Oil and condensate types in Cenozoic basins offshore Vietnam: Composition and derivation

The Vietnamese shelf contains several petroleum-producing Cenozoic rift basins, including the Song Hong, Cuu Long, Nam Con Son, and Malay-Cho Thu basins. This study describes geochemical data of oils and condensates from these basins, including new data from the Song Hong and Malay-Tho Chu basins, and demonstrates the variability in composition and source of the petroleum. The results contribute to a better understanding of oil types and petroleum-generating source rocks offshore Vietnam. All investigated samples contain angiosperm biomarkers (e.g. oleanane, bicadinane) indicating sourcing from post-Late Cretaceous source rocks. The Cuu Long Basin is the major oil-producing basin in Vietnam and the highly paraffinic oils have a lacustrine source (high TPP ratios, low C₃₁R/C₃₀ hopane ratios, Pr/Ph ratios of 2-4) and were generated from rocks within the early- to mid-oil window. Lacustrine oils are also known from wells at the northern margin of the Song Hong Basin, and analyses of offshore condensate discoveries show they were generated from relatively mature lacustrine source rocks. However, oil with a conspicuous coaly biomarker signature occurs in the Nam Con Son Basin. Further, a number of the condensates from the Song Hong Basin contain high proportions of higher land plant biomarkers suggesting significant contributions from coaly source rocks. Recently analysed oils from the Malay-Cho Thu Basin (northern margin of the Malay Basin) have likewise a geochemical composition strongly suggesting a coaly source (high wax, Pr/Ph ratios up to >10, dominance of C₂₉ steranes). Hence, new data together with previous data emphasize that coaly source rocks are important petroleum producers on the Vietnamese shelf. Coal-bearing strata in the syn- and post-rift successions in the Cenozoic basins should thus be considered as important additional source rocks offshore Vietnam.