Mineral and 3D Pore Characteristics of Rocks from the Fengcheng Formation in Mahu Sag, Junggar Basin, China

The Fengcheng Formation in the Mahu Sag of the Junggar Basin, China, represents a key target for unconventional oil and gas exploration. A comprehensive understanding of the mineral and pore characteristics is essential for revealing the physical properties of the reservoir rocks. Current research on the Fengcheng Formation primarily focuses on the northern and southern slopes of the Mahu Sag, while studies on the western slope remain limited. However, the western slope is a significant production area for both tight oil and shale oil resources. This study aims to address this research gap. Representative lithologies from the Fengcheng Formation in the FY-1 Well on the western slope were selected for analysis, including salt rock from the P ₁ f ₂ subformation and dolomitic sandstones from the P ₁ f ₁ and P ₁ f ₃ subformations. Mineralogical and pore characteristics were compared across these different lithologies and subformations using field emission scanning electron microscopy (FE-SEM) and focused ion beam-scanning electron microscopy (FIB-SEM). The results indicate that dolomitic sandstones exhibit relatively high contents of feldspar and dolomite, whereas the salt rock is characterized by an extremely high content of wegscheiderite. As for the porosity, the salt rock (P ₁ f ₂) exhibits the highest value, followed by the dolomitic sandstone in P ₁ f ₁, while the dolomitic sandstone in P ₁ f ₃ has the lowest. Among all the pores, although the mesopores (<50 nm) are the most numerous, macropores (50-1000 nm) and megapores (>1000 nm) dominate the total pore surface area and volume. The shape of the pores in dolomitic sandstone (P ₁ f ₁) is more spherical, while pores in salt rock (P ₁ f ₂) are flatter. Pore connectivity varies significantly among samples: interconnected pores only exist in one direction of the salt rock (P ₁ f ₂), and the corresponding permeability is 69.77 nD. As the analysis was conducted at the microscale, the measured petrophysical properties (especially porosity and permeability) can be regarded as attributes of the rock matrix. These parameters provide fundamental inputs for constructing multiscale geological models, thereby establishing a scientific basis for numerical simulations of hydraulic fracturing development.