Sequence Stratigraphic And Deep Water Sedimentaiy Characteristic In The Qingdongnan Basin

The South China Sea is a huge marginal sea at the western Pacific.The boundary of north is China continental margin;The boundary of west is near the Yinzhi continent;The boundary of south is near Kalimantan island;The boundary of east is near Marnila trench.The South China Sea is located in convergence parts of the Pacific plate、Indian-Australian plate and Eurasian plate.For the reason of subducting the Pacific plate to Eurasian plate,collision orogeny Indian-Australian plate to Eurasian plat, covergence Palawan and ReedBank to Kalimantan island. These tectonic activity cause the present tectonic framework of the South China Sea:the north is passive continental margin,the east is subduction belt, the south is covergence belt, the west is strike-slip shearing belt.The South China Sea stopped expanding from the end of the early Miocene(16.7Ma). During the expansion process, The South China Sea developed a wide range of depositional basin, which received a large amount of land-sourced material,riching in organic matter, and had good hydrocarbon source rock, transporting and storage of petroleum geological conditions. The deep water hydrocarbon prospective areas in the Northern South China Sea become the consensus of the international community.The Qingdongnan basin is located in the Northern South China Sea, which is a graben basin during Cenozoic. Its bottom half is graben basin, the top half is sag basin. The boundary between two half is seismic interface T60. Throughout the period of geological history, the South China Sea has main undergone rift epoch from Paleocene to early Oligocene, Rift fault-depression-squeeze reversed in late Oligocene and the sedimentation period from the early Pliocene to Quaternary.This paper is based on major national technology special project "The research on The evaluation about the potential rich hydrocarbon Sag in deepwater area of Northern South China Sea"-branch project:"The research of paleoenvironment and paleoclimate in deepwater area of Northern South China Sea"(2011ZX05025-002-03), basing on the paleontology information of the oil and gas well LS22-1-1、LS33-1-1and more than ten seismic profile. Analysing of the LS22-1-1、LS33-1-1well about the seismic interface icons of palaeontogical fossils data, log information, grain size of sediment, and then giving isochron sequence stratigraphic framework, analysis the sedimentary facies and depositional characteristics of deepwater area in Qingdongnan basin, give help to oilexploration in the future. The main conclusions are as follows:(1) Analysing of the LS33-1-1and LS22-1-1well about the seismic interface icons of palaeontological fossils data, and then giving paleontological stratigraphic division for the two wells. The main stratigraphic formation includes:eocene-lingtou formation; oligocene-yacheng and lingshui formation; miocene-sanya、meishan and huangliu formation; pliocene-yinggehai formation. The seismic interfaces are respectively T100、T80(32Ma)、T70(28.4)、T60(23.0Ma)、T50(16.0Ma) T40(11.6Ma)、T30(5.5Ma)、T20(1.8Ma). During32-1.8Ma, we recognise1primary sequence interface(T60),6sencondary sequence interface(T100、T80、T70、T60、 T50、T40、T30、T20), including2primary sequences、5sencondary sequences and18tertiary sequence stratigraphy, we recognise28low stand tract, transgressive stand tract and high stand tract.(2) Quantitative bio stratigraphic analysis methods were used to study the paleontological data of the LS33-1-1and LS22-1-1well, we determine the sequence interface age and the corresponding relation of age and depth,establish the biological sequence and isochronal sequence stratigraphic framework.(3) According to the slope system external form and its internal deposit formation, which can be divided into4classes from the west to the east:progradation slope, slumping slope, Channel change slope and Rolling monoclinic slope. Sedimentation type includes:Slope canyon、turbid flow、slumping、debris flow、slide and sediment waves.(4) The central valley is "s" type in the plane, extended direction NE. Originating from yinggehai basin to xisha trough,the central valley goes through ledong sag,lingshui sag, songtao sag, baodao sag and changchang sag.The sedimentary facies in The central valley includes5types,such as:turbidity channel, turbidite sand sheet,natural barrier and overtopping deposit, mass transport deposits, argillaceous sediment.(5) The analysis of the granularity of LS33-1-1and LS22-1-1well shows that, the average grain size of sediments is generally decreasing from yacheng formation to yingghai formation. Depositional environment of qiongdongnan basin changed from lake facies, near-shore facies, epeiric sea facies, to shallow sea-bathyal deposits, and it reflects that hydrodynamic condition is generally decreasing.