Geological Survey of Denmark and Greenland Bulletin 13, 2007
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Review of Survey activities 2006, 73-76 |
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A series of Cenozoic basins fringes the Vietnamese coastal
margin, often characterised by more than 10 km of sedimen-
tary infill (Fig. 1). Greater parts of the margin are still in an
early explorational state, although significant petroleum pro-
duction has taken place in all but the southern Song Hong
and the Phu Khanh Basins. This has increased the need for a
fundamental understanding of the processes behind the for-
mation of the basins, including analyses of potential source
rocks.
margin, often characterised by more than 10 km of sedimen-
tary infill (Fig. 1). Greater parts of the margin are still in an
early explorational state, although significant petroleum pro-
duction has taken place in all but the southern Song Hong
and the Phu Khanh Basins. This has increased the need for a
fundamental understanding of the processes behind the for-
mation of the basins, including analyses of potential source
rocks.
The basins fringing the Indochina
Block provide excellent evidence of the
geological evolution of the region, and
the basin geometries reflect the collision
of India and Eurasia and the late
Cenozoic uplift of south Indochina
(Rangin et al . 1995a; Fyhn et al . in
press). In addition, the basins provide
evidence of regional Palaeogene rifting
and subsequent Late Palaeogene through
Early Neogene sea-floor spreading in the
South China Sea. Apart from the regional
Cenozoic tectonic record, the basins con-
tain a high-resolution climatic record of
South-East Asia due to the high deposi-
tional rates, changing depositional styles
and large hinterland of the basin (Clift et
al . 2004).
geological evolution of the region, and
the basin geometries reflect the collision
of India and Eurasia and the late
Cenozoic uplift of south Indochina
(Rangin et al . 1995a; Fyhn et al . in
press). In addition, the basins provide
evidence of regional Palaeogene rifting
and subsequent Late Palaeogene through
Early Neogene sea-floor spreading in the
South China Sea. Apart from the regional
Cenozoic tectonic record, the basins con-
tain a high-resolution climatic record of
South-East Asia due to the high deposi-
tional rates, changing depositional styles
and large hinterland of the basin (Clift et
al . 2004).
Background
Since 1995 the Geological Survey of
Denmark and Greenland (GEUS) and
the Department of Geography and Geo -
logy, University of Copenhagen, have
operated jointly in Vietnam aiming to
improve the local geoscientific capa city.
The work is part of the ENRECA pro-
ject (Enhancement of Research Capacity
in Developing Countries), funded by
the Danish International Development
Agency (DANIDA). This part of the
ENRECA project focuses on an assess-
ment of the hydrocarbon potential of
the Vietnamese continental margin, and
Denmark and Greenland (GEUS) and
the Department of Geography and Geo -
logy, University of Copenhagen, have
operated jointly in Vietnam aiming to
improve the local geoscientific capa city.
The work is part of the ENRECA pro-
ject (Enhancement of Research Capacity
in Developing Countries), funded by
the Danish International Development
Agency (DANIDA). This part of the
ENRECA project focuses on an assess-
ment of the hydrocarbon potential of
the Vietnamese continental margin, and
has led to basin evaluations of the Song Hong and the Phu
Khanh Basins (Fig. 1), and to a series of both Vietnamese and
Danish M.Sc. projects (Nielsen et al . 1999; Nielsen &
Abatzis 2004; Andersen et al . 2005; Boldreel et al . 2005;
Fyhn et al . in press). The ongoing second phase of the project
focuses both on training Vietnamese M.Sc. and Ph.D. stu-
dents and on evaluating the hydrocarbon potential of the
Vietnamese part of the Malay and Khmer Basins, as well as
Khanh Basins (Fig. 1), and to a series of both Vietnamese and
Danish M.Sc. projects (Nielsen et al . 1999; Nielsen &
Abatzis 2004; Andersen et al . 2005; Boldreel et al . 2005;
Fyhn et al . in press). The ongoing second phase of the project
focuses both on training Vietnamese M.Sc. and Ph.D. stu-
dents and on evaluating the hydrocarbon potential of the
Vietnamese part of the Malay and Khmer Basins, as well as
Cenozoic evolution of the Vietnamese coastal margin
Michael B.W. Fyhn, Lars Henrik Nielsen and Lars Ole Boldreel
© GEUS, 2007.
