Dynamic Accumulation and Differential Enrichment Mechanism of Deep Coalbed Methane in the No. 8 Coal Seam of the Benxi Formation, Daji Block, Eastern Ordos Basin

【Objective】 The No. 8 coal seam of the Benxi Formation in the Daji Block, eastern Ordos Basin, exhibits marked differences in burial depth and present-day gas-bearing characteristics among different burial zones, which constrains the refined deployment of deep coalbed methane exploration. This study aims to clarify the differences in gas generation and preservation processes under different burial–preservation settings and to analyze the effects of historical gas generation and late-stage preservation adjustment on present-day gas-bearing differentiation. 【Methods】 Proximate analysis, maceral composition, organic geochemistry, reservoir physical properties, isothermal adsorption, and field gas-bearing data were integrated. Representative wells were selected from the deep-burial–deep-preservation, deep-burial–shallow-preservation, and shallow-burial–shallow-preservation zones for one-dimensional basin modeling, combined with two-dimensional section modeling, to reconstruct the burial, thermal, gas-generation, and pressure evolution of the No. 8 coal seam. 【Results】 Vitrinite is the dominant maceral in all three burial zones (52.30–92.60%). Their average porosities are 3.26%, 3.68%, and 4.06%, respectively, whereas the average gas contents are 25.31, 12.55, and 9.42 m³/t, and the average gas saturations are 96.94%, 75.60%, and 62.90%, respectively, showing an opposite trend between porosity and gas-bearing characteristics. The accumulation evolution of the No. 8 coal seam experienced five stages: initial burial during the Late Carboniferous–Permian, continuous subsidence and the first episode of gas generation during the Early–Middle Triassic, reduced subsidence and episodic uplift during the Late Triassic–Jurassic, renewed deep burial and major gas generation during the Early Cretaceous, and uplift-related pressure release and preservation adjustment since the Late Cretaceous. During the Early Cretaceous, the historical maximum burial depths of representative wells in the three zones were approximately 4, 300, 3, 842, and 3, 400 m, respectively, with simulated maximum vitrinite reflectance (R_{o, max}) decreasing successively. The maximum organic matter transformation ratio exceeded 90%, resulting in a relatively strong historical gas-generation basis in the deep-burial–deep-preservation and deep-burial–shallow-preservation zones. Since the Late Cretaceous, burial depths in the three zones have been reduced by approximately 1, 800–2, 100, 2, 000–2, 200, and 2, 100–2, 400 m, respectively. Uplift-related pressure release generally increased, further adjusting the preservation conditions of previously generated gas. Present-day gas-bearing characteristics are jointly controlled by differences in the gas-generation basis established during the major Early Cretaceous gas-generation stage and differences in gas preservation caused by subsequent differential uplift. Accordingly, three accumulation modes are recognized: “strong gas generation–favorable preservation, ” “strong gas generation–late-stage adjustment, ” and “relatively weak gas generation–relatively poor preservation.” 【Conclusion】 Favorable-area evaluation of deep coalbed methane should consider not only present-day burial depth, but also historical maximum burial depth, thermal maturity, late-stage uplift, and pressure-preservation conditions. These findings provide a theoretical basis for favorable-area evaluation of deep coalbed methane in the eastern Ordos Basin.