Microscopic Heterogeneity Characteristics and Movable Oil Characterization of Ultra-Deep Shale Oil in the Mahu Sag

The shale oil in the Fengcheng Formation of the Mahu Sag has become an extremely critical target for large-scale reserve growth in the Junggar Basin. Characterized by great burial depth, complex geology, and pronounced heterogeneity, the effective and economical development of shale oil “sweet spots” remains a key challenge. To clarify the characteristics and controlling factors of this heterogeneity, an integrated analysis combining geological context, well-log responses, and laboratory data was conducted, focusing on three aspects: litho-mineralogical composition, pore structure, and hydrocarbon content. This approach enabled a detailed characterization of the micro-scale heterogeneity of the Fengcheng Formation shale oil and identification of its primary controls. On this basis, a four-parameter coupled mobility index evaluation model was developed, and—by integrating formation test data and fracture intensity—a classification scheme for mobility ranges was established. The study reveals that the strong heterogeneity is intrinsically driven by the complex coupling among lithofacies assemblages, clay mineral distribution, pore structure, and oil saturation, while depositional environment and diagenesis act as the dominant macro-scale controlling factors. Field production data confirm that the proposed mobility evaluation method correlates well with actual performance: the mobility index exhibits a clear exponential relationship with average daily oil production (R 2 =0.90), and the mobility-based classification shows excellent agreement with well productivity, thereby providing a quantitative foundation for the effective development of ultra-deep shale oil sweet spots.