Abstract
Extensive submarine cold seep areas, i.e., the Beikang Basin and the Nansha Trough, were discovered on the southern continental slope of the South China Sea. Bottom-simulating reflections are widespread in these areas and show a close relationship to the cold seep system. High-resolution 2-D seismic data and multibeam bathymetry data have confirmed the existence of deep-routed conduits−mud volcanoes, diapirs, and gas chimneys. The geochemical characteristics of seep carbonates and headspace gas indicate that the fluid was mainly sourced from biogenic gas, with contributions from deep-rooted thermogenic gases. Additionally, negative pore water chloride anomalies and positive δ18O values (3.7‰ < δ18O < 5.0‰) of the seep carbonates provided indicators of hydrate water addition during carbonate precipitation. The negative δ13C excursion of planktonic foraminifera from the Nansha Trough indicated two methane release events, which occurred approximately 29–32 ka and 38–42 ka before present, and the driving mechanism for methane seepage in this area is possibly related to overpressure from the large sediment accumulation that occurred during sea level lowstands.
You have full access to this open access chapter, Download chapter PDF
Similar content being viewed by others
13.1 Introduction
Cold seepage systems are widely developed in the South China Sea (SCS), and more than 40 seep sites have been found along the northern slope of the SCS (Feng et al. 2018a). Various seep-related samples, including seep-derived authigenic carbonates, seep-impacted sediments, gas samples, and dead and living seep fauna, have been collected over the past decade. Ongoing studies provide information about the geomorphological characteristics and associated subsurface migration pathways of fluids, about biogeochemical processes, fluid origins and methane seepage histories, and about the macroecology of chemosynthesis-based ecosystems in the SCS (Feng et al. 2018a).
To date, using high-resolution 2-D seismic data, bottom-simulating reflections (BSRs) indicating the presence of gas hydrate and free gas had been identified in the offshore south Vietnam (Lee and Watkins 1998), the northwestern (NW) Sabah/Borneo (Behein et al. 2003; Gee et al. 2007; Laird and Morley 2011; Paganoni et al. 2016), the Nansha Trough (Berner and Faber 1990; Chen et al. 2007), the Nanweixi Basin (Deng et al. 2004), and the Beikang Basin (He et al. 2018; Huang et al. 2022). Various fluid seepage conduits, including mud volcanoes, gas chimneys, pockmark, diapirs, fluid-escape pipes, and paleo-uplift associated faults had also been identified by using 2D and 3D seismic analyses and multibeam bathymetry in some areas of the southern SCS (Traynor and Sladen 1997; Lee and Watkins 1998; Gee et al. 2007; Laird and Morley 2011; Paganoni et al. 2018; Yan et al. 2020; Zhang et al. 2020; Huang et al. 2022). Methane-rich fluid plumes and fluid seeps were found in the headwall region of a giant landslide offshore NW Borneo, indicating the presence of active fluid venting, which may provide a mechanism for weakening and triggering slope failure (Rehder and Suess 2001; Gee et al. 2007). The sediment pore water chemical composition in the Nansha Trough and Beikang Basin has been used to calculate the depth of the sulfate methane transition zone (SMTZ) and estimate the rate of anaerobic methane oxidation, revealing the occurrence of shallow hydrocarbon gas (Berner and Faber 1990; Feng et al. 2018b, 2021; Huang et al. 2022). Authigenic carbonates derived from oil/gas seepage were discovered on the seafloor in the offshore southern Vietnam (Traynor and Sladen 1997; Wetzel 2013), offshore NW Borneo (Warren et al. 2010) and the Beikang Basin (Huang et al. 2022; Zhang et al. 2023). Fluid geochemical compositions and their sources are acquired according to the headspace gas and seep carbonates (Huang et al. 2022). Sediment geochemistry indicated two methane release events in the southern SCS (Li et al. 2018, 2021). The history of seep activity in the Nansha Trough has also been reconstructed by using the stable carbon isotope characteristics of planktonic foraminifera (Zhou et al. 2020). In recent years, new discoveries and studies on seep activities in the southern SCS have been performed by Chinese teams in two areas with seep activities, namely the Beikang Basin and the Nansha Trough (Fig. 13.1).
