Structural controls on syn-rift sediments of the Northern Song Hong basin

PETROVIETNAM

PETROVIETNAM JOURNAL

Volume 6/2020, pp. 15 - 21

ISSN 2615-9902

STRUCTURAL CONTROLS ON SYN-RIFT SEDIMENTS

OF THE NORTHERN SONG HONG BASIN

Bui Huy Hoang¹, Nguyen Thu Huyen¹, Nguyen Tuan Anh¹, Tong Duy Cuong¹, Bui Viet Dung¹, Nguyen Trung Hieu¹

Nguyen Quang Tuan¹, Nguyen Thanh Tung¹, Trinh Xuan Cuong², Hoang Anh Tuan², Micheal Fyhn³, Lars Nielsen³

Ioannis Abatzis³, Jussi Hovikoski³, Mette Olivarius³

¹Vietnam Petroleum Institute (VPI)

²Vietnam Oil and Gas Group (PVN)

³Geological Survey of Denmark and Greenland (GEUS)

Email: huyennt@vpi.pvn.vn

Summary

The study area is located in the northern part of the Song Hong basin which is the largest basin along the western margin of the

East Sea stretching from north of Hanoi underneath the Song Hong Delta (Red River Delta) into the Gulf of Tonkin (Figure 1). The rift

system underneath the Song Hong basin constitutes a world-class example of how extrusion tectonics drives continental rifting and

transtensional basin development.

Many factors strongly inﬂuence the structural styles of rifting of the northern Song Hong basin: rifting occurred during later Eocene

- Late Oligocene time forced by ASRRSZ left lateral shearing; latest Oligocene - earliest Miocene transpression and inversion brought

rifting to a halt, after which left-lateral shearing decreased. Paleogene rift systems extended along the trail of the ASRRSZ are now

outlined by lower to mid-crustal metamorphic core complexes. Rift termination in the northern Song Hong basin and exhumation of

the metamorphic core complexes coincided with cessation of Paleogene rifting along the western margin of the East Vietnam Sea and a

common causal mechanism are speculated.

Structures within rifting aﬀect syn-rift depositional patterns by creating sites of uplift and erosion, by controlling pathways of

sediment transportation, by the accommodation space for sediment deposition and preservation. These resulting from the growth of

rifting may yield a tripartite stratigraphy such as ﬂuvial, deep lacustrine and shallow lacustrine.

Key words: Rifting, syn-rift sediments, extension, transtension, lacustrine, ﬂuvial, Song Hong basin.

1. Introduction

The study area is located in the northern part of Song

development trends and classiﬁcation schemes such

as a typical tripartite structural basin development with

an early, maximum and late rift evolution. The structural

relation between sediment input and formation of

accommodation space causes basins to be under ﬁlled,

balanced or overﬁlled, and a general discrimination of

two major types of environment facies associations of

ﬂuvial - lacustrine, lacustrine [2 - 4].

Hong basin which is the largest basin along the western

margin of the East Sea stretching from north of Hanoi

underneath the Song Hong Delta and into the Gulf of

Tonkin (Figure 1). Situated at the extension of the onshore

Ailao Shan - Red River Shear Zone (ASRRSZ), the formation

of the Song Hong basin is often considered to be linked

with Cenozoic continental-scale left-lateral motion taking

place across the shear zone [1].

Simultaneously, interpretation using 2D and

3D seismic data and well data during hydrocarbon

exploration and production activities has identiﬁed and

mapped the top syn-rift of the northern Song Hong basin.

Among seismic, well data within driven model of rifting

approaches have yielded valuable information about

the structural styles and depositional patterns of syn-rift

sediments in order to describe the variety of structures

The comprehensiveness of rift basin and their

petroleum systems manifests a number of characteristic

Date of receipt: 3/12/2019. Date of review and editing: 3 - 14/12/2019.

Date of approval: 5/6/2020.

