Mobility evaluation of continental shale oil of the Permian Fengcheng Formation in the Mahu Sag: Reservoirs characteristics, multiple influencing factors and conceptual models

Mobility is a crucial metric for assessing sweet spots of continental shale oil. However, due to the complexity of shale oil reservoirs characteristics and the lack of systematic analyses of factors influencing mobility, the difference in shale oil mobility under multiple lithofacies control remains unclear, causing significant challenges for mobility evaluation and sweet spot prediction. This study examines continental shales of the Fengcheng Formation in the Mahu Sag, employing scanning electron microscopy (SEM), nitrogen adsorption (NA), nuclear magnetic resonance (NMR), spontaneous imbibition (SI), and contact angle measurements (CAM) to investigate the pore structure, connectivity, and wettability properties of different lithofacies shale. Quantitative analyses of shale movable oil content and saturation were conducted using multistep temperature pyrolysis (MTP) and NMR centrifugation techniques. Furthermore, the influence of reservoir characteristics, geochemical characteristics, and lamination development on shale oil mobility were discussed. Results indicate that larger pore diameter, higher imbibition slopes, and lower fractal dimensions of movable fluid pores (D₂) correspond to higher movable oil saturation. Organic matter exerts a dual effect on shale movable oil content. When the TOC is below a threshold, the movable oil content gradually increases with TOC. Laminations exhibit favorable reservoir properties and light oil enrichment, enhancing shale oil mobility. Massive siltstone (MS) develops interconnected intergranular pores with the best pore structure and connectivity, the lowest D₂ values, and the highest shale oil mobility. Laminated felsic shale (LFS) and laminated calcareous shale (LCS) exhibit moderate mobility, where the development of microfractures enhances fluid flow by connecting isolated pores into pore-fracture networks. In contrast, massive felsic shale (MFS) and bedded felsic shale (BFS) primarily develop intragranular dissolution pores with more complex structures and poorer connectivity, resulting in weaker mobility. A more accurate approach for assessing shale oil mobility has been presented, taking into account both total oil content and movable oil saturation. More importantly, this study establishes a comprehensive conceptual model illustrating the potential relationships among shale lithofacies, reservoir characteristics, and movable oil flow space in the study area. This research not only provides a systematic approach for assessing shale oil mobility but also deepens the understanding of flow mechanisms of continental shale oil, offering theoretical guidance for optimizing sweet spots in the Fengcheng Formation shale oil reservoirs of the Mahu Sag.