image: Process I indicates that hydrocarbons undergo self-generation, self-storage and micromigration in felsic-rich laminated shale. Process II indicates that hydrocarbons undergo self-generation, self-storage and micromigration in carbonate-rich laminated shale. Process III shows that hydrocarbons undergo self-generation, self-storage and near-source migration and accumulation in combination with hybrid shale and siltstone. Process IV shows that hydrocarbons undergo self-generation, self-storage and near-source migration and accumulation in combination with hybrid shale and carbonate. Process V shows that hydrocarbons undergo self-generation, self-storage and near-source migration and accumulation in hybrid sedimentary rocks (organic-rich shales, sandy dolomitic, and dolomitic siltstone).
Credit: ©Science China Press
China’s vast nonmarine shale oil reserves hold immense potential to bolster national energy security, but unlocking these resources has long been hindered by extreme geological complexity. Unlike North America’s marine shale deposits, China’s formed in ancient lakes with wildly varying conditions, creating highly heterogeneous rock layers that make predicting oil-rich "sweet spots" challenging. Now, a breakthrough study reveals how shale oil accumulates in these intricate systems, providing a science-backed strategy for precise exploration.
A research team led by Prof. Quan-You LIU from Peking University has conducted a comprehensive geological study of shale oil in the Shahejie Formation, Bohai Bay Basin. Combining geochemical analysis, field-emission scanning electron microscopy (SEM), low-temperature nitrogen adsorption, mercury injection capillary pressure, and nuclear magnetic resonance (NMR), they discovered that calcareous shale and calcareous hybrid shale are the favorable lithofacies for shale oil accumulation. The favorable lithofacies exhibit not only higher free hydrocarbon content (S1 averages: 0.92 mg/g and 0.84 mg/g, respectively) and total organic carbon content (TOC averages: 2.35% and 2.60%, respectively), but more importantly, the study reveals that their superiority stems from the synergistic coupling of multiple key geological elements under specific paleosedimentary environments.
Specifically, the favorable lithofacies universally exhibit laminated structures characterized by high-frequency interbedding of organic-rich clay laminae and carbonate laminae. During diagenesis and hydrocarbon generation, such laminated configurations readily develop bedding-parallel fractures and microfractures. Concurrently, acidic fluids derived from hydrocarbon generation dissolve carbonates to form effective oil-storage pores, collectively promoting the development of multi-scale pore-fracture networks and micromigration-accumulation units. Pore-structure characterization confirms these lithofacies possess larger average pore diameters, higher proportions of macropores and microfractures critical for oil storage and mobility, lower N2-adsorption fractal dimensions, and enhanced 1D NMR T2 geometric means with stronger 2D free-oil signals. Paleosedimentary environments reconstruction indicates these favorable lithofacies were deposited mostly in specific paleosedimentary environments characterized by limited terrigenous clastic input, relatively arid paleoclimate, moderate paleosalinity, high paleoproductivity, and strongly reducing water column. This environment drove seasonal lamina development and organic matter enrichment, ultimately forging the "golden combination" of high TOC, laminated structures, and superior storage capacity. Shale oil primarily enriches through "self-generation and self-storage" within source laminae, followed by micromigration into carbonate laminae with optimal reservoir properties—a mechanism directly evidenced by SEM-observed trapped hydrocarbon infilling. Based on this understanding of lithofacies-paleoenvironment coupling, the team predicted and delineated the core shale oil "sweet spot" in the Shahejie Formation: the NW gentle slope zone (3,400–3,700 m burial depth). This prediction has been reliably validated by excellent pilot production from relevant wells, providing direct guidance for precise well placement.
Furthermore, based on their understanding of shale oil in the Raoyang Sag, the research team combined the chemical properties of sedimentary water bodies (fresh water, brackish water, saline water) with the microstructure of pores and fractures, favorable lithofacies, and paleosedimentary environments to systematically establish multi-scale, multi-factor accumulation models for nonmarine shale oil, categorizing five types (Fig. 1): (I) Felsic-rich laminated shale type (e.g., Qingshankou Formation, Songliao Basin); (II) Carbonate-rich laminated shale type (e.g., Shahejie Formation, Bohai Bay Basin); (III) Combined organic-rich hybrid shale and siltstone type (e.g., Yanchang Formation, Ordos Basin); (IV) Combined organic-rich hybrid shale and carbonate type (e.g., Shahejie Formation, Bohai Bay Basin); (V) Hybrid sedimentary rock type (e.g., Lucaogou Formation, Junggar Basin). This model establishes a unified conceptual framework for understanding the accumulation patterns of nonmarine shale oil, highlighting China's unique and diverse geological conditions. It is expected to provide a theoretical basis for the efficient, precise, and large-scale exploration and development of nonmarine shale oil.
Wei Y B, Liu Q Y, Lu S F, Zhao R X, Song Z J, Mou Y C. 2025. Accumulation mechanisms of nonmarine shale oil in China: A case study of the Shahejie Formation in Raoyang Sag, Bohai Bay Basin. Science China Earth Sciences, 55(7): 2268–2289, https://doi.org/10.1007/s11430-024-1585-1
Journal
Science China Earth Sciences