Review Article A Geophysical Review of the Seabed Methane Seepage Features and Their Relationship with Gas Hydrate Systems - Hindawi.com
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Hindawi
Geofluids
Volume 2021, Article ID 9953026, 26 pages
https://doi.org/10.1155/2021/9953026
Review Article
A Geophysical Review of the Seabed Methane Seepage Features
and Their Relationship with Gas Hydrate Systems
Jinxiu Yang ,1,2 Mingyue Lu,2 Zhiguang Yao,3,4 Min Wang,1,2 Shuangfang Lu,1,2 Ning Qi,2
and Ying Xia2
1
Key Laboratory of Deep Oil and Gas, China University of Petroleum (East China), Qingdao 266580, China
2
School of Geosciences, China University of Petroleum (East China), Qingdao 266580, China
3
Engineering Technology Research Company Limited, China National Petroleum Corporation, Tianjin 300451, China
4
Key Laboratory of Offshore Engineering, China National Petroleum Corporation, Tianjin 300451, China
Correspondence should be addressed to Jinxiu Yang; yangjinxiu@upc.edu.cn
Received 12 March 2021; Revised 8 July 2021; Accepted 6 September 2021; Published 12 October 2021
Academic Editor: Wei Zhang
Copyright © 2021 Jinxiu Yang et al. This is an open access article distributed under the Creative Commons Attribution License,
which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.
Seabed methane seepage has gained attention from all over the world in recent years as an important source of greenhouse gas
emission, and gas hydrates are also regarded as a key factor affecting climate change or even global warming due to their
shallow burial and poor stability. However, the relationship between seabed methane seepage and gas hydrate systems is
not clear although they often coexist in continental margins. It is of significance to clarify their relationship and better
understand the contribution of gas hydrate systems or the deeper hydrocarbon reservoirs for methane flux leaking to the
seawater or even the atmosphere by natural seepages at the seabed. In this paper, a geophysical examination of the global
seabed methane seepage events has been conducted, and nearby gas hydrate stability zone and relevant fluid migration
pathways have been interpreted or modelled using seismic data, multibeam data, or underwater photos. Results show that
seabed methane seepage sites are often manifested as methane flares, pockmarks, deep-water corals, authigenic carbonates,
and gas hydrate pingoes at the seabed, most of which are closely related to vertical fluid migration structures like faults,
gas chimneys, mud volcanoes, and unconformity surfaces or are located in the landward limit of gas hydrate stability zone
(LLGHSZ) where hydrate dissociation may have released a great volume of methane. Based on a comprehensive analysis of
these features, three major types of seabed methane seepage are classified according to their spatial relationship with the
location of LLGHSZ, deeper than the LLGHSZ (A), around the LLGHSZ (B), and shallower than LLGHSZ (C). These
three seabed methane seepage types can be further divided into five subtypes considering whether the gas source of seabed
methane seepage is from the gas hydrate systems or not. We propose subtype B2 represents the most important seabed
methane seepage type due to the high density of seepage sites and large volume of released methane from massive focused
vigorous methane seepage sites around the LLGHSZ. Based on the classification result of this research, more measures
should be taken for subtype B2 seabed methane seepage to predict or even prevent ocean warming or climate change.
1. Introduction and fluids involve hydrocarbon gas especially methane,
water, and even sediments [1]. Seabed seepage features are
In recent years, seabed methane seepage has gained attention often manifested as methane flares, pockmarks, deep-water
from all over the world as an important source of green- corals, authigenic carbonate, and gas hydrate pingoes at the
house gas emission which may affect climate change or even seabed, which are often related to vertical or subvertical fluid
global warming (Figure 1) [1–4]. Seabed methane seepage migration conduits [1, 5]. According to Talukder [1], there
features, also termed as “cold seeps,” refer to the seeping or are three elements for seabed methane seepage including
venting features of fluids at the seabed. Seeping often indi- source, plumbing system, and seabed methane seepage
cates slow fluid fluxes, venting indicates fast vigorous fluxes, structures. Seabed methane seepage is commonly used as
2 Geofluids
Gas hydrate
Cold seeps
(a)
Coral reef
(b)
Figure 1: (a) Global distribution of marine gas hydrates and cold seeps (modified from Klauda and Sandler [6]; German et al. [7]). (b)
Global distribution of deep-water reef-forming corals (from Roberts et al. [8]).
an indicator of subsurface gas reservoirs or gas hydrate sys- Gas hydrate systems are often regarded as an important
tems, which presents the leakage of methane from the shal- methane sink because of the large amounts of methane they
low lithosphere to the hydrosphere and atmosphere. captured, often occurring in shallow sediments at water
However, most case studies on seabed methane seepage at depths greater than a few hundred meters, and the base of
present often focus on description of the seepage features, gas hydrate stability zone varies in different research areas
and only a few studies investigated the source of methane with different latitudes controlled by the pressure and tem-
and the migration history from the source to the seabed perature conditions (Figure 1(a)) [9, 10]. Gas hydrates are
seepage sites. sensitive to environmental variations like sea level change,
Geofluids 3
seabed temperature change (also called ocean warming), sea- ever, seabed manifestations and related fluid conduits
bed bottom currents, salinity change of seawater, and ice provide clues for methane source information, such as the
sheet fluctuations, all of which can change the pressure and source depth, its relationship with the gas hydrate systems,
temperature conditions of gas hydrate stability, therefore and possible roles in material cycling, which is the objective
causing gas hydrate dissociation and probably subsequent of this study.
methane seepage at the seabed [3, 4, 11–15]. For example, In this paper, we mainly focus on the methane source of
NASA scientists observed millions of seabed methane seeps the seabed methane seepage and discuss its relationship with
in the Arctic region and it was proposed that the postglacial the gas hydrate systems by analysing their spatial relation-
climate warming and deglaciation caused the dissociation of ship. Global case studies from Niger Delta, offshore Maurita-
gas hydrates, therefore leading to the seabed methane release nia, and South China Sea and other multilatitude regions are
[16, 17]. Similar phenomena have also been observed in US discussed in this paper, which are featured by different geo-
Atlantic margin [18–20], offshore Svalbard [21–23], and logical conditions. For example, Niger Delta is located on the
Spitsbergen continental margin [21], where the seabed passive continental margin on the west coast of central
methane seepage features are all interpreted to be due to Africa, and the structural units of the Niger Delta can be
the dissociation of gas hydrates. divided into three domains, the extension domain, the trans-
However, not all seabed methane seepage features are lation domain, and the compression domain, featured by
related to gas hydrate systems and some may be from the growth faults, mud diapirs, shale diapirs, and folding-
deeper gas reservoirs [24]. Due to the proper temperature thrusting features [33, 34]. Offshore Mauritania is located
and pressure conditions for gas hydrate formation at the sea- on the West African passive continental margin, with lim-
bed, gas hydrate outcrops are often observed at the seabed ited faulting and fracturing development [10, 35, 36]. The
methane seepage sites, but this does not mean the gas source South China Sea is a large marginal sea characterized by
is directly from the gas hydrate systems. Therefore, it is continental rifting and seafloor spreading, with high heat
important to analyse the gas source of seabed methane seep- flow value [37, 38]. Additionally, large-scale gas chimneys
age and its relationship with the gas hydrate systems, which are common in South China Sea with faults and fractures
can help better understand the potential contribution of gas existing in the interior of the gas chimneys. In these offshore
hydrate systems or the deeper gas reservoirs for methane study areas, gas hydrate systems are often developed, with
leaking to the seawater or even the atmosphere. In this focused fluid migration providing gas source for hydrate for-
paper, we investigate the seabed methane seepage case stud- mation which may also have caused the seabed methane
ies all over the world, the relevant fluid migration pathways seepage features.
