Study on shale cross-bedding, coarse grains, and imbricate structure with their sedimentary dynamics in the Qingshankou Formation of Gulong Sag, Songliao Basin

The fabric of mudstone and shale is an important scientific issue and practical problem in the exploration and development. It is largely determined by the sedimentary mode, sedimentary environment and sedimentary dynamics. The exploration of shale oil in the Qingshankou Formation of Gulong Sag has achieved a major breakthrough, the oil and gas resources amount to 15.1 billion tons. Previously, the Qingshankou Formation shale was always regarded as the sedimentary product of a low-energy, quiet-water environment in deep-lake and semi-deep-lake settings. Recently, high-energy sedimentary structures, relatively large particles and gravel debris discovered in the Qingshankou Formation shale of Gulong Sag provide new evidence for reinterpreting its depositional environment. The shale of the Qingshankou Formation in Gulong Sag was studied through detailed core observation, thin section analysis, high-definition video microscopy and physical simulation experiments. The results reveal the development of numerous high-energy sedimentary structures, including cross-bedding, coarse grains (with a maximum diameter of 12 mm), as well as mudstone and sandstone chips over 1 cm thick, and imbricate structures. These findings reveal that the Gulong Sag was not a low-energy, quiet-water environment during the deposition of the Qingshankou Formation, but was frequently influenced by currents with velocities of 0.15~0.35 m/s. We conducted a simulation experiment to study the minimum vertical suspension velocity (Vmf) of particles. Nine particle sizes were selected: 0.125 mm, 0.25 mm, 0.50 mm, 1 mm, 2 mm, 10 mm, 20 mm, 30 mm and 40 mm. The experiment measured a critical suspension velocity slightly greater than 0.447 m/s for 12 mm silt-grained gravel and approximately 1 m/s for 40 mm sandstone spheres, attesting to intense storm-driven bottom currents in the Gulong Sag during the Qingshankou Period. Through this experiment, it was also found that the minimum suspension velocities calculated by the previous formulas for these nine particle sizes were much lower than the experimental values. If the results calculated using the previous formulas are used to assess the storm dynamics in the Qingshankou Formation of Gulong Sag, it would seriously underestimate the intensity of the storm's effect. Therefore, our suspension simulation experiment has greatly advanced the research on storm dynamics. The shale of the Qingshankou Formation of Gulong Sag was frequently influenced by high-energy water flows dominated by storms. The organic-rich shale developed under such high-energy conditions mainly benefits from the reducing environment formed by severe oxygen deficiency in the atmosphere at that time, rather than the deep-water, quiet-water reducing environment formed by the sag itself.