Quantifying the Pore Characteristics and Heterogeneity of the Lower Cambrian Black Shale in the Deep-Water Region, South China

Recently, significant breakthroughs have been made in the exploration of shale gas in the Lower Cambrian black shale of the Sichuan Basin, indicating a promising commercial extraction potential. However, there remains considerable controversy regarding the pore structural characteristics for this shale formation, especially in the deep-water region. To address this, this paper focused on core samples from two shale gas wells (Xa1 and Xb1) located in the slope-basin facies zone during the Early Cambrian. The analysis involved X-ray diffraction (XRD) analysis, microscopic imaging, and N₂-CO₂ adsorption-desorption experiments to qualitatively and quantitatively characterize the pore structural characteristics, pore size distribution, and pore types in the deep-water region. The results show that Niutitang shales can be classified into four lithofacies based on mineral composition: siliceous shale, muddy siliceous shale, mixed siliceous shale, and siliceous calcareous shale. Meanwhile, the black shale exhibits diverse pore types, predominantly organic pores, along with significant development of interparticle pores, intraparticle pores, intercrystalline pores, and a few microfractures. In the Niutitang Formation shale of deep-water regions, mesopores contribute the most to the pore volume, followed by macropores, exceeding 89%. Micropores contribute the most to the specific surface area, followed by mesopores, accounting for more than 98%. Each lithofacies shale displays a similar multipeaked distribution pattern, with siliceous shale generally having the most significant pore volume and specific surface area and the highest heterogeneity. Notably, the total organic carbon (TOC) content is the primary factor controlling the micropore structure of the Niutitang Formation shale in the study area, with a less significant impact on mesopores and macropores. Siliceous shale, which has a high average TOC content of 9.58 wt %, exhibits the greatest pore volume and specific surface area. Quartz contributes to the development of micropores in the shale of the study area, whereas clay minerals somewhat inhibit pore development. Overall, the mineral composition has a minor impact on pore development. These findings provide theoretical support for the evaluation of deep shale gas reservoirs and the prediction of favorable areas in the Lower Cambrian Niutitang Formation in Western Hunan.