Evaluation of Cenozoic source rock potential from drill cuttings in the northern Jeju Basin, offshore southern Korea

• Minor coal fractions markedly enhance hydrocarbon potential. • Coal-bearing source rocks show substantial generation of both oil and gas. • Late Eocene source rocks contribute more to the petroleum system than Oligocene. • Fragment proportions control bulk source rock quality. • Provides a robust framework for source rock assessment in drill-cutting-only basins. Uncertainties associated with mixed drill cuttings have limited quantitative assessments of hydrocarbon generation and the contribution of coal-bearing source rocks in the Jeju Basin. This study evaluated the source rock potential of the Jeju Basin by separating mudstone and coal fragments from the drill cuttings of the O well and integrating their proportions with the organic matter characteristics. Three mudstone-dominated source rocks (OM1, OM2, and OM3) and two coal-bearing source rocks (OC1 and OC2) were identified within the late Eocene and Oligocene sequences. Although coal fragments comprised less than 10% of the coal-bearing source rocks, their hydrocarbon generation potential (fair to good) is comparable to that of the mudstone-dominated source rocks. OM2 and OM3, characterized by high hydrogen index (HI) and the preservation of liptinite, are interpreted as oil-prone source rocks, whereas OM1 is oil/gas-prone. The coal-bearing source rocks, dominated by vitrinite (>90%) and relatively low HI values, were initially considered gas-prone. However, re-evaluated maximum HI values and abundant detrovitrinite indicate the potential for both oil and gas generation. The lower part of the late Eocene sequence, where OM1 and OC1 co-occur, exhibits the highest hydrocarbon generation potential. Considering both generation potential and source rock thickness, the late Eocene intervals (OM1 + OC1 and OM2) are expected to yield more hydrocarbons than the Oligocene intervals (OM3 and OC2). These results clarify the contribution of coal-bearing source rocks and refine understanding of the Jeju Basin petroleum system.