Paleoenvironmental reconstruction of lacustrine shale under high-frequency cyclonic constraints --an example from the upper subsection of the fourth section of the Shahejie Formation (Es4S) in the Dongying Depression

The Es4S section of the Paleocene in the Dongying Depression of the Bohai Bay Basin represents semi-saline lake-phase deposition, characterized by the development of thick shale layers spanning hundreds of meters. Due to paleoenvironmental fluctuations, the shale facies exhibit frequent and complex changes within the high-frequency lake-phase gyres, leading to significant heterogeneity in the shale reservoirs. This complexity poses challenges to the systematic exploration and development of shale oil resources. To investigate the relationships between shale lithological assemblages, high-frequency cycles (HF cycles), and paleoenvironmental changes, core samples from coring wells in the study area were analyzed using various methods, including core observation, XRF and GR scanning, thin-section analysis, XRD of whole-rock samples, and organic carbon and elemental geochemistry studies. The following insights were obtained: (i) Within the six-stage high-frequency sedimentary cycles, shale primarily deposits carbonate minerals during arid phases and clay minerals during wet phases. Grain layer thickness increases under conditions of extreme drought or extreme wetness. (ii) Shales showed an increased deposition of feldspathic minerals during periods of strong source supply, whereas carbonate mineral deposition was dominant during weaker source supply conditions. (iii) A palaeoclimate indicator and a palaeo-source indicator were developed based on mineral content ratios. Using these indicators, the Es4S section was classified into four four-level sedimentary cycles. The palaeoclimate evolution exhibited a progression from arid to semi-arid, semi-arid to semi-humid, and back to semi-arid conditions, accompanied by a gradual weakening of material source supply. This study introduces a novel approach to reconstructing palaeoenvironments in lacustrine shales, offering valuable insights for guiding shale facies development.