13.2 Geological Setting
The southern SCS is located in a region where three tectonic stresses (tensile, shear, and compression) are superimposed (Hutchison 2004). The study area includes the Beikang Basin and the Nansha Trough (Fig. 13.1). The Beikang Basin, which has a water depth of 1200 to 2000 m, is a Cenozoic fracture basin with sedimentary thickness ranging from 1000 to 12,000 m, and many fault terraces, mud diapirs, and fold-thrust belts have developed (Fig. 13.2; Liu et al. 2011). The tectonic activity, block migration, and South China Sea expansion influence basin development and sedimentary evolution, which provide good geological conditions for oil and gas accumulation. At present, several oil and gas fields have been found in the Beikang Basin (Liu 2005). In addition, seafloor BSRs have been identified in this area, indicating the presence of gas hydrates in unconsolidated sediments (He et al. 2018). Simultaneously, observed anomalies of mercury and natural aluminum in sediments are considered to be related to cold seepage (Chen et al. 2010; Liu et al. 2011).
The Nansha Trough is the largest trough developed along the SW‒NE rift in the southern SCS, and its sedimentary thickness reaches 2000 m. The tectonic activity since the Early Miocene induced the formation of hydrocarbon gases in source rocks, and the gases migrated through permeable migration channels, eventually accumulating abundant oil and gas resources (Ingram et al. 2004). In addition, BSR was found in this area, suggesting gas hydrate accumulation (Berner and Faber 1990).
13.3 Geochemical Constraints on Fluid Sources and Biogeochemical Processes
The depth of the SMTZ is an effective indicator of methane flux (Borowski et al. 1996). According to piston cores from the Beikang Basin, marine pore water depth profiles of methane and sulfate indicate that the SMTZ is located at approximately 3.3−6.6 m (Fig. 13.3; Feng et al. 2018b, 2021; Huang et al. 2022). The methane oxidation rate calculated by numerical simulation ranged from 27.5 mmol m−2 yr−1 to 43.1 mmol m−2 yr−1, and almost all the methane sourced from subsurface sediments was depleted within the SMTZ (Feng et al. 2018b), which indicated diffusive methane seepage.
Seep carbonate is formed as a result of increased pore water alkalinity due to the sulfate-driven anaerobic oxidation of methane (AOM), and the mineralogy and stable isotopic signature of seep carbonates can provide information on fluid sources and formation conditions (Peckmann and Thiel 2004). The negative δ13C values (−58.7‰ to −50.8‰) of seep carbonates from the piston core of the Beikang Basin suggest that the dissolved inorganic carbon is mainly derived from methane oxidation and possibly from biogenic methane (Huang et al. 2022; Zhang et al. 2023). This conclusion is also supported by the headspace gas data. The plot of δ13C1 versus C1/C2+ suggested that the methane mostly originated from biogenic gas, with some deep-sourced thermogenic gas (Fig. 13.4; Huang et al. 2022). The oxygen isotopic composition of seep carbonate is dependent on the fluid source and temperature of carbonate formation (Bohrmann et al. 1998). The positive δ18O values (3.7 to 5.0‰) of the seep carbonates from the Beikang Basin show disequilibrium with present-day bottom water temperatures, which suggests an influence of 18O-enriched fluids on carbonate formation (Huang et al. 2022; Zhang et al. 2023). Given the negative pore water chloride anomalies in the same core (Fig. 13.3), it is likely that these 18O-enriched fluids mostly originated from gas hydrate dissociation (Huang et al. 2022).
