Lithofacies-Related Variations in Hydrocarbon Abundance and Pore Characteristics in the Chang 9 Member, Ordos Basin: Evidence from Well Danye-1

Lithofacies heterogeneity may separate organic matter enrichment from the normalized oil-bearing response of lacustrine shale, complicating the identification of effective shale oil intervals. To examine this relationship, the Chang 9 Member of Well Danye-1 in the Ordos Basin was investigated through core observation, X-ray diffraction (XRD), total organic carbon (TOC) analysis, Rock-Eval pyrolysis, chloroform bitumen “A” extraction, vitrinite reflectance (Ro), scanning electron microscopy, and porosity–permeability measurements. The TOC values range from 0.07 to 9.62 wt.% and average 2.18 wt.%. The interval is thermally mature, with Ro values of 0.85–1.15% and a mean of 0.99%. Mud-rich groups generally contain higher TOC, S1, S2, and chloroform bitumen “A” content, and the positive correlations among these parameters indicate that the organic matter abundance exerts the primary control on the hydrocarbon generation potential and absolute hydrocarbon abundance. However, oil saturation index (OSI) maxima do not consistently coincide with TOC maxima: several samples with TOC above 4 wt.% have only moderate OSI values, while elevated OSI values occur locally in samples containing less than 2 wt.% TOC. The reservoir is generally tight, with porosity of 0.39–5.67% and permeability of 0.000170–0.0630 mD. Mud-rich groups are characterized by higher organic matter richness and hydrocarbon-related geochemical parameters; they contain abundant interlayer pores and organic-related nanopores that favor retention but restrict pore throat connectivity. Silt-rich rocks contain relatively abundant feldspar and locally preserve residual intergranular pores, dissolution pores, and bedding-parallel microfractures, indicating pore morphologies that differ from those observed in mud-rich rocks. The OSI-TOC geochemistry demonstrates that the low residual OSI in organic-rich mudstone reflects efficient hydrocarbon expulsion rather than insufficient hydrocarbon generation. The geological sweet-spot evaluation should therefore integrate hydrocarbon supply, the absolute oil content, and pore network effectiveness instead of relying on TOC or OSI alone.