Sequence Stratigraphy And Depositional System Of Es₃^s In Shulu Sag,jizhong Depression

Shulu sag is located in the south of Jizhong depression in Bohai Bay Basin,which is a single fault dustpan shaped oil-gas rich sag.According to logging data,Es₃^s is the best target bed for reservoir forming potential in this area.It is difficult to develop because of the large buried depth and unclear reservoir distribution.Guided by the classical sequence stratigraphy and sedime ntology research methods,ba sed on the research and analysis of logging,seismic,paleontolog y and other data,this paper comprehensively summa rizes the previous research results of Paleogene strata in Shulu sag,and studies the fourth-order sequence division and sedimentary syste m o f the main target la yer Es₃^s,so as to provide the basis for the prediction of favorable facies zone of reservoir.The main achievements are as follows:Based on the identification and division of sequence boundary,the upper Es₃^s in Shulu sag is divided i nto three fourth-order sequences(SSQ1,SSQ2,SSQ3).On this basis,the stratigraphic sequence framework is established in combination with the well connected sequence.Depositional system studies indicate that the Es₃^s is a dry and hot subtropical climate.The strata are generally characterized by the thickness of the East and the West.The y are mainly controlled by two major provenances in the West and the East.Alluvial fan,fan delta,braided river delta and lacustrine facies are developed,which can be further divided into fan root subfacies,fan subfacies,fan margin subfacies,braided river delta plain subfacies,braided river delta front subfacies,and pre Braided River Delta subfacies.The sedimentary microfacies include trench,side cap,overflow zone,main channel,distributary channel,distributary channel,underwater distributary channel,underwater distributary channel,estuary bar and so on.Combined with the analysis of seismic attributes,the western slope area in SSQ1 and the north and middle trough areas in SSQ2 and SSQ3 are well developed.Based on the above research results,we can draw the following conclusions:The fourth-order sequence SSQ1 in the Es₃^s shows a lake transgression-lake regression evolution law from bottom to top.The lake level reaches the maximum at the early stage of SSQ1,and then falls back.SSQ2 and SSQ3 are the evolution law of Lake regression.Es₃^s of four-order sequence sedimentary phase is equipped with inheritance.Short axis braided river delta is developed in t he west slope,long axis river controlled braided river delta is developed in the north depression,lacustrine facies is dominant in the middle and south depressions,and slope facies is developed in the steep zone.Among them,braided river delta plain di stributary channel microfacies,braided river delta front underwater distributary channel microfacies and mouth bar microfacies are favorable facies belts of reservoir.