Characteristics and the Main Controlling Factors of Micro-pore Structure of the Shale in Wufeng Formation-Longmaxi Formation in Jiaoshiba Area

Micro-porestructure is the key element influencing the storage capacity,permeability and determining whether the shale is of commercial value.Taking 15organic-rich shale samples from the Wufeng Formation-Longmaxi Formation of JY1 well as a target,this research studies the shale's pore volume,size and distribution by using total organic carbon,X-ray diffraction,high pressure mercury injection,helium porosity,low temperature N₂ adsorption and desorption technology and argon-ion polishing scanning electron microscope(SEM)combined with the the bitumen reflectance and natural gas carbon isotope date.Then this research also discusses the main controlling factors of the pore structure from both external and internal aspects.The research indicates that the pore types in organic-rich shale in the formation are mainly inorganic pores(intergranular,intragranular and inside pores),organic pores and microfractures.The results of mercury injection and adsorption experiments show that the shale pore structure is relatively complex as pores sized less than 50 nm are predominant and micropores occupy approximately 65% of the total surface area is provided by micropores,while about 57% of the specific pore volume is provided by mespores,dominated by parallel plate with all sides open exixting simultaneously with a variety of other forms of pores in the shale.During the process of extrusion,uplifting and erosion since Cretaceous,the change of tectonic stress or temperature and pressure may contribute to the formation of a large number of cracks.The thickness,distribution and the enrichment of organic cabon of the shale may be restrained by the difference of the sedimentary environment.Correlation analysis shows that specific surface area of micropores,mespores and volumes have strong relationship with total organic carbon content.The influence of clay minerals and quartz content on pore structure has reciprocal effect.When Rois less than 3.0%,specific surface area and volume present positive relationship,however,when it exceeds 3.0% the negative relationship is observed.It can be explained that the development of the nanoscale pore is restricted by thermal maturation,which results in the reduction of micropores.