Fine Evaluation Of Chang 7 Tight Sandstone Reservoir In Heshui Area,Ordos Basin

The Heshui area is located in the southwest of the Yishan slope of the Ordos Basin,and the Chang 7 oil layer group in the third member of the Upper Triassic Yanchang Formation（hereinafter referred to as"Chang 7"）is the key development member.The characteristics,influencing factors,and comprehensive evaluation methods of the Chang7 tight reservoir in Heshui area still need in-depth study.Under the guidance of theories such as oil geology and reservoir geology,combined with technical methods such as core observation,cast thin section observation,scanning electron microscope observation,and high-pressure mercury intrusion testing,this paper studies the petrological characteristics,reservoir characteristics,diagenesis,and pore evolution laws of the study area,and conducts a comprehensive evaluation of the reservoirs in the Chang 7 section of the study area.The average thickness of the Chang 7₁ and the Chang 7₂ sandstones in Heshui area is 13m,and the average thickness of the Chang 7₃ sandstones is 3m.Dark gray and gray feldspathic lithic sandstone and argillaceous siltstone mainly composed of main watercourses,branch watercourses,and branch watercourses lateral microfacies.The main pore types of the reservoir are feldspar dissolved pores and residual intergranular pores;According to the high-pressure mercury intrusion curve morphology and pore throat parameters,it is divided into three types of pore structures:I,II,and III;Mainly develops flaky and curved throat,with pore throat radius mostly less than 0.1μm.The average fractal dimension is 2.62,and the pore throat structure is relatively complex.The porosity of the reservoir is mostly between 7%and 13%,and the permeability is between0.1×10^-3μm²～0.2×10^-3μm²,belonging to low porosity and ultra-low permeability reservoir.The average brittleness index of the Chang7₁ and the Chang7₂ is about 50%,and the average brittleness index of Chang7₃ is 22%.The Chang 7₁ and the Chang 7₂layers mainly develope complex rhythms,with stratification coefficients of 6.76 and 6.31,and the sand ratio of the Chang 7₁ and the Chang 7₂ is 0.46 and 0.42.The porosity and permeability changes greatly in the plane,and the heterogeneity of in-layer,interlayer and plane is strong.The Chang 7 mainly experienced compaction,cementation,and dissolution,and is generally in the stage A of middle diagenetic.The diagenetic sequence is:compaction→secondary quartz→chlorite cementation→early calcite cementation→illite-montmorillonite mixed layer,illite cementation→feldspar particle dissolution→calcite dissolution,siliceous cementation→iron calcite,iron dolomite precipitation.According to the parameters such as sedimentary characteristics,physical properties,pore structure,and brittleness index,the Chang 7 reservoir is divided into three categories:I,II,and III.The Class I and Class II reservoirs of the Chang 7₁ and Chang 7₂ members are relatively developed,and the Chang 7₃ members are mostly Class III reservoirs.