Microscopic mechanisms of intrasource micro-migration and enrichment of lacustrine shale oil: A case study of Chang73 submember of Triassic Yanchang Formation, Ordos Basin, NW China

Abstract

To clarify the mechanism of differential enrichment of intrasource shale oil, taking the third of seventh member of the Triassic Yanchang Formation (Chang7₃ submember for short) in the Ordos Basin, NW China as an example, we integrated high-resolution scanning electron microscopy (SEM), optical microscopy, laser Raman spectroscopy, rock pyrolysis, and organic solvent extraction experiments to identify solid bitumen of varying origins, obtain direct evidence of intrasource micro-migration of shale oil, and establish the coupling between the shale nano/micro-fabric and the oil generation, migration and accumulation. The Chang7₃ shale with rich alginite in laminae has the highest hydrocarbon generation potential but a low thermal transformation ratio. Frequent alternations of micron-scale argillaceous-felsic laminae enhance the hydrocarbon expulsion efficiency, yielding consistent aromaticity between in-situ and migrated solid bitumen. Mudstone laminae rich in terrestrial organic matter (OM) and clay minerals exhibit lower hydrocarbon generation threshold but stronger hydrocarbon retention capacity, with a certain amount of light oil/bitumen preserved to differentiate the chemical structure of in-situ versus migrated bitumen. Tuffaceous and sandy laminae contain abundant felsic minerals and migrated bitumen. Tuffaceous laminae develop high-angle microfractures under shale overpressure, facilitating oil charging into rigid mineral intergranular pores of sandy laminae. Fractionation during micro-migration progressively decreases the aromatization of solid bitumen from shale, through tuffaceous and mudstone, to sandy laminae, while increasing light hydrocarbon components and enhancing OM-hosted pore development. The intrasource micro-migration and enrichment of the Chang7₃ shale oil result from synergistic organic-inorganic diagenesis, with crude oil component fractionation being a key mechanism for forming sweet spots in laminated shale oil reservoirs.