Fluid inclusion analysis and carbonate U–Pb dating reveal shale oil accumulation processes in the Qingshankou Formation, Songliao Basin, NE China

Abstract

Direct constraints on the absolute timing of shale oil accumulation and microscale migration within shale systems remain limited. Here we investigate ostracod limestone interbeds, calcite veins, and barite veins from the Qingshankou Formation in the Songliao Basin, integrating fluid inclusion petrography, diagenetic phase analysis of host minerals, and in situ U–Pb dating to reconstruct the timing and evolution of shale oil accumulation in Member 1 of the Qingshankou Formation. Hydrocarbon inclusions within the veins are dominated by yellow-green, blue-white, and predominantly blue fluorescence assemblages. Blue-fluorescing inclusions are characterized by fluorescence λ_max below 485 nm and CIE chromaticity coordinates (X, Y) lower than 0.3, indicating a relatively high level of thermal maturity. Homogenization temperatures of hydrocarbon inclusions and their coexisting aqueous inclusions show good correspondence with the diagenetic stages of host minerals, reflecting multiple episodes of fluid trapping. Trapping ages of hydrocarbon inclusions inferred from burial–thermal history reconstruction are broadly consistent with in situ U–Pb ages of calcite veins (65.3 ± 4.8 Ma and 58.2 ± 5.2 Ma), indicating that vein formation and hydrocarbon-bearing fluid activity were essentially synchronous at the study area scale. Image-based analysis of the hydrocarbon inclusion abundance index (IHI-AI) shows that blue-fluorescing inclusions are relatively enriched in the central Gulong Sag, and their spatial distribution closely matches the occurrence of high-maturity, light shale oil. Integrated interpretation of fluorescence spectral parameters, equivalent hydrocarbon inclusion reflectance derived from the image-based fluorescence CIE chromaticity (IF–CIE) method (EqHIR_o), homogenization temperature data, diagenetic evolution, and in situ U–Pb dating indicates that, after the Qingshankou Formation reached a high level of thermal maturity near the end of the Cretaceous, shale oil retained within the source rock continued to undergo intrasource microscale migration and redistribution from the middle to late Paleogene through the Neogene. Distinct fluorescence–chromaticity signatures of hydrocarbon inclusions indicate that shale oil accumulation was closely associated with continuous hydrocarbon generation and sustained intrasource micromigration. This study provides an integrated framework for constraining the timing and evolutionary pathways of shale oil accumulation in lacustrine shale systems.