Shale Lithofacies Characteristics and Distribution Patterns of High-Quality Lithofacies under Astronomical Cycle Constraints: A Case Study of the Second Member of the Kongdian Formation in the Cangdong Sag

【Objective】The lacustrine shale of the second member of the Kongdian Formation (Ek₂) in the Cangdong Sag, Bohai Bay Basin, is characterized by complex lithofacies types and strong heterogeneity, and the distribution patterns of high-quality lithofacies and shale oil sweet spots remain unclear. To improve the accuracy of lithofacies identification and the reliability of sweet-spot prediction in lacustrine fine-grained deposits, this study uses data from Well Guan 108-8 and surrounding wells to investigate shale lithofacies assemblages and the distribution of high-quality lithofacies under astronomical-cycle constraints. 【Methods】Core observations, thin-section identification, X-ray diffraction (XRD), X-ray fluorescence (XRF), well-log data, and organic geochemical data were integrated. The natural gamma-ray log was selected for cyclostratigraphic analysis to establish a high-resolution isochronous stratigraphic framework. A random forest algorithm was applied to predict mineral compositions and quantitatively identify lithofacies. In addition, reservoir quality, oil-bearing property, oil mobility, and fracability were jointly considered to evaluate favorable shale oil sweet-spot intervals and areas. 【Results】The results show that the Ek₂ shale records relatively stable astronomical signals, including the 405-kyr long eccentricity, ca. 100-kyr short eccentricity, ca. 38-kyr obliquity, and ca. 18-kyr precession cycles. The optimal sedimentation rate is 17.2 cm/kyr, and the target interval can be divided into six fourth-order sequences and 25 fifth-order sequences. Based on ternary mineral classification, three major lithofacies types and nine subtypes were identified, including felsic shale, mixed shale, and calcareous-dolomitic shale. Taking the ca. 38-kyr obliquity cycle as the basic division unit, three types of lithofacies assemblages were further recognized: laminated mixed–calcareous-dolomitic shale assemblage, laminated mixed–felsic shale assemblage, and laminated mixed–felsic–calcareous-dolomitic shale assemblage. The short-eccentricity cycle regulated the alternation between warm-humid and relatively dry-cold climatic conditions, thereby affecting terrigenous input, lake-water chemistry, and organic matter enrichment. During short-eccentricity maxima, terrigenous clastic and organic matter input increased, B-type and C-type lithofacies assemblages were well developed, and TOC and S1 contents were relatively high. In contrast, during short-eccentricity minima, carbonate precipitation was enhanced, and A-type lithofacies assemblages became dominant. The sweet-spot intervals mainly developed from the middle to late stage of the expanding systems tract to the early stage of the highstand systems tract. They are mainly hosted by organic-rich felsic shale and mixed shale, with a single sweet-spot interval reaching a thickness of up to 12.7 m. 【Conclusion】The formation and distribution of high-quality shale in the Ek₂ of the Cangdong Sag were jointly controlled by short-eccentricity-scale climatic cycles, lithofacies assemblage types, and tectono-paleogeomorphology. During short-eccentricity maxima, the transition zone from the deep sag to the gentle slope was characterized by moderate terrigenous input, strongly reducing conditions, and relatively high felsic mineral content, making it the most favorable area for the development of organic-rich B- and C-type lithofacies assemblages and Class I–II shale oil sweet spots. This understanding provides a refined geological basis, constrained by astronomical cycles, for shale oil sweet-spot prediction and favorable area selection in continental lacustrine basins.