Multiphase Extraction and Molecular Characterization of Residual Hydrocarbons in Cretaceous Qingshankou Shale: Implications for Movable Oil Assessment and Migration Mechanisms in Changling Sag, Songliao Basin

Quantifying movable oil is essential for shale oil resource evaluation. This study applies continuous residual hydrocarbon extraction to assess movable oil in the first member of the Qingshankou Formation and investigate hydrocarbon migration and accumulation. Chromatographic results show that medium-to-low-carbon hydrocarbons extracted by <i>n</i>-hexane and dichloromethane represent free retained hydrocarbons, validating residual hydrocarbon analysis as an effective method for movable oil quantification. Molecular geochemical data reveal distinct oil-source relationships between upper and lower intervals, with strong biomarker consistency between lower shale residual hydrocarbons and sandstone crude oil, as indicated by key terpane and sterane ratios. Variations in free hydrocarbon content and aromatic migration parameters (4-/1-MDBT and 4,6-/1,4-DMDBT) indicate a short-distance migration from shale to adjacent thin sandstones. High movable oil contents in three interbedded sandstone layers confirm their reservoir potential.