NMR-based investigation of wettability-driven oil and water occurrence behavior in lacustrine shale

Abstract: The occurrence and distribution of oil and water in lacustrine shale reservoirs significantly influence oil mobility and ultimate recovery. This study investigates the wettability-driven fluid behavior in two dominant lithofacies (organic-rich laminated calcareous shale and organic-lean massive mixed shale) from the Shahejie Formation in the Dongying Sag, Bohai Bay Basin, China. Using nuclear magnetic resonance (NMR) T₂ spectra and T₁-T₂ maps. we monitored in real time the fluid redistribution during spontaneous imbibition and dynamic displacement processes over periods 40 to 115 days. The results reveal distinct fluid occurrence patterns controlled by pore-scale wettability. Micropores (0.15-1 ms) exhibit weak water-wet/mixed-wettability, facilitating high oil displacement efficiency via capillary-driven water imbibition. In contrast, larger pores and microfractures (3-40 ms) show oil-wetting characteristics, acting as primary storage sites for residual oil after water flooding. Geological parameters such as porosity, permeability, tortuosity, and water contact angle jointly influence the fluid migration and recovery efficiency. Based on the NMR-derived fluid wettability analysis, a classification scheme for shale pore fluids is proposed, dividing the T₂ spectrum into four distinct occurrence domains: solid-like bound, mixed-wet, organic-adsorbed, and oil-wet binding. These findings provide critical insights into the microscopic oil-water distribution in heterogeneous shale matrices, offering a theoretical basis for optimizing production strategies and enhancing oil recovery in lacustrine shale reservoirs.