Paleosalinity and lake level fluctuations of the 3rd Member of Paleogene Shahejie Formation, Chezhen Sag, Bohai Bay Basin

Abstract

The Chezhen Sag, located in the north-western Jiyang Depression, is one of the most important oil-bearing sags in the Bohai Bay Basin. Due to the low degree of exploration in the sag, paleosalinity and sedimentary environment of the sag in the 3rd Member of Paleogene Shahejie Formation (Es3) is not clear. Recovering the paleosalinity and lake level fluctuations is helpful for understanding organic matter rich rocks sedimentation. Therefore, a detailed geochemical, mineralogical and paleontological analysis of the Es3 in the Chezhen Sag was conducted. Index like Sr/Ba ratios, B/Ga ratios, equivalent boron content and methods concluding Adams’ formula and Couch’s method were adopted to reveal the paleosalinity and lake level variations. The results indicate that the lower submember (Lower Es3) was deposited in a salt water with high salinity, accompanied by dry climate and transgression event. The middle submember (Middle Es3) and upper submember (Upper Es3) record a freshwater to brackish environment. The paleosalinity and paleoclimate changes are consistent with the global sea level variations. The type and content of sporopollen indicate a dry climate in Lower Es3, which further confirms the reliability of the reconstruction results of paleosalinity. Combined with the paleoclimate and previous marine paleontological evidence, we proposed that the high salinity period is associated with a high lake level and a large-scale transgression event in Lower Es3. According to salinities and corresponding Lake depths, we established a sedimentary environment variation model of the Es3 Member in Chezhen Sag.

Keywords

Chezhen Sag / paleosalinity / paleoclimate / boron lake level

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Acknowledgments

This study was supported by the Sinopec Shengli Oilfield Cooperation Project named “Reunderstanding the sedimentary system of the third to the fourth member of the Shahejie Formation in the Chezhen Depression and the distribution of hidden traps” and China Postdoctoral Science Foundation (No. 2021M700537). We thank the Sinopec Shengli Oilfield Geological Research Institute for the drilling core samples and analytical tests. We are very grateful to the reviewers for their constructive comments and helpful suggestions, which have greatly improved the quality of this manuscript.

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