The Nam Con Son Basin (also known as the Wanan Basin) formed as a rift basin during the Oligocene period. This basin is the southernmost sedimentary basin offshore of Vietnam, located within coordinates of 6°6'-9°45'N and 106°0-109°30'E in the East Vietnam Sea. It is the largest oil and gas bearing basin in Vietnam and has a number of producing fields.
Geologic Setting
Regional
The Nam Con Son Basin, located adjacent to the Cuu Long Basin, is approximately 90,000 km2. The Basin age ranges from Oligocene to Quaternary in age with sediment thickness of a maximum of 10 km.
While a majority of the basin is situated in less than 200 meters of shallow water, the bathymetry can go to deeper than 2000 meters to the north of the Nam Con Son Basin.
Structural
The Nam Con Son Basin was developed during the Tertiary by complex rifting of a poorly known basement. The geological formation of Nam Con Son Basin can be separated into two major structural elements: A Pre-Cenozoic strata basement and a Cenozoic sedimented cover.
The heterogeneous basement is composed of quartz diorite, granodiorite and Mesozoic metamorphic rocks. While the Pre-Tertiary basement shows an assemblage of volcanic, igneous and metasedimentary rocks. At various levels in the basement, faults cause subsidence troughs.
The cover is divided into the three sequences: The upper sequence consist of clastic and coastal marine carbonates from the Bien Dong Formation. The middle sequence have a composition of Miocene sediments of the Dua Formation, Thong-Man Cau Formations and the Nam Con Son Formation. The lower sequence which is composed from Paleogene sediments from the Cau Formation.
Sedimentology and stratigraphy
Cau Formation, Oligocene
This formation varies between 200m to 800m with the top formation composed of dark grey, greenish grey clay, intercalated with beds of sandstone, and siltstone.
The middle layer of the basin is composed of quartz sandstone intercalated with greyish black, firm claystone and a thick bed of dark grey clay intercalated with light coloured oil shale. The later of this basin is also locally fine grained and calcareous.
The bottom layer has a composition of brownish grey sandstones intercalated with several beds of silty clay.
Overall, the lithology of the Cau Formation is composed of lacustrine shale, alluvial sandstones and minor coals.
Dua Formation, Lower Miocene
Overlying unconformably in the Cau Formation, the Dua Formation is between 200m to 800m thick, composed of light and bright grey sandstone, interbedded with blackish-grey silty clay. The sandstone has characteristics of fine to medium grained, angular, quarts, with carbonate cement. This formation also has glauconite corresponding with fossil fragments.
The overall lithology of the Dua Formation in the Nam Con Son Basin consist of coastal plain sandstones and has claystones and interbedded minor coals.
Thong-Mang Cau Formation, Middle Miocene
Varying from 10m to 100m in the Thong-Man Cau formation, the upper formation has a composition of light grey, whitish/grey, locally reddish brown dolomitized carbonates and it is also intercalated with clays friable siltstones and fine grained, calcite-cemented sandstones.
The lower formation consists of sandstone and calcareous sandstones interbedded with clays and siltstones. It has fine to medium grained sandstones with carbonate cement, glauconite and fossil fragments.
As a whole, the lithology of the Thong-Man Cau Formation is made from sandstones and claystones with carbonate build up and platform.
Nam Con Son Formation, Upper Miocene
This formation has a thickness between 100m to 500m.The upper formation consist of Clay, calcareous clay and grey sandstones.
The lower formation has whitish grey carbonate and quartz sandstones.
The Nam Con Son Formation has a generally lithology of yellow claystone which are interbedded with siltstone. It is average cemented and have rich organic and fossil properties.
Bien Dong Formation, Pliocene-Quaternary
The Bien Dong Formation thickness varies from a few meters to 200m. In the Quaternary layer, moving from the bottom to the middle to the top, the Bien Dong Formation is composed of angular to round quartz sand which gradually changes to silty clay in the middle section to quartz sand imbedded with shell fragments at the top of the layer.
The Lower Pliocene layer has a composition of siltstones, friable claystones, and grey calcareous clays intercalated with white or light yellow quartz sandstone. This layer is also rich in carbonate and contains glauconite.