Nanopore structures and controlling factors of the Early Cambrian shale reservoir in the Middle Yangtze Platform, South China

At present, the characteristics and controlling factors of nanopore structure in shale reservoirs are still not well understood owing to the concurrence of both organic matter (OM) and inorganic mineral (IM) pores. This issue is more prominent for the Early Cambrian shale in South China owing to the greatly variable nanopore structures of the samples from different areas. In this study, a set of Early Cambrian shale samples was taken from an exploration well in the Middle Yangtze Platform, a newly-determined shale gas play with a great potential outside the Sichuan Basin, their OM-removed (OMR) samples were obtained by a high-temperature oxidation method, and the pore structures of the bulk shales and their OMR-samples were analyzed by low-pressure gas adsorption experiments. The OM nanopore structure parameters of shale were calculated, and the controlling factors of OM and IM nanopores in the shale were investigated. The results show that the shale nanopores are mainly mesopores, their OM nanopores, especially OM micropores, are obviously more developmental compared with the IM nanopores. The TOC content is the dominant influencing factor of the shale nanopores. The brittle minerals support the formation of interparticle pores, and provide protection to OM pores. The clay minerals provide a certain amount of nanopores, but the strong compaction result in rich micropores. The carbonate minerals decrease the shale nanopores owing to the strong cementation. The favorable area of shale gas in the Early Cambrian strata in the studied area was predicted by the TOC content and preservation conditions.