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Hoang Sa

islands

Truong Sa

islands

Figure 1. The northern Song Hong basin within structural outline emphasising the main faults, rift depressions and structural highs [5].

associated with continental rifting and to infer

the inﬂuence of these structural styles on the

depositional patterns within them.

This paper summarises the integration of

analytical results with all available geo‐scientiﬁc

knowledge to bring out an understanding of

regional structures within rifting that aﬀects

depositional patterns of syn-rift sediments in

the northern Song Hong basin.

2. Stratigraphic units of northern Song

Hong basin rifting

Song Hong basin is underlain by the

interconnected Beibuwan, Song Hong and

Qiongdongnan basins (Figure 1). There are

many factors strongly inﬂuencing the structural

Figure 2. Sequence stratigraphic column of the northern Song Hong basin (Modiﬁed from VPI).

styles of basin rifting: the mechanical behaviour

of the pre-rift and syn-rift packages, the

tectonic activity before rifting, the obliquity of

rifting, and the tectonic activity after rifting [6].

In the northern Song Hong basin, the rift onset

unconformity is subtle because of the limited

uplift and erosion which occurred during the

early stages of rifting. Thus, much of the pre-

rift package is preserved beneath the syn-rift

package and the most basic stratigraphic units

associated with the northern Song Hong basin

rifting are the syn-rift and post-rift packages

(Figures 2 and 3). A distinct unconformity caps

the Paleogene syn-rift. The Dinh Cao formation sub-cropping the

unconformity has been documented to include Upper Oligocene

strata and the Phong Chau formation immediately overlying the

unconformity has been more ﬁrmly deﬁned as Lower Miocene

based on both planktonic foraminifera, nanofossils and palynology

[1, 7]. Syn-rift sediments are clearly observed as four units which are

more prominent and widespread in these grabens. In the area of the

eastern and northeastern sectors, the syn-rift sediments seem to be

observed as 3 units because the rift climax stage prevailed during

deposition of the upper part of Unit 2 and Unit 3 (Figures 1, 5 and

6). The eastern sector, which was earlier connected to the western

sector deepest low at basement level, got well diﬀerentiated as

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the rift progressed and became shallower and localised

towards the southwestern margin. Moreover, Ham Rong

- Ky Lan - Bach Long Vi spur in the central part gradually

becomes more prominent and widespread towards NW,

bifurcating the two lows.

features deﬁned by several large-scale structural

components (Figure 1). Major NW-striking extensional

faults stretch along the axis of the central northern Song

Hong basin and delineate a major Paleogene syn-rift

depocenterofthesefaults.TheSongChayfaultliesindirect

continuation of the brittle Song Hong and Song Chay

faults that ﬂank the Con Voi mountains metamorphic core

complex [1] (Figures 1 and 3). The central part of the Song

Hong basin broadens southeastward. In the northwest,

the narrow central depression of the Song Hong basin

is limited to a roughly 10 km broad belt in between the

Vinh Ninh and the Song Chay faults. The basin broadening

reﬂects left-steps in the NW-SE-striking faults that border

the eastern basin margin. On the southwestern ﬂank of

the basin, imbricate extensional faults with a 10 - 20^o

counterclockwise angle to the Song Chay fault contribute

to the southeastward depocentre broadening (Figures

1 and 3). Along the ﬂanks of the Song Hong basin and

towards the Beibuwan basin, the base of the Cenozoic

is marked by a strong “hard-kick” reﬂector (Figure 4). The

pre-Cenozoic is acoustically transparent across much of

the northern Nam Dinh basement high located at the

western ﬂank of the Song Hong basin. In addition, subtle

stratigraphic sub-Cenozoic reﬂectors and mounded

3. Syn-rift sediments within structural components of

the northern Song Hong basin

The northern Song Hong basin is deﬁned as a

continental rift and characterised as elongate crustal

depressions bounded on both sides by basement-

involved normal faults that cut the basement (Figures 1, 3

- 6). These extensional features are up to 8 km deep, more

than 15 km wide, and around 80 km long (Figures 3 and 4).

