Astronomical control on the development of marine-continental transitional organic-rich shales during the Late Carboniferous-Early Permian at the Southeast Margin of the Ordos Basin

[Objective] The Late Carboniferous–Early Permian marine-continental transitional facies shale gas in the southeastern margin of the Ordos Basin boasts great exploration and development potential, yet the unclear mechanism of organic matter enrichment has severely restricted the sweet spot evaluation of such shale gas. [Method] In this study, a paleoclimatic proxy, the PL^Index, was constructed, and cyclostratigraphic analysis was performed on Well DJ70, a fully cored well in the Daji Block of the basin’s southeastern margin. [Results] The results show that: (1) Stable long eccentricity cycle signals are preserved in the Benxi and Shanxi Formations. Combined with the results of astronomical tuning and high-precision age tie points, an absolute astronomical time scale for the Benxi and Shanxi Formations was established, and 15.5 long eccentricity cycles were identified, corresponding to the division of 15.5 fourth-order sequences. Taking the troughs of the long eccentricity filtering curve as the fourth-order sequence boundaries, a high-resolution fourth-order sequence stratigraphic framework was built. (2) The enrichment of organic matter in the marine-continental transitional facies shales in the southeastern margin of the Ordos Basin is obviously controlled by astronomical cycles. The low-value periods of long eccentricity correspond to arid climates, during which volcanic ash from the Inner Mongolia Uplift could be transported over long distances and deposited by airfall in the Ordos Basin, significantly enhancing the paleoproductivity of the water body. Meanwhile, the sea level was relatively stable in these periods with a suitable water depth, and sediments could be deposited in the minimum value interval of dissolved oxygen saturation within the thermocline, which provided a favorable environment for organic matter preservation and thus resulted in high Total Organic Carbon (TOC) contents in the shales. In contrast, the high-value periods of long eccentricity correspond to warm and humid climates and the transgressive stages of fourth-order sequences, which are unfavorable for the long-distance aerial transportation of volcanic ash and the preservation of organic matter, leading to low TOC contents in the shales. [Conclusion] The differential enrichment of organic matter in the marine-continental transitional facies shales in the southeastern margin of the Ordos Basin is essentially attributed to the fact that astronomical cycles control paleoclimates, which, in conjunction with major geological events, jointly promote the organic matter enrichment process of shales with high productivity and excellent preservation conditions. This model provides a theoretical basis for the prediction of organic-rich shales in the Ordos Basin and North China Basin.