the evolution rule of the provenance system and the difference of the provenancetransformation development in the Paleogene rifting period in the South-North depression belts of the Pearl River Mouth Basin are clarified.There are obvious differences in provenance-transformation period between Zhu-I depression and Baiyun Sag: Provenance transformation in and out of Zhu-I depression mainly occurs in the late Wenchang lower Enping formation,while provenance transformation in and out of Baiyun Sag mainly occurs in the upper Enping formation.During the deposition period of Wenchang Formation,affected by the low uplift(low uplift)in the basin(such as Huixi low uplift,Xijiang low uplift,Huilu low uplift,Panyu Low Uplift,Dongsha uplift,Yunkai low uplift and Yunli low uplift,etc.),the internal depressions of Zhu-I depression and Baiyun Sag are generally separated,which are mainly provided by the local provenance system in the basin,and the depressions are generally separated.Typical migration type sequence(obvious migration of sedimentary center)is developed.In the depositional period of lower Enping formation,the sedimentary range of the North-South depression is generally expanded,but the Zhu-I depression is still separated from the Baiyun Sag in the south.At this time,the Zhu-I depression is mainly supplied by the regional provenance system of South China fold belt outside the basin and the local provenance system in the basin,while the Baiyun Sag is mainly supplied by the local provenance system in the basin The source system provides the sediments,and the sedimentary environment begins to transition to the marine environment,but the overall scale of the sedimentary system increases significantly.During the sedimentary period of the upper Enping formation,with the increase of transgression in the south of the basin and the further expansion of Zhu-I depression lake basin,Enping Sag and Baiyun Sag begin to connect.At this time,Zhu-I depression and Baiyun Sag are mainly provided by the regional provenance system outside the basin.The change of supply intensity of provenance system in and out of the basin in Paleogene in the South-North depressions is mainly controlled by the geomorphic blocking of the uplift in Panyu basin,the change of supply intensity of provenance in and out of the basin and the regional sea level evolution.On the basis of macro provenance tracing,based on 3D seismic data,the provenance channel types,combination patterns and the relationship between them and reservoir sand bodies in different depressions in the South and north of Pearl River Mouth basin can be further described in detail.According to the sedimentary paleogeomorphology and stratigraphic slice,the study area mainly develops three kinds of material source transport channels: incised valley,fault trough(single fault,double fault and fault plane troughs)and structural transition zone.Among them,the fault steep slope belt mainly develops incised valley and fault trough sediment-pathway and developed a series of fan delta and gravity flow fans(such as the north of Enping Sag,the north of Xijiang sag,the South and north of Huizhou sag,the southwest of Baiyun Sag,etc.),and the fault gentle slope belt mainly develops fault trough / structural transformation belt sediment-pathway with typical braided river delta sedimentation(such as the HZ25-7 structure of Huizhou sag,the east of Lufeng sag,the west of Baiyun east low uplift,etc.),while the slope belt mainly develops a wide shallow type grain sediment-pathway-proximal source short axis small braided river delta(such as Enping depression and south slope belt of Xijiang depression,etc.)or wide deep type ancient incised valley – far-transportation source long axis large braided river delta(north slope belt of Lufeng Sag,north slope belt of Baiyun Sag,etc.).According to the distribution of provenance and sedimentary areas,and the development characteristics of the S2 S system in Zhu-I depression and Baiyun Sag,we divide it into four types:(1)"convergent" S2 S system;(2)"divergent" S2 S system;(3)"structural transformation belt" S2 S system;(4)"migration" S2 S system.According to the characteristics of the specific boundary pattern,it can be further divided into three types: faulted steep slope type,faulted gentle slope type and slope type.Among them,Enping sag,Xijiang main sag,Panyu 4-xijiang 36 sags,Huizhou sag,Lufeng sag and Baiyun sag all belong to the "converging" S2 S system;while the central uplift of Xijiang sag,the low uplift in the south of XJ23 sag,Huilu low uplift and Yundong low uplift all belong to the "diverging" S2 S system;because of the development of two groups of fault systems in different directions,especially NWW.In the depression controlled by the fault system,there are generally structural transformation belt formed due to weak structural stress,which are typical of Huizhou 25-7 and Baiyun 9-1 structures.The Huilu low uplift and Lufeng low uplift in the south of Lufeng sag belong to the migration type source sink system.In this paper,the source sink systems such as Enping sag,Xijiang sag central low uplift,Xijiang 23 depression uplift,Yundong low uplift and Yundong low uplift are selected for fine characterization: watershed,pathway,convergence system and sedimentary facies model.Based on the analysis of the characteristics and differences of the source sink system in the South and North depressions of the Pearl River Mouth Basin,the main controlling factors of the source sink system in the study area are further clarified.Among them,the tectonic background and boundary conditions of the basin control the distribution and distribution of provenance sedimentary system,the important paleogeomorphic units and their evolution control the change of sedimentary pattern and the intensity of provenance supply,while the mode and intensity of provenance supply,climate conditions and lake level change and other factors comprehensively control the S2 S system coupling relationship and development differences in the study area.Based on the systematic study of Paleogene provenance system,transport pathway,distribution characteristics of sedimentary facies and evolution in the South-North depression zone of Pearl River Mouth Basin,this paper presents the structural paleogeomorphic pattern,sedimentary background evolution and the transformation difference of provenance system between the South-North depression and the SouthNorth depression of Pearl River mouth basin.The analysis is based on the comprehensive factors such as the structural paleogeomorphic pattern,sedimentary background evolution and the transformation difference of provenance system within and outside the basin.The S2 S system of the Paleogene Wenchang Enping formation in the depression belt can be divided into three different evolution stages,and the corresponding source sink system coupling model of the different stages of the Paleogene continental fault lake basin to the sea-land transitional phase evolution in the Pearl River Mouth Basin is established.That is: 1)Wenchang period,north-south separation,typical characteristics of rifted-lacustrine basin;2)lower Enping period,north-south separation,the Baiyun sag was transferred to the marine environment;3)upper Enping period,north-south connection,provenance outside the basin dominated.Through comprehensive study,it is clear that the secondary source sink system of faulted gentle slope type and slope type is an efficient coupling system,in which the physical properties of the reservoir sand body undergoing relatively distal sediment transportation are the best reservoir(for example,the braided river delta of the lower Wenchang Formation of the southern slope of Lufeng sagg,the braided river delta of the upper Wenchang Formation of the northern fault gentle slope,the large braided river delta and the braided river delta of the upper Enping formation of the northern slope belt of Baiyun sag,and the braided river delta of Wenchang Formation under the gentle slope of the west side of Yundong low uplift)can be served as the key oil and gas exploration target in the next stage of Paleogene in the Pearl River Mouth Basin.