and gas source are analysed, and its relationship with the gas
hydrate systems is used to classify the types of seabed meth- 2.2. Gas Hydrate Stability Zone (GHSZ). Marine gas hydrates
ane seepage. occur worldwide on continental active and passive margins,
marginal seas, and island slopes, and the gas hydrate stability
2. Geological Conditions zone (GHSZ) is mainly determined by factors including sea-
bed temperature, seabed depth, geothermal gradient, gas
2.1. Seabed Methane Seepage. Seabed methane seepage rep- composition, and pore water salinity, with temperature and
resents the transfer of methane from the geosphere to hydro- pressure being the primary factors affecting the phase equi-
sphere, biosphere, or even atmosphere, which is regarded as librium of gas hydrates [20, 39, 40]. Gas hydrates play dual
a process of the Earth’s carbon recycling [25, 26]. Seabed roles in upward methane migration during geological
methane seepage features can occur globally at different geo- periods, which can act as time-variable sources or sinks for
logical settings including active margins (e.g., convergent methane [41, 42]. On the one hand, methane can be cap-
margin, accretionary margin, erosive margins, and trans- tured in the GHSZ by forming gas hydrates during upward
form margins) and passive margins [25, 27]. Seepage fluid migration. On the other hand, hydrate dissociation will
composition often includes water, free gas (especially meth- release a great volume of methane which escapes to the shal-
ane), and some sediments, and they transfer from different lower sediments or even the seawater by seabed methane
sources upwards to the seafloor by different forcing mecha- seepage.
nisms such as sediment compaction, methane overpressure In seismic data, the base of GHSZ is often indicated by
caused by gas hydrate dissociation, biogeochemical reac- bottom simulating reflection (BSR), which is normally char-
tions, biological activities, overpressure, and facies changes acterized by strong amplitude, negative polarity, and cross-
[25, 26, 28]. Apart from seabed seepage manifestations cutting chronostratigraphic reflections. In high-resolution
(e.g., pockmarks, deep-water corals, and authigenic carbon- seismic data, BSR is often featured as aligned amplitude ter-
ate crusts on mounds or pavements), fluid conduit features minations [43–45]. Due to the shallow burial and weak sta-
(such as slope failures, faults, mud volcanoes, scars, scarps, bility of the gas hydrate system, the gas hydrate stability
and bulges) also play a big role in facilitating fluid escape conditions are easy to be affected by ocean currents, sedi-
and seabed methane seepage formation [29, 30]. The magni- mentation, diapirism, and other environmental changes,
tude of seabed methane seepage is difficult to determine resulting in the formation and decomposition of natural
because the seepage features, including bubble size, velocity, gas hydrates [46–48]. The methane captured and stored in
and concentration, often show significant temporal variabil- the gas hydrate systems is therefore prone to escape to the
ity on time scales from seconds to decades [31, 32]. How- seawater or even atmosphere through seabed seepage
4 Geofluids
features. As a result, it is necessary to better understand the alies transported from underneath by folding or faulting.
spatial relationship between seabed methane seepage and the The results are dependent on several assumptions including
gas hydrate systems, as well as related fluid migration velocities of seawater and sediments, as well as gas composi-
conduits. tion. Changes in these parameters would lead to different
However, the gas hydrate occurrence is not always indi- RGG values, which represent the uncertainties of this
cated by an obvious BSR in seismic data, and gas hydrates modelling. With the numerical modelling, the GHSZ range
may exist without BSR such as in Site 994 of ODP Leg 164 can be predicted which can help characterize the spatial rela-
[10]. When it exists, the BSR often decreases as the seabed tionship between gas hydrate systems and the seabed meth-
shallows until it intersects with the seabed, where it is known ane seepage features, therefore evaluating the role of gas
as the landward limit of gas hydrate stability zone (LLGHSZ) hydrate systems in forming seabed methane seepage.
representing the shallowest water depth of GHSZ. Due to
limited methane supply and the anaerobic oxidation of 3. Characterization of Seabed Methane
methane (AOM) at shallow depth, both BSR and LLGHSZ Seepage Manifestations
are rarely visible in the vicinity of the theoretical LLGHSZ
in seismic data in many gas hydrate occurrence areas [4, Seabed methane seepage is often characterized by seabed
35, 48–50]. In areas without BSR development, it is difficult manifestations such as methane flares, pockmarks, carbon-
to recognize the GHSZ which may hinder judging whether ate crusts on mounds or pavements, coral reefs, mud volca-
the methane source for the seabed methane seepage is noes, hydrate pingoes, and chemosynthetic biological
related with the gas hydrate systems or not. In these circum- communities [1, 4, 12, 52, 53]. The size and density of these
stances, the BSR can be numerically modelled in MATLAB seabed manifestations reflect the volume of methane trans-
software by assuming a proper gas composition and a rea- ferred from geosphere to hydrosphere or even the atmo-
sonable salinity for the modelled gas hydrate systems sphere, whose presence also indicates the existence of
(Figure 2). The temperature distribution is calculated by a underlying hydrocarbon reservoirs or gas hydrate systems
two-dimensional steady-state heat conduction model, with [1, 53]. The gas source of methane seepage at the seabed
related equation discretized in space using finite differences can be from shallow biogenic gas and deep thermogenic
and solved directly using MATLAB’s MLDIVIDE function. gas or from gas hydrate dissociation [25]. However, gas
The upper boundary condition is determined by fixing the hydrate outcrops are often observed at the seabed methane
temperature at the seabed to the seawater temperature at seepage sites regardless of their gas sources, which are
the depth of the local seabed surface (retrieved from the mainly determined by the proper temperature and pressure
World Ocean Database; Figure 2). The lower boundary con- conditions at the seabed. Therefore, we propose that the
dition is applied by fixing the geothermal gradient to a uni- presence of gas hydrate outcrops does not indicate that the
form value at 5 km below the seabed. This boundary gas source of seabed methane seepage is directly from the
geothermal gradient is therefore referred to as the regional gas hydrate systems. In this section, the detailed characteris-
geothermal gradient (RGG). The depth of the seabed is cal- tics of main seabed methane seepage features are introduced,
culated from the TWT time, which is converted to depth including pockmarks and carbonates, deep-water corals, and
using a water velocity of 1.5 km s-1. A pressure profile is mud volcanoes. Related fluid migration conduits are inter-
obtained by assuming a constant hydrostatic pressure gradi- preted, as well as nearby gas hydrate systems. Based on a
ent of 10.09 MPa km-1. Then, the depth of the BSR can be combined analysis of these factors, the specific gas source
determined using the hydrate stability curve which is suit- of seabed methane seepage can be determined which help
able for the studied gas hydrate system. accurately evaluate the role of different gas sources in affect-
For example, the gas hydrate stability curve for pure ing the carbon cycling by seabed methane seepage.