13.4 Origin and History of Methane Seepage
AOM increases pore-fluid alkalinity and favors the precipitation of authigenic seep carbonates with low δ13C values. Therefore, markedly low δ13C values of total inorganic carbon (TIC) in bulk sediments could serve as significant indicators of sulfate-driven AOM (Peckmann and Thiel 2004). In addition, intensive and long-term sulfate-driven AOM would result in a dissolved sulfate limitation; thus, iron sulfide minerals are typically more abundant within or near the SMTZ than in other zones and are enriched in 34S (Jørgensen et al. 2004).
Based on the decreased TOC/TS ratios, the positive sulfur isotopic value of chromium reducible sulfur (CRS), and the negative carbon isotopic value of TIC in the sediments, two methane release events were identified both in the Beikang Basin (Fig. 13.5) and the Nansha Trough (Fig. 13.6). Numerous studies have shown that foraminifera are one of the best carriers for recording seep activity. The negative excursion of the carbon isotopes of foraminiferal shells can be used to identify seep activity in geological history (Kennett et al. 2000). In a study of the sediments in the Nansha Trough in the southern SCS, it was found that the carbon isotopes of planktonic foraminifera in the sediments indicated two methane release events, roughly from 29–32 ka and 38–42 ka (Fig. 13.6). Since the seep carbonates also developed in a layer containing negative carbon isotopes of foraminifera, the lowest carbon isotope value reached −27.4‰. Therefore, in foraminiferal tests, the negative carbon isotopes of foraminiferal shells mainly result from the cement consisting of authigenic carbonates (Zhou et al. 2020).
Multiple driving mechanisms have been proposed to explain methane emissions from cold seepage, including bottom water warming, mass wasting processes, ice sheet dynamics, seismic activity, and low sea levels (Ruppel and Kessler 2017). For the Nansha Trough, from 29–32 ka before present (BP) and 38–42 ka BP, the site has been inside the methane hydrate stability field. Gas hydrates can reduce sediment permeability and cause overpressure build-up at the base of the gas hydrate stability zone. The resultant hydrofracturing forms fluid seepage conduits for overpressured fluids to migrate upward (Elger et al. 2018). Therefore, the driving mechanism for methane seepage in this area is possibly not related to gas hydrate dissociation due to thermodynamic instability outside the gas hydrate stability zone and is more likely due to the release of overpressured pore fluids due to sediment loading.
13.5 Summary
The development of methane seepage in the Nansha Trough and Beikang Basin in the southern South China Sea is due to the abundant oil and gas resources and extensive deep-routed fluid seepage conduits. The fluid is mainly sourced from biogenic gas, with contributions from deep-sourced thermogenic gas and water released by hydrate dissociation. Sediment pore water data in the southern SCS show that cold seeps may be active, but no definitive evidence has been put forth. Their presence could be confirmed in the future by submersible diving field investigations. Two methane release events that occurred at approximately 29–32 and 38–42 ka BP were identified in the Nansha Trough. It should be emphasized here that the age of a foraminifer is only a rough constraint for the timing of cold seep activities. The dating of seep carbonates in the future could allow the estimation of the chronology of seepage. Compared with the northern SCS, seeps in the southern SCS have been investigated less. Seafloor observations, sampling, and further work are therefore critical.
