Impact of Overpressure on the Deep Shale Pore SystemA Case Study of Wufeng-Longmaxi Formations from the Southern Sichuan Basin

This study investigatedthe effect of overpressure on the deepshale pore system in the Wufeng-Longmaxi Formation (WLF), awell-established shale gas reservoir in the southern Sichuan Basin,China. The Y1 well was drilled to explore deeper overpressured sectionsof the basin. Organic geochemistry, mineralogy analysis, field emissionscanning electron microscopy (FE-SEM), and gas physisorption experimentswere conducted to analyze the pore system. Results showed that despitestrong compaction, deep organic-rich shale retained large pores. Comparedto shallow shale, deep shale had a larger organic porosity with asmaller average pore size, although some pore sizes exceeded thosein shallow shale. Nitrogen (N-2) adsorption indicated thatthe abundance of organic matter (OM) affected mesoporous volume andspecific surface area (SSA), while carbon dioxide (CO2)adsorption experiments suggested that micropores were not influencedby OM abundance. Comparing calculated pore SSA and volume via gasadsorption of shallower and deep shale samples revealed that, undersimilar OM abundance, pore SSA was nearly identical, but deep shalehad a larger pore volume than shallow shale. The preservation of pores,particularly in deep shale, is attributed to overpressure, which protectsagainst collapse; additionally, generated shale gas during thermalevolution of OM serves as pore support.