Biomarkers of Oligocene lacustrine source rocks, Beibuwan-Song Hong basin junction, offshore northern Vietnam

Highly oil prone source rocks containing sapropelic Type I and mixed Types I and III kerogen are found in 233 m of the 500 m deep, fully cored ENRECA-3 well drilled on Bach Long Vi Island, located at the junction between the Beibuwan and the Song Hong basins offshore northern Vietnam. The cored succession is made up entirely by Oligocene, deep lacustrine deposits. The origin of the organic matter (OM) in 62 samples is evaluated on the basis of gas chromatography-mass spectrometry (GC-MS and GC-MS/MS). OM in 55 samples is sourced primarily from lacustrine organisms and contains abundant 4-methylsteranes, tetracyclic polyprenoids (TPP) and lycopane, and no markers for marine organic matter. The proportion of triterpanes derived from angiosperms is low in most saturated fractions and only traces of gymnosperm diterpanes are detected. In contrast, all aromatic fractions have a high proportion of compounds having an origin from both gymnosperms and angiosperms. Very high concentrations of hopanoids in both fractions suggest significant bacterial alteration of the original OM. Most samples have a high concentration of rare C₃₁ diaromatic 8,14-secohopanes with a fluorene moiety. Low pristane/phytane (Pr/Ph) ratios indicate that anoxic/suboxic bottom water conditions existed during OM accumulation. The presence of β-carotane, gammacerane and traces of arylisoprenoids could indicate a stratified water column. OM in a few samples with lower than average hydrogen index (HI) is mainly of terrestrial origin with a higher proportion of C₂₉ steranes, diterpanes and oleanoids and lower concentrations of both saturated and aromatic hopanes. Higher Pr/Ph, a higher C₃₄ isohopane ratio (IHR34) and a lower homohopane index (HHI) indicate that this low-HI OM was originally deposited under more oxic conditions and subsequently transported to deeper water. Low sterane and hopane isomerization ratios show that no samples have reached the oil-window. Some novel maturity parameters for immature lacustrine samples are proposed.