Pore Structure and Fractal Characteristics of Longmaxi Formation Shale in the Changning Region of Sichuan Basin

A study on shale gas reservoir of Longmaxi Formation of Lower Silurian in the Changning region of Sichuan Basin.Using low pressure nitrogen adsorption method and fractal theory,the pore structure parameters and fractal dimensions of shale were studied.Combining with total organic matter and mineralogical constituent testing,the influence factors of the pore structure and fractal characteristics were researched.The results show that the pore structure of Longmaxi Formation shale is complicated,and the pores mainly consist of cylinder pores with two open ends,and ink-bottle pores.The pore size distribution of shale is just concentrated in the range of 1-10 nm,and the peak of the pore size is near 1.86 nm.The average pore size of shale is between 1.807 nm and 4.343 nm.A negative correlation was found among TOC,silica and average pore size,whereas there is positive correlation between clay and average pore size,and which the TOC content has significant impact on.The BJH pore volume of shale is concentrated in the range of 0.010 76-0.025 04 cm³/g,and the BET pore specific surface area is between 5.416 m²/g and 25.958 m²/g.The BJH pore volume and the BET pore specific surface area were positive correlation with TOC,silica,and had reverse correlation with clay,carbonate.The fractal dimensions of pore structure of shale have the dual fractal feature.The fractal dimension values of the small pore structure are 2.091 8-2.653 7,and the fractal dimension values of the big pore structure are the 2.724 8-2.857 5.The big pore structure,with larger fractal dimension value,shows higher degree of complex than the small pore structure.There is a positive correlation among the fractal dimension values and specific surface area,pore volume.The fractal dimension values were positive correlation with TOC,silica,and had reverse correlation with clay,carbonate.Especially,it is found that the TOC,silica and clay have significant impact on the complexity of pore structure of shale.