Research On Tight Sandstone Oil Reservoir Microscopic Structure Of Chang 8 Member Of Triassic Yanchang Formation In Fuxian Area,Ordos Basin

Chang 8 Member of Triassic Yanchang Formation in Fuxian area belongs to delta front sedimentary system,there develops powder-fine sandstone with poor physical property.According to the predecessors' definition of tight oil reservoir,most part of Chang 8 reservoir in Fuxian area belongs to tight reservoir.Microscopic pore structure of tight reservoir is very complex;the enrichment regularity of oil is controlled by pore structure.We adopt field emission scanning electron microscopy,high-pressure mercury,computed tomography,nuclear magnetic resonance and casting thin sections method on recognize microscopic pore and throat characteristics of tight sandstone oil reservoir,the coupling mechanism of tight reservoir evolution and tight oil accumulation process,the enrichment regularity of Chang 8 tight sandstone oil.Then,we got the following new recognition.Chang 8 reservoir has porosity and permeability by an average of 8.59%and 0.42 ×10-3?m2,belongs to low porosity,low permeability type.Research by high-pressure mercury,field emission scanning electron microscopy,computed tomography,nuclear magnetic resonance analysis method on recognizing the average pore radius is 1.2 ?m,and the average throat radius is 0.1 ?m.So,Chang 8 reservoir is belonging to fine pore-micro throat.Chang 8 tight sandstone oil reservoir development of three kinds pore structure,? and ? kind of pore structure are the largest contribution to enrichment of tight oil.Digenesis has important influence for Chang 8 tight sandstone reservoir.Average pore loss rate caused by compaction is 62.35%,by cementation is 23.84%,for increasing porosity by dissolution is 6.56%.Compaction is the main cause of tight reservoir,and carbonate,chlorite membrane,illite,mica filler content further exacerbated the reservoir dense degree.According to mechanical balance method,core experimental analysis method,the upper limit of porosity,permeability,and pore-throat radius of Chang 8 tight reservoir is 11.2%,0.55 ×10-?m2,0.6?m respectively.With mechanics,logging analysis and effective pore-throat method draw a conclusion that the lower limit of tight oil accumulation porosity is 4.5%,lower limit of permeability is 0.04×10-3?m2,lower limit pore-throat diameter near source and reservoir is 15.77 nm,and lower limit of pore-throat inside the reservoir is 24 nm.According to burial history and porosity evolution history,the tight time of Chang 8 reservoir begins at terminal Middle Jurassic(157 Ma),and to the completely tight of early cretaceous(128 Ma).Analysis of the inclusion temperature measurement data,Chang 8 tight sandstone reservoir has two times of oil filling,respectively for 97 to 103 Ma and 112 to 121 Ma,corresponding to the end and middle of early cretaceous period.It indicates that the tight time of Chang 8 reservoir is before accumulation time.According to the configuration relationship between source rock and reservoir distribution of Chang 8 section,found that tight oil distribution is controlled by source rock and reservoir distribution.The oil-bearing characteristics of Chang 8 reservoir are directly controlled by the reservoir physical properties and pore structure.