Oil migration, retention, and differential accumulation in ?sandwiched? lacustrine shale oil systems from the Chang 7 member of the Upper Triassic Yanchang Formation, Ordos Basin, China

To extract hydrocarbons from unconventional shale systems, it is necessary to assess oil retention and accu-mulation. This study presents experimental findings on oil retention, migration, and accumulation in a lacustrine shale system from the Chang 7 member of the Upper Triassic Yanchang Formation, Ordos Basin. 68 core samples were collected from the target layer to investigate the migration behavior of trapped hydrocarbons by organic geochemical, petrological, and petrophysical methods. The amount of oil retention and generation potential was determined using total organic carbon and programmed pyrolysis analysis. The pore structure and pore con-nectivity were quantified using scanning electron microscopy and micro-computed tomography scanning. Shale samples were also fractionated to determine the chemical composition of extracted hydrocarbons using a ternary azeotropic solvent extraction system. The effects of lithofacies, mineral compositions, organic matter, and pore structure on oil retention and accumulation were studied based on these findings.The findings revealed that lithofacies have a significant impact on the composition and amount of oil stored in the sandwiched shale systems. As oil is sorbed by organic matter, organic-rich shale lithofacies retain a large amount of oil. However, due to the developed dissolution-related pores in feldspar, the interbedded organic-lean siltstones have a higher oil saturation index and contain more free oil. Pore diameters and pore connectivity are also larger in the thin organic-lean siltstones. Shale oil intrasource migration and charging are observed in the Chang 7 member lithofacies assemblages during primary migration: oil is generated in organic-rich source rocks and expelled in short distances to adjacent siltstones. The thin organic-lean siltstones act as reservoir units for petroleum that has migrated from organic-rich shale units. Simultaneously, oil in thin siltstone intervals enriches in aliphatic and aromatic compounds, while petroleum from source rocks enriched in asphaltenes and resins, due to migration fractionation. This research shows oil migration and differential accumulation in the sandwiched shale systems in the Chang 7 member of the Upper Triassic Yanchang Formation, enhancing our knowledge of shale oil accumulation.

Automated summary

This study investigates the retention, migration, and accumulation of oil in a lacustrine shale system from the Upper Triassic Yanchang Formation, Ordos Basin. The research findings demonstrate the impact of lithofacies on oil composition and amount stored in the shale systems. Organic-rich shale lithofacies retain a large amount of oil, while thin organic-lean siltstones act as reservoir units for migrated petroleum. The study also reveals the influence of pore structure on oil retention and migration.