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In tight shales, occurrence characteristics limit oil migration and recovery. This paper proposes an innovative approach based on experimental and theoretical models to investigate the oil occurrence mechanism in nanoporous shales. Thermogravimetry (TG) and nuclear magnetic resonance (NMR) tests were conducted to investigate the migrations and distributions of adsorbed, irreducible, and movable oils. Furthermore, the oil micro-occurrence characteristics within shales were quantitatively clarified using the theoretical equation for the adsorption ratio. Lacustrine shales with different lithofacies from Dongying Sag, Bohai Bay Basin, China, were evaluated as a case study. The results showed that adsorbed, irreducible, and movable oils occur in different pore sizes. The adsorbed oil is primarily saturated in micropores (<0.1 μm) and mesopores (0.1–1 μm) but hardly in macropores (>1 μm), characterized by an average density of around 1.4974 g/cm3 and a mean adsorption thickness of approximately 0.723 nm at a temperature of 60 °C. The irreducible oil predominantly occurs in mesopores, whereas the movable oil primarily exists in mesopores and macropores, and the weight ratio rapidly increases with increased pore size. Additionally, the NMR T 1 –T 2 patterns of the adsorbed, irreducible, and movable oils were reported, and these patterns can directly reveal the states of oil micro-occurrence in shale pore-fracture systems. This study gives substantial insights into the oil occurrence mechanisms in nanoporous shales. • An innovative method evaluating the micro-occurrence of shale oil was developed. • The mean density and adsorption thickness of adsorbed oil at 60 °C were disclosed. • The NMR T 1 – T 2 patterns of the adsorbed, irreducible, and movable oils were presented. [ABSTRACT FROM AUTHOR]

The occurrence and mobility of oil obtained from shales are primarily controlled by the pore–fracture networks of the shales. To clarify the distributions and characteristics of the type, morphology, and size in lacustrine shale pores, shale samples obtained from the Shahejie Formation, Dongying Sag, Bohai Bay Basin in China were examined using argon ion milling and scanning electron microscopy. In this study, pores were mainly classified as interparticle pores, intraparticle pores, and fractures and further classified into 14 types, comprising intraparticle pores (7 types), interparticle pores (6 types), and fractures, based on the differences in their components. Interparticle pores have more complex morphologies than intraparticle pores and fractures. Complex shapes characterize the interparticle pores between quartz granules and at the edges of feldspar granules. Intraparticle pores mainly contribute to mesopores and then to macropores, while interparticle pores behave in the opposite manner. Micropores are primarily derived from the intraparticle pores present in clay aggregates and dissolution pores present in calcite or dolomite granules. However, macropores mainly come from the interparticle pores present within or at the edges of quartz granules; consequently, siliceous-rich shales may be the favorable shale oil reservoirs called interlayer shale oil. Moreover, organic-rich calcareous shales with numerous large dissolution pores can become optimal self-generation and self-storage shale oil reservoirs. This study gives insight into the evaluation of shale oil exploration and development in the Dongying Sag in the Bohai Bay Basin. • Fracture, 7 types of intra- and 6 types of interparticle pores were observed. • The morphology and size of each type of pore were well revealed. • Micropores mainly come from intraparticle pores in clay and carbonate minerals. • Macropores primarily come from interparticle pores associated with quartz granules. [ABSTRACT FROM AUTHOR]