Micro-pore structure and fractal characteristics of deep shale from Wufeng Formation to Longmaxi Formation in Jingmen exploration area, Hubei Province, China

Abstract

To explore the pore structure and fractal characteristics of shale reservoirs in the Jingmen exploration area and quantitatively evaluate the heterogeneity and complexity of pores, taking Wufeng-Longmaxi formations of Well YT3 as the research object, the pore structure characteristics of shale reservoirs are analyzed by low-pressure nitrogen adsorption experiments, total organic carbon (TOC) content testing, X-ray diffraction analysis, etc., and the FHH fractal model is established based on the low-temperature nitrogen adsorption fractal geometry method. The relationship between fractal dimension and shale composition, pore structure, physical property, gas-bearing property, and burial depth is discussed. The results show that (1) The lower section of Longmaxi Formation and Wufeng Formation have significantly higher TOC concentrations than the upper section of Longmaxi Formation. With the increase of burial depth, the silicate minerals increase gradually, while clay minerals decrease. (2) The on-site desorption gas content of shale shows that the gas content of the upper section of Longmaxi Formation with low TOC abundance is lower than the lower section of Longmaxi Formation and Wufeng Formation. In terms of shale gas composition, nitrogen dominates the upper section of Longmaxi Formation, whereas methane dominates the Wufeng Formation and lower section of Longmaxi Formation. (3) The isothermal curve of shale under low-pressure nitrogen is identical to IUPAC type IV, while the adsorption hysteresis loop is similar to types H₃ and H₄. The pores are mainly micro medium pores distributed below 50 nm, and they are flat and slit. The lower section of Longmaxi Formation and Wufeng Formation have significantly larger adsorption volumes than the upper section of Longmaxi Formation, and the abundance of organic matter provides a large amount of organic pore storage space. (4) The BET specific surface area and BJH total pore volume in the lower section of Longmaxi Formation and Wufeng Formation with rich organic matter are significantly larger than in the upper section of Longmaxi Formation with low organic matter, whereas the average pore size is significantly smaller. (5) Shale pores have obvious fractal characteristics, with D₂ being higher than D₁, indicating that the pore structure is more complicated than that of the pore surface. Fractal dimension has a significant positive correlation with TOC content, BET-specific surface area, and burial depth, a weak positive correlation with quartz content and BJH total pore volume, a significant negative correlation with clay mineral content and average pore diameter, and almost no correlation with porosity and permeability. Many factors that affect the fractal dimension. Correlation analysis reveals that the mass fraction of TOC and clay mineral, specific surface area, average pore diameter, and burial depth are the main controlling factors. The fractal dimension can be utilized to quantify the complexity and heterogeneity of pore structure, which provides a concept for studying the distribution characteristics of shale pore structure and reservoir evaluation.