Lithofacies classification and microscopic pore characteristics of fine-grained sedimentary rocks in the Hetang Formation, Lower Yangtze region

The lithofacies and microscopic pore characteristics of fine-grained sedimentary rocks in the Lower Cambrian Hetang Formation in the Lower Yangtze region remain unclear, which restricts the exploration and exploitation of shale gas in this region. In this study, we obtain data using methods including ordinary thin section observation, scanning electron microscope (SEM) image analysis of argon-ion milled sample surface, physical property tests, X-ray diffraction (XRD) analysis of whole-rock and clay mineralogy, N₂ and CO₂ isothermal adsorption experiments, and focused ion beam-scanning electron microscope (FIB-SEM) image analysis. Using these experimental data, we develop a lithofacies classification scheme for fine-grained sedimentary rocks in the Hetang Formation and analyze the microscopic pore structure characteristics of various lithofacies. The results indicate the presence of five major lithofacies of the finegrained sedimentary rocks, namely massive siliceous mudstone, lamellar siliceous shale, massive clay-bearing siliceous mudstone, massive mixed siliceous-calcareous mudstone, and massive mixed calcareous mudstone, which differ greatlyin porosity and permeability. Main pores in the Hetang Formation include intergranular, intercrystalline, intragranular, and organic matter-hosted pores and microfractures, with pores of nano-to-micron scale dominating. Of these, mesopores range in size from 2.0 to 10.0 nm, while micropores range from 0.4 to 0.9 nm. The massive mixed siliceouscalcareous mudstone features high organic matter and brittle mineral contents, well-developed intergranular pores and microfractures, high porosity and permeability, excellent pore connectivity, and large specific surface area, which establish itself as the most favorable lithofacies for shale gas exploration and exploitation in the Hetang Formation. The massive claybearing siliceous mudstone is characterized by large specific surface area and substantial pore volume but lower brittleness index, porosity, and permeability compared to the massive mixed siliceous-calcareous mudstone, thereby identified as the secondary favorable lithofacies.