Coal Accumulation in Marine to Continental Transitional Sequence During Late Carboniferous—Early Permian in the Southeastern Ordos Basin, China

ABSTRACT Thick coal‐bearing strata developed in different environments in the Upper Palaeozoic Ordos Basin, providing high‐quality source rocks and reservoirs for coalbed methane fields. However, accurate prediction of coal occurrence and geometry requires a better understanding of coal seam characteristics within a marine‐to‐continental transitional sequence framework. Sequence stratigraphy, coal‐accumulating environment and coal distribution in SE Ordos Basin are analysed by integrating analysis of cores, well logs and seismic interpretation. Four types of depositional systems are developed in four fourth‐order sequences: barrier‐lagoon of SQ1, carbonate platform of SQ2, meandering river delta of SQ3 and braided river delta of SQ4. A complete transition from nearshore to terrestrial sequence occurred during the late Carboniferous to early Permian, shifting from high accommodation to low accommodation. SQ1 and SQ3 are dominant coal‐accumulating sequences, and the lagoon and distributary bay provide favourable environments for coal formation, forming regionally thick and thick‐thin continuous coal, respectively. SQ2 and SQ4 contain no to weak coal accumulation, with locally thin coal and carbonaceous mudstone. The differences in the accommodation of each sequence and the relative sea‐level changes within a sequence individually exert primary and direct control over coal accumulation. The thickest and most continuous coals were developed in the late highstand of the high accommodation sequence and the early highstand of the low accommodation sequence. The results of this study are important for predicting favourable areas for coal distribution in paralic settings worldwide and could be further applied to the exploration and development of unconventional resources in the Ordos Basin.