This rift is collections of stepping (Figure 2) and associated

with continental breakup, and rift systems can form in a

variety of tectonic settings [1, 6, 8]. It was formed through

Paleogene rifting followed by Late Cenozoic thermal

sagging [1, 2] and has been aﬀected by various Neogene

phases of extension and inversion with the most recent

inversion continuing to the present in the area (around

Bach Long Vi island) [1] (Figure 5).

The northern Song Hong basin rifting has complex

Figure 3. Seismic transect across the northern Song Hong basin illustrating the deep Paleogene rift and the strong Middle - Late Miocene inversion aﬀecting the central part of the basin.

EW-striking extensional faults continue into the lower Upper Miocene together with the transpressional structures. Karstiﬁed Upper Paleozoic carbonates sub-cropping the Cenozoic (wells

A and B).

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features often cored by a chaotic reﬂection pattern can be

distinguished especially in the southwest. On the eastern

basin ﬂank, distinctly reﬂected pre-Cenozoic stratigraphy

generally underlies the top of the pre-Cenozoic. In

some areas, the pre-Cenozoic is outlined by mounded

features with strong internal reﬂectivity documented as

karstiﬁed Devonian - Permian carbonates in Wells B and

C (Figures 3 and 4). The basin depth of the central Song

Hong basin decreases from more than 6,000 ms. TWT in

the southeast to a little more than 1,500 ms. TWT farthest

northwest (Figures 3 and 4). The central basin ﬁll is oﬀset

and deformed by long NW-SE-striking faults and folds. In

between the Vinh Ninh and the Song Chay faults, the Thai

Binh fault zone delineates a through-going fracture zone

in the northern central Song Hong basin (Figures 1 and

3). The fault zone is traceable within the Lower Miocene

and Upper Paleogene sections and must be rooted in the

basement, but the fault root is concealed underneath

the thick Cenozoic overburden. In addition, roughly EW-

striking extensional faults oﬀset strata in the basin centre,

but the deeper extent of these faults is unconstrained by

the seismic data due to the great basin depth (Figures

3 and 4). The base of the Cenozoic across the Nam Dinh

basement high is inclined towards the basin in the

northeast (Figure 1). The surface is sporadically faulted,

especially along the basin conﬁning margin, but is mostly

buried only by Neogene deposits except for a few isolated

minor, possibly Paleogene grabens (Figures 3 and 4).

Rifting was terminated by major latest Oligocene to the

earliest Miocene inversion reﬂecting a change to left-

lateral transpression. In comparison to the early phases

of shearing, the Neogene was aﬀected only by moderate

amounts of lateral displacement. This points towards

left-lateral shearing and extrusion tectonics initiating

sometimeduringtheLateEoceneandfurthercorroborates

a latest Oligocene or earliest Miocene slowdown of left-

lateral shearing and termination of ductile deformation

observed along the ASRRSZ onshore (Figure 4).

4. Structural controls on syn-rift sediments

The accommodation space created by faulting is

the primary control on the large-scale sedimentary

systems within the study area [1, 3, 6] (Figure 5). In this

paper, we present only structural controls on the syn-rift

sedimentary systems.

Rift basin stratigraphy commonly records an early

stage of major NW-striking extensional faults stretching

along the axis of the central northern Song Hong basin and

delineates a major Paleogene syn-rift depocentre [1 - 3].

