Coal-Derived Oil in the Upper Paleozoic Coals of the Ordos Basin: Occurrence, Origin, and Exploration Implications

With the recent breakthroughs in deep coalbed methane exploration in the Ordos Basin, Upper Paleozoic coals are widely recognized as strongly gas-prone, whereas their oil-generating capacity has long been underestimated. However, abundant light oil has been repeatedly observed in flowback fluids from Benxi Formation coalbed methane wells in the Linxing and adjacent areas, indicating these coals can act as both effective source rocks and oil reservoirs. Integrating coal petrography, organic geochemistry, and fluid inclusion microthermometry, this study re-evaluates the condensate potential of these marine-continental transitional coals. Crucially, we demonstrate that the Benxi coals host a high-maturity, strongly self-sourced primary condensate-wet-gas system, fundamentally challenging the traditional view that they contain only severely cracked residual oil. Liquid hydrocarbons occur primarily in two modes: free oil within structural microfractures (providing the dominant migration pathways) and adsorbed oil immobilized by capillary blocking within kaolinite aggregates. More importantly, we reveal a distinctive oil-generation mechanism unique to marine-continental transitional facies. Unlike strictly continental coals dominated by higher plants, the principal oil-prone parent materials here are significantly enhanced by marine transgressions. The incorporation of transgression-derived aquatic algal-planktonic organic matter, alongside terrestrial hydrogen-rich liptinite, provides highly efficient, aliphatic-rich, and high-H/C precursors. This synergistic dual-source mechanism significantly boosts condensate generation, offering a new perspective on the hydrocarbon systems of transitional coals and broadening the exploration targets for coal-derived liquids.