References
Behain D, Fertig J, Meyer H et al (2003) Properties of a gay hydrate province on a subduction-collision related margin off Sabah, NW Borneo (POPSCOMS). In EGS-AGU-EUG Joint Assembly (p. 10008)
Berner U, Faber E (1990) Hydrocarbon gases in surface sediments of the South China Sea. In: Ocean C (ed) Marine Geology and Geophysics of the South China Sea, Jin X L. Press, Beijing, China, pp 199–211
Bohrmann G, Greinert J, Suess E et al (1998) Authigenic carbonates from the cascadia subduction zone and their relation to gas hydrate stability. Geology 26(7):647–650
Borowski WS, Paull CK, Ussler W III (1996) Marine pore water sulfate profiles indicate in situ methane flux from underlying gas hydrate. Geology 24(7):655–658
Chen Z, Yan W, Huang C et al (2007) Geological settings and indicators of potential gas hydrates in the nansha trough area. South China Sea. Front Earth Sci 14(6):299–308 (in Chinese with English abstract)
Chen Z, Huang CY, Wu BH et al (2010) Discovery of native aluminum and its possible origin from prospective gas hydrate areas in the South China Sea. Sci China-Earth Sci 53(3):335–344
Deng H, Yan P, Liu H (2004) Seismic characteristics of gas hydrate in the Nansha waters. Mar Geol Q Geol 24(4):89–94 (in Chinese with English abstract)
Elger J, Berndt C, Ruepke L et al (2018) Submarine slope failures due to pipe structure formation. Nat Commun 9:715
Feng D, Qiu JW, Hu Y et al (2018a) Cold seep systems in the South China Sea: an overview. J Asian Earth Sci 168:3–16
Feng JX, Yang SX, Liang JQ et al (2018b) Methane seepage inferred from the porewater geochemistry of shallow sediments in the beikang basin of the southern South China Sea. J Asian Earth Sci 168:77–86
Feng JX, Li N, Liang JQ et al (2021) Using multi-proxy approach to constrain temporal variations of methane flux in methane-rich sediments of the southern South China Sea. Mar Pet Geol 132:105152
Gao HH, Yang XQ, Zhang JP et al (2019) Paleomagnetic records since−80 ka from the southern South China Sea. Chinese J Geophys-Chinese Ed 62(12):4750–4765 (in Chinese with English abstract)
Gee MJR, Uy HS, Warren J et al (2007) The Brunei slide: a giant submarine landslide on the north west borneo margin revealed by 3D seismic data. Mar Geol 246(1):9–23
He Y, Kuang Z, Xu M (2018) Seismic reflection characteristics and triggering mechanism of mass transport deposits of quaternary in Beikang Basin. Geol Sci Technol Inf 37(4):258–268 (in Chinese with English abstract)
Huang W, Meng MM, Zhang W et al (2022) Geological, geophysical, and geochemical characteristics of deep-routed fluid seepage and its indication of gas hydrate occurrence in the beikang basin, Southern South China Sea. Mar Pet Geol 139:105610
Hutchison CS (2004) Marginal basin evolution: the southern South China Sea. Mar Pet Geol 21(9):1129–1148
Ingram GM, Chisholm TJ, Grant CJ et al (2004) Deepwater north west borneo: hydrocarbon accumulation in an active fold and thrust belt. Mar Pet Geol 21(7):879–887
Jørgensen BB, Böttcher ME, Lüschen H et al (2004) Anaerobic methane oxidation and a deep H2S sink generate isotopically heavy sulfides in black sea sediments. Geochim Cosmochim Acta 68(9):2095–2118
Kennett JP, Cannariato KG, Hendy IL et al (2000) Carbon isotopic evidence for methane hydrate instability during quaternary interstadials. Science 288(5463):128–133
Laird AP, Morley CK (2011) Development of gas hydrates in a deep-water anticline based on attribute analysis from three-dimensional seismic data. Geosphere 7:240–259
Lee GH, Watkins JS (1998) Seismic sequence stratigraphy and hydrocarbon potential of the phu khanh basin, offshore central Vietnam, South China Sea. AAPG Bulletin 82(9):1711–1735
Li N, Yang XQ, Peng J et al (2018) Paleo-cold seep activity in the southern South China Sea: evidence from the geochemical and geophysical records of sediments. J Asian Earth Sci 168:106–111
Li N, Yang XQ, Peckmann J et al (2021) Persistent oxygen depletion of bottom waters caused by methane seepage: evidence from the South China Sea. Ore Geol Rev 129:103949
