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Oil-prone source rocks occurring in lacustrine syn-rift successions have generated significant amounts of hydrocarbons in many Cenozoic basins in SE Asia. As most exploration wells are located on structural highs, the source rock successions are seldom drilled and their initial composition and generation potential are poorly known. The inverted Bach Long Vi Graben is located at the intersection of the NW–SE trending Song Hong Basin (Yinggehai Basin) and the NE–SW trending Beibuwan Basin in the Gulf of Tonkin, offshore northern Vietnam. The uppermost part of the inverted graben is exhumed and exposed on Bach Long Vi island. In order to investigate the amount and source rock quality of the syn-rift mudstones, the ENRECA-3 well was drilled on the island and cored some 500 m of the syn-rift succession. The well provided excellent cores with a recovery of 99%, dominated by lacustrine mudstones interbedded with various gravity flow deposits. Organic petrography shows that the mudstones are thermally immature and contain sapropelic Type I and mixed Types I and III kerogen. Source rock screening data from more than 300 samples demonstrate that the lacustrine source rocks have an average TOC content of 2.88 wt% and an average Hydrogen Index of 566 mg HC/g TOC. The average Hydrogen Index of the reactive kerogen was determined to be 769 mg HC/g TOC. The Source Potential Index (SPI) is 9 tons HC/m2 and the mudstones will, upon full maturation, generate black oil with a gas-liquid ratio not exceeding ∼1700 scf/stb. The mudstones are thus highly oil-prone. In addition, several tens of metres of source rock within the overlying succession are exposed on Bach Long Vi island and in the surrounding seafloor, and the well did not reach the base of the source rock succession. Although the net-source rock thickness of the ENRECA-3 well is estimated to be 233 m, the net thickness of the entire source rock succession will be greater. The present study is the first organic geochemical assessment of a thick lacustrine source rock section in the petroliferous NE Song Hong Basin, and the promising results may be applied not only to other parts of the basin but also to other Cenozoic basins with syn-rift successions containing significant source rock intervals.