Geological Survey of Denmark and Greenland Bulletin
13, 73-76. Available at:
www.geus.dk/publications/bull
73
Fig. 1. Map showing major Cenozoic basins and oceanic crust and simplified Cenozoic structural
features.
A
: cross-section shown in Fig. 3.
B
: cross-section shown in Fig. 4. Modified from Fyhn
et
al
. (in press).
the Mesozoic strata underneath and shoreward of these
basins. Sampling of source rocks and oil seeps and drilling of
two 500 m deep, fully cored wells (ENRECA-1 and 2, Fig. 1)
as well as acquisition of shallow seismic data have been car-
ried out as part of the basin evaluations (Bojesen-Koefoed et
al . 2005; Petersen et al . 2005). Furthermore, a broader analy-
sis of the structure and stratigraphy of the entire Vietnamese
margin is being carried out as a separate Ph.D. study funded
by the University of Copenhagen.
basins. Sampling of source rocks and oil seeps and drilling of
two 500 m deep, fully cored wells (ENRECA-1 and 2, Fig. 1)
as well as acquisition of shallow seismic data have been car-
ried out as part of the basin evaluations (Bojesen-Koefoed et
al . 2005; Petersen et al . 2005). Furthermore, a broader analy-
sis of the structure and stratigraphy of the entire Vietnamese
margin is being carried out as a separate Ph.D. study funded
by the University of Copenhagen.
Tectonic models
The Indochina Block is situated immediately south-east of
the eastern Himalayan syntaxis. The Himalayan Orogeny
thus had a major impact on the structural evolution of Indo -
china, leading to major north-west-south-east crustal short-
ening in the north and to significant lateral movements along
shear zones transecting and bordering the Indochina Block
(Fig. 1; Morley 2002). Some of the largest shear zones are the
north-west-trending Red River, Mai Ping and Three Pagodas
Shear Zones. South-eastward displacement and rotation of
Indochina and adjacent areas produced a total left-lateral off-
set of several hundreds of kilometres along the three shear
the eastern Himalayan syntaxis. The Himalayan Orogeny
thus had a major impact on the structural evolution of Indo -
china, leading to major north-west-south-east crustal short-
ening in the north and to significant lateral movements along
shear zones transecting and bordering the Indochina Block
(Fig. 1; Morley 2002). Some of the largest shear zones are the
north-west-trending Red River, Mai Ping and Three Pagodas
Shear Zones. South-eastward displacement and rotation of
Indochina and adjacent areas produced a total left-lateral off-
set of several hundreds of kilometres along the three shear
zones (Hall 2002). Tapponnier
et al
. (1982) suggested that
the South China Sea and its marginal basins formed due to
complex pull-apart mechanisms in response to these left-lat-
eral displacements (Fig. 2A). Alternatively, Taylor & Hayes
(1983) suggested that the formation of the South China Sea
was a result of a southward subduction of ocean crust beneath
Borneo (Fig. 2B). One of the major differences between the
two models is that the subduction model predicts right-lateral
displacement across a large part of the Vietnamese margin,
whereas the pull-apart model is associated with a left-lateral
transform along the margin.
the South China Sea and its marginal basins formed due to
complex pull-apart mechanisms in response to these left-lat-
eral displacements (Fig. 2A). Alternatively, Taylor & Hayes
(1983) suggested that the formation of the South China Sea
was a result of a southward subduction of ocean crust beneath
Borneo (Fig. 2B). One of the major differences between the
two models is that the subduction model predicts right-lateral
displacement across a large part of the Vietnamese margin,
whereas the pull-apart model is associated with a left-lateral
transform along the margin.
