Lamina combination characteristics and hydrocarbon enrichment mechanism of continental shale formations: A case study of the second member of the Paleogene Funing Formation in the Qintong Sag, Subei Basin, China

Taking the shale of the second member of the Paleogene Funing Formation in the Qintong Sag, Subei Basin, China, as an example, this study integrates methods such as rock section imaging, optical and electron microscopy, micro-area mineral analysis and laser confocal in-situ observation, assisted by Wood's alloy impregnation and other auxiliary techniques, to systematically investigate lamina types and combinations, pore-fracture units and fracture systems, hydrocarbon occurrence and shale oil enrichment patterns. The following results are obtained. (1) Three basic lamina types, i.e. felsic, clay-rich, and carbonate, are identified in the study area. Their combinations are controlled by the interplay of climate, hydrodynamics, and tectonics, with vertical distribution influenced by lake-level fluctuations and event sedimentation. (2) Reservoir space is controlled by lithological composition, predominantly comprising intergranular pores and fractures within felsic laminae and intercrystalline pores and fractures within clay-rich laminae, which together with dissolution pores and organic matter pores form a matrix pore-fracture system. This system, combined with bedding fractures, structural fractures, and overpressure fractures, constitutes a hierarchical and three-dimensional transport network. (3) The “felsic + clay-rich + organic-rich” lamina combination exhibits an optimal pore-fracture configuration, serving as the preferred shale oil reservoir unit, continuously distributed in sub-members I-II. (4) A “hierarchical migration - dynamic sealing” in-source enrichment model is established. Specifically, hydrocarbon generation in clay-rich laminae creates overpressure, driving migration through nanoscale pore-fracture networks and forming localized accumulations; subsequent fracture formation from overpressure breaches lamina interfaces, allowing hydrocarbons to migrate under capillary pressure into micrometer-scale porous domains in felsic laminae; structural fractures connect multiple laminae to form a 3D seepage system, while cementation zones associated with micro-faults and lamina interfaces create dynamic sealing. Ultimately, shale oil accumulates in source via the coupling of pores, fractures and laminae.