Pore Structure Characteristics Of The Wufeng-Longmaxi Formation Shale From Southern Sichuan Basin And Their Controlling Factors

The micro-nano pore structure characteristics of shale reservoirs control the occurrence and migration of shale gas.Therefore,pore structure characterization of shale reservoirs is one of the core issues in shale gas exploration and evaluation,and it is also an important content to understand the shale gas accumulation mechanism.In this paper,the Wufeng-Longmaxi Formation shale in southern Sichuan Basin is used as the research object,and different lithofacies are divided based on total organic carbon(TOC)and mineral composition analyses.The pore structure of shale reservoir is characterized qualitatively and quantitatively by field emission-scanning electron microscopy(FE-SEM)and image processing software,and the organic matter(OM)is divided into five types according to the formation mechanism and pore structure characteristics:in-situ OMs with biological pores,migrated OMs with needle-shaped pores,migrated OMs with small bubble-shaped pores,migrated OMs with large bubble-shaped pores,and bioclastic OMs.The pore structure parameters of shale reservoir are obtained by N₂adsorption and high pressure mercury intrusion porosimetry experiments.The results show that the pores in shale samples are mainly mesopores.The well-developed OM pores provide the main specific surface area and pore volume.Clay minerals can also provide a certain specific surface area and pore volume.The pore structure of shale is mainly influenced by TOC content,mineral composition and OM types.The pore structure of shale samples from Changning-Weiyuan block dominated by migrated OMs with needle-shaped and bubble-shaped pores is the best,followed by shale samples from Luzhou block dominated by in-situ OMs with biological pores and migrated OMs with small bubble-shaped pores,and the pore structure of shale samples from Yuxi block dominated by OMs with needle-shaped pores and migrated OMs with small bubble-shaped pores is the worst.