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Highly Oil-Prone Source Rocks Offshore North Vietnam: 1st Results of the ENRECA-3 Core Well Drilled on Bach Long Vi Island, Gulf of Tonkin

Abstract

The present study is the first documentation of lacustrine source rock properties of syn-rift mudstones in the Vietnamese petroliferous Gulf of Tonkin. The stratigraphic ENRECA-3 well cored 500 m of lacustrine syn-rift sediments and proved the presence of minimum 250 m net-source rock on the Bach Long Vi Island located on the NE flank of the Song Hong Basin. The source rocks are thermally immature and thus contain the original petroleum generation potential. The approximately lower half of the drilled section consists of alternating greyish/brownish and black mudstones, whereas the mudstones in the upper drilled section are light grey. The black and the light grey mudstones are organic-rich and highly oil-prone containing Type I and Type I-II kerogen, while the greyish/brownish mudstones in the lower part of the well are of poorer source rock quality. The kerogen in the source rocks consists principally of alginite, mainly occurring as fluorescing AOM, but subtle organofacies variations are indicated by varying amounts of huminite and other terrestrial organic matter. The average TOC content is 2.91 wt.% and the average HI is 575 mg HC/g TOC, but if the non-reactive TOC is excluded the reactive TOC content is 2.70 wt.% and the reactive HI values is 637 mg HC/g TOC. The calculated minimum SPI is ∼10 tons HC/m2, and approximately 54% of the TOC can be converted to petroleum. Simple modelling using KinEx predicts that the lacustrine mudstones principally will generate black oil with an expelled GOR of maximum 1700 scf/stb within a vitrinite reflectance range of 0.7%Ro to 1.0%Ro.