抄録
Shale oil enrichment mechanisms and sweet-spot prediction in transitional lacustrine basins remain poorly understood because of the complex interactions among depositional environment, lithofacies evolution, and reservoir heterogeneity. Using the Paleogene Hetaoyuan Formation in the Biyang and Nanyang sags of the Nanxiang Basin as case studies, this study investigates the differential enrichment mechanisms of shale oil in saline–freshwater transitional lacustrine systems and establishes a unified genetic framework for shale oil accumulation. A multi-scale integrated workflow combining core observations, X-ray diffraction (XRD), petrographic and fluorescence thin-section analysis, field-emission scanning electron microscopy (FE-SEM), laser confocal microscopy, and comprehensive logging, seismic, and geochemical interpretation was employed to characterize lithofacies, reservoir architecture, hydrocarbon occurrence, and sweet-spot attributes. The results reveal two distinct shale oil enrichment patterns controlled by contrasting basin evolution. The Biyang Sag, developed in a closed saline deep-lake setting, is dominated by matrix-controlled enrichment within dolomitic-calcareous and felsic laminated shales, characterized by relatively high organic matter abundance, oil-bearing capacity, brittleness, and vertically continuous sweet-spot intervals. In contrast, the Nanyang Sag, formed in an open freshwater lacustrine environment, exhibits an interlayer-controlled enrichment pattern associated with alternating felsic–argillaceous shales and sandstone interbeds, where reservoir quality and hydrocarbon accumulation are strongly governed by interlayer development, resulting in vertically discrete sweet spots. Basin evolution, jointly regulated by boundary fault activity and paleowater-depth variations, controlled depositional environments and lithofacies architecture, whereas organic matter supply and preservation conditions determined source rock quality and hydrocarbon enrichment. These coupled processes define a differential genetic framework linking basin evolution, depositional environment, lithofacies assemblage, reservoir quality, and shale oil enrichment. This study establishes a unified genetic framework and multi-parameter sweet-spot evaluation methodology for transitional lacustrine shale oil systems, providing a transferable conceptual model for differentiated exploration and resource evaluation in continental lacustrine basins.
