Early Diagenesis Records and Pore Water Composition of Methane-Seep Sediments from the Southeast Hainan Basin, South China Sea

Several authigenic minerals were identified by XRD and SEM analyses in shallow sediments from the Southeast Hainan Basin, on the northern slope of South China Sea. These minerals include miscellaneous carbonates, sulphates, and framboidal pyrite, and this mineral assemblage indicates the existence of gas hydrates and a methane seep. The assemblage and fabric features of the minerals are similar to those identified in cold-seep sediments, which are thought to be related to microorganisms fostered by dissolved methane. Chemical composition of pore water shows that the concentrations of SO42-, Ca2+, Mg2+, and Sr2+ decrease clearly, and the ratios of Mg2+ to Ca2+ and Sr2+ to Ca2+ increase sharply with depth. These geochemical properties are similar to those where gas hydrates occur in the world. All results seem to indicate clearly the presence of gas hydrates or deep water oil (gas) reservoirs underneath the seafloor.

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Gas Hydrate Accumulation and Occurrence Associated with Cold Seep Systems in the Northern South China Sea: An Overview

Geofluids ◽

10.1155/2021/5571150 ◽

2021 ◽

Vol 2021 ◽

pp. 1-24

Author(s):

Wei Zhang ◽

Jinqiang Liang ◽

Qianyong Liang ◽

Jiangong Wei ◽

Zhifeng Wan ◽

...

Keyword(s):

South China Sea ◽

South China ◽

Gas Hydrates ◽

Gas Hydrate ◽

Northern South China Sea ◽

Cold Seep ◽

Cold Seeps ◽

Associated Gas ◽

China Sea ◽

Gas Bearing

Studying deep-water cold seep systems is of great significance to gas hydrate exploration due to their close relationship. Various cold seep systems and related gas hydrate accumulations have been discovered in the northern South China Sea in the past three decades. Based on high-resolution seismic data, subbottom profiles, in situ submergence observations, deep drilling and coring, and hydrate gas geochemical analyses, the geological and geophysical characteristics of these cold seep systems and their associated gas hydrate accumulations in the Qiongdongnan Basin, the Shenhu area, the Dongsha area, and the Taixinan Basin have been investigated. Cold seep systems are present in diverse stages of evolution and exhibit various seabed microgeomorphic, geological, and geochemical features. Active cold seep systems with a large amount of gas leakage, gas plumes, and microbial communities and inactive cold seep systems with authigenic carbonate pavements are related to the variable intensity of the gas-bearing fluid, which is usually derived from the deep strata through mud diapirs, mud volcanoes, gas chimneys, and faults. Gas hydrates are usually precipitated in cold seep vents and deeper vertical fluid migration pathways, indicating that deep gas-bearing fluid activities control the formation and accumulation of gas hydrates. The hydrocarbons collected from cold seep systems and their associated gas hydrate reservoirs are generally mixtures of biogenic gas and thermogenic gas, the origin of which is generally consistent with that of deep conventional gas. We also discuss the paragenetic relationship between the gas-bearing fluid and the seafloor morphology of cold seeps and the deep-shallow coupling of gas hydrates, cold seeps, and deep petroleum reservoirs. It is reasonable to conclude that the deep petroleum systems and gas-bearing fluid activity jointly control the development of cold seep systems and the accumulation of gas hydrates in the northern South China Sea. Therefore, the favorable areas for conventional oil and gas enrichment are also prospective areas for exploring active cold seeps and gas hydrates.

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Biogeochemical processes in continental slope sediments of the Dongsha Area, South China Sea

10.5194/egusphere-egu2020-4602 ◽

2020 ◽

Author(s):

Tiantian Sun ◽

Daidai Wu ◽

Ying Ye

Keyword(s):

