The Pore Characteristics Of Shale Of The Upper Cretaceous Nenjiang Formation In Songliao Basin And Its Relation To Diagenesis

The study of pore evolution characteristics of shale is of great significance to the exploration of shale oil and gas.The study object is the mudstone of the Upper Cretaceous Nenjiang Formation in the Songliao Basin.The 2D and 3D structural features of the pores were observed by high-resolution electronic instruments of Field Emission Scanning Electron Microscopy and Focused ion Beam Scanning Electron Microscopy.The pore evolution characteristics of shale with the increase of temperature and pressure are studied by the experiments of diagenetic physical simulation,high pressure mercury,nitrogen adsorption,TOC,pyrolysis of rock and total rock X-ray diffraction,and the main controlling factors and the relationship with diagenesis are discussed.The experimental results show that Nenjiang Formation shale mainly developed mineral intergranular pores,organic matter-mineral intergranular pores and clay minerals intergranular pores,and developed a small amount of organic matter pores and mineral Intragranular pore.FIB-SEM three-dimensional reconstruction showed poor pore connectivity,which calculated shale porosity of 0.89%.The porosity of organic matter is much smaller than that of total porosity,and TOC of shales is 6.39%.Therefore,there is no obvious relationship between organic porosity and TOC content.Nenjiang shale is generally concentrated in the distribution of the two regions,one is concentrated distribution within 100 nm,the peak is about 40nm;the other is concentrated distributionbetween 100?m and 200?m.During the simulation thermal evolution of shale,the pore system increases with the increase of temperature and pressure.The pore volume of the micro-pores decreases,the pore volume of the meso-pores increases and then decreases,and the pore volume of the macro-pores increases.From the relationship between pore evolution and mineral content,TOC and potential hydrocarbon production,in the process of shale hydrocarbon generation,the decrease of TOC and potential hydrocarbon production can increase macro-pores and decrease micro-pores,and the pyrolysis of pyrite can increase macro-pores,and the change of clay mineral content controls the volume of meso-pores.The reason for the increase of pore system during the thermal evolution of shale may be due to:(1)The over-pressure produced in the process of hydrocarbon generation slows down the porosity decline rate,which is the main factor that the porosity is relatively large in the middle-deep depth.(2)The acidic fluids formed during the hydrocarbon generation process dissolve the unstable brittle minerals,forming the acid-dissolving pores,providing K+ for the illitization.The conversion of illite can promote the dissolution of K-feldspar and cause the shrinkage of the mineral volume,thus promoting the increase of the reservoir space.(3)In this process,the effect of compaction on the pores is less than that of hydrocarbon generation.(4)For the high quality shale during middle-late gas generation period,nano-pores originated in hydrocarbon generation process is another important factor causing the organic-rich shale.