The Impact of Milankovitch Cycles on Coal Accumulation and Its Implications for Carbon Cycling and Carbon Sequestration: A Case Study of the Pinghu Formation, Area A, Xihu Depression

The Eocene Pinghu Formation in the Xihu Depression, East China Sea Shelf Basin, is a key coal-bearing unit for offshore China’s petroleum exploration. However, the mechanisms of coal accumulation controlled by astronomical cycles and the stacking patterns of coal seams remain underexplored. Recent studies using wavelet analysis have highlighted the need for further investigation into the role of Milankovitch cycles in coal formation. This study uses natural gamma-ray logging data from Well K and applies cyclic stratigraphy to investigate how astronomical orbital cycles control coal seam development, identifying the link between cyclic stratigraphy and coal accumulation, and the distribution patterns of coal seams across different cycle levels. The top of the Pinghu Formation was used as the astronomical anchor, and tuning was conducted from top to base following a “cycle identification–anchor tying–astronomical tuning” workflow. The resulting astronomical timescale indicates a depositional duration of 8.17 Ma. COCO/eCOCO analyses with 5000 Monte Carlo simulations (sedimentation-rate range: 7–11 cm/kyr; step: 0.1 cm/kyr) yield a mean sedimentation rate of 9 cm/kyr. Coal accumulation is influenced by Milankovitch cycles. High eccentricity periods correspond to warmer climates that promote coal development, while low eccentricity phases synchronize with optimal climatic conditions for coal formation. Based on these findings, this study proposes a coal seam development model for the Pinghu Formation in Area A of the Xihu Depression, offering insights for cyclic stratigraphy and coal accumulation research in similar basins and supporting sustainable development of coal-bearing strata in the East China Sea Basin.