Micro-migration and differential accumulation of hydrocarbon fluid within hybrid shales: Evidences from in situ biomarkers and inclusions

Clarifying the micro-migration and differential accumulation mechanisms of shale oil in organic-lean hybrid shale (OLHS) is crucial for enhancing continental shale oil accumulation theories. This study examines the second member of the Funing Formation (E1f2) hybrid shale interval in the Gaoyou Sag, Subei Basin, using integrated analytical approaches including organic geochemistry, total scanning fluorescence (TSF), x-ray diffraction (XRD), and fluid inclusion analysis to identify petroleum micro-migration phenomena and elucidate differential accumulation mechanisms. Results from organic geochemistry, TSF, and XRD analyses demonstrate that oil in the E1f2 OLHS exhibits favorable mobility, high oil content, indicating significant exploration potential. Clear petroleum micro-migration pathways exist between different components of the shale system with organic-rich dark laminae serving as hydrocarbon generation sites while light-colored carbonate laminae function as storage zones. Shale oil within source rocks displays dual accumulation mechanisms: in situ accumulation within source layers and non-in situ accumulation through micro-migrated hydrocarbon charging from adjacent source intervals. Differential organic geochemical analysis and fluid inclusion data reveal that shale oil in the E1f2 exploration wells primarily originates from underlying source rocks within a 10–30 m thick interval. In the vein-host rock systems, oils present in fractures, and pores show compositional differences compared to hydrocarbons generated from in situ source rocks, supporting combined in situ and non-in situ accumulation mechanisms. Within the hybrid shale, organic-rich laminae interbedded with carbonate-rich and sandstone laminae form microscopic source–reservoir assemblages where generated hydrocarbons either accumulate in situ or migrate laterally to adjacent layers. This enhances understanding of OLHS petroleum systems.