Sedimentary Microfacies And Reservoir Characteristics Of Chang8 In The Southeast Of Wanggoumen

This study takes Chang 8 Member of Yanchang Member in the southeastern area of Wanggoumen,Ordos Basin as the research object,and adopts logging,core analysis,scanning electron microscopy and other methods to comprehensively study the depositional system,single sand body overlay type,reservoir characteristics and diagenesis of low permeability sandstone reservoir in Chang 8 Member.Based on the optimized reservoir evaluation parameters,comprehensive evaluation of reservoir quality is carried out.The main achievements are as follows:Chang 8 member is divided into Chang 8₁ and Chang 8₂ oil-bearing layers with a thickness of about 40 meters,and the formation thickness is relatively stable.A set of delta front depositional system was developed,which experienced the cycle of lacustrine transgression-lacustrine recession-lacustrine transgression.There are mainly underwater distributary channel,mouth bar,underwater natural dike and interdistributary bay microfacies.By identifying markers such as mud interlayers and calcareous interlayers,the superimposed relationships of sand bodies can be divided into four types:isolation type,butt type,cut superimposed type and superimposed type.The sandstone types of Chang 8 member are mainly fine gray-brown feldspar sandstone and lithic feldspar sandstone.The average porosity and permeability are 8.6%and 0.54×10^-3μm^2.The planar distribution characteristics of physical properties are similar to that of sedimentary microfacies,and the reservoir physical properties of Chang 8₁ member are better than that of Chang 8₂ member.The pore types of the reservoir are mainly residual intergranular pores and dissolution pores.The structure types of the reservoir include classⅠhigh mercury intake saturation medium pore throat type,classⅡmedium mercury intake saturation fine pore throat type and classⅢlow mercury intake saturation micropore throat type.Compaction,cementation and dissolution are the main types of diageneses,which are currently in the middle diagenetic stage A and partly in the middle diagenetic stage B.The physical properties of sandstone reservoir in Chang 8 Member are mainly affected by the sedimentary environment and diagenesis.The underwater distributary channel sand body is a favorable sedimentary facies zone,followed by the estuarine bar and underwater natural embankment.According to the pore structure and pore evolution characteristics of the reservoir,it is calculated that the average original porosity of Chang 8 Member in the study area is 38.47%,the average porosity loss value caused by compaction is 22.65%,the average porosity reduction rate caused by late cementation and metasomatism is 10.72%,the porosity increase efficiency of dissolution is 3.79%,and the existing porosity of Chang8 Member in the study area is 8.6%.In this evaluation,seven evaluation parameters（sedimentary environment,superposition type of sand bodies,clay minerals,pore type,reservoir physical property,pore structure parameters,effective reservoir thickness）and one empirical parameter（diagenesis）were selected by multi-factor comprehensive analysis method,and Chang 8 reservoir was divided into three types:classⅠ,classⅡand classⅢaccording to their merits.ClassⅢreservoirs are most widely distributed in the study area.ClassⅠand ClassⅡreservoirs are mainly distributed along the provenance-source direction,underwater distributary channel sand bodies and some estuary bar thick-bed sand bodies,which are important exploration target areas in the study area.