Qualitative and quantitative study of micro-pore structures of Longmaxi Formation shale in Fuling area, Sichuan Basin

The pore structure of Longmaxi shale in Fuling area of the Sichuan Basin is so complicated that conventional single test technology fails to accurately characterize the microscopic shale pores. The multi-scale microcosmic pore structure of Longmaxi shale in Fuling area was studied using SEM of an argon ion polished surface, pressure mercury-nitrogen adsorption, and nuclear magnetic resonance (NMR) analyses. The shale reservoirs in Jiaoshiba mainly contain gray black to black argillaceous siltstones and silty mudstones with three pore types:organic and inorganic pores, and micro-cracks. The pore size distribution of shale is of multiple-scales, most of which are micro pores and meso pores. Pore diameter < 20 nm accounts for over 80% of the total. Quantitative NMR analysis showed that the main layer pore radius of Longmaxi Formation shale is significantly greater than the non-main layer. The pore size distribution increased with well depth. On the plane, the areas where the pore diameter peak value of the main layer shale is > 2 nm, the porosity is > 3%, and the water saturation of < 40% showed favorable reservoir preservation conditions.