Geological controls on focused fluid flow in the Song Hong Basin, offshore Vietnam

 Journal of Mining and Earth Sciences Vol. 62, Issue 3 (2021) 37 - 45 37 
Geological controls on focused fluid flow in the Song 
Hong Basin, offshore Vietnam 
Anh Ngoc Le * 
Faculty of Oil and Gas, Hanoi University of Mining and Geology, Vietnam 
ARTICLE INFO 
ABSTRACT 
Article history: 
Received 13th Feb. 2021 
Accepted 09th May 2021 
Available online 30th June 2021 
 This study uses 2D seismic lines located in the central Song Hong Basin, 
covering an area of c. 3900 m2, in the water depth of c. 100 m. Focused 
fluid flows are developed intensively and can be classified into two types: 
blow - out pipe and seepage pipe. They have similar seismic 
characteristics as a vertical zone of disturbed seismic reflections. The 
significant difference between them is the blow - out pipes associated with 
seafloor pockmarks and paleo - pockmarks which are absent in the 
seepage pipe. Besides, the scale of the blow - out pipe is larger, compared 
with the seepage pipe. The blow - out pipe is c. 500 m wide and 450÷3500 
ms TWT; the seepage pipe is smaller scale, c. 200 m wide and 500 ms TWT. 
Blow - out pipe is rooting from the sequence just above the diapir or 
deformation unit or deep lacustrine mudstones from the Late Eocene to 
Oligocene, and marine mudstones from the Early to Middle Miocene. The 
seepage pipe is rooting from the sequence above the diapir. The focused 
fluid flow is supposed to be controlled by the overpressured deep source 
layers and passive diapirism. The occurrence of focused fluid flow is an 
indicator for the active petroleum system in the study area. Intensive 
development of focused fluid flow proves a great hydrocarbon potential 
in the Song Hong basin. 
Copyright © 2021 Hanoi University of Mining and Geology. All rights reserved. 
Keywords: 
Blow - out pipe, 
Focused fluid flow, 
Seepage pipe, 
Song Hong Basin. 
1. Introduction 
Focused fluid migration in marine sediments 
is a widespread phenomenon that is increasingly 
gaining attention in the context of environmental 
discussions (Berndt, 2005). However, direct 
observations of hydrocarbon leakages are difficult 
for the obvious reason that they take place below 
the Earth’s surface (Løseth et al., 2011). Evidence 
of the active leakage of hydrocarbon and 
associated formation water is, therefore, most 
commonly found on the surface, seafloor, or in 
seawater (Cartwright et al., 2007; Løseth et al., 
2011), while remnants of subsurface paleo - 
leakage anomalies may be found in the outcrops 
(Parnell and Kelly, 2003). 
Focused fluid flow or pipe can best be defined 
seismically as columnar zones of disturbed
_____________________ 
*Corresponding author 
E - mail: lengocanh@humg.edu.vn 
DOI: 10.46326/JMES.2021.62(3).05 
38 Anh Ngoc Le/Journal of Mining and Earth Sciences 62(3), 37 - 45 
reflections that may or may not be associated with 
sub - vertically stacked amplitude anomalies. 
Before, these features are commonly ignored on 
seismic data because they tend to exhibit a 
vertical to sub - vertical geometry and can 
therefore be confused with seismic artifacts such 
as migration anomalies, scattering artifacts 
(Løseth et al., 2011). Therefore, care is needed in 
differentiating true pipes from seismic artifacts, 
which is best done by considering the structural 
and stratigraphic context of any probable pipe 
(Løseth et al., 2011). 
Cartwright et al. (2007) introduced the term 
seal bypass system, including pipes, mud 
volcanoes, sand injections, and gas chimneys. 
High fluid overpressure opening a hydrofracture 
Figure 1. Structural elements map of the Song Hong Basin (Nguyen - Hiep, 2019) and the location of the 
study area, indicated by four 2D seismic lines, marked from L1 to L4, covering an area of 3900 km2. 
 Anh Ngoc Le/Journal of Mining and Earth Sciences 62(3), 37 - 45 39 
through low permeable sediments is a common 
first phase of all these bypass structures. 
