Abstract:

Studying shale pores is the key to understand the mechanism of shale gas occurrence and accumulation. But there is little effective means to quantitatively depict micro/nano pore heterogeneity characteristics of high-maturity shales. This article quantitatively analyzed micro/nano pore heterogeneity and main controlling factors of the high-maturity Longmaxi Formation Shale in southeastern Chongqing through the low pressure N2 adsorption/desorption, high-pressure mercury injection (MIP), and field emission scanning electron microscopy (FE-SEM) experiment together with the use of the fractal dimension. The results show that micro/nano pores develop very well in high-maturity shale. Mesopores are mainly slit-shaped clay interparticle pores and ink-bottle-shaped organic pores. Fractal dimensions of pores between 2.0 nm and 4.5 nm in diameter have an average value of 2.853,4, while fractal dimensions of pores between 4.5 nm and 50.0 nm in diameter have an average value of 2.736,7. The strong heterogeneity is mainly controlled by organic matters. When total organic carbon (TOC) content is less than 0.9%, the pores provided by minerals play dominant role, while the TOC content is larger than 1.7%, organic pores play dominant role. In addition, the increase of clay mineral content will increase the mesopores heterogeneity in a certain extent. Macropores are mainly composed of slit-shaped interparticle pores and a small amount of ink-bottle-shaped organic pores. Fractal dimensions of macropores have an average value of 2.844,1, which show strong heterogeneity, indicating the pore structure is complex. The heterogeneity of macropores is mainly controlled by quartz and carbonate minerals. With the increase of carbonate minerals content, the degree of heterogeneity increases. As organic pores size is relatively small, they have no obvious influence on shale macropore heterogeneity.

Key words: heterogeneity, main controlling factor, fractal dimension, pore structure, high-maturity shale