Multidisciplinary characterization of migrated bitumen enrichment and pore network development in low-maturity Es4 shales of the Damintun Sag, East China

Low-maturity shale oil resources are widely developed in the Paleogene-aged Damintun Sag of the Bohai Bay Basin, yet the unclear pore architecture and reservoir space distribution have hindered its efficient exploitation. Focusing on the low-maturity Es₄ shale, this study integrates core observations, geochemical tests, X-ray diffraction, scanning electron microscopy (SEM), and mercury intrusion porosimetry (MIP) data from Well S352 to investigate lithofacies-controlled pore structures and residual oil characteristics. Three lithofacies reservoirs are identified: laminated felsic shale (LFS), mixed shale (MS), and massive dolomitic mudstone (MDM). LFS and MS are rich in Type I kerogen excellent hydrocarbon generation and retention. LFS acts as both source and reservoir rock, featuring abundant mesopores, strong pore connectivity, and bedding-parallel fractures, making it a favorable sweet spot for in situ generation and light oil expulsion. MS serves as a transitional facies with moderate generative and physical properties. In contrast, MDM, with low TOC and poor pore connectivity, lacks generative capacity and instead functions as a secondary reservoir that stores migrated oil from adjacent LFS. Thermal maturity assessments show high T_max values (up to 465 °C) indicative of peak oil generation, despite low measured R_o (∼0.6 %), likely suppressed by algal kerogen. Multiscale pore analyses reveal that LFS contains the highest cumulative pore volume and most developed pore–fracture networks, while MDM exhibits isolated, diagenetically altered pores. Geochemical zonation and gas chromatography fingerprinting distinguishes indigenous hydrocarbons in LFS/MS from migrated oils in MDM, confirming stratigraphically confined migration. Laminated shale—particularly those with interbedded MDM intervals—show optimal conditions for sweet spot development, as short-range migration enriches reservoir potential without requiring high maturity. This study underscores the viability of low-maturity shale oil systems and emphasizes the importance of integrating lithofacies, geochemistry, and pore network architecture to predict sweet spots for optimized exploration in lacustrine shale formations.