<![CDATA[ABSTRACT The Tanjung Formation in the Barito Basin, South Kalimantan, is a proven hydrocarbon-bearing interval; however, its source rock potential and controlling factors remain uncertain in the Martapura area. This study evaluates the influence of depositional environment on source rock quality, hydrocarbon generation potential, and thermal maturity within fluvio-estuarine sediments of the Lower Tanjung Formation. An integrated approach combining field observations, facies analysis, palynology, and geochemical data (TOC, Rock-Eval pyrolysis, and vitrinite reflectance) was applied to 49 samples from 11 locations. The formation is characterized by a transgressive, upward-fining succession of shale, mudstone, coal, and minor sandstone, deposited in fluvio-estuarine settings including floodplains, swamps, interdistributary bays, and oxbow lakes during the Late Eocene. Geochemical results show highly variable organic richness (TOC <0.5% to >70%) and hydrogen index (HI <100 to >500 mg HC/g TOC), indicating heterogeneous source rock quality in the Lower Tanjung Formation. Coal and coaly shale facies exhibit excellent oil-prone potential, whereas most mudstone and claystone facies are poor to moderate source rocks. Vitrinite reflectance values (Ro 0.38–0.46%) indicate immature thermal conditions. Our findings suggest that the coals in the study area constitute oil-prone source rocks characterized by very high TOC and HI values, and are interpreted to have been deposited in freshwater lacustrine to reducing floodplain environments associated with meandering river systems and oxbow lake development. Despite the presence of high-quality source facies, their limited thickness and lateral extent significantly restrict overall hydrocarbon generation potential.]]>
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