The Paleogene rift basin development with stratigraphic

information is controlled by two tectonic phases (Figure

2) and as following a model-driven within three stages of

syn-rift as initiation, climax and late. Four equivalent syn-

rift units bounded by these seismic markers were named

as Unit 1, Unit 2, Unit 3 and Unit 4, from older to younger

(Figures 2, 5 and 6).The tops of Unit 1 and Unit 4 have been

assigned as Phu Tien and Dinh Cao whereas Unit 2 and

Unit 3 lie within Dinh Cao (Figure 2). These identiﬁed units

have distinct seismic facies. The seismic facies within the

units indicate their depositional environment associated

with the stages of rifting. Unit 1 was deposited during the

Figure 4. The seismic section illustrating the deep Paleogene rift. The northern Song Hong basin broadening reﬂects left-steps in the NW-SE-striking faults that border the eastern basin

margin. The base of the Cenozoic is marked by a strong “hard-kick” reﬂector.

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Figure 5. The seismic section illustrating the rift system. The accommodation space created by faulting and fault related topography is the primary control on the large-scale sedimentary

systems within northern Song Hong basin.

initial rift stage and distributed dominantly in the western

part of the study area. However, no well records have been

seen yet. It is only observed on the seismic data (Figures

3 - 6). It is diﬃcult to observe Unit 1 in other remaining

parts. The climax rift stage persists during deposition

of Unit 2 and Unit 3. Whereas, Unit 4 was deposited

during the late rift stage (Figures 2 - 5). In cross sections

perpendicular to the border-fault system (Figures 5 and

6), syn-rift succession exhibits a variety of depositional

patterns. The scarcity of Paleogene syn-rift deposits on

the western margin of the Song Hong basin suggests that

this area behaved as an uplifted rift shoulder ﬂanking

the main basin [1]. In contrast, the base of the Cenozoic

along the northeastern margin of the Song Hong basin is

strongly rifted with horst structures and grabens and half

grabens ﬁlled by thick Paleogene alluvial and lacustrine

deposits intersected in wells (Figures 3 - 6).

climax. The phenomenon is well demonstrated in the

eastern sector (Ha Mai, Bach Long Vi graben, SE Bach Long

Vi trough) and northeastern sector (Kien An, Thuy Nguyen,

Cam Pha graben) as grabens and troughs bounded by

smaller faults seen on basement levels, ﬁnally switched

to two major troughs having main displacement fault

(Figures 3 - 6).

In the period of initial rifting, stretching increased,

the rate of fault displacement is relatively low while

during the peak rift stage, the rate of fault displacement

increases markedly with abundant sedimentation. The

succession begins with a ﬂuvial unit. In the initiation rift

stage, faulting is most active, and a signiﬁcant topography

is created. The patterns of lithofacies development are

surprisingly consistent. Sediment supply to the basin is

usually limited in this stage (observed only on the seismic

data on signiﬁcant highs, no well records seen yet)

and, where the fault-driven subsidence rate is high the

reﬂection geometry looks like an overall wedge-shaped

geometry. The hummocky discontinuous reﬂectors

The early stage of rift development is characterised

by numerous fault bounding basins with displacement

switching to major basin bounding fault during the rift

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Figure 6. The seismic section illustrating the syn-rift succession formed under three stages of syn-rift as initiation, climax within tripartite stratigraphy: ﬂuvial, deep lacustrine and shal-

low lacustrine.

show a ﬂuvial system. Prograding reﬂector geometry is

observed in the very lowest ﬁll, implying sedimentation

was able to inﬁll the space created through extension

(Figure 6). The similar pattern is seen in Unit 1 and the

lower part of Unit 2, which demonstrates the early rift

stage during deposition.

source rocks of excellent quality are commonly developed

[3, 4], which will be done in an attempt to predict the

gross depositional outline of the Oligocene in these

areas, and thereby contribute to the prediction of source

and reservoir rock intervals (Figure 6). The late peak rift

is characterised on seismic section as a draping reﬂector

that can be traced across the area onto the adjacent

footwall and hanging wall crests (Figure 6).