The offshore Red River Shear Zone
The most extensive of the Indochinese left-lateral shear zones
is the Red River Shear Zone that passes through South China
and northern Vietnam into the Song Hong Basin. Seismic
studies of the almost 20 km deep Song Hong Basin indicate
that the basin formed in response to major Palaeogene left-
lateral offset along the seaward continuation of the Red River
Shear Zone (Rangin et al . 1995a; Nielsen et al . 1999; Ander -
sen et al . 2005). Recent studies show that the shear zone con-
tinues along the Vietnamese coast in the Phu Khanh Basin
further south (Fig. 1; Fyhn et al . in press). The shear zone
runs along the western boundary of the Phu Khanh Basin
is the Red River Shear Zone that passes through South China
and northern Vietnam into the Song Hong Basin. Seismic
studies of the almost 20 km deep Song Hong Basin indicate
that the basin formed in response to major Palaeogene left-
lateral offset along the seaward continuation of the Red River
Shear Zone (Rangin et al . 1995a; Nielsen et al . 1999; Ander -
sen et al . 2005). Recent studies show that the shear zone con-
tinues along the Vietnamese coast in the Phu Khanh Basin
further south (Fig. 1; Fyhn et al . in press). The shear zone
runs along the western boundary of the Phu Khanh Basin
74
Fig. 2. Conceptual models of the two basic theories initially proposed
for the formation of the South China Sea (Tapponier
et al
. 1982; Taylor
& Hayes 1983). Later studies have suggested various integrations of the
two models (Hall 2002; Morley 2002; Fyhn
et al
. in press).
A
: The pull-
apart model suggests rifting and subsequent sea-floor spreading as a
result of a complex left-lateral pull-apart mechanism.
B
: The subduction
model suggests rifting and subsequent sea-floor spreading as a result of
the subduction of old oceanic lithosphere beneath Borneo. Note that
both models infer a transform zone along the central and south
Vietnamese margin but with opposite relative sense of motion. Modified
from Tapponier
et al
. (1982) and Taylor & Hayes (1983).
Fig. 3. Cross-section of the northern Phu Khanh Basin transecting the
offshore continuation of the Red River Shear Zone. Timing of the defor-
mations shows Palaeogene left-lateral movement followed by moderate
right-lateral inversion during the early Neogene. The structural cut-off of
the pre-rift sequence towards the shear zone is interpreted to be a result
of the large left-lateral movement along the zone (see Fig. 1 for loca-
tion).
forming a major rift structure filled by thick Palaeogene syn-
rift deposits (Fig. 3). Left-lateral transtension ended during
latest Oligocene time in the Phu Khanh Basin, but was fol-
lowed by earliest Miocene structural inversion. This is inter-
preted to reflect a change from intense left-lateral trans tension
to modest right-lateral movements along the seaward exten-
sion in the Red River Shear Zone in the basin, corroborated
by a study by Rangin et al . (1995b) showing that left-lateral,
coast-parallel wrench faults onshore have been inverted by
right-lateral movements. The latest Palaeogene termination
of left-lateral movement along the offshore part of the Red
River Shear Zone in the Phu Khanh Basin does not support
Neogene sea-floor spreading in the South China Sea as a
result of left-lateral pull-apart. Consequently, Neogene sea-
floor spreading cannot have been caused by left-lateral pull-
apart, but was probably forced by subduction of older
oceanic crust beneath Borneo. Palaeogene rifting along the
Vietnamese margin was, on the other hand, greatly influ-
enced by left-lateral transtension.
rift deposits (Fig. 3). Left-lateral transtension ended during
latest Oligocene time in the Phu Khanh Basin, but was fol-
lowed by earliest Miocene structural inversion. This is inter-
preted to reflect a change from intense left-lateral trans tension
to modest right-lateral movements along the seaward exten-
sion in the Red River Shear Zone in the basin, corroborated
by a study by Rangin et al . (1995b) showing that left-lateral,
coast-parallel wrench faults onshore have been inverted by
right-lateral movements. The latest Palaeogene termination
of left-lateral movement along the offshore part of the Red
River Shear Zone in the Phu Khanh Basin does not support
Neogene sea-floor spreading in the South China Sea as a
result of left-lateral pull-apart. Consequently, Neogene sea-
floor spreading cannot have been caused by left-lateral pull-
apart, but was probably forced by subduction of older
oceanic crust beneath Borneo. Palaeogene rifting along the
Vietnamese margin was, on the other hand, greatly influ-
enced by left-lateral transtension.
