Reservoir Architecture And Evaluation Of Tight Sandstone In The Chang 8 Interval,Midwest Ordos Basin,China

With the new change of the world energy supply,unconventional oil and gas have attracted more and more attention.The Chang 8 deposition period was characterized by a typical shallow water delta sedimentary condition in the Jiyuan area,Ordos Basin.At present the tight sandstone reservoir sandbodies with thick layer and sheet distribution have been developed,which is an important place for oil and gas exploration and development.This paper focuses on the analyses of the reservoir characteristics,mirostructure,and the controlling factores of the reservoir quality of the Chang 8Interval tight sandstone,which contribute to the efficient exploration and development,and the great realistic significance in the Jiyuan areaBased on the high resolution sequence stratigraphy and the principle of stratigraphic division,the Chang 8 Interval is subdivided into four layers.Meanwhile,the chararcteristics of the different level base cycles are identified combined with the drilling,logging,core observation,grain size analysis and paleoprovenance,which indicate that the Chang 8 sedimentary period experienced three stages:transgression-regression-transgression.The sedimentary paleowater was dominated by the brackish water with weak oxidation-reduction condition.And the stable structural condition and the abundant resource from the Yinshan ancient land contributed to the form of the shallow water delata.Three main microfacies including underwater distributary channel,underwater natural levee,tributary bay and asmall amount of the mouth bar have been identified in the Jiyuan area.The lithology of the Chang 8 Interval is characterized by the gray,light gray fine sandstone and siltstone,and small amount of the green fine sandstone and siltstone in local area.The average porosity and permeability of the Chan 8 Interval sandstone are8.9%and 0.75×10^-3?m²,respectively,and the sandstone physical property of the Chang8₂ subinterval is better than that of the Chang 8₁subinterval.Based on the thin section observation,mercury injection?Pressure-controlled/Rate-controlled mercury injection?,carbon and oxygen isotopes and fluid inclusions analyses,the main reservoir spaces are primary intergranular and secondary pores.The diagenesis is Mesozoic stage A,and stage B in local area.The sandbody superimposition pattern is characterized by four types,including,overlay,overlap,butt-joint and isolate types.Meanwhile,the reservoir architure model of the single channel sandbody of the Chang 8 Interval in different level base cycles has been established based on the analysis of the sedimentary features during the sedimentary period.Four types of the reservoir microstructure are identified,including type A:high mercury saturation-medium pore and thin throat,type B:middle mercury saturation-small pore and throat,type C:middle mercury saturation-thin pore and throat,and type D:low mercury saturation-thin pore and throat.Meanwhile,the quality of the Chang 8 sandstone is controlled by the reservoir throat radius and distribution based on the comprehensive analysis.The reservoir porosity evolution model is formed by the comprehensive analyses of the reservoir microstructure and the pore characteristics during the diagenetic process.The formation process of the carbonate cements can be subdivided into 3 stages,the micrite/microlite calcite cements was formed by the oversaturated alkaline lake water,and the formation of the sparry/eutectic calctie,ferrocalcite,and dolomite were affeceted by the organic acid and water-rock reaction.On the basis of the reservoir evaluation criteria,fuzzy mathematics and multifactor comprehensive analysis were used in this study.A total of 6 core evaluation parameters including avearage throat radius,maximum mercury saturation,porosity,permeability,displacement pressure and the effective thickness of the reservoir,and 2 empirical parameters including reservoir microstructure grade and the diagenetical facies were optimized to evaluate the reservoir quality of the Chang 8₁ and Chang 8₂ subinterval sandtone.Three types of the Chang 8 Interval reservoir are identified,and the type I and type II reservoirs mainly distribute in the near provenance and south central areas,which are the main exploration targets in the Jiyuan area.