2.1 Introduction

The South China Sea (SCS) is one of the largest marginal seas in the western Pacific Ocean and is located between the Eurasian plate, the Pacific Plate and the Indian Plate. The SCS is bounded by the South China mainland and Taiwan Island to the north, Indochina Peninsula and Malay Peninsula to the west, Sumatra and Kalimantan Islands of the Greater Sunda Islands to the south, and the Philippine Islands to the east (Fig. 2.1). The contour of the SCS is characterized by an irregular diamond shape, the longitudinal axis is approximately 3140 km long, the transverse axis is approximately 1250 km long, and the area reaches approximately 350 × l04 km2. The average water depth is 1140 m, and the depth of the central basin is ~ 4200 m. The seabed topography of the SCS can be divided into the continental shelf, continental slope and deep-sea basin. There are many continental islands on the continental shelf, which are mainly composed of pre-Quaternary magmatic, metamorphic and sedimentary rocks adjacent to the mainland. Terrigenous materials originating from the Pearl River, the Red River, the Mekong River and other major rivers have all been imported into the SCS, which has also transported abundant organic materials into the SCS.

Fig. 2.1
A satellite map highlights some of the ancient and active cold seep systems in the South China Sea, which lies between Indochina, South China, and Taiwan island. 2 inserts of an Asian continent and site F.

Distribution of cold seeps in the marginal basins of the South China Sea. QDNB: Qiongdongnan Basin; PRMB: Pearl River Mouth Basin; TXNB: Taixinan Basin; XSTB: Xisha Trough Basin; BKB: Beikang Basin

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The northern SCS is a passive continental margin in which the Neogene tectonic activity intensity is high, and there are two main structural layers composed of Paleogene rift sequences and Neogene depression sequences. The abundant terrestrial clastic rocks derived from the Southern China and the Indochina Peninsula have resulted in Neogene sediment deposits up to ~ 10 km thick at the center of several petroliferous basins. The thick stratigraphic deposits and intense hydrocarbon generation and expulsion processes of the deep source kitchens have led to high fluid activity, forming a series of NE-trending basins that are rich in oil and gas, with widely distributed mud volcanoes, mud diapirs, gas chimneys, and submarine cold seeps (Chen et al. 2010; He et al. 2016; Su et al. 2016; Liang et al. 2019; Wan et al. 2019; Zhang et al. 2019a, b; 2020a). Numerous acoustic surveys, including 2D and 3D seismic surveys, subbottom profiles, and multibeam investigations, as well as extensive deep-sea diving observations, sampling, and gas hydrate drilling and coring (Matsumoto et al. 2011; Wang et al. 2014; Feng and Chen 2015; Zhang et al. 2017a, b; Liang et al. 2019; Wei et al. 2019; Zhang et al. 2020a, b), have been conducted in the SCS.

The number of cold seeps and associated gas hydrate accumulations discovered is continuously increasing. Since the discovery of the first cold seep system in the northern SCS in 2004, more than 40 seeps have been discovered in the Qiongdongnan Basin (QDNB), Xisha area, Shenhu area, Dongsha area, Taixinan Basin (TXNB), and Beikang Basin (BKB) (Fig. 2.1; Han et al. 2013; Berndt et al. 2014; Feng et al. 2018). Since 2007, the Guangzhou Marine Geological Survey (GMGS) has performed eight gas hydrate scientific drilling expeditions in the Shenhu area and the Xisha area (GMGS1, GMGS3 and GMGS4), the Dongsha area (GMGS2), and the Qiongdongnan Basin (GMGS5, GMGS6, GMGS7 and GMGS8) in the northern SCS (Yang et al. 2008; Wu et al. 2010, 2011; Wang et al. 2014; Sha et al. 2015a, b; Yang et al. 2015, 2017a, b, c; Zhang et al. 2015; Su et al. 2016; Liang et al. 2019; Wei et al. 2019; Zhang et al. 2020a, b). Except for the Xisha area, gas hydrate samples have been recovered in several regions where cold seep systems have been discovered. In addition, the Chinese Academy of Sciences has found exposed seafloor gas hydrates related to a submarine active cold seep in the Taixinan Basin (Feng and Chen 2015; Zhang et al. 2017b). These gas hydrate accumulations, which are related to the development and evolution of cold seep systems, are mainly distributed in the vicinity of the low uplift, submarine ridge, and channel–levee system and on the seabed above deep faults, large mud diapirs, and gas chimneys. Therefore, the development and evolution of cold seeps and the precipitation and accumulation of gas hydrates may be closely related to deep strata and petroleum systems. The objective of this chapter was to document the distribution and variability of hydrocarbon seeps in the SCS.