The offshore Three Pagodas Shear Zone
Rifting in the Malay and Khmer Basins south-west of
Vietnam was originally linked to left-lateral transtension
across a seaward extension of the Three Pagodas Shear Zone
(Fig. 1; Tapponnier et al . 1982). Later models suggested
right-lateral faulting along the fault zone as the forcing mech-
anism (Polachan & Sattayarak 1989), or a combination of
forces related to the Indochina extrusion and extension
caused by subduction roll-back (Morley 2001), or mantle
plume emplacement (Ngah et al . 1996).
Vietnam was originally linked to left-lateral transtension
across a seaward extension of the Three Pagodas Shear Zone
(Fig. 1; Tapponnier et al . 1982). Later models suggested
right-lateral faulting along the fault zone as the forcing mech-
anism (Polachan & Sattayarak 1989), or a combination of
forces related to the Indochina extrusion and extension
caused by subduction roll-back (Morley 2001), or mantle
plume emplacement (Ngah et al . 1996).
Seismic structural analysis of the Vietnamese part of the
Malay and Khmer Basins indicates that rifting mainly took
place during the Palaeogene, and was controlled by a steep,
north-north-west-trending, downward steeping master fault,
which is flanked by smaller north-west-trending conjugate
normal faults (Fig. 4). The master fault offsets the basement
with up to more than 2 sec. TWT and transects the entire
study region striking towards the point at which the Three
Pagodas Shear Zone enters the Gulf of Thailand. The master
fault is therefore interpreted as an offshore fault strand of the
Three Pagodas Shear Zone. The fault characteristics indicate
Palaeogene left-lateral transtension and thus support a close
relation between extrusion of Indochina and rifting in the
two basins.
place during the Palaeogene, and was controlled by a steep,
north-north-west-trending, downward steeping master fault,
which is flanked by smaller north-west-trending conjugate
normal faults (Fig. 4). The master fault offsets the basement
with up to more than 2 sec. TWT and transects the entire
study region striking towards the point at which the Three
Pagodas Shear Zone enters the Gulf of Thailand. The master
fault is therefore interpreted as an offshore fault strand of the
Three Pagodas Shear Zone. The fault characteristics indicate
Palaeogene left-lateral transtension and thus support a close
relation between extrusion of Indochina and rifting in the
two basins.
Depositional trends
Sea-floor spreading in the South China Sea did not start until
the middle Oligocene, and Palaeogene syn-rift sedimentation
was therefore dominated by alluvial and lacustrine deposi-
the middle Oligocene, and Palaeogene syn-rift sedimentation
was therefore dominated by alluvial and lacustrine deposi-
tion. In the Song Hong Basin a gradual marine transgression
of the margin started after the onset of sea-floor spreading.
During initial transgression siliciclastic deposition in estua -
ries and narrow marine pathways dominated larger parts of
the basins, and carbonate growth took place on inundated
highs. Open marine conditions prevailed in most basins dur-
ing Neogene times as sea-floor spreading propagated to its max -
imum south-western extension. Extensive carbonate growth
took place on many intra- and interbasinal highs south of and
along the Vietnamese margin up to c . 16°N during the Neo -
gene, favoured by the open marine environment and climatic
conditions. In contrast, sediment supply kept pace with subsi-
dence in most parts of the Malay and Song Hong Basins, pre-
venting long-lasting periods of open marine sedimentation.
of the margin started after the onset of sea-floor spreading.
During initial transgression siliciclastic deposition in estua -
ries and narrow marine pathways dominated larger parts of
the basins, and carbonate growth took place on inundated
highs. Open marine conditions prevailed in most basins dur-
ing Neogene times as sea-floor spreading propagated to its max -
imum south-western extension. Extensive carbonate growth
took place on many intra- and interbasinal highs south of and
along the Vietnamese margin up to c . 16°N during the Neo -
gene, favoured by the open marine environment and climatic
conditions. In contrast, sediment supply kept pace with subsi-
dence in most parts of the Malay and Song Hong Basins, pre-
venting long-lasting periods of open marine sedimentation.
