Magnesian clay minerals and their influence on pores in the first member of Middle Permian Maokou Formation, southern Sichuan Basin

In recent years, multiple wells in the Sichuan Basin have produced industrial gas from the marlstone/marl-bearing limestone strata in the first member of the Middle Permian Maokou Formation (Mao 1), demonstrating promising exploration potential. Previous studies have suggested that the marlstone/marl-bearing limestone in Mao 1 exhibits self-generation and self-storage characteristics, forming low-porosity, low-permeability fracture-porosity type tight carbonate reservoirs with diverse storage spaces. Notably, diagenetic shrinkage pores and fractures formed during the transformation of sepiolite to talc contribute significantly to the storage space. To further investigate the genesis of clay minerals in Mao 1 and their significance for reservoir storage, rock samples were collected from six wells and one outcrop in the southern Sichuan Basin. A series of analyses, including microscopy, argon ion microscopy, X-ray diffraction (XRD), major and trace element analysis, strontium isotopic ratios (87Sr/86Sr), porosity tests, and nitrogen (N2) adsorption experiments, were conducted to study the characteristics of clay minerals in the marlstone/marl-bearing limestone of Mao 1. Microscopic observations and XRD results showed that the clay minerals mainly occurred as matrix minerals, irregular patches/spots, and replacements of biogenic calcareous shells. These minerals were primarily magnesian clay minerals such as talc and magnesium-rich montmorillonite, with minor amounts of sepiolite. Additionally, the marlstone/marl-bearing limestone in Mao 1 had relatively low Al2O3 contents and ΣREE concentrations, and its Y/Ho ratios and 87Sr/86Sr values resembled those of contemporaneous seawater, indicating limited terrigenous clastic input. This suggested that the magnesian clay minerals were originally authigenic clays formed during the deposition-early diagenesis period (in the sepiolite stage). The porosity/pore volume of the reservoir was positively correlated with the total clay mineral content and the magnesium-rich montmorillonite content, but no significant correlation or even a slight negative correlation was observed with the talc content. This indicated that the storage space in the marlstone reservoir was affected by the total clay content and the diagenetic evolution stage. The overall pore space increased with higher clay content. When thermal evolution was at the mature to highly mature stage and magnesium-rich montmorillonite was dominant, clay pores and fractures were developed, enhancing reservoir properties. However, excessive diagenetic evolution (over-mature thermal stage with higher talc content) was unfavorable for pore development.