Fractal Characteristics of Pores in the Longtan Shales of Guizhou, Southwest China

Abstract

The pore structure of marine-continental transitional shales from the Longtan Formation in Guizhou, China, was investigated using fractal dimensions calculated by the FHH (Frenkel-Halsey-Hill) model based on low-temperature N2 adsorption data. Results show that the overall D 1 (fractal dimension under low relative pressure, P / P 0 ≤ 0.5 ) and D 2 (fractal dimension under high relative pressure, P / P 0 > 0.5 ) values of Longtan shales were relatively large, with average values of 2.7426 and 2.7838, respectively, indicating a strong adsorption and storage capacity and complex pore structure. The correlation analysis of fractal dimensions with specific surface area, average pore size, and maximum gas absorption volume indicates that D 1 can comprehensively characterize the adsorption and storage capacity of shales, while D 2 can effectively characterize the pore structure complexity. Further correlation among pore fractal dimension, shale organic geochemical parameters, and mineral composition parameters shows that there is a significant positive correlation between fractal dimensions and organic matter abundance as well as a complex correlation between fractal dimension and organic matter maturity. Fractal dimensions increase with an increase in clay mineral content and pyrite content but decrease with an increase in quartz content. Considering the actual geological evaluation and shale gas exploitation characteristics, a lower limit for D 1 and upper limit for D 2 should be set as evaluation criteria for favorable reservoirs. Combined with the shale gas-bearing property test results of Longtan shales in Guizhou, the favorable reservoir evaluation criteria are set as D 1 ≥ 2.60 and D 2 ≤ 2.85 . When D 1 is less than 2.60, the storage capacity of the shales is insufficient. When D 2 is greater than 2.85, the shale pore structure is too complicated, resulting in poor permeability and difficult exploitation.

Pore structures and fractal dimensions in Early Permian Barakar shales of Rajmahal Basin, Jharkhand, India

“…The sufficient pore volume and larger surface area is effective for the substantial gas adsorption and gas storage, whereas the availability of a good pore networking and fractals are beneficial for gas transportation (Zhang et al, 2016;Huang et al, 2020). Likewise, the FHH plots of the studied samples indicate that shales have two separate fractal dimensions at different pressure ranges (low-pressure and high-pressure divisions), and the pore surface (D 1 ) is rough/rugged(Fig.…”

“…11) implying fractals in low-and highpressure range that leads to the ruggedness of pore surfaces and complex pore structures together (Chen et al, 2019). Similarly, two types of fractal dimensions have also been observed in the Longtan Shales (Middle Permian) of Guizhou, Southwest China (Huang et al, 2020), and Early Permian coal beds of West Bokaro, Jharkhand, India (Mendhe et al, 2017b).…”

Effect of Grain Size on Microscopic Pore Structure and Fractal Characteristics of Carbonate-Based Sand and Silicate-Based Sand

“…This indicates that as the grain size increases, the difference in particle shape has a more obvious impact on the pore structure. It is widely acknowledged that the determination of the fractal dimension plays an important role in predicting the physical and mechanical properties of geomaterials [45,46]. Huang et al [45] quantified the fractal characteristics of the pore structure of shale and found a significant relationship between the fractal dimension and permeability.…”

“…It is widely acknowledged that the determination of the fractal dimension plays an important role in predicting the physical and mechanical properties of geomaterials [45,46]. Huang et al [45] quantified the fractal characteristics of the pore structure of shale and found a significant relationship between the fractal dimension and permeability. Kong et al [47] established a fractal model of pore size distribution for porous geomaterials based on the fractal characteristics of pores, which could be used to study hydraulic properties.…”

Study of Fractal and Multifractal Features of Pore Structure in Tight Sandstone Reservoirs of the Permian Lucaogou Formation, Jimsar Sag, Junggar Basin, Northwest China

“… 8 In the Guizhou Longtan Formation, Huang et al examined the relationship between permeability and pores in marine and continental transitional shales. 9 To examine how different shale is from one another, Wang et al used SEM digital images, fractal and multifractal theory, and shale samples. 10 Using field emission scanning electron microscopy (FE-SEM), gas adsorption, and helium porosity measurements, Yang et al studied the pore structure of organic-rich shale.…”