High-frequency sequence patterns in lacustrine basins: Insights from delta-hyperpycnal flow systems in the Middle Jurassic shale interval, Sichuan Basin, China

Abstract

Unlike marine sedimentary basins, lacustrine sedimentary basins are more sensitive to small-scale climatic changes (fourth-order and higher sequences: spanning thousands to tens of thousands of years), and exhibit characteristics such as rapid sedimentary facies transitions and strong heterogeneity. Frequent changes in the sedimentary environment significantly influence the development of sedimentary systems and the formation of sequence structures. These factors present new challenges for understanding the sequence architecture and sedimentary evolution in deep-water zones in lacustrine depression basins. Taking the Middle Jurassic Lianggaoshan Formation in the eastern Sichuan Basin, China, as an example, in this paper, we integrate data from cores, well logs, element geochemistry, and 3D seismic surveys. By applying the transgression–regression cycle theory and high-frequency sequence stratigraphy of lacustrine depression basins, we analyze the genetic mechanisms and distribution patterns of different types of sandstone interlayers within the shale interval and investigate the effect of high-frequency climatic changes on sedimentary processes in lacustrine basins. A high-frequency sequence pattern for lacustrine depression basins is established. The results indicate that within the third-order sequence of the Lianggaoshan Formation, during the shale-deposition interval (maximum water abundance period), the lacustrine depression basins were controlled by the 405 ka (fourth-order sequence) climatic cycle, which still demonstrates basin-scale expansion and contraction. In the shale interval, climate-driven lake transgression-regression cycles within fourth-order sequences promoted frequent interbedding of high sedimentation rate, coarse-grained deposits (e.g., deltaic deposits and hyperpycnal flow deposits) and low sedimentation rate, fine-grained deposits (e.g., suspension settling: shallow-water mudstone and deep-water shale). This reveals a high-frequency sequence stratigraphy pattern that demonstrates the coupling mechanism between the sedimentary processes and climatic forcing in lacustrine depression basins. Within the fourth-order sequences, the early transgressive systems tract and late regressive systems tract are characterized by shallow water and arid conditions, which resulted in the development of organic-poor mudstones, while deltaic sandstones prograde into the center of the basin. In contrast, the late transgressive systems tract and early regressive systems tract are marked by deep water and humid conditions, which led to the formation of organic-rich shales, accompanied by hyperpycnal flow sandstones entering the center of the basin. This resulted in the formation of multiple sets of strongly heterogeneous interbedded organic-poor mudstone and organic-rich shale assemblages. The findings establish a high-frequency sequence pattern for these basins, elucidate their internal depositional and facies evolution processes, and provide scientific guidance for predicting sweet spot intervals in lacustrine shale oil and gas reservoirs.