BSRs, estimated heat flow, hydrate-related gas volume and their implications for methane seepage and gas hydrate in the Dongsha region, northern South China Sea

Li, Lun; Liu, Haojie; Zhang, Xin; Lei, Xinhua; Sha, Zhongli

doi:10.1016/j.marpetgeo.2015.07.008

Cited by 29 publications

(17 citation statements)

References 32 publications

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“…Within the GHSZ scope, there are few pockmark occurrence at the seabed compared to the huge amount of pockmarks around the LLGHSZ. The fluid source for these pockmark formation could be from both gas hydrate systems and deeper hydrocarbon reservoirs, which can be deduced by the vertical distribution of fluid migration conduits [58,59]. As shown in Figure 4(c), these two pockmarks were formed as a result of overpressure dissipation, and the overpressure was probably caused by the continuous accumulation of free gas below the base of gas hydrate stability zone [60].…”

Section: Characterization Of Seabed Methane Seepage Manifestationsmentioning

confidence: 99%

“…The fluid migration conduits may be not obvious, and small-scale vertical faults/gas chimneys/pockmarks may exist Methane source is independent of the hydrate system, transported vertically along fluid migration conduits such as faults or gas chimney to the seabed B1 Methane source is from the gas hydrate decomposition, or methane was transported along the base of hydrate stability zone laterally to the seabed (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]).…”

Section: Around Llghsz Bmentioning

confidence: 99%

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A Geophysical Review of the Seabed Methane Seepage Features and Their Relationship with Gas Hydrate Systems

Yang

Zheng

et al. 2021

Geofluids

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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.

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Section: Characterization Of Seabed Methane Seepage Manifestationsmentioning

confidence: 99%

Section: Around Llghsz Bmentioning

confidence: 99%

A Geophysical Review of the Seabed Methane Seepage Features and Their Relationship with Gas Hydrate Systems

Yang

Zheng

et al. 2021

Geofluids

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“…In particular, there is a large carbonate buildup named "Jiulong Methane Reef" at Site 3 that is characterized by abundant seep-related bivalve shells and shell fragments (Suess et al, 2005;Han et al, 2008). Both faults and mud diapirs are considered as fluid conduits at Site 3, allowing methane to migrate upward and causing free gas zones beneath BSRs, hydrate zones and seafloor methane seepages (Li et al, 2015). core was collected at the hydrate drilling site (Fig.…”

mentioning

confidence: 99%

Geochemical record of methane seepage in authigenic carbonates and surrounding host sediments: A case study from the South China Sea

Chen

Dong

et al. 2017

Journal of Asian Earth Sciences

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Sediments at marine methane seep sites provide potential archives of past fluid flow that serve to explore seepage activities over time. Three gravity cores (D-8, D-F, and D-7) were collected from seep sites on the northern slope of the South China Sea where gas hydrates were drilled in the subsurface. Various carbon and sulfur contents, δ 13 C values of total inorganic carbon (δ 13 C TIC), δ 34 S values of chromium reducible sulfur (δ 34 S CRS), trace element contents, grain size, and AMS 14 C dating of planktonic Foraminifera in the sediments were determined to explore the availability of related proxies at seeps and to trace past methane seepage activities. Evidence for the presence of methane seepage and consequently anaerobic oxidation of methane comes from the occurrence of 13 C-depleted authigenic carbonate nodules (δ 13 C values as low as-49‰) discovered at an interval of 150 cm-200 cm in core D-7. This finding is supported by high S/C ratios and molybdenum enrichment in the same interval. However, low contents of CRS and negative δ 34 S CRS values are present. It is suggested to reflect a transient methane seepage event, which continued for about 1 ka based on the 14 C ages. Cores D-8 and D-F have δ 13 C TIC values close to zero, low S/C ratios and CRS contents, negative δ 34 S CRS values, and no trace element enrichment, suggesting a negligible impact of methane-seepage on the sediments. The negative δ 34 S CRS values of the studied seep-impacted and background sediments suggest that the application of δ 34 S CRS alone as a proxy to identify AOM-related process may be insufficient. Sediment carbon-sulfur-trace element systematics and 14 C ages used here have the potential to be a promising tool to recognize transient methane seepages and constrain their timescales.

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“…We do not consider that the BSR reflections in our study area are related to the diagenetic boundaries since they show opposite polarity with respect to the seafloor (Figures 6c and 7c) and they occur at relatively shallower depths with 18 to 22 °C subsurface temperatures ( Figure 5). Considering their specific characteristics listed above, BSR reflections in the study area are interpreted to be associated with the base of the gas hydrate accumulations (BGHZ) as suggested by several researchers (e.g., Singh et al, 1993;Laberg and Andreassen, 1996;Andreassen et al, 1997;Diaconescu et al, 2001;Lee and Dillon, 2001;Pecher et al, 2001;Bohrmann et al, 2003;Krastel et al, 2003;Talukder et al, 2007;Li et al, 2015).…”

Section: Properties Of the Gas Hydratesmentioning

confidence: 86%

Seismic identification of gas hydrates: A case study from Sakarya Canyon, western Black Sea

Nasıf

Özel

Dondurur

2020

Turkish J Earth Sci

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Multichannel seismic, 3.5-kHz Chirp subbottom profiler and multibeam bathymetric data were collected along the western Black Sea margin, offshore Sakarya River, to investigate the bottom-simulating reflections (BSRs), free gas accumulations, and mud volcanoes. Geometries from the seismic data indicate widespread BSRs along the continental rise between 750 and 1950 m water depths, 70 to 350 ms below the seafloor. Seismic attribute analyses have been applied to the seismic data to reveal the acoustic properties of the gas hydrates. According to the results from such analyses, we conclude that there are acoustically transparent zones beneath most of the BSRs in the area, which are interpreted as free gas accumulations, and the gas hydrate-bearing sediments are acting as seals for the free gas in the underlying sediments. Stability analysis of the gas hydrates from different BSR zones in the area suggests that the gas composition in the gas hydrates may change locally. As we do not have ground truth data from BSR zones, the exact composition of the gas forming the gas hydrates is unknown. However, hydrocarbon productivity of the area, chromatography results of the shallow sediment samples nearby, and stability analysis of the gas hydrates indicate the possible existence of a thermogenic gas component in the gas hydrate composition, resulting in a mixture of gas hydrates.

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BSRs, estimated heat flow, hydrate-related gas volume and their implications for methane seepage and gas hydrate in the Dongsha region, northern South China Sea

Cited by 29 publications

References 32 publications

A Geophysical Review of the Seabed Methane Seepage Features and Their Relationship with Gas Hydrate Systems

A Geophysical Review of the Seabed Methane Seepage Features and Their Relationship with Gas Hydrate Systems

Geochemical record of methane seepage in authigenic carbonates and surrounding host sediments: A case study from the South China Sea

Seismic identification of gas hydrates: A case study from Sakarya Canyon, western Black Sea

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