Lithological Controls on Pore Structure and Their Implications for Deep Shale Gas Reservoir Quality in the Longmaxi Formation, Luzhou area, Southern Sichuan Basin, China
Jing Li, Qing Zhang, Wei Jiang, Hu Li, Li Li, Taofeng Xue, Qiang Wang, Zhi Gao
Abstract
This study investigates shale samples from the Longyi1 submember (SM) in the Luzhou area of the Sichuan Basin. A series of experiments, including organic matter content analysis, vitrinite reflectance, whole-rock mineral X-ray diffraction, carbon dioxide adsorption, nitrogen adsorption, high-pressure mercury intrusion, and argon ion polishing scanning electron microscopy, were conducted to characterize the full pore size distribution of different shale phases in the Longmaxi Formation (LMX FM). The study also explores the role of lithology in controlling the pore structure. The results show that the shale in the Longyi1 SM is overmature, with some well sections exhibiting semigraphitization. The main lithologies are overmature organic-rich siliceous shale and organic-rich mixed shale, with a small amount of semigraphitized organic-rich siliceous shale. Pore size distribution analysis reveals that mesopores contribute the largest proportion of pore volume (PV) (63.3%), followed by micropores (32.7%) and macropores (3.9%). Regarding specific surface area (SSA), micropores dominate (66.5%), while mesopores contribute 33.5% and macropores account for only 0.02%. The peak pore diameters of overmature organic-rich siliceous shale are in the ranges of 0.3–0.9, 4–15, and 20–40 nm, with PV ranging from 24.7 to 36.4 μL/g and SSA ranging from 35.7 to 50.4 m 2 /g. The PV and SSA of samples from the LMX FM are positively correlated with the total organic carbon content, siliceous minerals, and clay minerals. The relatively large volumes of mesopores and total pores in overmature organic-rich siliceous shale make it the most favorable reservoir phase. The top and bottom sealing layers create a barrier to fluid movement in organic-rich shale. The Longyi2 SM is primarily composed of poorly connected clay mineral pores, while the Longyi1 SM is dominated by organic matter pores. Differences in the pore structure between the two SMs create a sealing effect, isolating the Longyi2 SM from the Longyi1 SM.