Geological Survey of Denmark and Greenland Bulletin 26, 2012, 81-84


81© 2012 GEUS. Geological Survey of Denmark and Greenland Bulletin 26, 81–84. Open access:  www.geus.dk/publications/bull

The Cenozoic Song Hong and Beibuwan Basins, Vietnam

Michael B.W. Fyhn, Henrik I. Petersen, Lars Henrik Nielsen, Tran C. Giang, Le H. Nga, 
Nguyen T.M. Hong, Nguyen D. Nguyen and Ioannis Abatzis

The Vietnamese offshore margin holds a substantially un-

derexplored petroleum potential. The key to unravelling this 

potential lies in understanding the tectono-stratigraphic 

framework of the region including the Cenozoic mechanisms 

governing syn-rift and source rock deposition. This is essen-

tial for prediction of, for instance the presence and nature of 

source rocks in South-East Asia and possible reservoir inter-

vals in the syn-rift packages. The Vietnamese part of the Song 

Hong and Beibuwan Basins (Fig. 1) differs from other basins 

along the western margin of the South China Sea in that the 

Palaeogene syn-rift succession is sporadically exposed due 

to uplift and inversion. These exposures provide a unique 

glimpse into the Cenozoic syn-rift succession of the basin.

Considerable oil and condensate discoveries have recently 

been made in the Vietnamese part of the Song Hong Basin. 

This emphasises the need for improved geological models of 

the basin in order to unravel the local petroleum systems.

Since 1995, the Geological Survey of Denmark and 

Greenland and the Vietnam Petroleum Institute have con-

ducted exploration-related, foreign development aid projects 

in Vietnam funded by the Danish Ministry of Foreign Af-

fairs. The ongoing Danish–Vietnamese research coopera-

tion operates through an ENhanced REsearch CApacity 

(ENRECA) project. The ENRECA group consists of both 

Danish and Vietnamese researchers who, through the pro-

ject, have become the established experts in the region. The 

ongoing ENRECA work focuses on the geological develop-

ment of the greater Song Hong Basin (including the Viet-

namese part of the Beibuwan Basin). As part of the project 

activities, the ENRECA group has recently drilled a fully 

cored well into the syn-rift succession of the basin and is car-

rying out a comprehensive regional study combining seismic, 

well and outcrop information.

Evolution of the the Song Hong Basin and 
the greater Vietnamese margin
South-East Asia is tectonically one of the most complex re-

gions in the world, shaped by continental collisions and the 

creation and subduction of oceanic basins. A large portion of 

the deformation associated with the Indian–Eurasian plate 

collision was likely accommodated in the neighbouring part 

of South-East Asia during the Cenozoic (e.g. Leloup et al. 

2001). Indochina deformed in response to the collision and 

may have been squeezed hundreds of kilometres south-east-

Cuu Long B
asi

n

Malay

Basin

M
ae Ping Shear Zone

Pearl R
iver Mouth Basin

W.
Natuna
Basin

N
am

 C
on

 S
on

 B
as

in

Dangerous

Grounds

Malay

Basin

China

Borneo

Basins with Palaeogene lacustrine and
coaly syn-rift source rocks

Oceanic crust with numbered

Main Cenozoic strike-slip direction

6

M
al

ay
si

a

Cambodia

 

Beibuwan Basin

Bach Long Vi

Qiongdongnan Basin

Phu Khanh
Basin

Song H
ong Basin

Laos

Thailand

11

11 11

11

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10

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5E

5E

6A

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5E6A

6B

6B

6B

6B
6A

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6

6
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Vietnam

110°E

20°N

5°N

 

 

250 km

Red River Fault Zone

magnetic anomalies

Nha Trang

Quang
Ngai

South China Sea

Fig. 1. Map of Indochina and the western part of the South China Sea 

showing the outline of the main Cenozoic structures and basins with la-

custrine and coaly source rocks. The large red rectangle shows the study 

area, and the small rectangle shows the location of Fig. 3.



8282

wards from the Himalayan collision front along enormous 

strike-slip fault zones (Fig. 1). 

