Bulletin of the Geological Society of Malaysia, Volume 37, July 1995, pp. 33 – 54
Vietnam Petroleum Institute (VPI), Yen Hoa-Tu Liem, Hanoi
Abstract: Some offshore basins of Vietnam contain hydrocarbon accumulative traps not only in Tertiary sediments but also in fractured basement.
Pre-Cenozoic plays are related both to reservoirs of fractured basement and to source rock and topseal of Oligocene sediments. Consolidation of basement is demonstrated by Upper Jurassic-Eocene meta-molasses and plutonic-volcanic arcs (45-168 Ma) which are caused by the collision or convergence of the continents after the disappearance of the Tethys ocean.
Sedimentary basins on the continental shelf of Vietnam are filled by Oligocene-Miocene marine and deltaic sediments, related to rifting, spreading, sagging activities from which the East Vietnam Sea (or South China Sea) was formed and widened.
The Song Hong (Bacbo) rift basin is characterized by Oligocene-Miocene high-constructive deltaic sediments, which are rich in humic matter. Oligocene and Miocene plays are predominant in clastic sediments.
The Cuulong sag basin is composed of Oligocene-Miocene fine clastic sediments in high-destructive deltaic facies, which are rich in sapropel. Fractured plutonic-volcanic basement developed into good petroleum traps. But Oligocene-Miocene plays are predominant in clastic sediments.
The Nam Conson rift basin has a complicated structure. The lower part of this basin is filled by Oligocene and early Miocene marine clastic sediments, but the upper part is predominant in marine platform and reef carbonates. There are both Oligocene-Miocene clastic plays and Miocene carbonate ones in this basin.
The Pliocene-Quaternary deposits are sand, mud and carbonate of shelf facies in back-arc basins. Their great thickness enhanced the geothermal gradient and provided good conditions for maturation and trapping of hydrocarbons.