Abstract: Water depth plays a decisive role in the redox conditions of a basin and plays an important role in the enrichment and preservation of organic matter in sedimentary basins. Previous studies mainly relied on paleontology, element geochemistry,
stratigraphic thickness, and other methods for qualitative and semi-quantitative palaeodepth restoration. However, these methods have a high dependence on original samples and low prediction accuracy and are not suitable for Palaeogene Lake basin palaeodepth restoration in offshore basins. In this study, the depth of paleowater is mainly restored based on the thickness of the delta for deposits in seismic profiles. By using this method, the maximum palaeobathymetric during the sedimentary period of the third Member of the Shahejie Formation of the Eocene in Qinnan west subsag of Bohai Bay Basin is 595 m, and the maximum paleowater depth during the sedimentary period of Wenchang Formation in the southwest of Huizhou Sag of Pearl River Mouth Basin is 526 m. Combined with previous research results, it is found that the rate of sag-controlling fault activity is strong in the third Member of the Shahejie Formation period of Qinnan Sag and the Wenchang Formation period of Huizhou Sag, and they are during the strong rifting of the basin, which controls the formation of deep water lake basins. During this period, many Sags in Bohai Bay Basin and Pearl River Mouth Basin are in deep-water-ultra-deepwater environments, so it is inferred that the deep water environment is the result of strong fault activity. Then the redox conditions of the lake basin are controlled, and more organic matter is preserved in the lake basin to form high-quality source rocks. 

Key words: palaeobathymetric, delta foredeposit, Qinnan Sag, Huizhou Sag, Eocene