During Late Neogene time, central and southern Indo -
china were thermally uplifted, thus significantly increasing
the siliciclastic input to the marginal basins. The increased
terrigenous sediment supply inhibited widespread carbonate
growth off southern and central Vietnam and resulted in the
progradation of a distinct shelf slope, which has led to the
present outline of the margin.
the siliciclastic input to the marginal basins. The increased
terrigenous sediment supply inhibited widespread carbonate
growth off southern and central Vietnam and resulted in the
progradation of a distinct shelf slope, which has led to the
present outline of the margin.
Source rocks
One of the main risk factors regarding petroleum exploration
in the Vietnamese offshore basins is the presence of adequate
source rock intervals. Onshore data from the ENRECA-1
core through the Song Ba Trough in central Vietnam show,
however, that thick intervals of excellent oil- and gas-prone
lacustrine mudstone and humic coals may develop even in
in the Vietnamese offshore basins is the presence of adequate
source rock intervals. Onshore data from the ENRECA-1
core through the Song Ba Trough in central Vietnam show,
however, that thick intervals of excellent oil- and gas-prone
lacustrine mudstone and humic coals may develop even in
75
Fig. 4. Cross-section of the Vietnamese part of the Malay Basin which
transects a fault strand of the seaward continuation of the Three Pagodas
Shear Zone. The main offset along the major fault occurred during
Palaeogene times as left-lateral transtension forced by the indentation of
India into Eurasia (see Fig. 1 for location).
small basins characterised by high sediment input. Although
the Song Ba Trough is an order of magnitude smaller than the
Vietnamese offshore basins, seismic data in the latter show
apparent depositional similarities suggesting the presence of
similar high-quality source rocks in the offshore basins
(Nielsen et al . 2007; Fyhn et al . in press). In addition, seismic
facies analysis as well as oil and gas compositions indicate that
other source rock types, such as Neogene fluvio-deltaic coals,
carbonaceous shales and fore-reef marls are present in some
of the basins and thus testify to the great petroleum potential
of the Vietnamese margin (Bojesen-Koefoed et al . 2005;
Fyhn et al . in press).
the Song Ba Trough is an order of magnitude smaller than the
Vietnamese offshore basins, seismic data in the latter show
apparent depositional similarities suggesting the presence of
similar high-quality source rocks in the offshore basins
(Nielsen et al . 2007; Fyhn et al . in press). In addition, seismic
facies analysis as well as oil and gas compositions indicate that
other source rock types, such as Neogene fluvio-deltaic coals,
carbonaceous shales and fore-reef marls are present in some
of the basins and thus testify to the great petroleum potential
of the Vietnamese margin (Bojesen-Koefoed et al . 2005;
Fyhn et al . in press).
Acknowledgements
This study is a Ph.D. project funded by the Faculty of Natural Science at
the University of Copenhagen to the first author. Funding to the ENRECA
project was given by the Danish Ministry of Foreign Affairs through
DANIDA. Vietnam Petroleum Institute (PetroVietnam) is thanked for pro-
viding the seismic reflection and well data and giving permission to pub-
lish these.
the University of Copenhagen to the first author. Funding to the ENRECA
project was given by the Danish Ministry of Foreign Affairs through
DANIDA. Vietnam Petroleum Institute (PetroVietnam) is thanked for pro-
viding the seismic reflection and well data and giving permission to pub-
lish these.
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Authors' addresses
M.B.W.F. & L.O.B.,
Department of Geography and Geology, University of Copenhagen, Øster Voldgade 10, DK-1350 Copenhagen K, Denmark.
E-mail:
fyhn@geol.ku.dk
L.H.N.,
Geological Survey of Denmark and Greenland, Øster Voldgade 10, DK-1350 Copenhagen K, Denmark.
76
