Preliminary Study On The Rock Structure And Pore Structure Of The Poor Oil-bearing Part Of The Rocks With Different Oil-bearing Properties And The Cap Rock Characteristics Limiting Oil-gas Migration

Caprock is one of the important conditions for the formation of oil and gas reservoirs.A large number of studies have been carried out by predecessors.These studies mainly focus on conventional traps and caprocks that can prevent the upward migration of oil and gas in oil and gas reservoirs,but the study of caprocks that limit the migration of oil and gas in the process of oil and gas migration is not deep enough.This paper focuses on the cores with different oil-bearing properties from Paleogene to Neogene in Jiyang depression,analyzes the rock structure and pore structure characteristics of the poor oil-bearing part of the rocks with different oil-bearing properties through conventional porosity,permeability test,permeability change test before and after rock fracture,as well as cast thin section and fluorescent thin section data,and draws the conclusion that the limited oil-gas accumulation can be achieved On this basis,the characteristics of cap rocks that restrict hydrocarbon migration in the direction of the decrease of oil-gas driving pressure difference in the sag uplift combination structure of source rocks are preliminarily analyzed.The results are as follows:1.The characteristics of rock structure and pore structure of the poor oil-bearing part of the rocksIn the shale with no fractures,the oil-bearing property in the vertical plane is generally worse than that in the parallel plane.The lithologic heterogeneity in this direction is generally stronger,and the permeability in the vertical plane is generally lower than that in the parallel plane,which is generally between 0.001×10-3?m2—0.002×10-3?m2.The permeability of the former is generally 1/5-1/4 of that of the latter.In the fractured argillaceous rock,the oil-bearing property of the part without fractures is generally poor,and no pores can be seen in the conventional cast thin section.The permeability is generally between 0.040×10-3?m2—0.050×10-3?m2,and the permeability of this part of the rock is generally 1/1000-1/100 of that in the fractures.There are two types of sandstone with poor oil-bearing property.One is cemented tight sandstone,and a small amount of pores filled by casting can be seen in conventional casting thin section.Its permeability is generally between 0.040×10-3?m2—0.050×10-3?m2,and its permeability is generally less than one percent of non cemented tight sandstone.The other is fine grained sandstone with small pore volume A small amount of pores filled by the cast can be seen in the thin section of the gauge cast body.The permeability values are generally between 0.10×10-3?m2—5.00×10-3?m2,and the permeability is generally 1/100-1/10 of some sandstones with good oil bearing properties.In the sandstone argillaceous rock combination,the part with poor oil-bearing property has argillaceous rock with undeveloped fractures and sandstone with relatively fine grains and low permeability.The permeability of argillaceous rock with undeveloped fractures is generally less than one thousandth of that of sandstone with good oil-bearing property.The permeability of sandstone with relatively fine grains and low permeability is generally 1/100-1/10 of sandstone with good oil-bearing property.2.The basis of cap assessment for limiting oil and gas migrationThe difference of oil-bearing property of rocks is the comprehensive result of oil-gas migration.From the perspective of limiting oil-gas migration,the relationship between the limitation of oil-gas migration and the absolute value of pore size and permeability is not obvious,but the correlation between the limitation of oil-gas migration and the poor good combination of pore structure and permeability is greater.When there are differences in pore structure and permeability of rocks,the poor part of pore structure and permeability can restrict the migration of oil and gas.The greater the difference,the more obvious the restriction on the migration of oil and gas.Therefore,the cap rocks that restrict hydrocarbon migration can be evaluated according to the difference good combination of pore structure and permeability.3.Cap rock characteristics of oil and gas migration in the depression direction of oil and gas driving pressure difference in the sag-uplift combination structure of source rockIn the depression direction of oil-gas driving pressure difference in the sag-uplift combination structure of source rock,the cap rock characteristics restricting oil-gas migration are related to the vertical deformation structural belt and fault of lithologic combination.In the elastic-plastic and fractured structural belts,the caprocks limiting hydrocarbon migration in source rocks are stepped,that is,the vertical plane sealing to the parallel plane sealing,and the porous mudstone sealing to the stepped fractures.In the sandstone argillaceous rock combination,the caprock restricting oil and gas migration consists of two parts,that is,the sealing of fractures in sandstone argillaceous rock contact zone by porous mudstone and the sealing of better permeability by sandstone with poor permeability.In the fault rock combination structure,the cap rock limiting oil and gas migration is umbrella shaped,including the vertical sealing of the shallow plastic fault to the deep elastic-plastic fault and fracture fault,and the sealing of the porous argillaceous rock and sandstone with poor permeability on both sides of the fault.