Insight into the pore structure of Wufeng-Longmaxi black shales in the south Sichuan Basin, China

Based on total organic carbon (TOC) testing, X-ray diffraction (XRD) analysis, and low-pressure N-2 adsorption experiments, vertical change and controlling factors of pore structure and gas content of Wufeng - Longmaxi over mature shales were investigated. The results show that within the most favorable pay zones of shale gas, surface area and pore volume reveal an upward decreasing trend with maximum value occurred in Wufeng and the bottom of Longmaxi Formations. Vertically constant pore size distribution (PSD) and fractal dimensions indicating the dominant pore type remains the same in the studied samples. Increasing of pore volume with depth is mainly caused by volume change of mesopore, especially that with size > 4 nm. Surface area, however shows a good correlation with micropore area and is a function of micropore quantity. Pore structure variation of over mature shales is primarily controlled by TOC content and influence of minerals on pore structure is limited. For shales with similar preservation condition and thermal evolution, gas content is a function of reservoir storage capacity (surface area and pore volume) and gas generation. Organic matter acted both as the source of shale gas and as a storage medium and is the major control of gas content.