The Song Hong Basin is a northwest - 
trending transform extension basin, which began 
to develop in Mesozoic times on the passive 
continental margin of Vietnam (Figure 1) (Rangin 
et al., 1995; Zhenfeng and Baojia, 2008). Previous 
studies based on 2D and 3D multichannel seismic 
data have documented the occurrence of 
submarine gas seeps, focused fluid flow/pipe 
within the Song Hong Basin. However, the 
characteristic and occurrence of focused fluid 
flow are still limited. This paper presents a study 
of pipes from the central Song Hong Basin, 
describes the detail of the seismic pipe anomalies 
and investigates the geological controls on pipe 
distribution. 
2. Focused fluid flows 
The detailed structure of focused fluid flow is 
poorly understood at present and may be highly 
variable. In some cases, focused fluid flows consist 
of zones of deformed reflections related to minor 
folding and faulting. In others, they appear to 
consist of stacked pockmark craters or stacked 
localized amplitude anomalies that are likely to be 
small gas accumulations or zones of cementation 
but with no resolvable deformation (Cartwright et 
al., 2007) (Figure 2). According to these authors, 
there are four types of pipes, dissolution pipes, 
hydrothermal pipes, blow - out pipes, and seepage 
pipes. The latter two are closely related and can be 
distinguished by the presence or absence of 
surface features denoting a more dynamic flow
 regime along the pipe (Figure 2). 
Dissolution pipes form by rock dissolution 
occurring at depth to form subsurface cavities 
that promote instability in the overburden leading 
to collapse (Cooper, 1986) (Figure 2a). Thus, this 
type of pipe probable to occur in areas of 
evaporite or carbonate karst (Cartwright et al., 
2007). 
Hydrothermal pipes form by the release of a 
high flux of hydrothermal fluids associated with 
certain kinds of igneous intrusions, particularly 
mafic sills or laccoliths (Svense ... operties of the host rock. Blow - out pipes occur 
almost exclusively in fine - grained sealing 
sequences, whereas seepage pipes appear to 
occur in sand or silt - dominated sequences 
(Cartwright et al., 2007). 
An example of outcrops of blow - out pipes 
from Rhodes, Greece is shown in Figure 3, proving 
that vertical pipes form in relatively soft clays. 
Seismic modeling of an up - scaled 50 m wide pipe, 
which is filled with structureless material, proves 
to be similar to the seismic pipe anomalies 
offshore Nigeria (Løseth et al., 2011). 
3. Dataset and methodology 
The study purely used 2D seismic data in the 
offshore Song Hong Basin (from line 1 to line 4), 
covering an area of 3900 km2 (Figure 3). The area 
is located at a water depth of 100 m. The study 
interval ranges from Miocene to Pleistocene in age 
with a record time of 6.0 s Two Way Time (TWT). 
The seismic data is zero - phase and displayed in 
contrast color mode. 
The analysis focuses on fluid flows. The 
interpretation of the seismic data has primarily 
used the Schlumberger™ Petrel software. Because 
of the amplitude variation and discontinuous 
character of the focused fluid flow bodies, this 
vertical zone is, therefore, best detected in the 
chaos/RMS attribute, allowing to individualize 
the focused fluid flows from the substrate. Due to 
the lack of well data in the basin, the vertical depth 
will be quoted in two - way time. 
4. Results 
Seismic interpretation reveals the occurrence 
of focused fluid flows in the central Song Hong 
Basin. They are in general characterized as 
vertical zones with distorted seismic reflections, 
which are recognized below the seafloor craters 
(Figures 4, 5). In the distorted zone, various 
seismic reflection patterns are observed, from 
minor folding (Figure 6) to appear to consist of 
stacked paleo - pockmarks or dimmed reflections 
with variable continuity (Figures 4÷6). 
Figure 3. (a) Exposed pipe structure observed in the outcrop. The yellow measuring tape is 60 cm long. (b) 
The rim of the pipe is strongly sheared. When the center of the pipe (c) was breaked out it comprised 
angular clasts with black coating. The fractures surrounding the pipe commonly have slickensides (d) 
(Løseth et al., 2011). 
