Characterization of lacustrine basin paleosalinity and its control on calcareous lithofacies distribution: a case study in the eastern Dongying Sag, China

The lower and middle parts of the 3rd member of the Shahejie Formation (Es3x-Es3z) in the eastern Dongying Sag, southeast of the Bohai Bay Basin, China, are comprised of the turbidite sandstones and typical lacustrine source rocks that show good resource potential for lithologic and shale oil reservoirs. However, their significant calcareous-rich characteristics affect reservoir quality and geophysical prediction. A previous study of aqueous media in modern lakes showed that water salinity has a significant impact on the distribution of chemical sedimentary rocks. Therefore, there is a need to investigate the distribution of calcareous composition through paleosalinity restoration in Es3x-Es3z in the eastern Dongying Sag. In this study, the quantitative relationships between different methods were explored, and a comprehensive method of paleosalinity restoration was established for the study area. The results show that Es3x-Es3z is formed in a brackish-salt environment, and the paleosalinity in Es3x is higher than that in Es3z because the lacustrine type formed by climate and tectonic actions. The plane distribution of paleosalinity has two different variation trends, which can be closely related to paleogeomorphology. The paleosalinity distribution from the basin margin to sag shows a variable trend of low-high-low among different paleogeomorphology units, while that from the Chenjiazhuang Uplift in the north to sag shows a gradually decreasing trend due to the provenance of carbonate rocks. Based on core and thin section observations with mineral composition analysis, three types of calcareous lithofacies were identified. The plane distribution characteristics and vertical stacking patterns of these calcareous lithofacies were found to be significantly influenced by paleosalinity and detrital input. This study provides an insight into the relationship between paleosalinity and calcareous composition and provides a geological method with which to explore the distribution of calcareous composition in the lacustrine basin.