During the climax rift stage, the maximum rate of

displacement on fault causes sedimentation outpaced by

extension or exceeds subsidence. The basin topography

gradually becomes ﬁlled with lake deposits [2 - 6, 8]. This is

typically developed best in syn-rift cycles with a lacustrine

unit that demonstrates a rapid deepening-upward

interval to a lake highstand interval. On the seismic

section, the peak rift is characterised by an aggradation

reﬂector with divergent forms related to continue tilting

of the hanging wall during deposition (Figures 6). Units 2

and 3 were formed during rift climax with distinct seismic

facies (Figure 6). The onset of peak rift started during

the lower part of deposition of Unit 2; the whole upper

parts of Unit 2 and Unit 3 are rift climax which have been

deposited in mid and late peak rift stage and associated

with the point at which transgression of the hanging wall

slope occurs. Rift climax sections identiﬁed in the study

area based only on seismic sections and well data show

that shallow and deep lake ﬂuvio-lacustrine to lacustrine

The late rift stage corresponds to a period of waning

fault activity, tilting decreases and stops when sediment

supply keeps pace with subsidence resulting into the

deposition of well-sorted clastics which would act as good

reservoir. Seismic pattern is observed as more continuous

and parallel reﬂectors than the earlier sequences (Figure

6). Unit 4 has been deposited in the late syn-rift stage and

expected to have better reservoir characteristics.

5. Conclusions

The northern Song Hong basin is a continental

rift basin and possesses complex features deﬁned by

several large-scale structural components. Major NW-

striking extensional faults stretch along the axis of the

central northern Song Hong basin and delineate a major

Paleogene syn-rift depocentre.

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The rift onset unconformity is subtle because of

limited uplift and erosion occurring during the early

stages of rifting. The most basic stratigraphic units

associated with the northern Song Hong basin rifting are

the syn-rift and post-rift packages.

Henrik I.Petersen, Ioannis Abatzis, and Hoang Anh Tuan,

“Density ﬂow deposition in a fresh water lacustrine rift-

basin, Paleogene Bach Long Vi graben, Vietnam”, Journal

of Sedimentary Research, Vol. 86, No. 8, pp. 982 - 1007,

2016. DOI: 10.2110/jsr.2016.53.

The Paleogene rift basin development with

stratigraphic information is controlled by two tectonic

phases and as following a model-driven within three

stages of syn-rift as initiation, climax and late and

sediments were generated by successive collapses

triggered by tectonic instability in the syn-rift lake.

[4] Henrik I.Petersen, Michael Fyhn, Lars Henrik

Nielsen, Hoang Anh Tuan, Chu Duc Quang, Nguyen Thanh

Tuyen, Pham Van Thang, Nguyen Thi Tham, Nguyen

Kim Oanh, and Ioannis Abatzis, “World-class Paleogene

oil prone source rocks from a cored lacustrine syn-rift

succession, Bach LongVi island, Song Hong basin, oﬀshore

Northern Vietnam”, Journal of Petroleum Geology, Vol. 37,

No. 4, pp. 373 - 389, 2014. DOI: 10.1111/jpg.12591.

The syn-rift stratigraphic architecture is known as a

tripartite stratigraphy. The succession begins with a ﬂuvial

unit. and is overlain by lacustrine sediments. Lacustrine

deposited as a result of accommodation space exceeds

sediment supply in the rift climax sections with shallow

and deep lake ﬂuvio-lacustrine to lacustrine source rocks

of excellent quality commonly developed in the troughs.

[5] Bui Huy Hoang, Michael B.W.Fyhn, Tong Duy

Cuong, Nguyen Quang Tuan, William J. Schmidt, Lars O.

Boldreel, Nguyen T.K.Anh, Nguyen Thu Huyen, Trinh Xuan

Cuong, Paleogene structural development of the northern

Song Hong basin and adjacent areas: Implications for the

role of extrusion tectonics in basin formation in the Gulf

of Tonkin, Tectonophysics, Vol. 789, 2020. DOI: 10.1016/j.

tecto.2020.228522.

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