Lamination-Controlled Lithofacies Differentiation and Shale Oil Enrichment: A Case Study of the Qingshankou Formation, Songliao Basin

Organic-rich shales of Cretaceous age are widely developed in the Qian'an area of the Songliao Basin and represent an important exploration target for continental shale oil in China. However, due to complex depositional environments, diverse lithological assemblages, and strong lamination-related heterogeneity, the characteristics of reservoir space and their controlling factors among different lithofacies remain poorly constrained. This study aims to clarify the differences in reservoir space among various laminated shale lithofacies and to elucidate their formation mechanisms, thereby providing a geological basis for identifying favorable lithofacies in continental shale oil systems. Shales of the Qingshankou Formation in the Qian'an area were investigated through detailed core observations combined with thin-section petrography, scanning electron microscopy (SEM), X-ray diffraction (XRD), Rock-Eval pyrolysis, high-pressure mercury intrusion, low-temperature nitrogen adsorption, and laser confocal microscopy. Based on these integrated data sets, laminated shales were systematically classified into lithofacies, and the pore structure characteristics and controlling factors of different lithofacies were comparatively analyzed. The results indicate that the Qingshankou Formation shales are lithologically heterogeneous and well laminated and are generally characterized by low porosity and low permeability, with pronounced variations in reservoir properties under different depositional settings. Using an integrated hierarchical classification scheme based on lithology/mineral composition, sedimentary structure, and total organic carbon (TOC) content, eight lithofacies were identified, including clay-rich shale with high-density lamination and high TOC, felsic-rich shale with high-density lamination and high TOC, felsic-rich shale with low-density lamination and moderate TOC, felsic-rich shale with low-density lamination and low TOC, massive mud shale with low TOC, bedded siltstone with low TOC, bedded bioclastic limestone with moderate TOC, and bedded dolostone with moderate TOC. Significant differences in pore types, pore structure, and oil-bearing properties are observed among the identified lithofacies. Among them, the low-TOC bedded siltstone lithofacies exhibit the most favorable reservoir performance, characterized by abundant interparticle pores, dissolution pores and fractures, well-developed pore connectivity, and relatively high oil saturation. The formation and evolution of shale reservoir space are jointly controlled by mineral composition, organic matter abundance, and lamination development. TOC exerts a persistent control on micropore development throughout thermal evolution, whereas rigid detrital grains such as quartz and feldspar within silt-rich laminae form grain-supported frameworks that effectively preserve interparticle pores during compaction, thereby increasing the proportion of meso- to macropores and promoting shale oil enrichment. These results demonstrate that the enrichment potential of laminated shale oil systems is controlled by a multiscale coupling process linking lamina attributes, lithofacies architecture, and formation-scale source-reservoir configuration, rather than by organic matter abundance alone. This insight provides valuable implications for the identification of favorable lithofacies and shale oil exploration and development in the Qingshankou Formation and other continental shale oil basins.