乌石凹陷古近系层序地层特征及充填演化

依据层序地层学、沉积学理论方法,综合地震剖面、钻测井、岩心、古生物、地球化学等资料研究认为;乌石凹陷古近系可识别3种级别不整合面,划分为7个三级地震(测井)层序,是一个进积(加积)—退积—加积序列。构造沉降是控制层序几何形态,充填叠置方式、演化及沉积体系分布的最重要因素:凹陷由多个半地堑在不同层序发育时期以不同方式叠置联结而成;受断层控制,层序外部形态有箱状-楔形、楔形-席状披覆型,边界终止方式有双断终止型、单断终止超覆型;不同构造演化阶段层序内部充填的沉积体系各具特征,初始断陷阶段的SQ1沉积冲积层序,强烈断陷阶段的SQ2—SQ4充填湖相层序,晚期断陷阶段SQ5—SQ7内部充填浅湖和河流三角洲沉积;同沉积断层陡坡带沉降迅速、可容纳空间快速增长,是凹陷的沉积和沉降中心,断陷期主控断裂活动时间和空间的变化导致了凹陷沉积和沉降中心的迁移。

Basin Filling Features and Evolution of Sequence Stratigraphy of Paleogene in Wushi Sag

By means of sequence stratigraphy,sedimentology and with comprehensive study of core section,cuttings,well-logging,seismic informations,fossils and geochemistry data the authors recognized three kinds of unconformities and classified the Paleogene into seven seismic sequences of third grade,vertical section of which is transgression,regression and aggradation respectively.Tectonic subsidence play dominant role in their geometric shapes,basin fillings,sequence evolution and sediment distribution in the Wushi Sag.Outline of the sequences are in shape of box-wedge or sheet-wedge and boundaries of sequences are varied from bilateral fault to monoclinal fault overlap,so that the sequences show distinct semigrabens which overlay in different ways.Seven sequences were formed in different stage of tectonic evolution.The sequence of SQ1 formed in initial rift stage and deposited alluvial sequence.Sequences SQ2-SQ4 developed in intense rift stage and sediment lacustrine sequence.Sequences SQ5-SQ7 formed in late period of rift before thermal subsidence and deposit mere and delta sediment.The place adjacent to the synsedimentary fault is the subsidence center and depocenter;meanwhile,controlling fracture migrates in horizontal made subsidence and sedimentary center of the sag changed.