Sedimentary Evolution Of The Late Permian-Early Triassic In The Central Uplift Of The South Yellow Sea Basin

Although the Mesozoic and Paleozoic strata in the South Yellow Sea Basin have good hydrocarbon accumulation conditions,no industrial hydrocarbon resources have been found.Mesozoic and Paleozoic strata in the central uplift are the exploration target of oil and gas in the South Yellow Sea Basin.However,the drilling core is lack,so the Mesozoic and Paleozoic stratigraphic development,the types and evolution of sedimentary facies in the central uplift are at the speculative stage,which seriously restricts oil and gas exploration.This paper takes the first well-CSDP-2 full core well(the deepest well in the global ocean science drilling)in the central uplift of the South Yellow Sea Basin as research object,and comprehensively utilizes core observation,paleontology identification,thin section identification,logging and seismic data analysis.And this paper combines the Mesozoic and Paleozoic tectonic and sedimentary evolution background of the Lower Yangtze Plate in the Northern Jiangsu and South Yellow Sea Basin.This paper makes a systematic study on the stratigraphic division,characteristics,sedimentary facies types and evolution of the Upper Permian-Lower Triassic in the central uplift of the South Yellow Sea Basin.The Upper Permian-Lower Triassic in the central uplift of the South Yellow Sea Basin developed the Upper Permian Longtan Formation and the Dalong Formation and the Upper Triassic Qinglong Formation from the bottom up.The bottom interface of the Upper Permian Longtan Formation is the lithologic transformation surface of the mudstone and Gufeng Formation siliceous rock,which is set at 1635.10 m.The interface between the Longtan Formation and the Dalong Formation is the transition plane from the progradation to the retrogradation,and the boundary is set at 914.80 m.The interface between the Upper Permian Dalong Formation and the Lower Triassic Qinglong Formation is the first appeared belt of the Hindeodus parvus,and the lithologic transformation of the Dalong Formation Bioclastic dolostone and the Qinglong Formation mudstone can be identified.and the boundary is set at866.20 m.The top interface of the Lower Triassic Qinglong Formation is the unconformity of the Indosinian on the area.The Upper Permian Longtan Formation is a set of silt clastic rock with thinning coal seams,and the stratum thickness is thinned in three directions: north,south and west.The Upper Permian Dalong Formation is the sediment of bioclastic dolomite and ash-bearing fine sandstone,and the stratum is thin in the area.The Lower Triassic Qinglong Formation is mainly composed of mudstone and gray mudstone.Because the top of the stratum is generally erosional,the thickness of the strata varies greatly in the area.The Upper Permian-Lower Triassic in the central uplift of the South Yellow Sea Basin is mainly composed of sea-land transitional facies(the delta facies and the tidal-flat facies),the carbonate platform facies,and the shallow sea shelf.The sedimentary facies of the Upper Permian Longtan Formation is the vertical evolution sequence of the tidal flat-shallow sea shelf-delta,and the depth of sedimentary water firstly increases and then decreases.The Upper Permian Dalong Formation mainly developed tidal-flat facies deposition.The sedimentary facies of the Lower Triassic Qinglong Formation is a vertical evolution sequence of the restricted platform-opening platform,and the depth of water deepens upward.Through the Longtan Formation,the Northern Jiangsu and South Yellow Sea Basin developed delta,tidal,and lagoon facies(Nanjing),tidal flat and lagoon facies(N5 well),tidal flat,shallow sea shelf,and delta facies(CSDP-2 well,WX5-ST1 well)from southwest to northeast,the water depths of the whole area are not much different.According to the distribution of sedimentary facies,the depth of water is deep in the southwest while shallow in the north east,which is considered to be the result of the rapid differential uplift of the Northern Jiangsu and South Yellow Sea Basin during the Soochow Movement period.