Paleosalinity Reconstruction for the Paleocene Sequence of Lishui Sag in the East China Sea Shelf Basin

The Lishui Sag on the southwest margin of the East China Sea Shelf Basin (ECSSB) is a relatively new oil and gas exploration area, with the Paleocene being one of the most potential oil-bearing intervals in the sag. In this study, combined with the principles of element geochemistry and mineralogy, the salinity characteristics of the paleowater body are reconstructed and discussed using the Sr/Ba ratio, B/Ga ratio, equivalent boron content and Couch's method. The results show that (1) the highest salinity indicated by Sr/Ba and B/Ga ratios appears in the Upper Lingfeng Formation; (2) the equivalent boron content ranges from 62 to 254 ppm, with an average of 122 ppm, showing the existence of fresh water and brackish deposition settings; and (3) the paleosalinity calculated quantitatively by Couch's method is between 4.9 and 13.1 parts per thousand. To ensure the reliability of the study, paleosalinity results from the different methods are cross-plotted, and the effects of provenance and diagenesis on the paleosalinity analysis are negligible. The comparison results show that Couch's method, equivalent boron content and B/Ga ratio are more reliable among all paleosalimeters. The identified paleosalinity type of paleowater body is mainly brackish water, which is exactly mesohaline water. The paleosalinity of the Paleocene stratigraphic interval in Lishui Sag is consistent with the sedimentary facies distribution. Our comprehensive comparison provides a basis for inferring the source of fresh water, the genesis of transitional water bodies and the evolution of sedimentary facies.