 Anh Ngoc Le/Journal of Mining and Earth Sciences 62(3), 37 - 45 41 
4.1. Blow - out pipes 
Most of the pipes observed in the study area 
are classified as blow - out pipes. The significant 
difference between the blow - out pipes and 
seepage pipes is the occurrence of surface 
pockmarks and stacked paleo - pockmarks 
(Figures 4, 5). The observation of modern 
pockmark and paleo - pockmarks associated with 
a pipe leading the interpretation of blow - out pipe 
in the area. The pipes have a deep source, high 
energy, large scale or shallow source, lower
energy, and smaller scale. 
Deep - sourced pipe is characterized as a 
vertical column of highly disturbed reflections 
(Figures 4 & 7). Local deformation of reflections is 
observed in the upper pipe and lower pipe. These 
are interpreted to be paleo - stacked pockmarks. 
The pipe created a crater on the seafloor; craters 
are about 10÷15 m. They are underlain by 
interpreted cones and seismic pipe anomalies that 
can be traced down to deep source zones at 3500 
ms TWT below the seafloor. The seismic pipe 
anomalies are 500÷1600 m wide and almost.
Figure 4. Seismic expression of blow - out pipe observed in line 3. This seismic profile showing vertically 
stacked paleo - pockmarks (cone zone) proving for longevity of blowout pipe activity. This blow - out 
pipe is rooting from a deep Oligocene overpressured unit. 
Figure 5. Seismic expression of blow - out pipe in line 1. This seismic profile showing vertically stacked 
pockmarks in the upper pipe. This shallow blow - out pipe is rooting from the high deformation unit. 
42 Anh Ngoc Le/Journal of Mining and Earth Sciences 62(3), 37 - 45 
Figure 6. Seismic expression of seepage pipe and blow - out pipe in line 3. Seepage pipe appears as 
columnar zones of disrupted reflections with localized amplitude anomalies. The pipe derives from a 
small fold on the lower section. The lack of any pockmark located at the upper termination of the pipe 
suggests low fluid flux and possible seepage. 
Figure 7. (a) A full seismic line (line 3) expressed of focused fluid flows and a diapir. The blow - out pipes 
have deep source in Oligocene/Miocene sequence; (b) Chaos amplitude attribute of line 3; (c) RMS 
attribute of line 3. The diapir seems to reveal better in the RMS than Chaos attribute. In the Chaos 
attribute the boundary of diapir is hard to be detected. 
 Anh Ngoc Le/Journal of Mining and Earth Sciences 62(3), 37 - 45 43 
vertical. This blow - out pipe roots from deep 
lacustrine mudstones from the Late Eocene to 
Oligocene, and marine mudstones from the Early 
to Middle Miocene 
A shallow - sourced pipe is observed as a 
vertical zone, characterized as disturbed seismic 
reflections (Figures 5, 6). The pipe terminated on 
the seafloor in form of a pockmark. Underlying the 
seafloor pockmark is stacked of paleo - 
pockmarks which are dominated on the upper 
part of the pipe, giving the confidence for the 
interpretation of blow - out pipe. This pipe has a 
smaller scale compared with the deep source 
pipe. The pipe is about 400 m wide and 450 ms 
TWT long, rooting from the high deformation unit 
in Pliocene. 
4.2. Seepage pipes 
Seepage pipe is distributed above the 
deformation unit (Figure 6). The pipe has a similar 
form to the blow - out pipe. The pipe is defined 
seismically as a columnar zone of disturbing 
reflections, except small folded reflections 
observed at the lower pipe. It is about 200 m wide 
and 500 ms TWT long. There is no clear image of 
the crater on the seafloor at the pipe termination 
and also the paleo - pockmarks along the pipe. 
This indicates the lack of blow - out craters 
diagnosing an explosion of fluid at the upper pipe 
termination. This pipe, therefore, is defined as a 
seepage pipe. 
The pipe is rooting from the thermal sag unit 
just above the diapir. Diapir acts as the main 
conduit for the upward movement of fluids 
released from the deep high - pressure system 
(Figure 7). Abnormally high pressure provides a 
significant driving force for gas migration 
(Zhenfeng and Baojia, 2008). The diapir is in the 
passive phase creating a thermal sag mini basin 
above the diapir, with a total thickness of 300 ms 
TWT. The remaining high pressure in the core of 
the diapir is transferred to the thermal sag 
sequence, resulting in the leaking fluid and 
creating the seepage pipe observed above the 
diapir. 