2.2 Distribution of Seeps

Based on comprehensive interpretation and analysis of 2D/3D seismic data, subbottom and multibeam data, and manned deep submersible and/or unmanned deep submersible (ROV) observations and sampling data, several regions of cold seep systems and associated gas hydrate accumulations have been discovered, and our understanding of cold seep systems in the SCS has been greatly improved (Schnürle et al. 2004; Suess 2005; Huang et al. 2006; Jiang et al. 2006; Liu et al. 2006; Han et al. 2008; Chen et al. 2014a, b; Feng and Chen 2015; Zhong et al. 2017; Feng et al. 2019; Wan et al. 2020; Cao et al. 2021).

In 2004, China and Germany jointly conducted the SO-177 Expedition in the deep water area near Dongsha in the eastern Pearl River Mouth Basin (PRMB), and for the first time, giant distributions of carbonate rocks, namely, the Jiulong Methane Reef, formed via seepage from cold seeps were discovered (Suess 2005; Han et al. 2008). In addition, the Jiulong Methane Reef was later confirmed as a gas hydrate accumulation area through deep gas hydrate drilling by the GMGS in 2013 (Sha et al. 2015a, b; Zhang et al. 2015; Zhong et al. 2017). In the same year, the Jiaolong manned deep submersible conducted its first scientific research voyage in the active cold seep area of the Site F in the TXNB, and intermittent activity of this cold seep system was initially identified (Feng and Chen 2015). In the deep waters of the Taixinan Basin, scholars have not only collected authigenic carbonates recording cold seep activity at five sites on the Gaoping Slope (Huang et al. 2006) but have also discovered very active cold seeps on the seafloor of the Formosa Ridge. In 2015, a giant active cold seep, namely, the Haima Cold Seep, was discovered by the GMGS in the southwestern QDNB. In 2018, a new submarine cold seep was discovered in the eastern QDNB. In addition, geophysical and geochemical evidence of cold seep activity was found in the BKB in the southern SCS through a geological survey (Huang et al. 2022).

Although dozens of cold seeps and associated gas seepages have been found in the SCS, active cold seeps have only been discovered in certain regions. Detailed results of the investigation of cold seeps in different regions are presented in Table 2.1 and addressed below.

Table 2.1 Variability of the seeps discovered in the South China Sea
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2.3 Variability of Seeps

There are usually a series of microtopographic seabed features in cold seep and gas hydrate accumulation areas, such as slumps, pockmarks, mounds, mud volcanoes, depressions, and platforms (Kvenvolden 1993; Demirbas 2010; Matsumoto et al. 2011; Minshull et al. 2020). Through a comprehensive study of the seismic, subbottom profile, and ROV image data for the target area for cold seep and gas hydrate exploration in the SCS, various seismic and acoustic reflection features related to fluid seepage/leakage were recognized, including gas plumes, seabed mounds, pockmarks, bright spots, acoustic blanking, and acoustic turbidity (Chen et al. 2010; Shang et al. 2013, 2014; Wang et al. 2014; Zhang et al. 2019b, 2020a, b; Cheng et al. 2020; Wu et al. 2020). In addition, microbial communities, bacterial mats, methane biochemical reefs, and carbonate crusts associated with cold seeps have been found in the QDNB, PRMB, and TXNB in the northern SCS and the BKB in the southern SCS (Fig. 2.1) (Chen et al. 2007; Feng and Chen 2015; Zhang et al. 2017b; Wei et al. 2020a, b; Zhang et al. 2023). These seafloor seepage phenomena, especially the discovery of gas plumes, suggest that a sufficient gas source exists in the study area. The appearance of many bivalves and bacterial mats is often a sign of a cold seep with active methane seepage, while carbonate crusts may indicate the cessation of methane seepage. These signs further indicate that gas hydrates are likely to accumulate in cold seep areas.

2.3.1 Qiongdongnan Basin

Based on fine interpretation of a 3D seismic profile in the QDNB, many geophysical anomalies closely related to gas seepage were identified in the deep-water area (Fig. 2.2). In addition, a large number of seabed pockmarks, mounds, and acoustic blanking reflections, which indicate the presence of free gas and gas hydrates, were identified in subbottom and seismic profile (Fig. 2.2a, b). The reflection of acoustic blanking suggests gas-bearing strata, which is consistent with the local structural high, indicating the accumulation characteristics of gas at high points. Small mounds, i.e., tens to hundreds of meters in diameter, were interpreted atop the observed acoustic blanking, indicating the accumulation and seepage of gas from depth. The presence of large flourishing biological communities often indicates the occurrence of current methane seepage, which is likely related to the dissociation of gas hydrates precipitated in shallow strata.

