Research On Structural Evolution And Sedimentary Facies In North Depression Of South Yellow Sea Basin

South Yellow Sea Basin is a multicycle superimposed basin forming on the pre-Sinian metamorphic rock basement of the Lower Yangtze Platform.North Depression is the secondary structual unit of South Yellow Sea Basin,making Qianliyan Uplift as the northern boundary.Its southern stratum overlaps the Middle Uplift of South Yellow Sea Basin,and on the whole it downfaults to north and overlaps to the south,with a total area of about 51,000 km2.North Depression can be devided into 9 third-level structural units,including 6 negative structural units(Northeast Sag,North Sag,Middle Sag,West Sag,South Sag and East Sag) and 3 positive structural units(North Salient,West Salient and South Salient).Since well Huang 2 was drilled in 1975 in North Depression of South Yellow Sea Basin,it has been almost 30 years already.China has found liquid crude oil in the crack of Taizhou Formation in North Sag,and Korea has found gas show in its 2 wells,but neither of the two countries get commercial gas and oil flows so far.On the base of structural geology,sendimentology and petroleum geology,geophysical data such as seismic profiles and wells are used to study the structural evolution,sedimentary characteristics and hydrocarbon potential of this region by means of structural style analysis,balanced cross-section restoration,subsidence history analysis and sedimentary facies analysis.The result shows that the structural evolution of North Depression of South Yellow Sea Basin can be divided into three stages:the marine basin stage,the continental rift basin stage and depression basin stage. Basement cover of Yangtze style was deposited in North Depression of South Yellow Sea Basin in the marine basin stage.The sequence of the strata was well developed and there was basically no hiatus. Owing to the development of tensional faults,active continental rift basion with limited distribution was developed in the continental rift basin stage. The depression basin stage was the regional subsidence stage of North Depression of South Yellow Sea Basin.There are mainly two kinds of structural styles in the North Depression of South Yellow Sea Basin:compressive block and basement-involved thrust style and tensional block style.Both of them are basement-involved type.The basement-involved thrust is mainly located in the southern edge of Qianliyan Uplift,and the tensional blocks are widely developed.Both positive inversion tectonics and negative inversion tectonics can be identified in the North Depression of South Yellow Sea Basin.The negative inversion tectonics are mailly distributed to the south of Qianliyan Uplift.The positive inversion tectonics are mainly distributed in northeast sag,north sag,middle sag and south sag.The inversion period is Oligocene and late Pliocene.The inversion is related to the convergence rate between Paciffic plate and Eurasia plate.The subsidence curve of North Depression of South Yellow Sea Basin can be divided into two sections.The steep section corresponds to the rift phase in which subsidence rate is high,and the slow section corresponds to the depression phase in which subsidence rate is low.The rift phase can further be diveded into initial rifting stage(Late Cretaceous),high rifting stage(Palaeocene),rifting atrophy stage(Eocene) and inversion-uplifting stage(Oligocene).On the whole it shows the characteristic of episodic rift basin cycle evolution under the control of extensional action.The sedimentary evolution characteristic of North Depression of South Yellow Sea Basin from Late Cretaceous to Eocene can be summarized to river facies-lacustrine facies-river facies.Sedimentation of river facies was well developed in Late Cretaceous.The Late Cretaceous Taizhou Formation which developed sedimentation of river facies and lacustrine facies was districted only in North Sag and Northeast Sag.With the deepening of water,the sedimentary facies of Palaeocene Funing-1 Member to Palaeocene Funing-4 Member gradually changed from river facies to lacustrine facies.The Eocene Dainan Formation developed sedimentation of river facies in a smaller area compared with Funing Formation. The Eocene Sanduo Formation developed sedimentation of river facies after planation.The structural framework has control action on depositional filling of the basin.In the initial rifting stage and high rifting stage the subsidence rate is high and the sedimentation is thick.The sedimentation is distributed in EW trending belts controlled by synsedimentary faults.The depocenter is mainly located in steep slope zone of the northern border fault of north sag and northeast sag.The differential activity and evolution of the synsedimentary fault makes the transfer of depocenter.The sedimentation of river facies is well developed in the initial rifting stage. With the deepening of water,the sedimentary facies gradually changes to lacustrine facies. In the rifting atrophy stage the subsidence rate is low and the sedimentation is thin and wide.Owing to wide-ranging denudation,the difference of local structural activities,secondary horsts and grabens can make obvious differerce of local residual thickness.The river facies are well developed in the rifting atrophy stage.