Pore structure and fractal characteristics of deep shale gas reservoirs in the Permian Dalong Formation, northeastern Sichuan Basin

Abstract

The characteristics of shale reservoirs play a crucial role in the enrichment and exploitation of shale gas. The Upper Permian Dalong Formation in the northern Sichuan Basin is a significant marine-derived high-quality hydrocarbon source rock that requires further detailed investigation. This study conducted qualitative and quantitative analyses on the pore structure of deep shale reservoirs within the Permian Dalong Formation in northeastern Sichuan. Various methods such as high-resolution field emission scanning electron microscopy, CO2 adsorption, N2 adsorption, and high-pressure Hg injection measurements were employed. Fractal fitting using the V-S model based on CO2 adsorption, the FHH model based on N2 adsorption, and the fractal geometry model based on high-pressure Hg injection was utilized to characterize the pore structure of deep shale. The findings revealed that deep shale reservoirs in the Dalong Formation in northeastern Sichuan Basin contain abundant nano-scale organic pores and limited inorganic pores. The development of organic pores exhibits strong heterogeneity influenced by maceral differences and organic matter distribution. Similar to Longmaxi Formation shale, the pores in the Dalong Formation are predominantly micropores and mesopores, comprising over 90% of the total pore volume and primarily influenced by organic matter abundance. The research on fractal dimension characteristics suggests that macropores in deep shale exhibit stronger heterogeneity compared to mesopores and micropores. Due to their smaller size and minimal diagenetic impact, micropores and mesopores have simpler structures with lower fractal dimensions. On the other hand, macropores display significant heterogeneity due to their larger size, wide distribution range, diverse genesis, and susceptibility to diagenesis. Due to their abundant reservoir space and strong self-similarity, micropores and mesopores in deep shale are conducive to the occurrence, seepage and exploitation of shale gas.

Keywords

• heterogeneity
• fractal dimension
• pore structure
• shale reservoir
• dalong formation
• permian
• northeastern sichuan basin