Fig. 2.2
4 contour plots and 2 photos. The profiles a to c and f highlight the mound, pockmark, and acoustic chaotic blanking of the mounds, buried mounds and unconformities, piston core, pull-ups, migration pathway, B S R, and acoustic chaotic blanking of the mounds, among others. d is a microbe photo, and e is hydrates recovered from the Haima Cold Seep.

a Subbottom profile and b seismic profile showing seabed microgeomorphological features related to gas seepage in the deep-water Qiongdongnan Basin. c Seismic profile showing hydrocarbon migration and seepage features of the Haima Cold Seep. d Microbial communities and e massive gas hydrates recovered from the Haima Cold Seep (modified from Liang et al. 2017). f Seismic profile showing deep structural features below the Haima Cold Seep in the southern low uplift area of the Qiongdongnan Basin. A large range of acoustic fuzzy zones resulting from gas chimneys can be observed in the seismic profile, indicating upward migration of deep hydrocarbons, supplying materials for gas seepage and gas hydrate accumulation

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The bottom-simulating reflector (BSR) was clearly identified by the seismic anomalies resulting from gas migration along the low uplift, with a fuzzy seismic reflection zone below the BSR (Fig. 2.2c, f). Gas seepage pathways extending from the BSR to the seafloor were identified in the high-precision 3D seismic profile, which shows the pulled-up features of the events on both sides of the vertical pathways (Fig. 2.2c). Pulled-up reflectors are usually caused by the presence of a material that is harder than the surrounding strata since this hard material could cause velocity anomalies (Yoo et al. 2013). The gas hydrate drilling and sampling campaigns conducted during GMGS expeditions 5 and 6 demonstrated that hard materials, including gas hydrates and authigenic carbonate rocks precipitated within seepage/migration pathways, corresponding to pulled-up features (Liang et al. 2019). Piston and push coring processes also recovered gas hydrates and authigenic carbonate rocks from subsurface sediments at seepage sites in the QDNB (Liang et al. 2017). In addition, seepage pathways usually correspond to cold seep vents on the seafloor, and this phenomenon has been commonly observed in the Haima Cold Seep (Fig. 2.2c–e). Anomalies with various amplitudes associated with seafloor seepage were observed in the seismic profiles, mainly including high-amplitude bright spots associated with BSRs and large areas of blanking or chaotic reflection zones due to mud diapirs and/or gas chimneys, which promote gas migration and accumulation in deep strata (Zhang et al. 2019b).

The cold seeps in the QDNB developed atop strata overlying the low uplift of the pre-Paleogene basement, which resulted from intense magmatic intrusion (Wang et al. 2018a; Wei et al. 2020a). In addition, the faults and fractures formed by the uplift of structures generated active plumping systems (Wang et al. 2018a; Wei et al. 2020a) connected to the deep Paleogene source rocks, transporting deep hydrocarbons upward and resulting in the formation of gas chimneys and associated fluid leakage under overpressure conditions. Many thermogenic and biogenic gases migrated along these gas chimneys into the GHSZ to form fracture-filling gas hydrates in mass transport deposits (Liang et al. 2019). Analysis of the gas hydrate petroleum system revealed the close relationship between these shallow gas hydrates and deep conventional petroleum reservoirs. The deep central channel sand gas reservoir and deep source strata exhibit a direct vertical coupling relationship with the distribution of gas hydrates. The geochemical characteristics of the gas hydrates drilled during the GMGS5 expedition indicate that the thermogenic gas in the shallow gas hydrate accumulation area is consistent with the origin of natural gas in deep reservoirs such as LS17-2, LS22-1, and Y8-1 (Zhang et al. 2017c, 2019c; Lai et al. 2021; Zhang et al. 2020a, c). Therefore, it is believed that shallow gas hydrates are homologous with deep gas reservoirs. The gas-bearing fluid provided by the development and evolution of source rocks in the deep depression, the migration system comprising the low uplift due to tectonic activity, the associated faults, and the gas chimneys jointly controlled submarine cold seep system formation and gas hydrate accumulation in the QDNB.