South China Sea ◽

South China ◽

Planktonic Foraminifera ◽

Major And Trace Elements ◽

Cold Seep ◽

Biogeochemical Processes ◽

Methane Seep ◽

China Sea ◽

The Impact ◽

The Last Glacial

Serving as an indicator or fluid seepage from seabed sediments, cold seeps are ubiquitous along continental margins worldwide. In this study, a 14 m long sediment core (# 973-4) from the Dongsha Area on the northern continental slope of the South China Sea, was investigated to trace the cold seep activity and sedimentary paleo-environmental changes and its consequence for sediment mineralogy, contents of major and trace elements, total organic and inorganic carbon and total TRIS (total reducible inorganic sulfur) and &#948;34S of sulfide minerals. In addition, planktonic foraminifera were selected for accelerator mass spectrometer carbon 14 (AMS14C) dating [1]. Furthermore, we identified the strength and effects of cold seep activity and its impact on the underlying seawater redox condition, and finally elucidated the derived force and paleoenvironment constraints of cold seep activity. C-S-Fe geochemistry, &#948;34S of sulfide minerals and major and trace elements suggest that anaerobic oxidation of methane (AOM) occurred at 619-900 cmbsf (centimeters below seafloor). The 34S enrichments (up to 23.6 &#8240;), abundant TRIS contents, high S/C ratios close to the seawater, together with high enrichments of Mo indicate temporal sulfidic methane seep events. Lithological distribution and AMS14C dating of planktonic foraminifera show that a turbidite (~35ka) is related to a foram-rich interval (440-619 cm) and increased carbonate productivity during the Last Glacial Maximum (LGM). An enrichment of Mo and U was observed accompanied by low contents of other trace and major (Al, Ti, V, Ni, Fe, Mn and Cu) in this interval. The foram-rich interval of cold seep sediments was probably linked to the phenomenon of inconsecutive sedimentary sequence due to the turbidites, which resulted in the lack of Fe, Mn and Ba. Based on the new results, it can be speculated that this area has experienced several episodes of methane seep activity and aerobic oxidation occurring alternatively in the last glacial period which may have been caused by fluctuating non-steady conditions. Further exploration of AOM should focus on the impact of rapid deposition, especially the impact of turbidites on sedimentary biogeochemical processes.[1] Zhang Bidong, Pan Mengdi, Wu Daidai etc. Distribution and isotopic composition of foraminifera at cold-seep Site 973-4 in the Dongsha area, northeastern South China Sea. J. Asian Earth Sciences.&#160;The research supported by the Institution of South China Sea Ecology and Environmental Engineering, Chinese Academy of Sciences (no. ISEE2018YB03) and the special project for marine economy development of Guangdong Province (no. GDME-2018D002).

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Deeply Buried Authigenic Carbonates in the Qiongdongnan Basin, South China Sea: Implications for Ancient Cold Seep Activities

Minerals ◽

10.3390/min10121135 ◽

2020 ◽

Vol 10 (12) ◽

pp. 1135

Author(s):

Jiangong Wei ◽

Tingting Wu ◽

Wei Zhang ◽

Yinan Deng ◽

Rui Xie ◽

...

Keyword(s):

South China Sea ◽

South China ◽

Methane Flux ◽

Qiongdongnan Basin ◽

Cold Seep ◽

Carbonate Precipitation ◽

Methane Seep ◽

China Sea ◽

Methane Seepage ◽

Sediment Layers

Cold seep carbonates are important archives of pore water chemistry and ancient methane seepage activity. They also provide an important contribution to the global carbon sink. Seep carbonates at three sediment layers (3.0, 52.1, and 53.6 mbsf) were collected at site W08B in the Qiongdongnan Basin of the South China Sea. This study investigated the mineralogy, microstructure, stable carbon and oxygen isotopes, trace elements, and U-Th dates of these carbonates to identify the relationship between methane flux and authigenic carbonate precipitation. The results showed that the δ13C and δ18O values of all carbonates are similar, indicating that the carbon source for shallow carbonates and deep carbonates has remained constant over time and included biogenic and thermogenic methane. Although carbonates were found in three sediment layers, the two main stages of methane seepage events were discernible, which was likely caused by the dissociation of gas hydrates. The first methane seep took place at 131.1–136.3 ka BP. During a dramatic drop in the sea level, the seep carbonate at 52.1 mbsf formed at 136.3 ka BP through the anaerobic oxidation of methane (AOM). The carbonate at 53.6 mbsf resulted from the vertical downward movement of the sulfate-methane transition zone with decreasing methane flux at 131.1 ka BP. This is the reason for the age of carbonates at 52.1 mbsf being older than the age of carbonates at 53.6 mbsf. The second methane seep took place at 12.2 ka BP. Shallow carbonate formed at that time via AOM and is now located at 3 mbsf. Moreover, thin-section photomicrographs of deep carbonate mainly consisted of matrix micrite and biological debris and acicular aragonite occurred as vein cement filling the pore spaces between the matrix micrite. The acicular aragonite was mainly influenced by the timing of the carbonate precipitation of minerals. This research identified a long history of methane seep activity reflected by the vertical distribution of carbonates.

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