The Song Hong Basin is the largest Vietnamese sedimen-

tary basin and covers close to one third of the western margin 

of the South China Sea. Its formation has been attributed to 

large-scale, left-lateral movements along the Vietnamese mar-

gin (e.g. Leloup et al. 2001), whereas a pull-apart model for 

the basin remains controversial (e.g. Hall & Morley 2004). 

Understanding the basic mechanism behind the formation 

of the Song Hong Basin is crucial for understanding the 

overall establishment of the western half of the South China 

Sea. The Song Hong Basin is likely to hold important infor-

mation on the opening mechanism of the South China Sea.

Whether the greater Vietnamese margin constitutes a 

transform margin or a more classic oblique rift margin is 

fundamental to the structural concepts applied when inter-

preting geophysical data across basins along the margin. So 

far it has not been possible to evaluate this satisfactorily due 

to the scarcity of seismic data tied to a limited number of 

wells offshore Vietnam. However, recent studies of relatively 

densely spaced seismic profiles tied to wells offshore central 

and southern Vietnam have led to an increased understand-

ing of the tectonic evolution of the margin (Fyhn et al. 2009). 

It is clear from these studies that the Song Hong Basin is cru-

cial for reconstructing the overall tectonic development of 

the western South China Sea.

The current ENRECA study is based on a comprehensive 

geoscientific database of the Vietnamese portion of the basin 

and aims to unravel the tectonic and depositional evolution 

of the basin in a regional context. This will allow a detailed 

stratigraphic understanding of the basin fill and a compre-

hensive structural analysis of the basin and the adjacent areas.

The Song Hong Basin forms a depocentre in the north-

western part of the South China Sea and is filled with up 

to 15–20 km of sediments. Sediment dispersal to the north-

western part of the South China Sea has largely occurred as 

spill from the Song Hong Basin. The distribution and style 

of sediments in the surrounding basins are therefore closely 

linked to the depositional pattern in the Song Hong Basin. 

Deposition evolved rapidly in the basin and varied in time 

and space from lacustrine and fluvial to marine sedimen-

tation. Sedimentary environments ranged from carbonate 

platform, siliciclastic and lava deltas to shelf, slope and basin 

floor and were subject to relative sea-level changes. Com-

bined with high sedimentation rates this makes the basin 

well suited for studies of Cenozoic depositional trends along 

the western South China Sea.

The depositional development in the basin is investigated 

through seismic stratigraphic and facies analyses, and well 

data are closely integrated in the analysis in order to extract 

age and lithologic information. Palaeogeographic recon-

structions illustrate the gross-sedimentary evolution of the 

basin and form a primary result of this part of the study 

(Fig.  2). Information on the regional geological evolution 

can be obtained from the depositional style of the basin in 

addition to information on the establishment of the South 

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 ■■ 

  

 
 

 
 

 
 

Quang Ngai

100 km

Intra-Miocene

Nha Trang

Tuy Hoa

Qui Nhon

Carbonate platform

(predominant)

Carbonate detritus

apron

Carbonate ramp

Alluvial – neritic

(siliciclastics)

Bathyal

(siliciclastics)

Slope

(siliciclastics)

Volcano/basalt delta

Geology concealed

underneath volcanics

Subaerially exposed

non-deposition

Present land with

major cities

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Active fault

 

 

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13°N 13°N

Fig. 2. Intra-Miocene depositional map for the southern part of the Song 

Hong and the Phu Khanh Basins. Carbonate deposition took place on 

structural highs, but was affected by siliciclastic input sourced from on-

shore uplift and local volcanism.



83

China Sea, regional uplift history, sediment transport and 

Cenozoic climate.

