Abstract
The systematic understanding of the coupling relationship between shale lithofacies and pore structure remains unclear, hindering in-depth analysis of shale oil enrichment mechanisms. This study focused on the shale reservoirs in the Second Member of the Kongdian Formation in the Cangdong Sag, Bohai Bay Basin. A multi-scale pore structure characterization system was established by integrating X-ray diffraction (XRD), total organic carbon (TOC) analysis, field emission-scanning electron microscopy (FE-SEM), nitrogen physisorption (NP) analysis, mercury intrusion porosimetry (MIP), advanced mineral identification and characterization system (AMICS), modular automated processing system (MAPS), and focused ion beam-scanning electron microscopy (FIB-SEM). The results revealed five distinct lithofacies: laminated felsic shale (LF), laminated mixed shale (LM), massive mixed shale (MM), laminated carbonate shale (LC), and massive carbonate shale (MC). These shale lithofacies exhibited diverse storage spaces, including inorganic pores, organic matter-hosted pores, and microfractures, with significant variations in pore structures. The dominant pore size range for all shale lithofacies is 3–200 nm, indicating that nanoscale pores serve as the primary contributors to storage capacity. Notably, LF and LM lithofacies are identified as favorable lithofacies due to their superior pore structures, their larger average pore size, combined with the presence of macropores and microfractures, along with the development of laminae, results in superior connectivity compared to other shale lithofacies. Fractal dimension analysis based on NP and MIP data demonstrated that mesopores exhibit lower fractal dimensions than micropores, suggesting simpler pore structures. Increased micropore content enhanced structural complexity, while micrometer-scale pores and microfractures also contributed significantly to pore volume. The deposition and evolution of organic matter and mineral components jointly govern pore system modification: (1) the compressive resistance of brittle mineral frameworks facilitates pore preservation; (2) dissolution pores are widely developed in LC and MC lithofacies, but mineral cementation restricts porosity and connectivity; and (3) moderate TOC content and thermally generated aggressive fluids migrating along lamina interfaces and microfractures likely play critical roles in differentiating the reservoir performances of the shale lithofacies. These findings may provide reference and guidance for the selection of targets in shale oil exploration and development.
Paper Information：
Guozheng Feng, Liqiang Zhang, Jihua Yan*, Shiyue Chen, Xiugang Pu, Wenzhong Han, Zhannan Shi, Wei Zhang. Lithofacies-based analysis of pore structure characteristics and controlling factors of shale reservoirs: A case study of the second member of the Kongdian Formation in the Cangdong Sag, Bohai Bay Basin, China[J]. Marine and Petroleum Geology, 2025, 182: 107575. https://doi.org/10.1016/j.marpetgeo.2025.107575.
