Mismatch between oil abundance and apparent displacement response in lacustrine shales: The role of wettability in the Triassic Chang 73 shale, Ordos Basin

Oil abundance alone does not fully describe displacement behavior in lacustrine shales. Here, we examine how this behavior varies with wettability. We investigated 37 core samples obtained from the third submember of the Chang 7 Member (Chang 7 3 ) of the Upper Triassic Yanchang Formation in the Ordos Basin, northern China, encompassing laminated felsic shale, silty massive felsic shale, and massive felsic shale. Oil abundance was characterized using quantitative grain fluorescence on extract (QGF-E), whereas multistage spontaneous imbibition experiments coupled with nuclear magnetic resonance (NMR) were used to derive the NMR wettability index (NWI) and heavy water (D 2 O) apparent displacement efficiency (DE). Oil abundance decreases from laminated felsic shale through silty massive felsic shale to massive felsic shale. In contrast, NWI is lowest in laminated felsic shale, intermediate in silty massive felsic shale, and highest in massive felsic shale. Most laminated felsic shale samples yield low apparent DE values despite showing the highest oil abundance. For the unextracted samples, the NWI–apparent DE envelope is inverted U-shaped, with the highest observed responses at NWI of approximately 0–0.35. After oil cleaning, this relationship shifts to a broadly increasing trend. Oil-cleaned NWI correlates positively with feldspar content but negatively with oil‑abundance parameters. Silty massive felsic shale, with moderate oil abundance and near-neutral to weakly water-wet conditions, provides a favorable balance between oil abundance and apparent displacement response. Our findings demonstrate a lithofacies-scale mismatch between oil abundance and apparent displacement response and highlight the role of wettability in interpreting this contrast.