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 similar to0.75% R-o. 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. (C) 2004 Elsevier Ltd. All rights reserved.