Pore structure characteristics and controls of Lower Cambrian Niutitang Formation, western Hubei Province

The pore structure features of shale in the Lower Cambrian Niutitang Formation in the western Hubei Province were examined using low-temperature CO₂ and N₂ adsorption tests, Field Emission-Scanning Electron Microscopy (FE-SEM) observations, TOC content measurements, and X-ray diffraction analyses. The TOC content is high in the Niutitang Formation, and the dominant pore types are organic pores and intra-particle pores, displaying complex pore morphology. XRD analysis suggests that the mineral components are dominated by quartz and clays. The CO₂ and N₂ adsorption experiments show that the pore size distribution displays multi-peaks, and the size of mesopores was mostly within the range of 2-25 nm. The pore volumes and specific surface areas of Niutitang shale are mainly provided by micropores and mesopores. Three shale lithofacies can be identified including siliceous shale, mixed shale and muddy shale. The role of TOC content and minerals on pore development is different in different lithofacies of Niutitang Formation. The porosity development of siliceous shale is mainly affected by the contents of TOC and biogenic silica. The porosity development of mixed shale is mainly affected by the contents of TOC and clay minerals.