To extract hydrocarbons from unconventional shale systems, it is necessary to assess oil retention and accumulation. 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 connectivity 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.

为了从非常规页岩系统中提取碳氢化合物，有必要评估油的保留和积累。本研究介绍了鄂尔多斯盆地上三叠统延长组长 7 段湖相页岩系统的油滞留、运移和聚集的实验结果。从目标层采集了 68 个岩心样品，通过有机地球化学、岩石学和岩石物理方法研究了被困烃的运移行为。使用总有机碳和程序热解分析确定油的保留量和生成潜力。使用扫描电子显微镜和微型计算机断层扫描对孔结构和孔连通性进行量化。还对页岩样品进行分馏，以确定使用三元共沸溶剂萃取系统萃取的碳氢化合物的化学成分。在此基础上，研究了岩相、矿物组成、有机质和孔隙结构对石油滞留和聚集的影响。

研究结果表明，岩相对“夹层”页岩系统中储存的石油的组成和数量有重大影响。由于石油被有机质吸附，富含有机质的页岩岩相保留了大量的石油。但由于长石中溶蚀相关孔隙发育，互层有机贫质粉砂岩含油饱和度指数较高，游离油含量较多。稀薄有机质粉砂岩的孔径和孔隙连通性也较大。长7段岩相组合在初次运移过程中观察到页岩油源内运移充注：油生成于富有机质烃源岩中，并在短距离内排到邻近粉砂岩中。稀薄的有机质稀薄粉砂岩充当从富含有机质页岩单元运移的石油的储层单元。同时，由于运移分馏，薄粉砂岩层段的石油富含脂肪族和芳香族化合物，而来自烃源岩的石油则富含沥青质和树脂。本研究揭示了上三叠统延长组长 7 段“夹层”页岩系统的石油运移和差异聚集，增强了我们对页岩油聚集的认识。