| Junggar Basin is one of the large superposed petroliferous sedimentary basins in China.There are multiple sets of source rocks in the basin.The southern margin is located in the Junggar Basin and the Tianshan Mountains.It mainly consists of three sub-structural units,namely,Qipaleofault-fold belt,Huomatu structural belt and Hutubi anticline belt.In this area,Jurassic-Cretaceous sand conglomerate oil and gas reservoirs are developed,with burial depth between 7000-7900 m.In 2019,well Gaotan-1 achieved oil and gas breakthrough in the accumulation combination in the lower southern margin,indicating that Jurassic in this area has good exploration potential.However,due to the complex reservoir structure,the distribution of reservoirs is difficult to predict,and the scale has not been developed yet.Taking the Lower Association(Kalaza Formation and Qingshuihe Formation)in the southern margin of Junggar Basin as the research object,guided by the theories of sedimentology and reservoir geology,this paper makes full use of geological,well logging and seismic data to study the distribution of sedimentary facies and favorable reservoir facies belts,and obtains the following understandings:(1)Based on the analysis of outcrop development characteristics in the southern margin of Junggar Basin,four major lithofacies characteristics of the lower association in the study area are identified based on the double attributes of lithology and sedimentary structure,which are conglomerate facies,sandstone facies,siltstone facies and mudstone facies.On the basis of lithofacies,10 lithofacies associations were identified,and two sedimentary facies,fan delta and lake,developed in the study area,were further identified.The fan delta-lake sedimentary model was established,and the sedimentary evolution law of the lower association Kalaza Formation and Qingshuihe Formation was clarified by combining with the analysis of paleoenvironment.(2)Under the guidance of outcrop sedimentary model,4 major types of underground lithofacies were identified,with a total of 17 lithofacies.Eight kinds of sedimentary microfacies,such as braided channel,mouth bar and sheet sand,were divided by the overlapping of different lithofacies on a single well.Two types of sedimentary facies,namely fan delta of Kalaza Formation and lake of Qingshuihe Formation,were identified.(3)The Jurassic Kalaza Formation in the southern margin of Junggar Basin identified one mid-base level half-cycle and 3-4 short-term base level cycles;The Qingshuihe Formation of Cretaceous has one half-cycle of medium base level and four to five short-term base level.(4)Using the method of comprehensive analysis of sedimentary facies by combining well vibration and multiple constraints,and the wave impedance inversion technique of poststack geostatistics,the thin-layer sand bodies in the study area were studied,the distribution of sedimentary microfacies was characterized,and the fan delta depositional system of Kalaza Formation was established.(5)The reservoir properties of Kalaza Formation and Qingshuihe Formation are studied by means of well logging,analysis and laboratory data.On the basis of observing the microscopic characteristics of the reservoir,the distribution characteristics of the internal physical properties of different sedimentary microfacies are studied,and the main controlling factors of the reservoir quality in the study area are discussed.Based on the latest exploration results,the comprehensive classification and evaluation criteria of reservoir are established,the distribution characteristics of favorable reservoir facies zones are predicted,and the development locations of high-quality reservoirs are pointed out.This study has enriched the fan delta-lacustrine sedimentary model,and has certain reference value for the exploration and development of Jurassic-Cretaceous ultra-deep tight oil and gas reservoirs in the southern margin of Junggar Basin. |
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