Neogene basalt volcanism affected large parts of the Viet-

namese margin from the Nam Con Son Basin in the south 

to the Song Hong Basin in the north, and acted coeval with 

widespread carbonate platform growth along the margin 

(Fig. 2). Magmatism was associated with regional onshore 

uplift and denudation, which seems to have had a signifi-

cant derivative effect on contemporary deposition (Fyhn et 

al. 2009). Increased siliciclastic input to the offshore basins 

and elevated inorganic nutrient concentrations in platform 

areas associated with onshore uplift and denudation led to 

a lowering of long-term carbonate production and deposi-

tion. As a result, carbonate platforms drowned and became 

buried underneath kilometre-thick, prograding siliciclastic 

sediment wedges during the Late Neogene. In the southern 

Song Hong Basin, volcanics and carbonate platforms are 

nicely revealed by seismic data and are drilled and dated in 

wells (Fig. 2).

Syn-rift and source-rock deposition
Cretaceous and Cenozoic organic-rich lacustrine mudstones 

and humic coals, which were deposited during periods of 

rifting, form world-class source-rock intervals in South-East 

Asia (Fig.1). These source rocks frequently occur adjacent to 

coarse-grained, reservoir-quality sedimentary bodies derived 

from nearby elevated hinterland areas. Moreover, source-rock 

intervals are often located adjacent to fractured basement or 

pre-rift successions with effective reservoir characteristics. 

Understanding these rift systems, including the occurrence 

and genesis of lake systems and the sedimentary successions 

capable of generating and storing hydrocarbons, is critical 

when evaluating local petroleum systems over larger parts of 

South-East Asia.

It has long been recognised that stratigraphic wells 

through syn-rift successions are useful to  unravel the deeper 

depositional systems including source-rock units (e.g. Sladen 

1997). This is particularly true offshore northern Vietnam, 

where syn-rift lacustrine and coaly-sourced petroleum main-

ly occur in fractured and karstified pre-rift carbonates. Even 

so, little is known about the source-rock system, and deep 

wells drilled into the syn-rift succession have not provided 

information on the source rocks of the area. To address this 

problem, the ENRECA group drilled a 500 m fully cored 

well into the inverted syn-rift succession on the island of 

Bach Long Vi (Figs 1, 3) where highly oil-prone source-rock 

intervals are widespread. The core demonstrates a thick la-

custrine succession within the area. At the time of writing, 

cores are transported to a core depository to be analysed in 

detail. Bach Long Vi has low relief and dense vegetation, and 

hence well-exposed outcrops on the island are limited to lo-

cations along the shore, and a satisfactory study of syn-rift 

sedimentation and source-rock quality is not possible based 

on outcrops alone. However, combined with the cored strati-

graphic well, the geology of Bach Long Vi provides a unique 

opportunity to study syn-rift deposition and source-rock 

composition. The core provides a regional type section of 

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Do Son
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Chi Linh

Tien Lang

20 km
Palaeogene
rift system

N

Bach Long Vi

Fig. 3A

NW SE

0

500

1000

1500

Post-rift

Inverted
syn-rift

5 km

Bach Long Vi well projected

c. 5 km along structural strike

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ay
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(s

ec
)

Major fault

Seismic line

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Eocene–Oligocene syn-rift

Basement and Palaeozoic–Mesozoic pre-rift

Basement/syn-rift prospects

Fig. 3. A: Geotransect across the inverted Palaeogene graben, which is exposed on the island of Bach Long Vi a few kilometres from the profile. The ap-

proximate well trajectory is projected onto the section. The transect is based on the seismic line 89-1-103 (partly shown on Fig. 3B, red, dotted line). B: Map 

showing the outline of the primary Paleogene syn-rift depocentre and syn-rift structural highs in the north-eastern Song Hong Basin. Oil and condensate 

discoveries in the area are sourced from lacustrine and coaly syn-rift source rocks. Bach Long Vi forms a window to a graben directly linked with the syn-rift 

kitchen areas of the recent discoveries made in fractured and karstified Palaeozoic carbonates in the area.



8484

the Palaeogene syn-rift succession. Sampled source-rock in-

tervals are studied through general source rock screening and 

biomarker analysis combined with more advanced analysis of 

selected source-rock intervals.

Situated at the intersection of the Beibuwan and the Song 

Hong Basin, Bach Long Vi occurs in the centre of a newly 

discovered petroleum province offshore Vietnam (Fig. 3). 

