Eocene Statistics of the Semi-Deep-Deep Lake-Level Evolution of the Jijuntun Formation in the Fushun Basin, Northeast China.

The fluctuation of lake levels in semi-deep and deep lake environments has long been a central topic in the study of ancient lake evolution. This phenomenon has a significant impact on the enrichment of organic matter and the overall ecosystem. The study of lake-level changes in deep lake environments is hindered by the scarcity of records in continental strata. To address this issue, we conducted a study on the Eocene Jijuntun Formation in Fushun Basin, specifically focusing on the LFD-1 well. Our study involved finely sampling the extremely thick oil shale (about 80 m), which was deposited in the semi-deep to deep lake environment of the Jijuntun Formation. The TOC was predicted by multiple methods, and the lake level study was restored by combining logging INPEFA and Dynamic noise after orbital tuning (DYNOT) techniques. The oil shale of the target layer is type I kerogen, and the source of organic matter is basically the same. The γ ray (GR), resistivity (RT), acoustic (AC), and density (DEN) logging curves are in the normal distribution, indicating that the logging data are better. The accuracy of TOC simulated by improved Δlog , SVR, and XGBoost models is affected by the number of sample sets. The improved Δlog  model is most affected by the change of sample size, followed by the SVR model, and the XGBoost model is the most stable. In addition, compared with the prediction accuracy of TOC by improved Δlog , SVR, and XGBoost models, it is shown that the improved Δlog  method has limitations in the prediction of TOC in oil shale. The SVR model is more suitable for the prediction of oil shale resources with small sample size, and the XGBoost model is applicable when the sample size is relatively large. According to the DYNOT analysis of logging INPEFA and TOC, the lake level changes frequently during the deposition of ultra-thick oil shale, and the lake level has experienced five stages of rising-stabilizing-frequent fluctuation-stabilizing- decreasing. The research results provide a theoretical basis for revealing the plane change of stable deep lake lakes and provide a basis for the study of lake levels in faulted lake basins in Paleogene Northeast Asia.