Fractal Characteristics of Nanopores and Their Controlling Factors of the Lower Cambrian Shale in the Western Hubei Area, South China

Abstract

The Lower Cambrian shale is one of the most important marine shale reservoirs in South China, which has huge potential for shale gas resources. However, the Lower Cambrian shale displays strong pore heterogeneity, influencing shale gas exploration. Based on the low-pressure N2 adsorption (LPNA) of the Lower Cambrian shale samples and their isolated kerogens in the western Hubei area, South China, fractal dimensions and their controlling factors of shale nanopores have been investigated. In this study, the pore specific surface area (SSA) and pore volume (PV) of isolated kerogen samples are mainly contributed from micropores and non-micropores, respectively, which are consistent with those of whole-rock samples. The average fractal dimensions of whole-rock samples are higher than those of isolated kerogen samples, suggesting that organic matter (OM) controls the shale pore structure and exerts major influences on fractal characteristics of nanopores and inorganic mineral (IM) enhances the complexity of shale nanopores. The correlations between fractal dimensions and total organic carbon (TOC) and IM contents of whole-rock samples further indicate that the OM simultaneously affects the heterogeneities of the pore surface and structure, while the IM mainly affects the heterogeneity of the pore structure. They both control the fractal characteristics of nanopores in the Lower Cambrian shale. Compared with the Upper Carboniferous–Lower Permian marine-continental transitional (MCT) shale and the Lower Silurian marine shale, the D 1 and D 2 of the studied marine shale display relatively lower and higher values, respectively, revealing that the Lower Cambrian shale has relatively weak adsorption capacity, as well as poor permeability and connectivity.

Comparative Study of the Characteristics of Lower Cambrian Marine Shale and Their Gas-Bearing Controlling Factors in the Middle and Lower Yangtze Areas, South China

“…Contents of brittle minerals (such as quartz, feldspar, and calcite) are important factors in controlling the degree of fracture development, which directly affects the reservoir space and percolation channels [80]. It also forms a rigid supporting framework, which is beneficial for the preservation of organic and inter-crystalline pores in clay minerals [43]. In addition, an increase in brittle mineral contents makes it more likely that network cracks will form, which is conducive to fracturing [81].…”

“…The data for the JR [85]. The data for wells WD-1 and WD-2 were from reference [43], the data for the ZZK and JSL sections were from reference [66], and the data for wells CY-1 and WY-1 were from references [46,58], respectively. The data for the JR and PM1 sections were from references [67,86], respectively.…”

“…However, a significant positive correlation was observed between TOC and BET specific surface area (c) and between TOC and BJH pore volume (d) in the shales of the Lower Yangtze area. The data for wells WD-1 and WD-2 were from reference [43], the data for the WN section were from reference [39], and the data for well RDZ01 were from reference [72].…”

“…An increase in carbonate content will also lead to more complex pore structures [43]. The lower Cambrian marine shale in the Middle Yangtze area had a higher carbonate content (see Table S2, Figures 4 and 7a), and thus, it had a more complex pore structure.…”

“…However, the interplay between these factors is, as yet, poorly understood, and further research is needed to investigate their combined controlling effect. Research on the mechanisms of organic matter enrichment, silicon sources, and gas-bearing properties in the Lower and Middle Yangtze regions has mainly focused on individual wells or sections, and thus, there is a lack of comprehensive regional studies [41][42][43][44][45][46]. This study aimed to combine individual-well and regional-scale comparative research based on individual-well data and regional characteristics.…”