Thermal Evolution and Gas Occurrence in the No. 5 Coal Seam Revealed by Three‐Dimensional Basin Modelling, Daning–Jixian Block, Ordos Basin

ABSTRACT

Understanding the thermal evolution and gas‐occurrence evolution of deep coalbed methane (CBM) systems is essential for assessing gas generation and preservation potential in overmature basins. This study investigates the No. 5 coal seam (1000–2600 m) of the Lower Permian Shanxi Formation in the Daning–Jixian block, eastern Ordos Basin, through an integrated approach combining geological characterisation and three‐dimensional (3D) basin modelling. The No. 5 coal is a high‐rank, Type III gas‐prone source rock ( R _o  = 1.56%–2.78%, average vitrinite content > 80%), with high fixed carbon and low volatile matter, indicating strong thermogenic potential. Basin modelling reconstructs the burial and thermal histories, revealing five major evolutionary stages, with maximum burial and peak maturity attained during the Early–Late Cretaceous. Hydrocarbon generation modelling identifies two primary gas‐generation phases: a moderate Triassic–Jurassic stage and an intense Cretaceous peak, yielding an average generation potential of 14.36 × 10 ⁸  m ³ /km ² . The evolution of gas occurrence shows a transition from early adsorption‐dominated storage to mixed adsorbed–free gas accumulation driven by pressure increase during the Early Cretaceous deep‐burial stage. Presently, deep seams contain 21–32.7 m ³ /t of gas, over twice that of shallow seams. The overall hydrocarbon expulsion history follows a ‘slow–accelerated–peak–stable’ trend, consistent with tectono‐thermal evolution.