Sedimentary environment of lacustrine mud shale and its control over lithofacies development: A case study of Boxing Sag, Jiyang Depression, Bohai Bay Basin, China

The sedimentary environment and organic matter enrichment patterns of organic-rich shale are of great significance for the exploration and development of shale oil and gas resources. By using thin section identification, X-ray diffraction, argon ion polishing, scanning electron microscopy, and major and trace element geochemistry experiments, the mineralogy, geochemistry, and paleoenvironmental evolution characteristics of shale rocks in the lower of the third member and upper of the fourth member of the Shahejie Formation of the Boxing Depression were studied. The influence of sedimentary environments such as paleoclimate, paleosalinity, redox conditions, and paleowater depth on lithofacies was discussed.

The research results indicate that:① The shale in the study area is mainly composed of calcite, quartz, and clay minerals, accompanied by dolomite, feldspar, and pyrite. According to the three end element division scheme, the lithofacies of the study area can be divided into six types: rich organic layered calcareous mudstone, rich organic layered calcareous mudstone, containing organic layered calcareous mudstone, containing organic block calcareous mudstone, rich organic layered mudstone, and containing organic layered mudstone② The paleosalinity and paleoclimate determine the macro environment during the sedimentary period, while paleoproductivity and redox conditions control the generation and preservation of organic matter. By using XRF to test the vertical variation of characteristic element content ratio, it can be concluded that the vertical evolution of shale in Boxing Depression has obvious stages in terms of ancient climate, ancient salinity, ancient water depth, water redox properties, and terrestrial input degree. The upper of the forth member of the Shahejie Formation shows that the water body first deepens and then shallows, the climate changes from drought to humidity, salinity gradually decreases, and the terrestrial supply first increases and then decreases; The lower of the third member of the Shahejie Formation is that the water body deepens, the salinity decreases, and the climate becomes humid.③ The type of lithofacies is closely related to the sedimentary environment: the entire stratum  in the upper of the fourth member of the Shahejie Formation of the Boxing Depression experienced lake basin expansion and contraction, which led to the evolution of lithofacies towards a direction where the calcite content first increased, then decreased, and then increased again. The terrestrial detrital minerals first increased and then decreased, and the organic matter content first increased and then decreased, but the overall direction was lower; The lower third member of the Shahejie Formation has experienced rapid subsidence, and the climate and water depth have undergone multiple stages of changes, leading to a trend of increasing terrestrial detrital minerals, decreasing carbonate minerals, and slightly increasing clay minerals in the evolution of lithofacies. This indicates that environmental changes control the evolution of lithofacies.Therefore, paying attention to the evolution of shale sedimentary environment and exploring the response relationship between rock and sedimentary environment can provide new ideas for unconventional oil and gas exploration.

Keywords: lacustrine mud shale; Organic geochemistry; Elemental geochemistry; Paleoenvironmental characteristics; Boxing Depression