Study On The Difference Of Reservoir Characteristics Between Chang2 And Chang6 Member Of Yanchang Formation In Zhidan Area,Ordos Basin

There are good hydrocarbon shows in the Upper Triassic Yanchang Formation from Chang1 to Chang10 oil-bearing group.However,several years of exploration practice shows that the hydrocarbon enrichment rules of each oil-bearing group are different.Taking Chang2 and Chang6 reservoirs in Zhidan area as the research object in this paper,compare and discuss the differences of petrological characteristics,diagenesis,micro pore-throat structure and seepage characteristics between Chang2 and Chang6 reservoirs by using the experiments of casting slice,X-ray diffraction,scanning electron microscopy(SEM),electron probe,conventional mercury injection,nuclear magnetic resonance(NMR)and microscopic displacement of real sandstone model,and finally explore the hydrocarbon accumulation regulations in Chang2 and Chang6 reservoirs.The Chang2 and Chang6 in the study area are developed low-amplitude nose-shaped uplifts which descend from east to west.There are some differences in sedimentary microfacies: Chang2 is delta plain subfacies and high-quality reservoirs are mainly located in distributary channel microfacies.Chang6 is delta front subfacies and high-quality reservoirs are mainly located in underwater distributary channel microfacies.Chang2 reservoir is medium-fine feldspar and lithic feldspar sandstone,while Chang6 reservoir is fine feldspar sandstone.Chang2 sandstone is slightly higher than Chang6 sandstone in component maturity.The textural maturity is equivalent between Chang2 and Chang6.But the fillings are quite different.The content of carbonate minerals is different.Clay mineral of Chang2 is dominated by chlorite and kaolinite,while Chang6 is dominated by chlorite.The physical property of Chang2 reservoir is superior to that of Chang6 reservoir.The main reason for the difference of physical property is the difference of diagenesis.It has little difference for the comprehensive effect of compaction and cementation on porosity.The main factors that affect the connectivity of pore and throat are strong compaction,high cement content and chlorite film thickness of Chang6 reservoir.The dissolution of Chang2 reservoir is strong that the average increase of dissolution pore is 4.2%.But the average increase of dissolution pore of Chang 6 reservoir is only 1.12%.The strong dissolution also improves the permeability of reservoir.The pore-throat structure of Chang2 and Chang6 reservoirs is quite different.The pore of Chang2 is mainly composed of residual intergranular pores and dissolved pores.The throat of Chang2 is mainly lamellar and curved.Chang6 is mainly composed of residual intergranular pores and very few corrosion pores.The throat of Chang6 is mainly dot-shaped and curvedflake.The typical samples of conventional mercury injection and NMR show that the pore throat radius and connectivity of pore throat in Chang2 reservoir are better than that in Chang 6 reservoir.The displacement efficiency of Chang2 reservoir is obviously higher than that of Chang 6 reservoir.Therefore,the micro pore-throat structure of the reservoir is the main factor affecting the permeability of the reservoir.According to the standard of comprehensive evaluation,the physical property of Chang2 reservoir is obviously better than Chang6 reservoir.The Chang 2 mainly develop reservoir of type I and II,while Chang 6 mainly develop reservoir of type II and III.