Mineralogical and Lithofacies Controls on Gas Storage Mechanisms in Organic-Rich Marine Shales

This study examines the lithofacies characteristics of the Silurian Wufeng–Longmaxi Formation black shale in the southern Sichuan Basin and their influence on gas content. The analysis integrates core observations, organic matter content measurements, X-ray diffraction (XRD) analyses, and field gas data. Seven shale lithofacies were classified based on mineral composition and total organic carbon (TOC) content. Vertically, the bottom section of a single well is dominated by organic-rich siliceous shale interbedded with thin mixed shale, characterized by considerable thickness, high TOC content, high gas content, and stable lateral distribution. The middle section features organic-rich clay-siliceous shale interbedded with siliceous shale, while the upper section consists primarily of organic-rich siliceous shale and clay-siliceous shale with lower organic matter and gas content, as well as frequent vertical lithofacies changes that lead to strong lateral heterogeneity. In the lower deep-water organic-rich siliceous shale, biogenic silica and organic matter evolve together. Siliceous support enhances hydrocarbon formation and preservation, leading to a gas enrichment model dominated by adsorbed gas. This results in high total gas content, a large proportion of adsorbed gas, and a brittleness index favorable for shale gas storage and hydraulic fracturing. Due to the relatively high organic matter content, the organic-rich clay-siliceous shale lithofacies also displays high gas content, coupled with an increased proportion of free gas. Based on four distinct lithofacies-based shale gas enrichment models, the organic-rich siliceous shale and organic-rich clay-siliceous shale facies are recognized as the most favorable targets for shale gas storage and development.