Study On Reservoir Characteristics Of Chang 7 Formation In Zhijing-Ansai Area

In this paper,Chang 7 reservoir formation in Zhijing-Ansai area is taken as the target section,based on all kinds of test data,the sedimentary characteristics,petrological characteristics,reservoir physical properties,micro pore structure,heterogeneity and diagenetic evolution in the study area are deeply studied,the influencing factors of reservoir performance are identified,and the reservoirs are classified and evaluated.The delta-deepwater sedimentary system is mainly developed in Chang 7 oil formation in Zhijing-Ansai area,in which underwater distributary channel and gravity flow turbidite sand bodies are important reservoir sand bodies in the study area.three types of sand bodies are developed:Ⅰ-multi-stage sand body superimposed thick layer type,Ⅱ-thick sand type,thin mud interbed type,Ⅲ-thin sand type and mud interbed type.The lithology is mainly fine-grained sandstone and shale,and the reservoir is mainly feldspar sandstone and feldspar lithic sandstone.The grinding roundness is mainly subangular,the sorting is medium,the cementation type is mainly pore type,the clastic composition has the characteristics of high feldspar and low quartz,and the average content of interstitial material is 19.03%,mainly chlorite and Illite.The sediments in the study area mainly experienced diagenesis such as compaction,cementation,metasomatism and dissolution.According to the standards of rock and mineral change,distribution and formation sequence,and the crystallinity of Illite,it is considered that the diagenetic evolution stage in the study area is middle diagenetic A stage.It is divided into four kinds of diagenetic facies:clay mineral cementation diagenetic facies,carbonate cementation diagenetic facies,chlorite cemented film diagenetic facies and feldspar dissolution diagenetic facies.The porosity of Chang 7 reservoir in the study area is mostly distributed in the range of60.8%,and the permeability is mainly distributed in the range of（0.05～0.2）×10³μm²,in which Chang 7₁ layer porosity is relatively high,the flat porosity is 8.24%,and the average permeability is 0.12×10³μm².The main pore types are intergranular pore and feldspar solution pore,and the average total area porosity is 2.58%.The results of conventional mercury injection show that the samples of Chang 7 reservoir formation in the study area have more threshold pressure-micro-throat type,mainly fine pore throat,medium pore-throat connectivity,complex pore-throat type,strong distribution heterogeneity and so on.Based on the above research,it is considered that the sedimentary system and diagenesis are the main factors affecting the reservoir performance,in which the sedimentary system controls the reservoir performance,and the later diagenesis affects the reservoir quality,and the reservoir evaluation criteria and types are divided according to various parameters.Among them,the development of Chang 7₁ reservoir is obviously better than Chang 7₂ and Chang 7₃.