Lithofacies Characteristics and Its Effect on Pore Structure of the Marine Shale in the Low Silurian Longmaxi Formation, Southeastern Chongqing

The favorable facies classification for shale has not been established so far, and it is a key issue in shale gas evaluation to characterize the full scale pore size distribution by reasonable relating of different results. In this study, lithofacies classification is established based on TOC and XRD experiments on cores. There are 9 types of lithofacies, namely the Organic-rich siliceous shale (ORS), Organic-rich mixed shale (ORM), Organic-rich argillaceous shale (ORA), Organic-fair siliceous shale (OMS), Organic-fair mixed shale (OMM), Organic-fair argillaceous shale (OMA), Organic-poor siliceous shale (OPS), Organic-poor mixed shale (OPM), and Organic-poor argillaceous shale (OPA) develop in southeastern Chongqing. Low pressure nitrogen adsorption and high pressure mercury intrusion experiments are conducted to quantitatively characterize the full scale pore size distribution. It is found that spectrum of pore size distribution for ORS have multiple peaks at 2-3nm, 70-90nm and 200-300nm, and the peak shifts to the small pore size when the TOC value increases. Mesopores and macropores have the largest proportion in pore volumes, whereas mesopores and micropores take the largest part of the pore surface areas. The pore volume percentage of micropore, mesopore and marcropore is 12%, 53%, and 52% respectively, and the pore surface area percentage of micro pore, mesopore and marcropore is 47%, 57%, and 11% respectively. The volume of 200-400nm pore obviously increases when clay mineral content increases. The contribution to the surface area is over 90% for micropores and mesopores. The contribution to the pore volume is over 90% for mesopores and marcropores. The organic-rich siliceous shale, which has the largest pore volume and surface area, is the most favourable lithofacies for shale gas enrichment.