Pore Characterization And Control Factors Of Shanxi Shale In Eastern Ordos Basin

There are many suits of organic rich shale in Ordos Basin,among them,the Shanxi shale are widely distributed with great cumulative thickness,which will be one of the most important exploratory development target for shale gas in North China.Based on the drilling core data,through the field emission scanning electron microscopy,high-pressure mercury injection,CO₂ adsorption and N₂ adsorption experiment technology,the shale pore types and appearance characteristics are qualitatively observed directly and the characteristics of different lithofacies shale pore structure from micropore to macropore are full scale quantitatively characterized;According to the fractal theory the different scale pore fractal characteristics have been characterized,on the basis of the evaluation of pore structure parameters,the advantage of shale pore division standard is determined in the study area;the main controlling factors of the different scale shale pores are revealed,and build the shale porosity evolution model of Shanxi formation in eastern ordos basin.Research shows that there are mainly five types of shale pores in the study area:clay interlayer pores,intragranular pores,intergranular pores,organic matter pores and micro cracks,and the clay interlayer pores are mostly developed,while the organic matter pores are poorly developed compared with the marine shale;The full pore size distribution is multiple kurtosis,the main pore size distribution ranges are 0.45⁰.95nm,3⁵nm,10⁴0nm,10³0um,and the shale pore volume is mainly consists of mesopores and macropores?46.64%and 46.66%,respectively?,and the shale pore surface area is mainly consists of micropores and mesopores?38.61%and 61.24%,respectively?,mainly develop slit type plate pores,conical tube pores;There is high bound water content in Shanxi shale,an average of 95.7%,the effective porosity is low,an average of 0.76%;Macropore fractal dimension is higher,an average of 2.8,the fractal dimension of mesopore is relatively lower,an average of 2.67,so the structure of macropores is much more complex than that of mesopores;with the increasing of the fractal dimension,the porosity,permeability,pore volume and pore diameter is decreasing,while the specific surface area is increasing;Class I shale pores have the best pore structure parameters,the porosity>3.5%,pore volume>0.035ml/g,specific surface area is higher than 12m²/g;Class II shale pores have the middle pore structure parameters,the porosity between 2.5³.5%,pore volume between0.018⁰.035 ml/g,specific surface area between 9¹2 m²/g;Class III shale pores have the worst pore structure parameters,the porosity<2.5%,pore volume<0.018 ml/g,specific surface area<9m²/g;And the high organic clay siliceous shale pore structure parameters in high maturity area is the most optimal.The micropores,mesopores and macropores are respectively controlled by organic matter content,quartz and clay mineral content;With the increase of shale organic matter maturity,the shale porosity and pore size is not a linear increase,when Ro is more than 3.1%,shale porosity and pore volume are in decreasing trend;The shale porosity evolution can be divided into five stages:Fast compaction stage,Pore dissolution stage,Degradation stage,Secondary cracking stage,Carbonation compaction stage,and the Secondary cracking stage is the most beneficial shale pore development stage in the study area.