Characterisation of oil and potential source rocks in the northeastern Song Hong Basin, Vietnam: indications of a lacustrine-coal sourced petroleum system

Oil discovered in the B10-STB-1x well in the northeastern Song Hong Basin, Vietnam, has typical lacustrine-coaly geochemical features, indicating the existence of a principally lacustrine sourced petroleum system with minor contributions from a coal source. The presence of lacustrine source rocks in the basin is indicated by distinct high-amplitude seismic reflectors in undrilled half-grabens and outcrops of Oligocene immature mudstones and humic coals at Dong Ho and on Bach Long Vi Island. The mudstones from Dong Ho generally have a TOC content of 8–17 wt.% and HI values >500 mg HC/g TOC. The organic matter is principally composed of fluorescing amorphous organic matter and liptodetrinite; alginite with Botryococcus-morphology is present. The organic matter corresponds to type I kerogen. The Bach Long Vi Island mudstones contain 2–7 wt.% TOC and have HI values from 200–700 mg HC/g TOC. The organic matter is largely similar to that in the Dong Ho mudstones, but in addition may contain some marine organic material. The B10-STB-1x oil shows features common to both the Dong Ho and Bach Long Vi Island mudstones, but was probably principally generated from a source similar to the Dong Ho mudstones with contributions from higher land plant organic matter. A prominent activation energy peak dominates the activation energy distributions of the mudstones, and upon artificial maturation (hydrous pyrolysis) the mudstones show extensive hydrocarbon generation over a narrow temperature range. Hydrocarbon expulsion has been estimated to occur at a vitrinite reflectance of 0.75%Ro. The source of the B10-STB-1x oil may be located in the Thuy Nguyen Graben, and maturity modelling shows that the lacustrine mudstones will be thermally mature in this graben.