Effect of Shale Lithofacies on Pore Structure of the Wufeng-Longmaxi Shale in Southeast Chongqing, China

Total organic carbon (TOC), optical microscopy, field emission scanning electron microscopy, X-ray diffraction, and nitrogen gas adsorption analyses were performed on shale samples from the Upper Ordovician Wufeng Formation and the Lower Silurian Longmaxi Formation (Wufeng Longmaxi Formation) in order to explore the effect of shale lithofacies on pore structure. The results show that the Wufeng Longmaxi shale consists of four types of pores: organic matter pores, interparticle pores, intraparticle pores, and microfracture pores. A total of eight kinds of shale lithofacies were identified on the basis of TOC and the ternary diagram of siliceous minerals (quartz and feldspar), clay, and carbonate. The geological properties have substantial heterogeneity in these shale lithofacies. The volumes of organic matter (OM) and clay-related and brittle mineral related pores are 0.00119-0.01262, 0.00080-0.00260, and 0.00122-0.00215 cm(3)/g, with an average proportion of 51.85%, 26.44%, and 21.71%, respectively. The specific surface area (SSA) values derived from OM, clay, and brittle minerals range from 2.35 to 24.70 m(2)/g, from 1.89 to 6.12 m(2)/g, and from 0.21 to 0.37 m(2)/g, with a mean percentage of 60.18%, 37.50%, and 2.32%, respectively. The pore volume and SSA of organic-rich siliceous shale (ORSS) and organic-rich mixed shale (ORMS) are primarily contributed by OM, while those of organic-lean siliceous, organic-lean mixed, and organic-lean argillaceous shales (OLSS, OLMS, and OLAS, respectively) are mainly contributed by clay. OM is the dominant positive controller of pore volume and SSA, and its control level declines with decreasing OM content. However, clay is weakly negatively correlated with both pore volume and SSA. The micropores (<2 nm) and mesopores (2-50 nm) principally originate from OM, whereas the development of macropores (>50 nm) is related to inorganic minerals. ORSS is the most favorable facies for shale gas storage due to the substantial SSA and pore volume resulting from the high OM content and consists mainly of micropores and mesopores, while OLSS, OLMS, and OLAS have a much lower shale gas storage capacity corresponding to relatively more macropores due to the very small SSA and small pore volume resulting from the low OM content.