Study On The Characteristics Of Chang 8₁ Reservoir In Triassic Yanchang Formation,Xuecha Area,Ordos Basin

Xuecha area is located in the southwest of Wuqi oilfield in Ordos Basin.It is an oil-bearing enrichment area with multiple oil layers superimposed.10 horizontal wells have been drilled.Due to the weak research on Chang 8₁reservoir in the study area,it is still in the evaluation stage and has not been developed on a large scale.Therefore,this paper comprehensively discusses the static characteristics of the reservoir in the study area by using cast thin section,scanning electron microscope,high-pressure mercury injection,visual real sandstone model,oil-water two-phase seepage and other experiments,analyzes the seepage law under the condition of the static characteristics of the reservoir,analyzes the main controlling factors of the reservoir,optimizes the reservoir evaluation parameters,and carries out the comprehensive classification and evaluation of the reservoir,in order to provide theoretical support for the subsequent development of Chang 8₁reservoir group in Xuecha area.The main achievements are as follows.（1）The lithology of Chang 8₁reservoir in the study area is mainly lithic arkose.The average interstitial content is 15.8%,the average porosity is 8.5%,and the average permeability is 0.24×10^-3μm².It belongs to tight sandstone reservoir.The average face rate is 2.27%.The pore types are mainly intergranular pores and feldspar dissolved pores,with an average pore diameter of 24.5μm.The throat is mainly flaky and curved.（2）The high-pressure mercury injection curve can be divided into three types:classⅠ,classⅡand classⅢ.the pore throat structure gradually deteriorates from classⅠto classⅢ.The seepage characteristics are controlled by the reservoir physical properties,pore throat size,pore throat connectivity and pore throat heterogeneity.The better the physical properties,the larger the pore throat radius,the smaller the mean coefficient,the greater the sorting coefficient,the higher the maximum mercury saturation,the wider the oil-water two-phase seepage area,the higher the water drive oil efficiency and the stronger the seepage capacity.（3）The diagenesis types of Chang 8₁reservoir in the study area include compaction,pressure dissolution,dissolution,cementation and metasomatism.The main diagenetic stage is in phase A of medium diagenetic stage and a small part is in phase B of medium diagenetic stage;The porosity loss rate of the reservoir caused by compaction and pressure solution is 46.3%,and the porosity loss rate of the study area caused by cementation is42.2%.The dissolution effectively improves the porosity of the reservoir,and the porosity growth rate is 11.3%.（4）The average interlayer distribution frequency of Chang 8₁reservoir in the study area is0.11 strips/m,the average interlayer distribution density is 0.17,the average permeability differential is 3.1,the average permeability breakthrough coefficient is 1.4,the average permeability variation coefficient is 0.3,and the heterogeneity in the formation is weak.The stratification coefficient is large,with an average of 6.5,the average density of sandstone is 0.49,the thickness of single well compartment and single compartment is large,with an average of 12.2m and 5.5m respectively,and the sand body has poor vertical continuity and weak connectivity.On the plane,the continuity of sand body and physical properties along the provenance direction is relatively good,and the continuity perpendicular to the provenance direction is relatively poor.（5）The reservoir quality is mainly affected by sedimentation,diagenesis,micro pore throat structure and heterogeneity.Using the multivariate classification coefficient method,the Chang 8₁reservoir in the study area is divided into four types:classⅠ,classⅡ,classⅢand classⅣ,of which classⅡand classⅢreservoirs account for the most and classⅠreservoirs are the least.From classⅠto classⅣ,the reservoir quality gradually deteriorates,and the average daily output gradually decreases in the initial stage of production.