四川盆地长宁构造地区龙马溪组页岩孔隙结构及其分形特征

以四川盆地长宁构造地区古生界下志留统龙马溪组页岩为例,利用低压氮气吸附测试手段和分形法理论,分析了页岩的孔隙结构参数及分形维数,结合有机质含量和矿物组成分析,研究了页岩孔隙结构特征及其分形特征的影响因素。结果表明:龙马溪组页岩孔隙结构复杂,孔隙主要以两端开口的圆筒状孔、墨水瓶状孔等开放性孔为主,孔径主要在1~10nm之间,孔径主峰位于1.86nm附近;页岩平均孔径在1.807~4.343nm之间,与w(TOC)、石英含量呈负相关,与黏土矿物含量呈正相关,其中w(TOC)对其影响显著;页岩总孔容在0.010 76~0.025 04cm3/g之间,比表面积在5.416~25.958m2/g之间,页岩总孔容、比表面积与w(TOC)、石英含量呈正相关,与黏土矿物、碳酸盐含量呈负相关;页岩具有双重分形特征,大孔隙分形维数D₁为2.091 8~2.653 7,小孔隙分形维数D₂为2.724 8~2.857 5,说明大孔隙结构的复杂程度大于小孔隙;页岩孔隙的分形维数与比表面积和总孔容呈正相关,而与平均孔径呈负相关;与w(TOC)、石英含量呈正相关,与黏土矿物、碳酸盐含量呈负相关,其中w(TOC)、石英和黏土矿物含量对页岩孔隙结构复杂性影响较大。 

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.