Sedimentary Evolution and Sand Transport Mechanisms of the Paleogene Kongdian Formation in the Cangdong Sag, Bohai Bay Basin

【Objective】In recent years, significant breakthroughs have been achieved in the exploration of shale oil and gas as well as conventional oil and gas within the Paleogene Kongdian Formation of the Cangdong Sag, Bohai Bay Basin, demonstrating promising exploration prospects. However, due to the limited number of deep drilling and the relatively low level of exploration, there is currently no unified understanding regarding the sedimentary evolution and sand transport mechanisms of the Kongdian Formation in this region.【Methods】Based on the analysis of the prototype basin in the Cangdong Sag, this study adopts a research approach combining "point-line-surface" and systematically investigates sequence stratigraphy, sedimentary evolution, and sand transport mechanisms by integrating core, well logging and seismic data.【Results】The results indicate that the Kongdian Formation can be subdivided into three third-order sequences: the Member 3, Member 2, and Member 1 of the Kongdian Formation. Multiple types of sedimentary systems were developed, including alluvial fans, alluvial plains, fan deltas, braided river deltas, offshore subaqueous fans, and lakes, exhibiting an overall "lacustrine transgression-lacustrine regression" sedimentary cycle. During the deposition of Member 3, the lake basin was limited in scope, primarily featuring alluvial fans and alluvial plains. During the deposition of Member 2, the lake basin expanded to form a unified lake basin with a semi-deep lake environment, developing a delta-offshore subaqueous fan sedimentary system. During the deposition of Member 1, the lake basin shrank, provenance supply increased, and alluvial fans and delta systems predominated.【Conclusions】During the sedimentary period of the Kongdian Formation, the Cangdong Sag exhibited a tectonic geomorphological pattern of "two sags sandwiching one uplift." Belt-like sedimentary fans developed in the footwalls of the western Cangdong Fault and the eastern Xuxi Fault. The segmented activity of faults and their associated multiple transfer zones controlled the development of large-scale sedimentary fans, forming a model of "transfer zones controlling gullies and nose uplifts controlling sand transport." In summary, the delta fans in the footwall of the sag-controlling faults and the shale strata in the deep depression area of Member 2 possess immense hydrocarbon resource potential, serving as key directions for future exploration.