Development Characteristics and Reservoir Significance of Laminae in the Cambrian Qiongzhusi Formation Shale in the Southern Sichuan Basin

The Cambrian Qiongzhusi Formation shale in southern Sichuan is a promising new marine shale gas exploration target, often considered the next major potential source following the Silurian Longmaxi Formation. Clarifying its reservoir characteristics of shale is crucial for identifying shale gas sweet spots. As the most distinctive structure feature in shale, laminae development plays a vital role in the formation and evolution of shale reservoirs. Based on core samples, thin sections, and a variety of test data, this study investigates the laminae development characteristics and reservoir significance of the Qiongzhusi Formation shale in the southern Sichuan Basin, yielding the following conclusions: (1) A three-level classification and nomenclature system for shale laminae in the Qiongzhusi Formation is proposed based on mineral composition and stacking patterns, dividing laminae into single laminae, lamina sets, and lamina series. The study area exhibits diverse lamina types, including four types of single laminae, three types of lamina sets, and seven types of lamina series. (2) The vertical heterogeneity in lamina series is pronounced. Within the organic-rich interval, the lithology transitions upward from organic-rich massive shale, through organic-rich argillaceous–felsic laminae, to organic-lean argillaceous–felsic laminae. In the low-TOC interval, increasing water depth corresponds to a transition from massive sandstone to predominantly organic-lean argillaceous–felsic–calcareous laminae and organic-lean argillaceous–felsic laminae. (3) Lamina development exerts a significant control over reservoir properties, with marked differences observed between various lamina series and massive shale. Among them, the organic-rich argillaceous–felsic lamina series exhibits the most favorable reservoir characteristics, including the highest total organic carbon (TOC) content, porosity, and gas content, representing the optimal shale reservoir type.