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).

Fig. 13.1
A satellite map of the South China Sea. It highlights the Nansha Trough and Beikang Basin. Both seep sites are located in regions that fall between 50 and 500 meters.

Location of the two seep sites in the southern South China Sea. The map was generated with ocean data view

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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).

Fig. 13.2
A map highlights the mud volcanoes. Most of the volcanoes fall within the 0 to 500 range. All the mud volcanoes are marked with an ellipsis.

Multibeam image showing the locations and topography of mud volcanoes in Beikang Basin (from Huang et al. 2022). Reprinted from Marine and Petroleum Geology, 139, Huang 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, 105,610, Copyright (2022), with permission from Elsevier

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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.

Fig. 13.3
8 graphs plot depth versus 2 of each of C H subscript 4, S O subscript 4 superscript 2 negative, A L K, and C l. 4 follow an increasing trend and 4 follow a decreasing trend.

(Modified from Huang et al. 2022). Reprinted from Marine and Petroleum Geology, 139, Huang 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, 105,610, Copyright (2022), with permission from Elsevier

Depth profiles showing the variations in the methane content, Cl− and SO42− concentrations, and alkalinity (ALK) of the headspace gas and pore water from the Beikang Basin

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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).

Fig. 13.4
A graph plots c subscript 1 slash C subscript 2 positive versus delta superscript 13 C subscript 1. The point of (negative 50, 100) encompasses the region of thermogenic gas while the point (negative 60, 1000) encompasses the microbial gas. It also plots points for 6 particles with most of them plotting in the microbial gas region.

(Modified from Huang et al. 2022). Reprinted from Marine and Petroleum Geology, 139, Huang 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, 105,610, Copyright (2022), with permission from Elsevier

Plot of δ13C1 versus C1/C2+ for the headspace gas from piston cores in the Beikang Basin

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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).

Fig. 13.5
6 graphs plot depth versus T O C, T S, T O C slash T S, delta superscript 13 C subscript T I C, delta superscript 34 C subscript C R S, and open parenthesis N R M subscript A F 600 to N R M subscript A F 800 close parenthesis slash N R M. All follow a decreasing trend between 0 and 8.5 meters.

(Modified from Li et al. 2018). The shaded areas indicate the sections influenced by the anaerobic oxidation of methane as characterized by high TS/TOC ratios. Reprinted from Journal of Asian Earth Sciences, 168, Li et al. (2018) Paleo-cold seep activity in the southern South China Sea: Evidence from the geochemical and geophysical records of sediments, 106–111, Copyright (2018), with permission from Elsevier

Depth profiles of elemental, isotopic, and magnetism data in core 58S from the Beikang Basin

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Fig. 13.6
6 graphs plot depth versus T O C, T S, T O C slash T S, delta superscript 34 C subscript C R S, delta superscript 13 C subscript T I C, and delta superscript 13 C subscript planktonic. All lines follow an increasing and decreasing trend between 0 and 80.

(Modified from Gao et al. 2019; Zhou et al. 2020; Li et al. 2021). The shaded areas indicate the two carbon isotopic excursions of planktonic foraminifera. Reprinted from Ore Geology Reviews, 129, Li et al. (2021) Persistent oxygen depletion of bottom waters caused by methane seepage: Evidence from the South China Sea, 103,949, Copyright (2021), with permission from Elsevier

Depth profiles of elemental and isotopic data in core 2PC from the Nansha Trough

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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.