methane with a salinity of 35 wt. % given by Moridis [51]
is used to predict the BSR locations in two seismic profiles 3.1. Pockmarks and Carbonates. Pockmarks are crater-like
from Niger Delta and Qiongdongnan Basin of South China depressions developed in fine marine or lacustrine sediments
Sea (Figure 3). The depth of the BSR is determined by in various geological environments, probably due to over-
hydrate stability conditions and can be calculated by numer- pressure discharge of subsurface fluids at the seabed
ical modelling through assuming proper controlling param- [54–56]. As the result of fluid migration through sediments
eters. Regional geothermal gradient (RGG) is considered as a and expulsion at the seabed, pockmarks were often inter-
calibration parameter in this context, and an optimal fit preted to overlie hydrocarbon reservoirs or gas hydrate sys-
between the modelled and observed BSR was obtained by tems (including gas hydrates and underlying free gas zone)
adjusting the RGG value in the geothermal gradient range which are the possible fluid sources [1, 53]. Limited data
of the study area. Results show that optimal fit between the has been known about the process of fluid venting which
modelled and observed BSR can be obtained by setting a formed and maintained the pockmarks, but several mecha-
RGG value of 4.2°C/hm for the seismic profile from Niger nisms have been raised as drivers of fluid migration includ-
Delta and by setting a RGG value of 3.5°C/hm for the seis- ing migration from overpressured hydrocarbon reservoirs
mic profile from Qiongdongnan Basin (Figure 3). For both along vertical conduits like faults, fractures, and gas chim-
seismic profiles, the BSRs at the crest of the anticlines neys, sediment compaction, overpressure dissipation via
(Figure 3(a)) or above the focused cluster of faults focused vertical fluid migration conduits or salt/mud diapirs,
(Figure 3(b)) are vaulted, probably due to the thermal anom- gas hydrate dissociation, and related methane seepage
Geofluids 5
0 0
500
1000 500
Depth (m)
1500
Depth (m)
1000
2000
30 31 32 33 34 35 36
Salinity (psu)
1500
2000
0 10 20 30
Temperature (degrees°C)
(a) (b)
22°N
20°N
Hainan
18°N
16°N
a
a Se
Vietnam in
14°N Ch
h
ut
So
12°N
106°E 108°E 110°E 112°E 114°E 116°E
(c)
Figure 2: Data used in numerical modelling of the BSR. (a) Scatter plot of the sea water salinity with varying seabed depths in the northern
South China Sea. (b) Scatter plot of the sea water temperature with varying seabed depths in the northern South China Sea. (c) Location map
of the data points.
around the LLGHSZ. Pockmarks occur in both random and fluid migration conduits since the gas source is directly from
nonrandom distribution patterns, probably controlled by in situ hydrate dissociation.
both the distribution of subsurface fluid conduits and the In global case studies, there are different kinds of pock-
overpressured fluid source [53]. However, at the LLGHSZ marks. For example, giant pockmarks formed in the Faroe
where gas hydrate dissociation often occurs, pockmarks Shetland Basin due to a sudden expulsion of a methane res-
may occur in linear patterns without obvious subsurface ervoir which is deeper than the GHSZ, probably driven by
6 Geofluids
Pockmark
3400
Observed BSR
3600 Seabed
3800 Modelled BSR
TWT (ms)
4000
Fold Fold
4200
Fault
4400
Fault
4600
3 km Fault
(a)
Pockmark
Pockmark
1500 Pockmark Seabed
1600
Observed BSR
TWT (ms)
1700
HAAs
1800
1900 Modelled BSR
2000 Faults
3 km
(b)
Figure 3: Numerical modelling results showing the observed BSR (yellow line) and the modelled BSR (blue line). (a) A seismic profile from
Niger Delta showing faults and seabed pockmark. (b) A seismic profile from Qiongdongnan Basin showing faults and seabed pockmarks.
natural reservoir overpressure [57]. The overpressure was around the LLGHSZ. Gas hydrate dissociation due to differ-
interpreted to be mainly trigger by tectonic compression ential sediment loading and global warming was proposed to
because other causes are too gradual and modest, which be responsible for the formation of these pockmarks, which
opened the vertical conduits by exploiting the weakness in caused pore fluid overpressure and vertical fluid migration
the seal. Figure 4 shows the relationship between pockmarks [19]. Within the GHSZ scope, there are few pockmark
and gas hydrate systems in the U.S. Atlantic continental occurrence at the seabed compared to the huge amount of
margin, and there are two types of pockmarks considering pockmarks around the LLGHSZ. The fluid source for these
their relationship with the gas hydrate systems [18, 19]. pockmark formation could be from both gas hydrate sys-
The pockmarks much shallower than the LLGHSZ are tems and deeper hydrocarbon reservoirs, which can be
caused by the overpressure of the shallow gas reservoir, irrel- deduced by the vertical distribution of fluid migration con-
evant to the gas hydrate systems (Figure 4(a)). In duits [58, 59]. As shown in Figure 4(c), these two pockmarks
Figure 4(b), more than 5000 pockmarks were observed in a were formed as a result of overpressure dissipation, and the
1100 km stretch along the strike of the U.S. Atlantic conti- overpressure was probably caused by the continuous accu-
nental margin, and most of the pockmarks are located mulation of free gas below the base of gas hydrate stability
Geofluids 7
0
Pockmark 5 km
s
500
C
1000 Zone of high chimney probability
TWT (ms)
Pockmark
s
1500
GHSZ
2000
Free Gas
Zone of high chimney
2500
probability (Eocence and
Cretaeous strata)
(a)
Seafloor
Inactive Poc
kmarks
~10 m
300 m
Gas hydrate zone
Site 1073
Chimney
Base of
Pleistocene
Free gas
zone
~300 m
~100 m
(b) (c)
Figure 4: (a) Schematic diagram showing pockmarks, related gas chimneys, and nearby gas hydrate stability zone (modified from Brothers
et al. [19]). Pockmarks in the updip of the GHSZ are interpreted to be due to the overpressure of the shallow gas reservoir, which is irrelevant
to the gas hydrate system. Additionally, the pockmarks within the GHSZ scope were caused by the overpressure of the free gas zone which
triggered the formation of gas chimneys and pockmarks. (b) 3D view of seismic data showing the landward limit of GHSZ and nearby
numerous pockmarks which were the result of gas hydrate dissociation due to the seaward retreat of the GHSZ during global warming
or sea level change (modified from Brothers et al. [19]). (c) Seismic data showing the inactive pockmarks, gas chimneys, and the free gas
zone below the base of the gas hydrate stability zone (modified from Plaza-Faverola et al. [60]).
zone [60]. The pockmarks became inactive shortly after with sediments and bacterial mats first and complete block-
transient initial expulsion, which may be reactivated when age by carbonate precipitation which builds up a carbonate
the free gas accumulates again and generates required over- crust [1, 67]. Additionally, authigenic carbonates are often
pressure [57, 60]. Besides the present pockmarks, there are regarded as associated with the gas hydrate systems [68,
also paleopockmarks which are buried pockmarks indicating 69]. A case study from the southern summit of the Hydrate
the fluid escape features in the geological past [61] Ridge of the Cascadia margin shows that the carbonate for-
(Figures 5(c) and 5(d)). mation may be related to the dynamic formation and disso-
Authigenic carbonates often extensively coexist with ciation of gas hydrates due to its fragile stability [41].
