Geological Conditions And Play Evaluation Of Yanchang Formation-chang-7 Tight Oil In Huachi Area,ordos Basin

A comprehensive evaluation,based on the tight sand classification and shale layers definition,is carried out to clarify the reservoir-source assemblage of Chang-7 reservoir group in the area of Huachi,Ordos basin.Furthermore,the controlling factors of tight oil accumulation are identified,the main types of play are defined,the genetic relation of the controlling factors is analyzed,and the oil accumulation favorable area is proposed.Two types of sedimentary facies present in the study area include braided river delta and deep-water lake facies,which dominant the framework of reservoir and hydrocarbon source rock deposition.In this paper,the reservoir is evaluated by the method of grey correlation and classified as I-IV reservoir.The type?,?and IV reservoir are widely distributed in the study area.Intergranular dissolved pore is primary,secondary pore exist,microfracture accidental present,illite and chlorite films are the main clay consists,diagenetic in middle stage,including phase A and phase B.Mercury injection analysis shows that there are multiple throat types in the reservoir.The permeability is in the range of(0.1-0.81)×10^-3?m².Vertical differentiation of the shale layers plays a different and even opposite effects for tight oil accumulation in Chang-7.The first type shale,as a high-quality hydrocarbon source rock,provided the abundant of material supply and the overpressure of oil expulsion,to overcome the high-level displacement resistance into the tight reservoir.Therefore,based on the accumulation mechanism and through TOC and overpressure logging interpretation,the shale layers in Chang-7 are defined as 3 types,as the High expulsion Source rock(H.S),Low expulsion Source rock(M.S)and L.S.The H.S is generally considered as a high-quality hydrocarbon source rock with the TOC>6%and overpressure>5MPa,which is the deep-water lake deposition.The L.S is a tight shale layers with the TOC<2%and Thickness>3m,normal formation pressure,predelta-depositon.The M.S,as an intermediate type,is effective source rock also.Multiple contact models are generalized in Chang-7,with sedimentary link inside,thus be defined as 6 types of play.Source-dominant I Play is the type that I-III reservoir and the H.S direct contact,and the reservoir microfacies is underwater distributary channel or mouth bar,normally oil accumulation.Source-dominant II Play is the type that II-III reservoir direct contact with the relative high permeability rock and indirect contact with H.L or M.S.through a short distance secondary migration.Source-dominant III Play is the type that II-III reservoir direct contact with the L.S,with high risk for accumulation,form water bearing sand body.The Physical I Play is the type that relative good permeability turbidite direct contact with H.S.It forms a thick layer or massive oil layers.The Physical I is the most favorable target.The Physical II and Physical III Play is the types that tighter reservoir IV,in the microfacies of braided river delta front and the turbidite respectively,contact with H.S and/or M.S.These two plays are long-term potential reservoir stimulations targets in the study area.The controlling role of high-quality source rock and the mechanism of high-generation high-expulsion is most emphasized in this classification program.The reservoir-source assemblage and microfacies link are also consider serious.In combination with all the factors above,the favorable area for tight oil accumulation is proposed from the study.