Lithofacies-Controlled Pore Development Patterns of the Lower Cambrian Shuijingtuo Formation Shale in the Middle Yangtze Region, China

Lithofacies and pore structure strongly influence shale reservoir quality and shale gas enrichment. To characterize pore development in the Lower Cambrian Shuijingtuo Formation shale of the Middle Yangtze region, core samples from well EYY1 were analyzed using total organic carbon (TOC) measurements, whole-rock X-ray diffraction (XRD), optical microscopy, field-emission scanning electron microscopy (FE-SEM), energy-dispersive spectroscopy (EDS), and nuclear magnetic resonance (NMR). The results show that: (1) Four mineralogical lithofacies were identified, including carbonate mudstone (CM), mixed mudstone (MM), argillaceous mudstone (AM), and siliceous mudstone (SM). The studied interval is dominated by SM, MM, and CM, whereas AM is rare. SM has the highest TOC content, followed by MM and CM. (2) The pore systems are dominated by micropores and small pores, with the main pore-size peaks concentrated at approximately 10–30 nm. SM has the highest porosity and the most pronounced organic-pore development. MM contains diverse pore types, including intergranular pores, intragranular clay-mineral pores, microfractures, and minor organic-related pores, whereas CM is mainly characterized by carbonate-related dissolution pores and microfractures. (3) Pore development is jointly controlled by TOC content, mineral composition, and lithofacies framework. SM represents the most favorable reservoir lithofacies, while MM also exhibits considerable reservoir potential. These results indicate that shale gas evaluation of the Shuijingtuo Formation should integrate TOC content, lithofacies, mineral composition, and pore-development characteristics.