methane seepage features in different geological settings, However, the relationship of gas hydrates and carbonates is
representing the removal of methane and inhibiting its emis- in substance related to the fluid source, and it was proposed
sion to the sea water or even the atmosphere [62–64]. It is that prolific methane seepage will lead to increased AOM
commonly believed that the biological process of anaerobic efficiency [63]. For example, researchers found that 10 m
oxidation of methane (AOM) couple to sulphate reduction high ridges of methane-derived authigenic carbonate rocks
(SR) is mediated by anaerobic methanotrophs and developed within a large pockmark in mid-Norway [70]
sulphate-reducing bacteria in anaerobic environments, with (Figures 5(a) and 5(b)). The amount of authigenic carbon-
authigenic carbonates being the main by-product of the ates is comprehensively determined by the methane supply
AOM-SR process [63–66]. According to previous studies, and the rate of AOM and the duration time [71]. Not all
the formation of carbonates will eventually seal the seeping pockmarks are accompanied by authigenic carbonates. For
feature, which is characterized by partial seepage clogged example, in the northern Carnarvon basin offshore
8 Geofluids
N
o ⁎
100 m
(a) (b)
NNW-SSE Faults Paleo-pockmarks
West East
200 ms
Paleo-Pockmarks
1 km Pipes
1 km
N
(c) (d)
Carbonate
reef mounds
200 ms
N 2 km East
West
2.5 km
(e) (f)
Figure 5: (a) Photo of carbonates at the seabed observed by ROV located within a large pockmark as shown in (b) (modified from Hovland
et al. [70]). (b) A large pockmark with carbonates developed within it in mid-Norway (modified from Hovland et al. [70]). (c) Plan view map
showing the linear distribution of paleopockmarks in Exmouth Plateau, Northern Carnarvon Basin (modified from Velayatham et al. [61]).
(d) A seismic profile showing the paleopockmarks in Exmouth Plateau, Northern Carnarvon Basin (modified from Velayatham et al. [61]).
(e) Plan view map showing the carbonates in Exmouth Plateau, Northern Carnarvon Basin (modified from Velayatham et al. [61]). (f)
Seismic profile showing the positive relief of carbonates in Exmouth Plateau, Northern Carnarvon Basin (modified from Velayatham
et al. [61]).
northwest Australia, the pockmarks in the east are not the gas hydrate system. Compilation shows that 62.2% pock-
accompanied by carbonate rocks, while the pockmarks in marks are accompanied by authigenic carbonates, and the
the west are accompanied by carbonate rocks [61] preponderance of pockmarks coexists with the gas hydrate
(Figures 5(c)–5(f)). systems or around the LLGHSZ. According to previous
In Table 1, we supplement the pockmark compilation of studies, the pockmark density and total number of pock-
Panieri et al. [53] by referring to more published global case marks are the largest around the LLGHSZ which has been
studies, displaying information on water depth, pockmark observed in Arctic continental shelves [72, 73], US Atlantic
formation mechanism including relevant fluid pathways margin [18–20], and the West Spitsbergen continental mar-
and gas source, nearby carbonates, and relationship with gin [21].
Geofluids 9
Table 1: Compilation of pockmark and carbonate locations around the world showing formation mechanisms and relationship with the gas
hydrate systems (modified from Panieri et al. [53]).
Relationship with
Water
Pockmark location Formation mechanisms Carbonates the gas hydrate References
depth
system
Arctic Ocean (Vestnesa Fluid pathways: faults, chimneys, and fractures Carbonates of Deeper than the [60, 74,
1200 m
Ridge) Gas source: deep reservoir and shallow free gas various shapes LLGHSZ 75]
Fluid pathways: tectonic controls (faults)
Arctic Ocean (Spitsbergen 110- Shallower than
Gas source: thermogenic gas from the organic- No report [76]
fjords) 130 m the GHSZ
rich shales
Fluid pathways: gas pressure and faults
200- Authigenic Gas hydrates at
Barents Sea Gas source: thermogenic gas, hydrate [77–80]
1270 m carbonates different depths
dissociation
< Fluid pathways: faults and fractures Shallower than
Bering Sea (Alaska) No report [81]
200 m Gas source: biogenic and thermogenic gas the GHSZ
Norwegian Sea (Troll Fluid pathways: gas hydrate dissociation Authigenic Shallower than
300 m [82]
region) Gas source: gas hydrate carbonates the GHSZ
Norwegian Sea Fluid pathways: overpressure, chimneys, and Shallower than
300-
(continental shelf and fractures Carbonate ridges the GHSZ [83]
800 m
slope) Gas source: gas reservoir and free gas Around LLGHSZ
Around LLGHSZ
Norwegian Sea (Nyegga 500- Fluid pathways: chimneys and faults Carbonates of
Deeper than the [84]
region) 1500 m Gas source: gas reservoir and free gas various shapes
LLGHSZ
Around LLGHSZ
400- Fluid pathways: gas hydrate dissociation
NE Atlantic margin No report Deeper than the [85]
1500 m Gas source: gas hydrate
LLGHSZ
Strait between Norway and 100- Fluid pathways: clay diapir and faults Carbonate Shallower than
[86]
Denmark (Skagerrak) 200 m Gas source: biogenic and thermogenic gas cemented the GHSZ
Fluid pathways: fault formation inside iceberg Authigenic
130- Shallower than
Canada (Pacific margin) ploughmarks carbonates, [87, 88]
250 m the GHSZ
Gas source: hydrocarbon gas carbonate ridges
Fluid pathways: degradation of subseabed
North Sea, UK sector 100- Shallower than
permafrost ice No report [89]
(Witch Ground Basin) 150 m the GHSZ
Gas source: biogenic and thermogenic gas
Atlantic Ocean (Belfast Fluid pathways: structure Shallower than
30 m No report [90]
Bay, Maine) Gas source: biogenic natural gas and pore water the GHSZ
Biogenic
Pacific Ocean (Big Sur, 900- Fluid pathways: no report Deeper than the
carbonate [91]
California) 1200 m Gas source: free gas LLGHSZ
fragments
Shallower than
100- Fluid pathways: fault Carbonate
Gulf of Mexico the GHSZ [92]
500 m Gas source: thermogenic gas buildups
Around LLGHSZ
Fluid pathways: faults and salt diapirism Around LLGHSZ
Atlantic Ocean (Santos 400-
Gas source: salt diapirs and hydrocarbon No report Deeper than the [52, 93]
Basin) 900 m
province LLGHSZ
Fluid pathways: faults, overpressure, hydrate
1100- Carbonate rock Deeper than the
SW Africa (Niger Delta) dissociation [94]
1250 m crust LLGHSZ
Gas source: hydrate dissociation
SW Africa (Congo deep- Fluid pathways: faults, chimneys, and fractures Deeper than the
3160 m Carbonate crusts [95]
sea channel) Gas source: no report LLGHSZ
Fluid pathways: fault zones, salt diapirs,
SW Africa (Northern Carbonates of Deeper than the
3100 m polygonal faults [96, 97]
Congo Fan) various shapes LLGHSZ
Gas source: biogenic gas
South China Sea (N 600- Fluid pathways: listric faults, chimneys Carbonate Deeper than the
[98]
Zhongjiannan Basin) 1400 m Gas source: hydrocarbon-rich fluids cements LLGHSZ
South China Sea (Xisha Fluid pathways: faults, chimney Deeper than the
700 m No report [99]
Uplift) Gas source: deep reservoir LLGHSZ
10 Geofluids
Table 1: Continued.
