This study focuses on the saline lacustrine shales of the Shahejie Formation in the Jiyang Depression, Bohai Bay Basin. Using a range of techniques, including scanning electron microscopy (SEM), cathodoluminescence (CL), and energy-dispersive spectroscopy (EDS), we systematically investigate the characteristics of authigenic minerals in the shales and their influence on micro-pore evolution. The results indicate that coarse-grained calcites or calcites with epitaxial overgrowth precipitated substantially in an alkaline diagenetic environment, leading to significantly reduced porosity. The diagenetic dolomites, in spite of low content and scattered distribution, also indirectly hindered pore preservation due to the weakened reworking by acidic fluids during cementation in an alkaline environment. Ankerites are mostly formed in a Fe²⁺-rich acidic environment, frequently associated with illitization and albitization. Albites and ankerites exhibit pronounced positive correlations with thin-section porosity, although they are not direct pore carriers themselves. This relationship reflects the synergistic effects of acidic fluid-induced reworking and mineral transformation. Illites act as important pore carriers, and their platy or fibrous aggregates facilitate the formation and interconnectivity of intergranular pores, bedding-plane pores, and microfractures in an acidic environment. Authigenic quartz, except for some tiny quartz grains that fill pores, can occur as distinctive vein-like aggregates that provide rigid support for surrounding pore structures, thereby mitigating the damage to pores caused by subsequent compaction and cementation. Overall, the pore evolution in the shales of the Shahejie Formation is dominated by alternating dissolution-induced porosity enhancement in an acidic environment and cementation-induced porosity reduction in an alkaline environment. Furthermore, various authigenic mineral assemblages and their spatial distributions can serve as important indicators for assessing the potential for pore preservation in shale reservoirs.

Breakthroughs have been achieved in the exploration of lacustrine shale oil. However, the dominant geological factors controlling the enrichment and high yield of shale oil remain unclear, restricting its efficient exploitation. The fine characterization of various laminae can reveal the major factors controlling shale oil enrichment. This study focuses on the carbonate-rich shales of the Paleogene Shahejie Formation in the Jiyang Depression. Using thin section observations and field emission scanning electron microscopy (FE-SEM), we analyze the reservoir spaces and oil-bearing properties of typical laminae and reveal the controlling effects of the lamina assemblages of shales on oil enrichment. The results indicate that carbonate-rich shales in the upper sub-member of the 4th member of the Shahejie Formation (Es₄^U) within the Jiyang Depression contain five lamina types: fibrous calcite laminae, micritic calcite laminae, very fine crystalline calcite laminae, clay-mineral-rich laminae, and mixed laminae. Major lamina assemblages include the combination of micritic calcite laminae, clay-mineral-rich laminae, and mixed laminae, and the combination of fibrous calcite laminae, very fine crystalline calcite laminae, clay-mineral-rich laminae, and mixed laminae. The clay-mineral-rich and mixed laminae, among others, exhibit high organic matter content, serving as the material basis for shale oil enrichment. Meanwhile, the very fine crystalline calcite laminae, with well-developed reservoir spaces, act as preferential storage media for shale oil. Additionally, the superposition of clay-mineral-rich laminae with very fine crystalline or micritic calcite laminae forms the optimal lamina configuration for shale oil enrichment. Therefore, the carbonate-rich shales with the clay-mineral-rich laminae and very fine crystalline or micritic calcite laminae assemblage serve as an important target for shale oil exploration and exploitation.

The swift shifts in sedimentary water bodies in continental petroliferous sedimentary basins result in frequent intercalation of sandstone and mudstone layers. Various sandstone-mudstone intercalation patterns (sand-mud assemblages) lead to significant differences in the storage spaces in sandstone reservoirs. Focusing on the lower submember of the 3^rd member of the Paleogene Shahejie Formation in the Linnan sub-sag, Huimin Sag, Bohai Bay Basin, we first analyze the spatial assemblages and single-layer thickness of sandstone and mudstone layers. Using casting thin section observations, physical property tests, and micro-CT scanning, we systematically elucidate the physical properties, pore types, and pore structures of sandstone reservoirs with different sand-mud assemblages. As indicated by the findings, the lower submember contains nine types of sand-mud assemblages, namely thick mudstone interbedded with thin sandstone, medium mudstone interbedded with thin sandstone, thick mudstone interbedded with medium sandstone, intercalated thin sandstone and thin mudstone, intercalated medium sandstone and medium mudstone, intercalated thick sandstone and thick mudstone, medium sandstone interbedded with thin mudstone, thick sandstone interbedded with medium mudstone, and thick sandstone interbedded with thin mudstone. The ionic interactions between sandstones and mudstones lead to strong heterogeneity in the storage capacity of sandstone reservoirs with different sand-mud assemblages. For sand-mud assemblages with low net-to-gross ratios, mudstones supply ample CO₃^2-, Ca²⁺, Fe²⁺, and Mg²⁺ to sandstones, and sandstones are completely filled with cements. Consequently, the sandstone reservoirs become extremely tight. However, when this ratio rises, mudstones cannot provide sandstones with sufficient ions mentioned above. In this case, sandstones near the sand-mud interfaces exhibit strong carbonate cementation, forming extremely tight reservoirs. In contrast, the interior of the sandstones shows weak carbonate cementation, with a small number of primary pores present. Additionally, the sand-mud assemblages with relatively thick sandstones promote organic acid infiltration, enhancing reservoir quality through the formation of numerous intergranular dissolution pores. Based on the differences in sand-mud assemblages, we reveal the influence of sand-mud assemblages on the evolutionary path and model of the storage spaces in sandstone reservoirs. Our insights are pivotal for predicting sweet spots in tight sandstone reservoirs.