Occurrence mechanism of shale oil in laminated shale reservoirs: A review

Shale oil is an important unconventional resource. In-depth exploration of the shale oil occurrence mechanism is beneficial for the selection of sweet spots and the innovation of production technologies. Shale oil in laminated reservoirs shows commercial potential, yet systematic reviews on its occurrence mechanism remain scarce. This article provides a comprehensive summary of the occurrence mechanism of shale oil from four aspects. These include occurrence states, occurrence space, influencing factors and enrichment patterns. In addition, the formation, classification, and controlling factors of laminae, and their significance for shale oil enrichment are discussed. Finally, the challenges facing shale oil occurrence mechanism are described, and the next research work is prospected. The formation of laminae in fine-grained sedimentary rocks is associated with processes such as suspension settling, gravity flow, bottom currents, volcanism, hydrothermal activity, and microbial processes. Two-dimensional nuclear magnetic resonance (NMR) offers the advantages of being non-destructive and rapid in characterizing the occurrence state of shale oil, as well as providing information on pore structure and fluid distribution within shale. Due to the influence of sedimentary tectonics and diagenesis, there are significant differences in the occurrence spaces and formation mechanisms of various types of laminated shales. Compared to the massive reservoirs, laminated shale reservoirs have larger macropore volume, better connectivity, and more developed microfractures. Pure shales and organic-rich laminated shales mainly accumulate adsorbed oil within organic matter nanopores, exhibiting high hydrocarbon generation potential. In contrast, rigid laminated shales tend to accumulate free oil in intergranular mineral pores and fractures, demonstrating greater development potential.