Chronologically Constraining Multiphase Secondary Oil Migration Pathways Using Integrated Geochemistry, Fluid Inclusions and In Situ U–Pb Dating of Calcite: Insights From the Tarim Basin, NW China

Abstract

Understanding the pathways and timing of multiphase oil migration is critical for accurate hydrocarbon exploration, particularly in tectonically complex and deeply buried sedimentary basins. In superimposed basins, traditional molecular geochemical indicators often struggle to resolve multiphase hydrocarbon charge histories due to the oil mixing, limiting their effectiveness. This study integrates carbazole-based geochemical indicators and thermal maturity modelling of source rocks with in situ U–Pb dating of calcite cements that contain primary oil inclusions, to reconstruct the secondary migration history of crude oil in the Ordovician carbonate reservoirs of the Akeyasu area, Tarim Basin, NW China. This integrated approach constrains the timing, direction, and mechanism of oil migration within a chronological framework. Three distinct oil charge episodes include Middle Caledonian (~458–448 Ma), Hercynian (332–252 Ma), and Himalayan (< 20 Ma) are identified, all originating from the same source rocks. The first episode involved long-distance lateral migration along an unconformity surface, whereas the subsequent two were dominated by vertical migration along reactivated fault systems. This work resolves long-standing uncertainties regarding multiphase oil mixing and migration in superimposed basins and establishes a transferable workflow for dating and decoding complex petroleum system evolutions globally. The findings not only clarify the spatial and temporal evolution of petroleum accumulation in the Tarim Basin but also offer a methodological blueprint for unravelling complex migration histories in other ancient and structurally complex sedimentary basins worldwide.