Pore characteristics differences between clay-rich and clay-poor shales of the Lower Cambrian Niutitang Formation in the Northern Guizhou area, and insights into shale gas storage mechanisms

To study organic matter and pore characteristics of the Lower Cambrian Niutitang organic-rich shales, 166 shale samples from the Niutitang Formation were collected from the studied well, drilled in Kaiyang county in the Northern Guizhou area, Upper Yangtze region. All of the 166 shale cores were analysed for total organic carbon (TOC), total sulfur (TS), and the stable carbon isotope of kerogen (δ13Corg). Shale samples with varying TOC values were chosen for mineral composition analysis, helium porosity measurement and low pressure nitrogen adsorption measurement. The Lower Cambrian Niutitang Formation features high organic matter content, type I oil-prone organic matter, ultra-thick organic-rich shales, and an abundance of brittle minerals exhibiting favourable characteristics for the Lower Cambrian shale gas exploration in the Upper Yangtze region. Helium porosity measurement and low pressure nitrogen adsorption measurements were performed on selected shale samples with varying TOC content in order to analyse and characterize pore characteristics. Significant differences in mineral composition and pore characteristics were found between clay-rich and clay-poor Niutitang shales. Quartz weakly correlated with TOC content in clay-rich shales, which contained minor biogenically contributed quartz and performed better in terms of reservoir properties than clay-poor shales, and exhibited higher porosity, maximum nitrogen quantity adsorbed, pore volume, and specific surface area. In clay-rich shales, the maximum nitrogen quantity adsorbed, total pore volume, and specific surface area positively correlated with TOC content (R2 = 0.77, 0.79, and 0.75, respectively). The pore volume of clay-rich shales was dominated by mesopores, and both micropore and mesopore volume of clay-rich shales correlated significantly with TOC content. In clay-poor shales, quartz positively correlated with TOC content, indicating biogenic contribution to quartz. Micropore volume and mesopore volume of clay-poor shales were weakly positively correlated with TOC content. Pore volume, especially mesopore volume, may be affected by the recrystallization of biogenic silica, exhibiting identical micropore volume with clay-rich shales but lower mesopore volume.