Relationship with
Water
Pockmark location Formation mechanisms Carbonates the gas hydrate References
depth
system
300- Fluid pathways: faults, chimney, and diapir Authigenic Shallower than
E Atlantic (Gulf of Cadiz) [100]
400 m Gas source: biogenic and thermogenic gas carbonates the GHSZ
Central Mediterranean
110- Fluid pathways: faults Authigenic Shallower than
(Zannone Giant [101]
130 m Fluid pathways: no report carbonates the GHSZ
Pockmark)
Fluid pathways: episodic dissociation of gas-
W Mediterranean Sea Around LLGHSZ
500- hydrate-rich sediments, tectonic and climate/
(TASYO field, Gulf of Carbonate crusts Deeper than the [102]
2000 m oceanographic factors
Cadiz) LLGHSZ
Gas source: petrogenic gas
SW Mediterranean Sea 200- Fluid pathways: faults, sediment lithology Shallower than
No report [103]
(Gulf of Cadiz) 400 m Gas source: biogenic and thermogenic gas the GHSZ
E Mediterranean Sea 20- Fluid pathways: faults, earthquake Shallower than
No report [104]
(Patras Gulf, Greece) 80 m Gas source: no report the GHSZ
E Mediterranean Sea Fluid pathways: gas venting and groundwater
20- Shallower than
(Patras and Corinth gulfs, seepage No report [105]
40 m the GHSZ
Greece) Gas source: underground water
E Mediterranean Sea Fluid pathways: faults Shallower than
70 m Buried carbonate [106]
(Iskenderun Bay) Gas source: no report the GHSZ
NW Black Sea (Dnepr Fluid pathways: no report Carbonate Shallower than
90 m [107]
paleodelta) Gas source: free gas cemented the GHSZ
100- Fluid pathways: local subsidence, faults Hydrates at
E Black Sea No report [108]
2000 m Gas source: shallow gas different depths
Fluid pathways: deep-seated faults, collapse
20- Shallower than
Western Indian margin structures, dissociation of gas hydrates No report [109]
260 m the GHSZ
Gas source: thermogenic gas
Crusts of
NW Shelf of Australia
50- Fluid pathways: preexisting fractures and dykes cemented Shallower than
(Yampi Shelf and Timor [110]
100 m Gas source: no report bioclastic the GHSZ
Sea region)
material
Northwest Australia 20- Fluid pathways: faults, overpressure Carbonate reef Shallower than
[61]
(Exmouth Plateau) 95 m Gas source: no report mounds the GHSZ
Fluid pathways: faults and diapir, coarse-grained
South China Sea 10- Shallower than
sediments, unconformity, fractures No report [111]
(Yinggehai Basin) 50 m the GHSZ
Gas source: hydrocarbons in the sediments
South China Sea (SW of 600- Fluid pathways: gas chimney and faults Authigenic Deeper than the
[112]
Qiongdongnan Basin) 1600 m Gas source: free gas, thermogenic gas carbonates LLGHSZ
Western margin of the Fluid pathways: extensional fault, the thermal
900- Carbonate reef Deeper than the
South China Sea subsidence fault, and uplifting fault [113]
1100 m mounds LLGHSZ
(Zhongjiannan Basin) Gas source: deep reservoir, free gas
South China Sea (northern 1600- The fractures in the seepage pathways provided Authigenic Deeper than the
[114]
Qiongdongnan Basin) 1800 m direct evidence of vertical fluid flow carbonates LLGHSZ
Northeast Atlantic 900- Fluid pathways: faults, overpressure Deeper than the
No report [115]
(northern Rockall Trough) 1000 m Gas source: pore water LLGHSZ
3.2. Deep-Water Corals. Deep-water corals, also known as 0 to 13.6°C [118]. Till now, deep-water corals have been dis-
cold-water corals, were first described in the 18th century covered at many locations all over the world (Figure 1(b);
[116], but until the late 20th century were the deep-water Table 2), such as off mid-Norway [118], North Sea [119],
corals generally delineated with the development of sub- South China Sea [120], Mediterranean Sea [121], Atlantic
mersibles [117]. They often live without light in relatively Ocean [122, 123], and offshore Mauritania [124]. Most stud-
nutrient-rich seawater and are independent of symbiotic ies of deep-water corals focused on their biology and role in
algae. Deep-water corals commonly occur at water depth global climate change, while limited has been done about
ranging from 39 to over 2000 m, which cover the latitude their formation mechanisms. Hovland and Thomsen [118]
ranging from 70°N to 60°S, with temperature ranging from proposed that the deep-water corals in off mid-Norway areGeofluids 11
Table 2: Compilation of deep-water coral locations around the world showing information on water depth, size, coral types, relationship
with the gas hydrate system, and related fluid seepage.
Relationship
Deep-water coral Water Related fluid
Size Types with gas References
location depth seepage
hydrate system
Norway (northern, 39- Height: 45 m, width: Shallower than
L. pertusa Hydrocarbon seeps [140]
mid, southwest) 400 m >100 m the GHSZ
Gas seeps
Authigenic
New Zealand 756- Desmophyllum, Deeper than
Coral ridge carbonates [141, 142]
(Hikurangi Margin) 1120 m Caryophyllia the LLGHSZ
Chemosynthetic
fauna
“Invasions” of the
Gulf of Cádiz
800- L. pertusa Deeper than warmer
(Guadalquivir No report [143]
1100 m M. oculata the LLGHSZ Mediterranean
Diapiric Ridge)
fauna
Active hydrocarbon
Height and width: 1.5-
Gulf of Mexico (De 434- Around seepage
2 m, length: 3-4 m, L. pertusa [144]
Soto Slope) 530 m LLGHSZ Carbonate knolls
diameter: >1.5 m
and ridges
Shallower than
39- Height: 2-33 m, the GHSZ
Norwegian coast L. pertusa Iceberg plough [145]
550 m diameter: 50-500 m Around
LLGHSZ
Canada (Jordan 154- Gas hydrates at Hydrodynamic
No report L. pertusa [146]
basin) 1400 m different depths regime
Height: 100-140 m,
NE Atlantic 798- L. pertusa Deeper than
width: 650 m, length: Carbonates [147]
(Porcupine Seabight) 942 m M. oculata the LLGHSZ
1.3 km
Irish margin (Rockall 100- Gas hydrates at
No report No report Hiatuses [123]
Bank) 1000 m different depths
SE U.S. (Blake 640- Deeper than
Height: 146 m L. pertusa No report [148]
Plateau) 869 m the LLGHSZ
SE U.S. (Straits of 490- Around Strong bottom
Height: 15-23 m Octocorallia [148]
Florida) 550 m LLGHSZ currents
SE U.S. (eastern Gulf Deeper than
780 m Height: 140 m No report No report [148]
of Mexico) the LLGHSZ
Shallower than
Gulf of Mexico
315- the GHSZ
(upper continental No report L. pertusa Gas seeps [149]
535 m Around
slope)
LLGHSZ
Uruguayan (outer 167- Shallower than
Height: ~35 m No report Gas seeps [122]
shelf and slope) 326 m the GHSZ
982- Corallium, Paragorgia, D. Deeper than
SW Australia No report No report [150]
2395 m dianthus the LLGHSZ
New Zealand (region 200- Oculina, M. oculata, O. Gas hydrates at Strong bottom
No report [151]
seamounts) 2850 m virgosa different depths currents
Anthomastus spp.
Bay of Biscay 880- L. pertusa Deeper than Strong bottom
No report [152, 153]
(Whittard Canyon) 3300 m Octocorals the LLGHSZ currents
Primnoa spp.
800- Deeper than Strong bottom
Mid-Atlantic Ridge No report Octocorals [154]
2400 m the LLGHSZ currents
Central 100- Gas hydrates at
Height:12 Geofluids
Table 2: Continued.