Liu HL, Yao YJ, Deng H (2011) Geological and geophysical conditions for potential natural gas hydrate resources in southern South China Sea waters. J Earth Sci 22(6):718–725
Liu ZH (2005) Distribution of sedimentary basins and petroleum potential in southern South China Sea. Geotect Metall 29(3):410–4l7 (in Chinese with English Abstract)
Paganoni M, Cartwright JA, Foschi M et al (2016) Structure II gas hydrates found below the bottom-simulating reflector. Geophys Res Lett 43:5696–5706
Paganoni M, Cartwright JA, Foschi M et al (2018) Relationship between fluid-escape pipes and hydrate distribution in offshore Sabah (NW Borneo). Mar Geol 395:82–103
Peckmann J, Thiel V (2004) Carbon cycling at ancient methane-seeps. Chem Geol 205(3–4):443–467
Rehder G, Suess E (2001) Methane and pCO2 in the kuroshio and the South China Sea during maximum summer surface temperatures. Mar Chem 75(1–2):89–108
Ruppel CD, Kessler JD (2017) The interaction of climate change and methane hydrates. Rev Geophys 55(1):126–168
Traynor JJ, Sladen C (1997) Seepage in vietnam—onshore and offshore examples. Mar Pet Geol 14(4):345–362
Warren JK, Cheung A, Cartwright I (2010) Organic geochemical, isotopic, and seismic indicators of fluid flow in pressurized growth anticlines and mud volcanoes in modern deep-water slope and rise sediments of offshore brunei darussalam: implications for hydrocarbon exploration in other mud-and salt-diapir provinces. In: Wood L (ed.), Shale Tectonics. AAPG Memoir Vol. 93, pp. 163–196
Wetzel A (2013) Formation of methane-related authigenic carbonates within the bioturbated zone—an example from the upwelling area off Vietnam. Palaeogeogr Palaeocl 386:23–33
Yan W, Zhang G, Zhang L et al (2020) Focused fluid flow systems discovered from seismic data at the southern margin of the South China Sea. Interpretation 8(3):555–567
Zhang K, Guan Y, Song H et al (2020) A preliminary study on morphology and genesis of giant and mega pockmarks near andu seamount, nansha region (South China Sea). Mar Geophys Res 41:1–12
Zhang W, Chen C, Su P et al (2023) Formation and implication of cold-seep carbonates in the southern South China Sea. J Asian Earth Sci 241:105485
Zhou Y, Di PF, Li N et al (2020) Unique authigenic mineral assemblages and planktonic foraminifera reveal dynamic cold seepage in the southern South China Sea. Minerals 10(3):1–13
Acknowledgements
We are grateful to the Guangzhou Marine Geological Survey (GMGS) for the geochemical data. Sponsorship is by the National Natural Science Foundation of China (41976061, 42202174), the Major Program of Guangdong Basic and Applied Research (Grant: 2019B030302004), and the Guangdong Basic and Applied Basic Research Foundation (2019A1515110306, 2022A1515011822).
Author information
Authors and Affiliations
Corresponding author
Editor information
Editors and Affiliations
Rights and permissions
Open Access This chapter is licensed under the terms of the Creative Commons Attribution 4.0 International License (http://creativecommons.org/licenses/by/4.0/), which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license and indicate if changes were made.
The images or other third party material in this chapter are included in the chapter's Creative Commons license, unless indicated otherwise in a credit line to the material. If material is not included in the chapter's Creative Commons license and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder.
Copyright information
© 2023 The Author(s)
About this chapter
Cite this chapter
Li, N., Feng, J. (2023). Cold Seepage in the Southern South China Sea. In: Chen, D., Feng, D. (eds) South China Sea Seeps. Springer, Singapore. https://doi.org/10.1007/978-981-99-1494-4_13
Download citation
DOI: https://doi.org/10.1007/978-981-99-1494-4_13
Published:
Publisher Name: Springer, Singapore
Print ISBN: 978-981-99-1493-7
Online ISBN: 978-981-99-1494-4
eBook Packages: Earth and Environmental ScienceEarth and Environmental Science (R0)