Diagenetic Facies Characteristics And Comprehensive Evaluation Of Chang 6 Reservoir In Wuqi Area, Ordos Basin

The reservoir of Chang 6 Member in Yanchang Formation in Wuqi area of Ordos Basin is highly anisotropy with many paradox in the production and construction process.Therefore,more detailed understanding of reservoir characteristics are urgently needed.In this paper,the basic geological characteristics,diagenesis and pore evolution of the Chang 6 reservoir in the Wuqi area are studied using basic physical data,cast thin section,scanning electron microscopy and X-ray diffraction to divide the reservoir diagenetic facies type.The characteristics of micro-pore structure and seepage of each diagenetic facies are studied by high pressure mercury intrusion,constant velocity mercury intrusion,nuclear magnetic resonance and oil-water phase infiltration experiments.The Chang 6 reservoir is classified by cluster analysis.Comprehensive evaluation of different types of reservoirs are carried out in combination with production dynamics.Main understandings are as follows:The Chang 6 reservoir in the Wuqi area is nearly horizontal with minimum tectonic deformation.The sedimentary sub-facies are delta front.with microfacies of subaqueous distributary channels,estuary dams,and diversion bays.The lithology is feldspathic sandstone with main cement of carbonate minerals and matrix of clay minerals.The reservoir is a typical low to extra low porosity?8.58%in average?and extra to super low permeability(0.33×10^-3?m² in average)reservoir.The reservoir has undergone a complex process of diagenetic deformation.The compaction destroys the physical properties of the reservoir,resulting in a decrease in porosity of 20.14%;The cementation includes chlorite,illite,carbonate,and siliceous cementation,resulting in a decrease in porosity of 9.88%;The dissolution improved the reservoir properties,resulting in an increase in porosity of 2.84%.According to diagenetic minerals,pore structure,diagenetic comprehensive coefficient and different diagenesis degree,Chang 6 reservoir in the Wuqi area is divided into five types of diagenetic facies:?1?chlorite cemented intergranular pore facies,?2?feldspar dissolution facies,?3?illite cemented pores-intergranular pore facies,?4?siliceous cemented pores-intergranular pore facies,and?5?carbonate cements the dense facies.The main pore types of the Chang 6 reservoir in the Wuqi area are intergranular pores and dissolved pores,and the main throat types are flaky,curved and bundled.The chlorite cemented intergranular pore facies have the best physical properties with large pore throat radius,good pore throat sorting,good pore throat connectivity,high movable fluid saturation,and small oil-water two-phase seepage resistance,which is the most favorable diagenetic facies type.The carbonate cements the dense facies have the worst physical properties with small pore throat radius,large pore throat ratio,poor pore throat connectivity,low movable fluid saturation,and high oil-water two-phase seepage resistance,which is the worst quality diagenetic facies type.The Chang 6 reservoir is divided into four types of reservoirs from good to bad by cluster analysis method.The reservoirs have a good correspondence with the dynamic characteristics of production.