Shale‐Oil Micromigration and Sweet Spot Evaluation of the Shahejie Formation in the Dongying Sag, Bohai Bay Basin

ABSTRACT Strong heterogeneity of lacustrine shales in China hinders high‐precision shale‐oil sweet spot delineation, whereas the conventional “hydrocarbon generation‐retention” paradigm fails to account for widespread hydrocarbon micromigration. Here we systematically characterize micromigration in the Shahejie Formation shales from the Well NY1 of the Dongying Sag, Bohai Bay Basin, using Rock‐Eval pyrolysis, total organic carbon (TOC), and x‐ray diffraction (XRD) analysis, and establish a micromigration hydrocarbon ( δQ ) evaluation model. Results show widespread micromigration across the studied succession, with δQ values ranging from −523 to 559 mg/g TOC. Hydrocarbon intra‐micromigration (HIM) intervals account for 30.9% of the section and exhibit significantly higher oil content ( S 1 ) and oil mobility (quantified by the oil saturation index [OSI]) relative to hydrocarbon extra‐micromigration (HEM) intervals. Multiple geochemical evidences support micromigration, including suppressed T max values, abnormally elevated production index at low thermal maturity, and selective enrichment of light saturated hydrocarbon fractions in HIM intervals. Carbonate‐rich lithofacies are identified as preferential sites for hydrocarbon charging, whereas intervals with high TOC and high original hydrogen index tend to act as hydrocarbon expulsion zones. Mineral assemblages exert primary controls on micromigration pathways and efficiency via modulation of pore structure, capillary pressure, and rock brittleness. On this basis, we develop an improved sweet spot criterion by integrating the micromigration parameter δQ with conventional indicators ( S 1 and OSI). Application of this criterion identifies four Type I sweet spot intervals (3382–3394, 3408–3413, 3444–3468, and 3480–3500 m) in the Well NY1. This study confirms the role of hydrocarbon micromigration in differential shale‐oil enrichment in lacustrine basins and provides a quantitative method for micromigration type sweet spot prediction.