Tectonic Evolution And Paleogene Prototype Restoration Of Songnan Baodao Sag In Qiongdongnan Basin

Songnan Baodao sag is located in the deep water area in the east of Qiongdongnan Basin(the present water depth is more than 300m),which is another hot exploration field with great potential for oil and gas resources after Yacheng uplift.Its structural evolution,structural style and prototype basin restoration are of guiding significance for the analysis of the distribution range and maturity of source rocks and the understanding of hydrocarbon accumulation rules.Based on the interpretation of seismic data and drilling data,this paper establishes the skeleton section of the study area,clarifies the tectonic evolution stages,analyzes the distribution law of the extensional structural style of Songnan Baodao sag,and restores the characteristics of Paleogene prototype basin.According to the passive continental margin basin model,the structural evolution of Songnan Baodao sag can be divided into three stages: pre rift stage,synrift stage and post rift stage.The rift stage can be further divided into three stages: early rift stage(leading stage),middle rift stage(Yacheng stage)and late rift stage(Lingshui stage),The post rift stage can be divided into two stages: thermal subsidence(Sanya Meishan stage)and accelerated subsidence(Huangliu Yinggehai Ledong stage);According to the differences of single-stage tectonic deformation and multi-stage combination in the extensional background,structural styles can be divided into three categories: contemporaneous deformation styles,contemporaneous style combination and multi-stage style combination,and further subdivided into 14 sub categories.Based on the analysis of Songnan Baodao sag and its adjacent prototype basins,the original distribution of eight Paleogene dustpan like fault depressions has been restored.The development of these dustpan like fault depressions is characterized by the echelon stagger of dip facies.Finally,through the analysis of the maturity history of source rocks,the oil and gas migration model and the optimization of favorable zones in Songnan Baodao sag are discussed.It is considered that the main source rocks in Songnan Baodao sag,the leading formation of lacustrine source rocks and the Yacheng formation of marine source rocks,have a high degree of thermal evolution,have reached a high maturity stage as a whole,and have entered the peak period of hydrocarbon generation;The favorable exploration zones are fault zone,slope zone and peripheral uplift zone,mainly including Songtao low uplift in the north,Lingshui low uplift in the West and Songnan low uplift in the East.