Study On The Sedimentary Facies And Reservoir Differences Of The Lower Jurassic Yangxia Formation In The East Of Kuqa Depression

The research object of this study is the Lower Jurassic Yangxia Formation in the east of the Kuqa Depression.The research content is sedimentary facies and reservoir characteristics.Core,logging,thin section,and mercury injection data were used in this paper.The Yangxia Formation in the study area can be divided into four oil layer groups:J₁y₁,J₁y₂,J₁y₃ and J₁y₄,which are further subdivided into seven sand groups:J₁y₁,J₁y_2-1,J₁y_2-2,J₁y_3-1,J₁y_3-2,J₁y_4-1 and J₁y_4-2.The sedimentary system of Yangxia Formation is braided river delta-lacustrine system.Among them,the shallow lacustrine subfacies is developed during the deposition of J₁y₁;the braided river delta plain is developed during the deposition of the J₁y_2-1,J₁y_2-2,J₁y_4-1 and J₁y_4-2 sand groups.The braided river delta front is developed during the deposition of the J₁y_3-1 and J₁y_3-2.The provenance of Yangxia Formation are the South Tianshan structural belt and the south of Yangxia Sag.The fans are mainly distributed in the piedmont area of Tianshan Mountains and the south of the Yangxia Sag from mountain to the deposition center of basin.The reservoir space in Yangxia Formation are mainly pores and fractures;fractures are dominant in the west of the study area;and fractures and dissolution pores are dominant in the center of the study area;primary intergranular pores and dissolution pores are dominant in the east of the study area.The pore structure of the reservoir in Yangxia Formation can be divided into four types based on mercury injection parameters,porosity and permeability.For type I,the entry pressure is less than 1 MPa;the maximum connected pore throat radius is between0.81-1.5μm;the average porosity is 8.28%,and the average permeability is 1.49 m D.For type II,the entry pressure is less than 1 MPa;the maximum connected pore throat radius is between 1.5-8.54μm;the average porosity is 5.75%,and the average permeability is 0.46m D.For type III,the entry pressure is between 1-5 MPa;the maximum connected pore throat radius is between 0.16-0.74μm;the average porosity is 3.58%,and the average permeability is 0.1 m D.For Type IV,the entry pressure is greater than 5 MPa;the maximum connected pore throat radius is 0.01-0.15μm;the average porosity is 1.28%,and the average permeability is 0.02 m D.The physical properties of the pore structure from type I to type IV gradually deteriorated.The diagenetic facies can be divided into compacted facies,carbonate cemented facies,dissolution facies and fracture facies based on diagenesis,diagenetic minerals and physical properties of the reservoir.Comprehensively considering the various factors causing reservoir differences,the Yangxia Formation reservoir in the study area is divided into four types.The first type of reservoir is mainly developed in the underwater distributary channels.Its reservoir property is good after reformed by dissolution facies and the reservoir space is composed of primary intergranular pores and dissolution pores.The second type of reservoir is distributed at the edge of the underwater distributary channel and at the distributary channel;its diagenetic facies mainly develops fractured facies and the reservoir space is composed of dissolved pores and micro-fractures.The third type of reservoir is distributed in the mouth bar and the edge of the channel;it mainly consists of fine sand or silt sand,with development of compacted facies and carbonate cemented facies.The four type of reservoir is distributed in the inter-channel bay,where the fine rock grains are mainly silt or argillaceous silt,with development of compacted facies or carbonate cemented facies.Based on the sand thickness,sand-to-ground ratio and the dominant diagenetic facies,it is believed that the favorable reservoir of the Yangxia Formation maily distribute in the J₁y_3-1,where ratio of fractured facies and dissolution facies was greater than 50%and the sand thickness is between 30 and50m.