Geological characteristics and exploration suggestions for shale in Paleogene Hetaoyuan Formation of Biyang and Nanyang sags, Nanxiang Basin

The shale layers in the Paleogene Hetaoyuan Formation of the Biyang and Nanyang sags in the Nanxiang Basin show significant geological differences from the shale strata in the eastern rift basins where breakthroughs have been achieved. The limited understanding of the formation mechanism of shale oil and its geological characteristics in this area constrains its exploration and development. Through core analysis, X-ray diffraction (XRD), thin-section observation, scanning electron microscopy (SEM), and confocal laser scanning microscopy (CLSM), the geological differences of the shale in the Hetaoyuan Formation across various sags in the Nanxiang Basin were systematically studied, and suggestions for future exploration were proposed. The Hetaoyuan Formation in the Nanxiang Basin mainly develops laminated mixed shale. Particularly, the Biyang Sag mainly develops laminated dolomitic and felsic mixed shale and is abundant in matrix-type shale oil. The Nanyang Sag mainly develops laminated felsic and argillaceous mixed shale with well-developed sandstone interlayers, primarily containing sandwich-type shale oil. The shale in the Biyang Sag exhibits normal pressure, medium to low evolution degree, and medium to high organic matter abundance, with medium to good reservoir property, good oil-bearing capacity, good compressibility, and medium mobility. The shale in the Nanyang Sag exhibits normal pressure, medium evolution degree, and medium to low organic matter abundance, with medium reservoir property, medium to good oil-bearing capacity, and medium compressibility and mobility. Comparative analysis revealed that boundary faults and paleo-water depth controlled the types of lake basins in different sags. Lake basin types and the paleo-depositional environment collectively affected shale lithofacies types. The organic matter components and hydrocarbon-generating organism types controlled the abundance of organic matter in shale. The sandwich-type shale oil with medium to low organic matter abundance in the freshwater lake basin of the Nanyang Sag and the matrix-type shale oil under normal pressure in the salt lake basin of the Biyang Sag possess favorable reservoir-forming conditions. For the shale layers in the Biyang Sag, which have good oil-bearing properties and relatively concentrated sweet spots, it is recommended to strengthen research on low-cost engineering technologies and explore efficient development models for sandwich-type shale oil through vertical and deviated wells. For the shale layers in the Nanyang Sag, which feature large vertical spans and complex faults, highly deviated wells are recommended for large-scale, multi-layer vertical fracturing.