Relationship
Deep-water coral Water Related fluid
Size Types with gas References
location depth seepage
hydrate system
L. pertusa, M. oculata,
Solenosmilia variabilis,
Desmophyllum
Off southwest Ireland 200- Height: 200 m, width: Gas hydrates at
Coral mound Bottom currents [156]
(Porcupine Basin) 2000 m 1 km, length: 5 km different depths
Around
Southern Rockall 500- Height: 350 m, width: LLGHSZ
Coral mound Bottom currents [157]
Trough 1200 m 2 km Deeper than
the LLGHSZ
Shallower than
Agdenes morainic 150- the GHSZ
Width: 100 m L. pertusa Gas chimneys [158]
ridge 500 m Around
LLGHSZ
233- Height: 45 km, length: Shallower than
Sula Ridge L. pertusa Gas chimneys [159]
330 m 13 km the GHSZ
Off mid-Norway (east Shallower than
280 m No report L. pertusa Gas seeps [158]
Draugen field) the GHSZ
Norwegian The largest: height: 15 m,
300- Shallower than
Concession Block length: 700 m, width: L. pertusa Gas chimneys [158]
330 m the GHSZ
(island Træna) 150 m
Off mid-Norway
310- The largest: height: Shallower than
(Kristin hydrocarbon No report No report [70]
385 m 3.5 m, length: 90 m the GHSZ
field)
Between Norway and
Shallower than
UK (Langeled 280 m No report L. pertusa No report [158]
the GHSZ
pipeline)
200- Height: 20 m, width: 150- Shallower than
Vestera° lsgrunnen No report Bottom currents [158]
270 m 200 m the GHSZ
Shallower than Gas seeps, bacterial
West Skogsøy 200 m No report Floholman reefs [158]
the GHSZ mats
300- Width: 3 km, length: Shallower than Gas seeps,
Storegga slope L. pertusa [158]
400 m 35 km the GHSZ pockmarks
Swedish and
75- Width: 200 m, length: Shallower than
Norwegian (eastern L. pertusa No report [158]
160 m 1.2 km the GHSZ
coast of Kattegat)
275- L. pertusa Gas hydrates at
Bill Bailey bank Height: 30 m No report [158]
1020 m M. oculata different depths
Around
NW Porcupine Bank 500- The largest: height: LLGHSZ
L. pertusa No report [158]
(Rockall Bank) 1200 m 100 m, length: 5 km Deeper than
the LLGHSZ
Northernmost 900- Deeper than
Height: 5 m, width: 75 m L. pertusa Gas seeps [115]
Rockall Trough 1060 m the LLGHSZ
Campos Basin off 570- Deeper than Hydrocarbons from
Covers about 600 km2 L. pertusa [52]
Brazil 850 m the LLGHSZ sediments
400- Height: 60 m, width: Around Salt diapirs or salt
Gulf of Mexico L. pertusa [158]
600 m 1.2 km LLGHSZ stocks
Height: 160 m, width: Deeper than Gas seeps,
Blake Ridge 700 m No report [158]
800 m the LLGHSZ pockmark
Canada (east 1900- Height: 200 m, width: Deeper than
No report No report [158]
Newfoundland) 2100 m 2 km the LLGHSZ
South of the Charlie Deeper than
1340 m Diameter:Geofluids 13
Table 2: Continued.
Relationship
Deep-water coral Water Related fluid
Size Types with gas References
location depth seepage
hydrate system
Height: 30 m, width: 200- Shallower than Gas seeps, chimney,
Lower Congo Basin 400 m No report [158, 160]
400 m the GHSZ salt diapir
related to hydrocarbon seeps, on which the bacteria and systems and offers insights into diagenetic processes like
other microorganisms thrived and provided the deep-water mineral dissolution and transformation, degradation of
corals with substantial food source. Deep-water corals were organic material, and high pressure-temperature reactions
observed at the edge of pockmarks in Santos Basin with coral [129–131]. The geometry and size of the mud volcanoes
rubble filling the surrounding sediments of the pockmarks, are variable in different areas, often circular or subcircular
and it was proposed that the pockmark-related seabed in plane view. The height may range from several meters
hydrocarbon seepage may have fertilized the water by to hundreds of meters due to the emission scale, which is
increasing the concentration of nutrients which was favor- mainly controlled by tectonic activity, rapid sediment load-
able for sustaining the corals [52]. Strong currents and low ing, the presence of thick fine-grained sediments, and
sediment supply are also regarded as favorable conditions enriched hydrocarbon accumulation [55, 132]. It was pro-
for the deep-water coral and associated benthic fauna. posed that compression and rapid sedimentation are mainly
Recent studies show that the formation of deep-water responsible for the initiation and maintenance of mud volca-
corals is controlled by two major factors, one is suitable hard noes, while weak argillaceous sediments under compression
substratum for coral colonization such as authigenic carbon- are prone to experience mineral alteration and dehydration
ates and the other one is nutrient supply provided by processes, therefore causing brittle deformation such as
dynamic bottom current, seabed methane seepage, or gas faulting [130]. Additionally, the preexistence of fault zones
hydrate systems [120, 125]. As a result, methane seepage fea- may also act as favorable condition for the formation of
tures at the seabed often sustain a certain density of benthic mud volcanoes [133]. Different from mud volcano, there is
life which is comparatively flourished compared to sur- another similar feature called mud diapir which has risen
rounding areas [53, 126]. In seabed methane seepage areas, from the deep sediments into shallower sediments [129]
the process of methane anaerobic oxidation at the subsurface (Figure 7). When it reaches and pierces the seabed, mud vol-
forms authigenic carbonates, and released methane feeds the cano will occur.
microorganisms which can act as food for the deep-water Many mud volcanoes in marine environments are asso-
corals, both favorable for the formation of deep-water corals ciated with gas hydrate systems [129] (Figure 7). The pres-
(Figure 6(a)). Additionally, gas hydrate systems may help ence of mud volcanoes is capable of changing the stability
fertilize the deep-water corals as well [120, 127]. A case study conditions of gas hydrates and causing an upvaulted BSR,
from Porcupine Basin shows that methane released from gas and the fluid migration associated with the mud volcanoes
hydrate dissociation, which is glacially controlled as a result can supply methane for the formation of gas hydrates and
of global warming, may have played a big role in the forma- underlying free gas zones [134, 135]. Additionally, gas
tion of deep-water coral reefs [127]. hydrate dissociation in response to sea level change, global
Table 2 is the compilation of published data on deep- warming, or local elevated heat flow may lead to the over-
water corals showing information on water depth, coral size, pressure of the pore fluid and deformation of the sedimen-
relationship with gas hydrate system, and related fluid seep- tary overburden, which eventually cause the expulsion of
age. Results show that most of the deep-water corals occur at mud volcanoes [136, 137]. In these processes, fluid migra-
a depth of 200-1000 m, and their locations rely largely on tion plays a key role in both gas hydrate formation and
hard substratum. According to the case studies in Table 2, mud volcano formation.