5. Discussion 
Overpressured fluid, generated from the 
combined factors of under compaction, 
hydrocarbon generation, hydrothermal 
pressurization, and tectonic compression, is the 
driving force of the evolution of diapir (Wan et al., 
2019). Blow - out pipes tend to be localized at 
natural leakoff points for overpressured pore 
fluids, for example, at the crests of structures, 
above gas reservoirs, or at the updip limits of 
aquifers (Cartwright et al., 2007). The genetic 
process of a high - pressure gradient drives the 
fluid flow in such a violent way as to produce the 
columnar conduit. The distribution of the focused 
fluid flow is related to the overpressure in the 
muddy sequence of Oligocene and Miocene age 
and associated with the diapir occurrence. 
Kilometer scale focused fluid flow roots directly 
from the overpressured layer in the deep section, 
piercing the upper sequence. Its termination 
created a seafloor pockmark. In addition, hundred 
- meter scale focused fluid flows are developed 
from the top of the diapir as a result of passive 
diapirism. The distribution of mud diapir is 
possibly related to the population of the focused 
fluid flows. Mud diapir was intensively developed 
in the central Song Hong Basin and documented in 
both Vietnam and China parts (Wan et al., 2015; 
Zhenfeng and Baojia, 2008; Nguyen - Hiep, 2019; 
Le and Ha, 2020; Le, 2021). 
The blow - out pipes in the area has two 
sources. They are either rooting from a deep 
source or a shallow source. The deep source blow 
- out pipe has much larger scale compared with 
the shallow blow - out pipe. It roots from the same 
source with the diapir in the area and thus has 
high energy to form large‐size blow - out pipe. The 
shallow source blow - out pipe developed above 
the diapir, as a result of passive phase of diapism 
thus has much lower energy, resulting in small 
scale pipes. However, the occurrence of seepage 
pipes above the diapir indicates the unequal 
energy distribution of the diapir itself below. 
In the study area, the seismic characteristics 
of the pipes and also associated geological feature 
indicate the existence of blow - out pipes and 
seepage pipes. The shallow pipes developed from 
the late Miocene to Pleistocene. There is no 
carbonate or evaporate in these sequences; 
therefore, there is no chance to procedure the 
dissolution pipes. Moreover, the deep - source 
pipe is rooting directly from the 
Oligocene/Miocene sediments; thus, this pipe has 
44 Anh Ngoc Le/Journal of Mining and Earth Sciences 62(3), 37 - 45 
no relationship with magma composition, and it is 
impossible to be the hydrothermal pipe. 
The occurrence of focused fluid flows is 
considered to be associated with hydrocarbon 
generation, thus, suggests the active petroleum 
system in the study area. This possibly produces a 
commercial reserve for the area. This 
interpretation is supported by the occurrence of 
an active petroleum system discovered in the part 
of China. The Dongfang 1 - 1 (DF1 - 1) mud diapir 
field is the largest gas field discovered in the Song 
Hong basin up to now (Zhenfeng and Baojia, 
2008). The distribution of focused fluid flows in 
the study area suggested its relationship with the 
overpressured layer in the deeper part and the 
occurrence of diapir in the area. 
6. Conclusions 
Focused fluid flows in the central Song Hong 
basin are developed intensively at different scales. 
There are two types of focused fluid flows, 
including blow - out pipes and seepage pipes. The 
blow - out pipes are rooting from either a deep 
source or shallow source; meanwhile, the seepage 
pipe is rooting from the shallow one. Blow - out 
pipes are terminated on the seafloor in form of 
seafloor pockmarks at about 500 m wide and 450 
to 3500 ms TWT. It is characterized as stacked of 
paleo - pockmarks along the pipe or dominantly in 
the upper part of the pipe. Seepage pipe is a 
smaller scale, c. 200 m wide and 500 ms TWT, 
characterized as a disturbed zone with minor 
folding at the low part of the pipe. The occurrence 
of focused fluid flows indicates an active 
petroleum system which proves a great 
hydrocarbon potential in the study area, and 
groups are as different as possible from others. 
Author contributions 
The author contributes 100% to the article. 
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