Abstract: The First Member of Qingshankou Formation in the Changling depression of the Songliao basin is a major exploration target for shale oil, but the current level of research on its paleoenvironment is relatively low. This paper studies the paleoenvironment changes based on elemental geochemical methods and analyzes the impact of environmental factors on organic matter enrichment, which is of great practical significance for clarifying the main controlling factors of shale oil enrichment and selected-area evaluation. 73 shale samples from the P1-P3 layers of the Upper Cretaceous Qingshankou Formation in the Changling depression were tested for total organic carbon and major and trace elements. The organic matter abundance and major and trace element characteristics of the samples in the study area were discussed. The paleoclimate, paleoproductivity, paleowater depth, paleosalinity, and paleooxidation of the target layer were restored using elemental geochemical indicators, and the paleoenvironmental change characteristics of the First Member of Qingshankou Formation in the study area were reconstructed. The results indicate that the organic matter abundance of shale in the study area is relatively high, with an average TOC of 2.14%. From the Daqingzijing to the Yuzijing to the Tahucheng work areas, as well as vertically from the P1 to P3 layers, the lake water deepens, and the paleoproductivity and organic matter abundance increase. The overall paleoclimate in the study area are warm and humid, with a slightly warmer and wetter climate to the P3 layer. The paleowater is characterized by an oxygen deficient reduction environment of fresh to brackish water, with high salinity forming salinity stratification, making the reduction environment in the study area less susceptible to damage and conducive to the preservation of organic matter. The source of organic matter and paleosalinity are the key factors that lead to the differential enrichment of organic matter in the study area. The input of organic matter and paleosalinity in lakes increases from Daqingzijing to Yuzijing to Tahucheng work area, organic matter is relatively enriched. Vertically, the P1 to P3 layers also exhibit similar patterns of change, forming a set of high-quality black shale in the lower part of the First Member of Qingshankou Formation, which is a favorable interval for shale oil enrichment in the study area.

Key words: Songliao basin, Changling depression, First Member of Qingshankou Formation, mud shale, element geochemistry, paleoenvironment, shale oil