42.5% of the deep-water corals are related to obvious fluid In addition to these seabed methane seepage manifes-
seepage features and 70% of the deep-water corals are tations we introduced above, gas hydrate pingoes are also
located within the GHSZ scope and nearby LLGHSZ area. commonly observed at seabed methane seepage sites
Generally, case studies of the pockmarks, authigenic carbon- [138, 139]. Gas hydrate pingoes, also known as gas
ates, and deep-water corals from all over the world indicate a hydrate mounds, are the result of gas hydrate growth at
genetic link, with seabed methane seepage playing a key role the very shallow subsurface sediments. Adequate gas sup-
in the formation of carbonates and deep-water corals. ply along faults, gas chimneys, and other fluid migration
conduits will maintain the continuous formation of gas
3.3. Mud Volcanoes. Compared to other seabed methane hydrates, triggering the overlying sediments to swell and
seepage manifestations, submarine mud volcanoes are rela- therefore causing gas hydrate pingoes. Generally, the for-
tively anonymous, mysterious, and seldomly discussed, mation of gas hydrate pingoes also depends on fluid
which are topographically expressed seabed edifice due to migration which represents the plumbing system connect-
mud and fluid eruption [129]. The presence of mud volca- ing the deep hydrocarbon fluids and shallower gas
noes often indicates deep petroleum assets or gas hydrate hydrate reservoirs.14 Geofluids
Sea-level
Coral reef
Carbonates
ed
Seab AOM
High methane flux
Gas hydrate
dissocation
(a) (b)
(c) (d)
(e) (f)
Figure 6: (a) Schematic diagram showing the formation mechanism of deep-water corals at a seabed methane seepage site. Authigenic
carbonates were formed by anaerobic oxidation methane (AOM) which provided hard substratum for corals (modified from Deng et al.
[120]). (b, c) Fragments of large sediments with white arrows indicating deep-water corals and red arrows indicating authigenic
carbonates. (d) Large carbonate rocks at seabed methane seepage site in northern South China Sea (modified from Feng and Chen
[128]). (e, f) Large carbonates and mussels suggesting that methane seepage in this area is sustainable (modified from Feng and Chen [128]).
4. Relevant Fluid Migration Conduits may have reactivated preexisting fractures or have triggered
new fractures, therefore facilitating fluid migration [3, 12].
The detailed characterization of the seabed methane seepage The gas source of fluid migration could be from overpres-
features shows that seabed methane seepage is often related sured hydrocarbon reservoirs or gas hydrate systems, which
to subsurface focused fluid migration conduits such as faults can be deduced from the vertical extension of the fluid
(Figure 3), gas chimneys (Figures 4, 5, and 8), unconfor- migration conduits. A seismic profile from Niger Delta
mities (Figure 9), mud volcanoes or diapirs (Figure 7), and shows that the gas chimney extends deeply into the deep
permeable sandstone layers. The stress change associated stratum, and we propose that the gas source for the seabed
with factors like tectonic events or rapid sediment loading methane seepage is probably from the deep hydrocarbonGeofluids 15
Pockmark
d
Seabe
R
BS
FGZ
Faults
200 ms
FGZ
Mud diapir
1000 m
Figure 7: A seismic profile from offshore Mauritania area showing a pockmark above a mud diapir structure. The diapir structure and the
overlying related faults provide an advantageous migration channel for the overpressured fluids, which formed pockmarks at the seabed.
FGZ: free gas zone which is underlying the base of GHSZ.
2500 Seabed methane seepage
Seabed
BSR
3000
TWT (ms)
3500 Fluid migration pathway
2 km
Figure 8: A seismic profile from Niger Delta, showing the seabed methane seepage feature and related underlying gas chimney. The gas
chimney acted as fluid migration conduit, transporting the deeper fluids (free gas, water, etc.) upward through the GHSZ to the seabed.
reservoir, rather than the shallower gas hydrate systems ing from the seabed of the West Spitsbergen continental
(Figure 8). However, another seismic profile from Niger margin, and it was proposed that the gas source is from
Delta shows that the unconformity and faults may act as hydrate dissociation with released methane migrated along
fluid conduits which have transported methane from the the base of GHSZ updip to the seabed [21].
free gas zone (FGZ) underlying the BSR or from gas reser-
voirs at the deep laterally to the updip, eventually forming 5. Discussions
seabed methane seepage at the seabed (Figure 9). Besides,
the base of GHSZ, as well as permeable sandstone layers, On the basis of above characterization of the seabed meth-
can also act as fluid migration conduits which play roles in ane seepage features and related fluid migration conduits,
forming the seabed methane seepage features by supplying we divide the seabed methane seepage features into three
fluid source (Figure 4(b)). According to Westbrook et al., types according to its spatial relationship with the landward
hundreds of gas bubble plumes have been observed emanat- limit of gas hydrate stability zone (LLGHSZ), deeper than16 Geofluids
Seabed methane seepage
Seabed
3000
BSR
TWT (ms)
Fluid migration pathway
3500
2 km
Faults
Figure 9: A seismic profile from Niger Delta, showing the unconformity surface and faults which acted as fluid migration conduits. The free
gas underlying below the BSR migrated along the unconformity surface or fault to the seabed, forming methane seepage features.
Table 3: Relationship of GHSZ and seabed methane seepage.
Spatial relationship of
seabed methane seepage Related fluid migration pathways Source of methane Code
and the GHSZ
Methane source is irrelevant to the gas hydrate system
Fault/gas chimney/diapirism/mud volcano/ A1
Deeper than LLGHSZ and is from deeper gas reservoirs
unconformity surface/permeable sandstone
A Methane source is from the gas hydrate system, like gas
layer/pockmark A2
hydrate decomposition or underlying free gas zone
Methane source is independent of the hydrate system,
transported vertically along fluid migration conduits B1
The fluid migration conduits may be not such as faults or gas chimney to the seabed
Around LLGHSZ
obvious, and small-scale vertical faults/gas
B Methane source is from the gas hydrate decomposition,
chimneys/pockmarks may exist
or methane was transported along the base of hydrate B2
stability zone laterally to the seabed
Fault/gas chimney/diapirism/mud volcano/ Methane seepage sites are out of the gas hydrate stability
Shallower than LLGHSZ
unconformity surface/permeable sandstone zone, and methane source is independent of the gas C1
C
layer/pockmark hydrate system
the LLGHSZ (A), around the LLGHSZ (B), and shallower A2, B1, B2, and C1 (Table 3), which will be explained
than LLGHSZ (C). In plane view, the LLGHSZ is a linear in the following sections.
