Main Controlling Factors Of Tight Sandstone Reservoir In Chang 8 Of Yanchang Formation In N Area Of Ordos Basin

With the deepening of oil and gas exploration,new discoveries have been made in tight reservoirs of Chang 8 reservoir in the lower Yanchang formation in recent years.Previous studies have mainly focused on reservoir characteristics,sedimentary facies,reservoir-forming characteristics,but the degree of research on its formation mechanism and reservoir enrichment is relatively low.On the basis of stratigraphic division,this paper studies the structural characteristics,distribution characteristics of hydrocarbon source rocks,sedimentary facies and sand body distribution characteristics of Chang 8 subgroups in N area;based on the data of lithology,porosity and permeability,cast thin section and high pressure mercury injection of sandstone of Chang 8 oil formation,the plane distribution map of physical properties of target layer is drawn and its physical properties are analyzed comprehensively;by compiling the plane distribution map of target caprock,the thickness and distribution of caprock are determined.By using the physical and electrical lower limits of the established effective thickness,the oil layer,water layer and dry layer of the target layer are interpreted,and the distribution of the reservoir plane and profile of Chang 8 oil layer in the study area is determined;the excess pressure obtained by the equilibrium depth method is used to determine the dynamic type and magnitude;then,the channel type is determined according to the fracture development and sand body connectivity,and the reservoir-forming period is judged according to previous studies,so as to determine its reservoir-forming mechanism.Combined with the reservoir-forming conditions,reservoir-forming mechanism and reservoir distribution characteristics,the main controlling factors of Chang 8 reservoir in the study area are summarized,and the reservoir-forming model is further established.The study shows that the sedimentary facies of Chang 8 oil formation are mainly delta front subfacies,and underwater distributary channel and interdistributary bay microfacies are developed,in which underwater distributary channel sand body is the main reservoir;Chang 8reservoir lithology is mainly fine siltstone,mineral composition is mainly feldspar,quartz is the second,porosity is concentrated in 3%⁸%,permeability is concentrated in?0.03×0.5?×10^-3?m²;The main reservoir space is intergranular solution pore,the pore throat structure type is mainly fine pore-fine throat type and micropore-fine throat type,and the reservoir can be divided into three types:?_a^?,among which,?_a and?_b reservoirs are better,developed in the relatively thick and good area of sand body,type V reservoir is developed in the contact part of sand body and mudstone,located on the flank of underwater distributary channel,and is generally ultra-low permeability-tight reservoir.The sand body plane of Chang 8 reservoir group is mainly distributed in NE-SW direction;high-quality reservoirs mainly develop small pores and fine pores,and the porosity is generally more than 6%;the reservoir-forming power is abnormal high pressure,buoyancy and capillary force caused by hydrocarbon generation and unbalanced compaction of Chang 7 and Chang 9 source rocks,and the migration channels are mainly sand bodies with relatively high porosity and permeability,and fractures play an improvement role;the reservoir-forming period is at the end of late Jurassic and early Cretaceous.The superimposed oil-bearing area of Chang 8 formation is 83.93 km²,in which the oil-bearing area of Chang 8¹and Chang 8²is 40.75 km²and 43.18 km²,respectively.Chang 8 is a lithologic reservoir,which is mainly controlled by lithology and physical properties.comprehensive analysis shows that the main factors controlling the enrichment of Chang 8 reservoir in the study area are good sedimentary facies zone and good physical properties,and the reservoir-forming models are upper source and lower reservoir and lower source and lower reservoir.Through the above research,in order to further understand the distribution law of tight oil and provide guidance for tight oil exploration.