Bach Long Vi sits at the top of a prominent structure associ-

ated with inversion of a Palaeogene graben. The island there-

fore provides a window into the nature of the Palaeogene 

syn-rift succession of the area. Highly oil-prone lacustrine 

mudstones with an organic content ranging from c. 2 to 7 

wt.% total organic carbon were sampled from scattered out-

crops on Bach Long Vi and from sub-sea outcrops around the 

island (Fig. 4). ENRECA analyses of these excellent source 

rocks confirm that petroleum-prone mudstones occur at var-

ious stratigraphic levels that crop out over the entire area (e.g. 

Petersen et al. 2004 and new unpublished data).

A dense net of seismic data cover the area offshore Bach 

Long Vi and reveals a N–S to NE–SW-trending, inverted 

Eocene–Oligocene graben. The graben fill sub-crops at and 

closely beneath the seafloor and forms the core of Bach Long 

Vi (Fig. 3A). The graben is part of a larger rift system that 

actively subsided during Palaeogene time (Fig. 3B). The de-

posits, including the sampled source rocks on and around 

Bach Long Vi, therefore form part of a regional syn-rift sys-

tem, which is directly continuous with the syn-rift kitchen 

areas sourcing the recent oil and condensate discoveries in 

the region (Fig. 3). The preliminary results suggest that rich 

source-rock intervals could be present throughout much of 

the syn-rift succession in the basin, highly encouraging for 

further exploration.

Acknowledgements
The ENRECA project is funded by the Danish Ministry of Foreign Af-

fairs via Danida. PetroVietnam co-funded the ENRECA 3 well and is 

thanked for providing data and for permitting publication of this paper.

References
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ment of the central and south Vietnamese margin: Implications for the 

establishment of the South China Sea, Indochinese escape tectonics 

and Cenozoic volcanism. Tectonophysics 478, 184–214.

Hall, R. & Morley, C.K. 2004: Sundaland Basins. In: Clift, P. et al. (eds): 

Continent–ocean interactions within the East Asian marginal seas. 

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can Geophysical Union.Leloup, P.H., Arnaud, N., Lacassin, R., Kienast, 

J.R., Harrison, T.M., Trong, T.T.P., Replumaz, A. & Tapponnier, P. 

2001: New constraints on the structure, thermochronology and timing 

of the Ailao Shan-Red River shear zone, SE Asia. Journal of Geophysical 

Research 106, B4, 6683–6732.

Petersen, H.I., Nytoft, H.P. & Nielsen, L.H. 2004: Characterisation of 

oil and potential source rocks in the northeastern Song Hong Basin, 

Vietnam: indications of a lacustrine–coal sourced petroleum system. 

Organic Geochemistry 35, 493–515. 

Sladen, C. 1997: Exploring the lake basins of east and Southeast Asia. In: 

Fraser, A.J., Matthews, S.J. & Murphy, R.W. (eds): Petroleum geology 

of Southeast Asia. Geological Society Special Publications (London) 

126, 49–76.

20°14´N

20°13´N

Bach Long Vi

107°72´N 107°74´N500 m

Bach Long Vi

5 km

Fig. 4. Map of the island of Bach Long Vi. Red dots show sites of source-

rock samples. The inset map shows the nearest industrial seismic lines in 

the area. For location see Figs 1 and 3.

Authors’ addresses

M.B.W.F., H.I.P., L.H.N. & I.A., Geological Survey of Denmark and Greenland, Øster Voldgade 10, DK-1350 Copenhagen K, Denmark.

E-mail: mbwf@geus.dk

T.C.G. & L.H.Ng., Vietnam Petroleum Institute, 173 Trung Kinh Str., Yen Hoa Wrd., Cau Giay Dist., Hanoi, Vietnam.

N.T.M.H., Hanoi University of Mining and Geology, Dong Ngac, Tu Liem, Hanoi, Vietnam.

N.D.N., Hanoi University of Science, 334 Nguyen Trai-Thanh Xuan-Hanoi, Vietnam.