Pore Fractal Characteristics Of Lacustrine Shale Of Upper Cretaceous Nenjiang Fm From The Songliao Basin,NE China

Shale oil is an oil resource dominated by shale,which occurs in shale pores and fractures.The study of shale pore characteristics plays a positive role in the development of shale oil.Sixteen lacustrine shale core samples of late Cretaceous Nenjiang formation in Songliao Basin were tested by scanning electron microscope,nitrogen adsorption,high pressure mercury injection,X-ray diffraction(XRD)and rock pyrolysis.At the same time,the concept of fractal dimension is introduced to quantitatively characterize the shale pores of the first and second member of Nenjiang formation.The relationship between Total Organic Carbon(TOC)and S2 is obtained by rock pyrolysis and TOC measurement.The kerogen type of Nenjiang formation shale is mainly I-lipophilic type,so the evolution degree of shale in the first and second member of Nenjiang formation is low and is still in oil-generating stage.According to the results of long hair scanning electron microscope,a large number of mineral intergranular pores,a small amount of organic pores and mineral intragranular pores are mainly developed in the shale of the first and second member of Nenjiang formation.Through the analysis of scanning electron microscope images by PCAS software,it is found that the shape factors of organic and inorganic pores are both about 0.5,and the pores have a certain regular shape,and the probability entropy of organic pores is 0.6 and that of inorganic pores is 0.9,which shows that the pores almost have no consistent direction and the pore connectivity is poor.The fractal dimension obtained by high pressure mercury injection experiment can only characterize pores larger than 200 nm,but the pores of Nenjiang formation shale are mainly micropores and mesopores,so mercury injection experiments can not be used as an effective means to quantitatively characterize the distribution of pores.at the same time,it can also show that the heterogeneity and complexity of pores are strong,and complex pores can provide more storage space for free oil.Therefore,the Nenjiang formation shale has a certain oil generation and storage capacity.The samples of the first and second member of Nenjiang formation mainly contain clay minerals and quartz.Clay minerals control the structural characteristic parameters of shale pores,and quartz has no obvious relationship with pore size,specific surface area and specific pore volume,so clay may be the main factor controlling pore development,while quartz may not.In this paper,the calculation of fractal dimension is mainly based on the data obtained from nitrogen adsorption experiments,and the average fractal dimensions of pores obtained are D1 ~ 2.65546 and D2 ~ 2.532 respectively.There is a significant negative correlation between fractal dimension(D1 and D2)and TOC content.The smaller the TOC is,the lower the fractal dimension is.The size of the fractal dimension can characterize the content of shale oil: the smaller the fractal dimension,the more free oil.Therefore,combined with the relationship between fractal dimension and TOC,when the content of TOC is higher,more large pores can be produced due to hydrocarbon generation expansion,so that the pore diameter increases,the fractal dimension of pores becomes smaller,and more free oil can be stored at this time,so the larger the area of TOC and the lower fractal dimension is,it may be the most suitable area for exploitation.At the same time,fractal dimension is also an important parameter of average shale reservoir.