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

Abstract: The second member of the Kongdian Formation (Ek₂) in the Cangdong Sag, Bohai Bay Basin, China, develops thick organic-rich shale sequences with significant resource exploration potential. However, a systematic understanding of the coupling relationship between shale lithofacies and pore structure remains unclear, hindering in-depth analysis of shale oil enrichment mechanisms.

To clarify the microscopic pore structure characteristics of different shale lithofacies, this study takes the Ek₂ shales in the Cangdong Sag as the research subject, the samples were collected from wells GX, G, G1, GD, and GY in the Cangdong Sag. Multiple techniques, including X-ray diffraction (XRD), total organic carbon (TOC) analysis, field emission-scanning electron microscopy (FE-SEM), gas adsorption (N₂ and CO₂), advanced mineral identification and characterization system (AMICS) mineral quantitative analysis, and focused ion beam-scanning electron microscopy (FIB-SEM) 3-D reconstruction, were employed for multi-scale characterization of the microscopic pore structure.

The results indicate: (1) Five shale lithofacies types are developed in the study area: laminated felsic shale, laminated mixed shale, massive mixed shale, laminated carbonate shale, and massive carbonate shale. (2) Different lithofacies exhibit various reservoir space types, including inorganic pores, organic matter pores, and micro-fractures, with significant differences in pore structure. The dominant pore size range for all shale lithofacies is 2–200 nm, indicating that nanoscale pores serve as the primary contributors to storage capacity. Among them, the laminated felsic shale and laminated mixed shale lithofacies possess larger pore volumes due to the presence of macropores and micro-fractures. The connectivity of organic-rich laminated shale facies is superior to other shale lithofacies. (3) Syngenetic organic matter, interstitial organic matter, and organic matter-clay composites exhibit different morphologies and contact relationships with minerals, leading to differential contributions to pore volume, connectivity, and development. Syngenetic organic matter in high-frequency laminated shales can enhance pore structure. (4) The deposition and evolution of organic matter and mineral components control the modification of the reservoir pore system: the pressure resistance of the felsic mineral framework favors pore preservation; dissolution pores are widely developed in laminated carbonate shale and massive carbonate shale lithofacies, but mineral cementation restricts their porosity and pore connectivity; moderate TOC content and corrosive fluids generated during thermal evolution migrating along lamina interfaces and micro-fracture channels are significant factors causing differences in reservoir properties among different lithofacies.

Keywords: Shale lithofacies; Pore structure; Controlling factors; Second member of Kongdian Formation; Cangdong Sag