feature in theory, but it often exists as a narrow zone due
to the dynamic shift of the LLGHSZ in response to geo- 5.1. Deeper Than LLGHSZ (A). Seabed methane seepage dee-
logical events such as glacial unloading, sea level change, per than LLGHSZ is defined as type A in this paper, which is
and global warming [12]. These three types of seabed located in the range of GHSZ, but deeper than the landward
methane seepage can be further divided into five subtypes boundary of the gas hydrate stability zone (Figures 3, 4(c), 7,
based on the gas source of seabed methane seepage, A1, 8, and 9). Considering the gas source, type A of seabedGeofluids 17
(a) (b)
Seafloor
1490
TWT (ms)
BSR 1200
1620 1300
1400
Deeper than LLGHSZ 1760
1000 m 1500
Disturbed reflectivity
1600
Costa Rica (G.J. Crutchley et al., 2014)
1700
(c)
1800
TWT(ms)
Gas Seepage 1900
BSR
1400
2000
Seafloor
s 2100
1500 sed free ga
Diffu 2200
TWT (ms)
Free
Gas
2300
1600 BSR
2400
Gas Hydrate
2500
1700 F2
2600
on F3 F4
oriz F1
h as
1800 Gas Fre
eG F5 Northern South China Sea (Lun Li et al., 2015)
Krishna-Godavari offshore basin(S. Gullapalli et al., 2019)
(d) (e)
400
ft of GHSZ
Seasonal shi
470
Upper limit
TWT (ms)
750 of GHSZ
TWT (ms)
(warm season) Seasonal methane
540
capacitor
78°35' N
MASOX
610
9° 30' E
GHSZ
Around LLGHSZ
9° 20' E
The northern US Atlantic margin(A. Skarke et al., 2014) Offshore Svalbard (Bénédicte Ferré et al., 2020)
(f)
Sea-level
0
400
800 SMT(1.3mbsf)
1200 Dissc SMT (3.0mbsf)
TWT (ms)
ociati Sulphate reduction
ng h zoneSMT (6.5m Seafloor
1600 ydra bsf)
Hydrate-free zone te zo
2000 ne
Stable hyd SMT (14mb
2400 rate zone sf)
Active flare BSR (300m
2800
Difusive methane flux bsf)
3200
Methane migration pathway
3600
Upper slope of the Rio Grande(Marcelo Ketzer et al., 2020)
(g) (h)
25 20 mbsl
Permafrost
gas flares
TWT (ms)
670
TWT (ms)
35 Gas hydrate
stability zone
Pe
acoustic acoustic
rm
MIS 1
afr
45 MIS 4-MIS 2 turbidity turbidity
os
Continuous
t
?-MIS5d
Shallower than LLGHSZ
discontinuous permafrost
55 PLF permafrost
South Kara Sea shelf (Alexey Portnov et al., 2013) microbial gas
released from gas hydrate
released CH4 Gas hydrate stability zone
Mediterranean Sea(Sukru Merey et al., 2018)
(I) 80 (j) 50
A A A Seafloor
85 E 60
E B E H porkmark
90 70
G
D
95 80
D
TWT (ms)
100 C
90
TWT (ms)
105 D F
100
110 Gas chimney
110
115 Paleo chennel
120
120
0 0.05 0.10 0.15 0.20 0.25 0.30 0.35 0.40 0.45 0.50 130
Southern Baltic Sea (Jakub Idczak et al.,2020)
Yellow Sea Basin of China (Xiaoyu Liu et al., 2019)
Figure 10: Compilation diagram showing different types of seabed methane seepage based on global case studies. (a) Seabed methane
seepage in Costa Rica located at deeper than the LLGHSZ, with gas migrated along gas chimneys from gas reservoirs deeper than the gas
hydrate systems to the seabed, defined as subtype A1 (modified from Crutchley et al. [58]). (b) Seabed methane seepage in northern
South China Sea located at deeper than the LLGHSZ, with gas migrated along faults from gas reservoirs deeper than the gas hydrate
systems to the seabed, defined as subtype A1 (modified from Li et al. [59]). (c) Seabed methane seepage in offshore Krishna-Godavari
Basin located at deeper than the LLGHSZ, with gas migrated along faults from the gas hydrate systems to the seabed, defined as subtype
A2 (modified from Gullapalli et al. [161]). (d–f) Seabed methane seepage features at around the LLGHSZ in the northern U.S. Atlantic
margin, offshore Svalbard, and upper slope of the Rio Grande, defined as subtype B2. The gas sources of these methane seepage sites
were all from the gas hydrate system, especially due to the gas hydrate dissociation caused by the dynamic shift of the GHSZ (modified
after Skarke et al. [20]; Ferré et al. [15]; Ketzer et al. [4]). (g–j) Seabed methane seepage features at shallower than the LLGHSZ in the
South Kara Sea shelf, permafrost of Mediterranean Sea, southern Baltic Sea, and Yellow Sea Basin of China, defined as subtype C1. The
gas sources of these seabed methane seepage sites are irrelevant to the gas hydrate system, can be from thermogenic methane from the
deep, microbial methane generated in shallow sediments, or newly released from frozen soil melting (modified from Portnov et al. [163];
Merey and Longinos [164]; Liu et al. [24]; Idczak et al. [165]).18 Geofluids
CMP
10300 10400 10500 10600 10700 10800
V.E.=~6
300 m GHSZ at 2°
(depth 370 m)
Shallow scattered
gas pockets within
500 hydrate retreat area Gas seepages
GHSZ at 3°
(depth 410 m)
TWT(ms)
/km
°C
78°40'N
600
60
500 m
m
C/k
60°
Gas in marine
78°35'N
hemipelagic beds
700
9° E 9° 10'E 9° 20'E 9° 30'E
Figure 11: A seismic profile from offshore Svalbard, showing seabed methane seepage features at around the LLGHSZ. The LLGHSZ shifted
seawards due to seasonal temperature change from 370 m at 2°C to 410 m at 3°C, and related gas hydrate dissociation caused the formation
of seabed methane seepage, belonging to subtype B2 (modified from Sarkar et al. [162]).
methane seepage can be further divided into two subtypes, hydrate systems, transported vertically along fluid migration
A1 and A2. The methane source of A1 subtype is irrelevant conduits such as faults or gas chimney to the seabed. Sub-
to the gas hydrate systems and is probably from deeper gas type B1 has rarely been observed, and most of the seabed
reservoirs, while methane source of A2 subtype is from the methane seepage at the LLGHSZ is closely related to the
gas hydrate system, either from gas hydrate decomposition gas hydrate systems, either from gas hydrate decomposition
or underlying free gas zone. For example, the gas source of or methane transported laterally along the base of hydrate
seabed methane seepage in a case study from Costa Rica stability zone to the seabed which is defined as subtype B2.
was interpreted to be from the gas reservoir which is located The gas hydrate stability zone is a wedge-shaped zone with
at deeper depth than the gas hydrate system, defined as A1 the BSR shallowing updip as the seabed depth decreases
subtype [58] (Figure 10(a)). Similar seabed methane seepage and eventually intersecting the seabed, and the intersection
features were also observed in the South China Sea where location is recognized as the landward limit of the GHSZ
faults act as fluid migration conduits transporting gas from (LLGHSZ) [47]. On the U.S. Atlantic continental margin,
the gas reservoir deeper than the gas hydrate systems to Skarke et al. [20] observed 570 previously undiscovered gas
the seabed, also defined as A1 subtype [59] (Figure 10(b)). plumes at water depth ranging from 50 to 1700 mbsl, of
In offshore Krishna-Godavari Basin, the gas source of meth- which about 440 are located near the present LLGHSZ
ane seepage was interpreted to be transported along faults (Figure 10(d)). It was proposed that these seabed methane
from the gas hydrate systems and the underlying free gas seepage features were caused by hydrate dissociation as a
zone to the seabed, defined as A2 subtype [161] result of dynamic shift of the GHSZ due to the seabed water
(Figure 10(c)). For both subtypes, the vertical fluid migra- temperature changes, therefore defined as B2 subtype in this
tion along focused conduits provided gas source for the for- paper [20, 162] (Figures 11 and 12). Similar phenomena
mation of seabed methane seepage feature, which was have also been observed in offshore Svalbard (Figure 10(e))
probably driven by overpressure associated with tectonic and upper slope of the Rio Grande (Figure 10(f)), where
compression or continuous accumulation of fluids [18, 19]. the gas sources of seabed methane seepage were from gas
hydrate dissociation [4, 15]. Many seabed pockmarks and
5.2. Around LLGHSZ (B). Seabed methane seepage located gas plumes distributed along the linear LLGHSZ zone are
nearby the LLGHSZ is defined as type B in this paper, which caused by the periodic formation and decomposition of gas
is located at the landward boundary of the gas hydrate stabil- hydrate, probably due to the weak stability of gas hydrates
ity zone (Figures 4(b), 11, 12, and 10(d)–10(f)). The methane and the variable environmental conditions at the seabed
source of subtype B1 is defined as independent of the [20, 21, 72]. However, we cannot rule out another possibilityYou can also read
