Coal-Measure Gas Charging Stages and Accumulation Controls in the Upper Paleozoic of Yan’an Gas Field, Ordos Basin

Deep coal-measured gas has emerged as a pivotal unconventional resource, with the Upper Paleozoic in the Ordos Basin, representing a key exploration target. Recent discoveries in the Yan’an gas field, southeastern Ordos Basin, have challenged traditional models, yet a comprehensive understanding of its gas accumulation mechanisms remains limited. Our study employs geochemical analysis (TOC, Rock-Eval, stable isotopes), fluid inclusion microthermometry, and reservoir characterization, and the results delineate that (1) two discrete hydrocarbon charging episodes: an initial phase during the Early-Middle Jurassic (J1–J2) and a principal, likely tectonically influenced phase in the Late Jurassic-Early Cretaceous (J3–K1). The efficient gas accumulation in the tight sandstone reservoirs is controlled by a critical pore-throat lower limit. (2) The gas, characterized by high dryness and widespread carbon isotope reversal (δ13C1 > δ13C2), is confirmed to be a postmature, coal-derived product primarily sourced from the Shanxi and Benxi Formation source rocks. (3) An accumulation model is proposed: intrasource accumulation within the Shanxi and Benxi Formations and near-source accumulation in the overlying Shihezi Formation (“lower-generating/upper-storing”). (4) Crucially, the presence of an isolated compaction system formed by cap-reservoir interaction is identified as a key factor for effective preservation. These findings systematically clarify the genetic model and enrichment mechanisms, providing a robust geological foundation for optimizing exploration strategies and predicting favorable zones in the Yan’an gas field and analogous basins.