Lithology, Reservoir Characteristics, Formation Mechanism, and Significance of Permian Shale in the Hongxing Area, Sichuan Basin, China

Exploration of Permian shale gas in the Hongxing area has expanded the Sichuan Basin’s potential, but transitional shale reservoir research is scarce, impeding the assessment of prospective shale gas zones. Here, we employ geochemical analysis, low-temperature CO2 and N2 adsorption, nuclear magnetic resonance, and field emission scanning electron microscopy to characterize lithofacies in the Permian shale. We identified four shale lithologies: carbon-rich siliceous shale (RS), carbon-rich mixed shale (RM), carbon-rich calcareous shale (RC), and carbon-poor calcareous shale (LC). RS and RM display superior pore volumes (14.33 cm3/g and 16.58 cm3/g, respectively) and specific surface areas (19.21 m2/g and 16.58 m2/g), highlighting their potential as the most promising lithofacies for shale gas development. Overall, the shales in the Hongxing area show poor pore connectivity, with liquid mobility rates of 37.84%, 24.33%, 18.75%, and 29.48% for RS, RM, RC, and LC, respectively. The corresponding T2cutoff values are 2.86, 1.74, 3.47, and 4.57 ms. Liquid mobility is primarily influenced by siliceous and calcareous minerals, while T2cutoff is controlled by clay minerals and pyrite. Organic matter (including its type, abundance, and maturity) predominantly affects micropore development, while mesopores are mainly influenced by siliceous minerals, clay minerals, and pyrite. Macropore development is primarily controlled by calcareous minerals. Depositional environments significantly influence lithofacies development: RS and RM were formed in reducing saline to brackish conditions, while RC and LC originated in oxidizing freshwater settings. Reducing estuarine and lagoonal settings with saline to brackish conditions represent the favorable depositional facies for shale development.