Environmental reconstruction for the paleo-lake of ZhuⅠdepression and the depositional model for high-quality source rocks

High-quality source rocks are well occurred in the Paleogene lacustrine deposits of the ZhuⅠdepression. Element geochemistry and isotope analysis for mudstone cuttings from the Wenchang Formation and Enping Formation is carried out by this paper in order to optimize the study of element content and elements ratio which are sensitive to paleoenvironment reconstruction. Paleoenvironmental parameters for the source rocks in ZhuⅠdepression are systematically analyzed, which include paleosalinity, paleowater depth, redox conditions, paleoclimate, degree of environmental closure and so on. Upon the basis, we revealed the quantitative relationship between the organic matter content and the ratio of environmental sensitive elements, and established the depositional model for the high-quality source rocks in the Wenchang Formation. The results show that under the warm and slightly dry climate conditions during the Wenchang period, the ZhuⅠdepression was a medium to deep and open to semiclosed lake dominated by fresh water and brackish water under a reductive or strongly reductive environment, which was conducive to the preservation of organic matter and the formation of high-quality source rocks. The quality of source rocks is positively related to salinity, water depth and reduction conditions. In conclusion, in a warm and slightly dry lake with slightly salty deep water as the ZhuⅠdepression, the strong reduction and open-semiclosed environment jointly control the development of high-quality source rocks in ZhuⅠdepression.