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

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 $D1$ (fractal dimension under low relative pressure, $P/P_0\le 0.5$ ) and $D2$ (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 $D1$ can comprehensively characterize the adsorption and storage capacity of shales, while $D2$ 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 $D1$ and upper limit for $D2$ 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 $D1\ge 2.60$ and $D2\le 2.85$ . When $D1$ is less than 2.60, the storage capacity of the shales is insufficient. When $D2$ is greater than 2.85, the shale pore structure is too complicated, resulting in poor permeability and difficult exploitation.