Evolution Of Reef-bank System In The North And South Continental Margins Of South China Sea And Their Difference

The reef-bank system is one of the most important petroleum-bearing reservoirs in the South China Sea.The development and evolution of the reef-bank system and the analysis of its controlling factors have always been frontier hot spots in geological research.Carbonate platforms and biological reefs are widely developed in the southern and northern parts of the South China Sea.However,there are very few drilling cores in the Island Reef,which greatly restricts the study of carbonate platform and reef-bank system.This thesis takes the core XK1 well in the north of the South China Sea and the NK1 well in the south of the South China Sea as the research objects.Based on core description,thin slice observation and a large number of test data and analysis,the XK1 and NK1 well reef-bank system are finely characterized.The structure effects,sea level changes,East Asian monsoon and upwelling current,dolomite development and global climate event response on the development and evolution of the reef-bank system.The carbonate factory composition of the reef-bank system in the southern and northern parts of the South China Sea are further identified.As a result,the difference in the development and evolution law of the reef-bank system between the north and south continental margins in the South China Sea are summarized,which provides the basis for the prediction of the distribution of carbonate reef reservoirs in the South China Sea.The research results obtained by the thesis mainly include the following aspects:(1)Based on the delicate study of XK1 Well,the sedimentary facies composition,sequence unit division,and vertical variation rule of carbonate association in the XK1 well since the Miocene are summarized: Five sedimentary facies and 29 carbonate association belts in the XK1 well have been identified since the Miocene,the development and evolution characteristics of the reef-bank system are systematically summarized in the XK1 well.There are five types of sedimentary facies in the reef-bank system of XK1 Well,namely,reef cap microfacies,reef core microfacies,inner shoal microfacies,outer shoal microfacies,and lagoon microfacies.According to the developmental environment of coral reefs in Well XK1,12 Heterozoan carbonate association belts,6 Heterozoan-dominated carbonate association belts,and 11 Photozoan carbonate association belts are divided.On this basis,the vertical strata since the Miocene has been divided into 16 third-order sequences and the evolution of various carbonate component combinations in the reef-bank system since the Miocene is summarized.(2)2.Based on the delicate study of XK1 Well,the sedimentary facies composition,sequence unit division,and vertical variation rule of carbonate association in the XK1 well since the Miocene are summarized: There are five types of sedimentary facies in the reef-bank system of XK1 Well,namely,reef capmicrofacies,reef core microfacies,inner shoal microfacies,outer shoal microfacies,and lagoon microfacies.Combining paleontology and strontium isotope data,five key exposed surfaces in 807.2m,543.1m,464.6m,383.6m and 210.3m were proposed to be corresponding to the top interfaces of Oligocene,Lower Miocene,Middle Miocene,Upper Miocene,and Upper Pliocene.This work provides a strong basic knowledge for the classification of the strata in this well.According to the identification of exposed surface markers in the reef-bank system from NK1 Well,22 significant exposed surfaces were identified.The vertical strata since the Oligocene has been divided into 17 third-order sequences.Based on various aspects of geochemical analyses,the paleo-oceanic environment changes in the development and evolution of the NK1 well reef were revealed.The test results of C,O isotopes,mineral composition and geochemical elements of carbonate rocks have well revealed the changes of sea level,paleo-productivity and nutrient conditions since the Oligocene.According to the developmental environment of coral reefs in Well XK1,17 Photozoan carbonate association belts,12 Heterozoan-dominated carbonate association belts,and 7 Heterozoan carbonate association belts are divided.A large number of coral components in this well form the main body of the NK1 Well.The combination of geochemical analysis and sedimentology research helps to establish a model for the development and evolution of NK1 well reef-bank system.(3)Using comparative sedimentology methods,a detailed description of the outcrop profile of Jiantianba reef in the Late Permian in western Hubei revealed the three-dimensional spatial combination characteristics of the reef sedimentary facies in the outcrop area: 8 lithologic facies and 4 sedimentary association are identified,revealing the reef that used sponges as building organisms.Although the late Permian Jiantianba reef and the Cenozoic reef in the South China Sea have different reef-building organisms,the three-dimensional combination characteristics of the reef system summarize the combination and evolution characteristics of various lithologic facies during the vertical evolution of the reef-bank system,and provide an analogy example for the lithologic facies and spatial association of the underground reef system.(4)Multiple factors significantly affected the development and evolution of the reef bank system in Wells XK1 and NK1,such as,tectonic effects,sea level changes,East Asian monsoon,upwelling current and global climate events.The difference in tectonic activities between the south and the north of the South China Sea not only leads to the difference in the development time of carbonate rocks in the continental margin basins,but also the difference in the thickness of the frequently exposed surface and the biological reef-shoal system due to the difference in structural subsidence.Relative sea level fluctuations control the development and evolution of biological reefs.Although they are still affected by local tectonic effects,they are comparable to global sea level changes,which affect the periodic cycle of the development and evolution of the reef-bank systems.The monsoon and upwelling currents carry rich nutrients to the northern Xisha uplift zone,leading to changes in the composition of the carbonate composition.It restrains the development of the XK1 coral reef in the northern continental margin,while the reef bank system in thesouthern continental margin had not been affected by the East Asian monsoon activity influences.XK1 and NK1 Well both experienced significant dolomitization in the Late Miocene,and the law of this synchronization is related to global paleoclimate events.(5)The differences between the Cenozoic carbonate factory in north and south reef island of the South China Sea were identified,and the differences in the main controlling factors of the changes of carbonate factory in different periods were revealed: During the Oligocene period,the change mechanism of carbonate factory in the South China Sea is a paleo-climatic control type,and the Heterozoan carbonate association was developed in Well NK1.By the end of the Oligocene,the global climate was warming and coral reefs appeared in Well NK1;During the Early Miocene,the warming climate in the South China Sea was suitable for coral reef development.At this time,the change mechanism carbonate factory in the South China Sea is a sea-level control type.The conversion of the Photozoan to Heterozoan carbonate factory was mainly controlled by the relative sea-level fluctuation;During the Miocene period,the change mechanism of the carbonate factory in the South China Sea was dominated by tectonic and monsoon activities.Well XK1 was transformed from the Photozoan to the Heterozoan carbonate association due to the influence of monsoon enhancement and eutrophication sources.The Heterozoan carbonate association is the main component of the Well XK1,while the NK1 well is hardly affected by the monsoon to form the Photozoan carbonate association;During the late Miocene,change mechanism carbonate factory in the South China Sea is dominated by sea-level fluctuation and enhanced monsoon activity.Since the end of the expansion of the South China Sea due to the Sabah orogenic process,the entire South China Sea during the late Miocene has gone through the heat sinking stage.The sea-level rise became the main factor controlling the carbonate factory types in the South China Sea.Similarly,well XK1 showed the characteristics of the transition from Photozoan to Heterozoan carbonate factory during the late Miocene monsoon enhancement period,while Well NK1 was dominated by the Photozoan carbonate factory;The Pliocene-Quaternary carbonate factory change mechanism is dominated by sea-level changes.The transgressive systems tract of Wells XK1 and NK1 is dominated by the Heterozoan carbonate factory,and the high-stand systems tract is dominated by the Photozoan carbonate factory;Overall,the reef island in the southern South China Sea is dominated by the Photozoan carbonate association,while the reef island in the northern South China Sea is dominated by the Heterozoan carbonate association.Although there are certain differences in control factors such as tectonic subsidence,relative sea-level change,and monsoon effects in the south and north reefs Island,resulting in the difference in carbonate association types,the development and evolution of the reef-bank system have favorable comparability.