Sequence stratigraphic architecture of the Quaternary Ledong Formation in the deep-water area of the Qiongdongnan Basin and its significance to gas hydrate exploration

The Quaternary Ledong Formation in the deep-water area of the Qiongdongnan Basin is a proven gas hydrate enriched zone in the northern South China Sea. The sequence stratigraphic architecture, depositional patterns, and their influences on the gas hydrate stability, migration and accumulation have become hot topics internationally and attracted great attention from the geological society of the region. Based on the theory of sequence stratigraphy, and combined with the high-resolution seismic surveys in the working area, a detailed sequence stratigraphic framework of the Quaternary Ledong Formation is established in this paper, which includes 4 second-order sequence boundaries and 13 third-order sequence boundaries. In addition, the maximum flooding surface (MFS), first flooding surface (FTS) and maximum regression surface (MRS) are also identified. The third level of sequence is further divided into system tracts. Since 1.8 Ma, the Red River provenance system has supplied sufficient sediments to the basin, which led to the progradation of deltaic deposits of Lower Ledong Formation, and a sequence of H-type was formed. In the upper part of Ledong Formation, however, the stratigraphic sequence is dominated by an accretionary architecture, and the sequence style was mainly of L-H type. Finally, this paper established the depositional model under the constraints of the sequence framework, and discussed the deltaic-gravity flow related deposits in the study area. This study has provided an important clue for prediction and exploration of natural gas hydrate in the future.