2.3.2 Shenhu Area in the Middle PRMB

Similar to the QDNB, the seismic profiles of the Shenhu area indicated that there exists a large range of fuzzy reflection zones formed by diapirs and/or gas chimneys, which have a columnar or mushroom-shaped appearance along the vertical direction (Wang et al. 2014; Su et al. 2017; Liang et al. 2019; Zhang et al. 2020b). Bright spots were observed at the top and edges of the gas chimneys, indicating the presence of active gas-bearing fluids in the Shenhu area. BSRs were widely distributed in the study area, and very high-amplitude BSRs occurred in the upper parts of the gas chimneys (Wang et al. 2014; Chen et al. 2016; Cheng et al. 2020; Zhang et al. 2020b). A set of amorphous enhanced reflections oblique to the BSR usually occurs immediately above the BSR. Drilling confirmed that these enhanced reflections indicate the presence of gas hydrates (Wang et al. 2014; Zhang et al. 2020b) (Fig. 2.3). With the acquisition and interpretation of high-resolution 3D seismic data, large-scale listric faults connecting deep source kitchens, medium-deep petroleum reservoirs, and shallow GHSZ were observed in the gas hydrate accumulation zone (Chen et al. 2016; Su et al. 2017; Cheng et al. 2020). In addition, it was found that the gliding faults in the GHSZ are partially connected with the BSRs and extended upward to the seabed, constituting possible pathways along which gas could enter the GHSZ or escape and leak into the water column after gas hydrate dissociation due to slope failure. Although no active cold seeps have been confirmed by ROV observations at present, gas seepage and paleo-cold seep activity have been confirmed through geophysical detection and geochemical analysis of sediments recovered from the Shenhu area (Xu et al. 2012; Chen et al. 2014a, b; Lin et al. 2016; Hu et al. 2020).

Fig. 2.3
3 contour plots. a highlights elements like slides, initially and highly deformed slumps, and so on. Graphs b and c depict two-way travel times highlighting the gas chimney, with layers from T 1 from T 5, B S R, and E R Ss for the former and acoustic blanking, gas reservoir, fault, 2 E Rs for the latter.

a Overlapping relationship between the gas chimneys, mud diapirs, mud volcanoes, pockmarks, BSRs, and gas reservoirs in the Shenhu area (modified from Chen et al. 2016). b and c Seismic profiles showing geophysical reflection characteristics of the gas hydrate drilling sites in the Shenhu area. ERs: Enhanced reflections

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2.3.3 Dongsha Area in the Eastern PRMB

Gas chimneys with chaotic seismic reflections and pull-down features indicating hydrocarbon migration and accumulation, as well as BSRs and associated seismic anomalies indicating seeps, have been identified by interpreting quasi-3D seismic data for the Dongsha area, northern PRMB (Sha et al. 2015a, b; Zhang et al. 2015). In addition, a large number of slumps and listric faults have been identified in shallow strata. The above characteristics indicate the occurrence of gas seepage in the area and the possibility of the development of various submarine microgeomorphologies associated with gas seepage. A large range of blanking zones was recognized in subbottom profiles (Fig. 2.4a; Shang et al. 2013, 2014), indicating that there may exist mud diapirs with a high strata pressure or faults conducive to fluid migration in this area. Based on analysis of subbottom profiles, acoustic blanking reflection anomalies indicating the presence of mud volcanoes and gas accumulation in shallow sediments were observed (Fig. 2.4b), and it was found that many pockmarks that may be caused by submarine gas leakage (Fig. 2.4c–h; Shang et al. 2013, 2014). When the deep gas-bearing fluid is blocked by shallow strata and cannot reach the seafloor, the upper strata are deformed due to overpressure and can form dome features in the seafloor, which is also one of the macroscopic manifestations of gas seepage below the sea floor (Fig. 2.4d).

Fig. 2.4
8 profiles. The acoustic chaotic is highlighted in a, while the mud volcano and acoustic blanking are depicted in b. c, d, e, f, and g highlight the pockmark, the pockmark and acoustic blanking, the gas chimney, the pockmark, and the seabed. h depicts pockmark and carbonate mound.

Subbottom profile and seismic profile showing the microgeomorphological features associated with the cold seep and gas hydrate accumulation in the Dongsha area (modified from Shang et al. 2013, 2014). The pockmarks are closely related to gas-bearing fluid migration from the deep strata, which are indicated by acoustic blanking and chaotic features in the subbottom and seismic profiles. The submarine mound with authigenic carbonate pavement resulted from gas hydrate dissociation and hydrocarbon seepage

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Gas chimneys are commonly observed in the seismic profile of the Dongsha area (Kuang et al. 2018; Wang et al. 2018b). In general, the internal reflections of these gas chimneys are disordered and chaotic or are characterized by acoustic blanking. In addition, continuous reflection events on both flanks are suddenly interrupted and terminate at the edge of the chimney zone. The pulled-down phenomenon of events can commonly be observed at the top of the gas chimney, and the bright spots on both sides and the top of the upper part are distinct (Fig. 2.4). Low-amplitude, pulled-down features are mostly caused by low-velocity anomalies created by gas charging. Bright spots are the enhanced reflections of free gas accumulations. Most of the gas chimneys extend from Miocene to Quaternary strata. In plan view, the gas chimneys are mainly located along the eastern and western submarine ridges, indicating that the local structural high controls hydrocarbon migration and accumulation (Fig. 2.5). Vertically, the large gas chimneys in the drilling area nearly terminate below the BSR, indicating that the gas-bearing fluid is enriched at the base of the gas hydrate stability zone (BGHSZ) during upward migration and that gas hydrates are precipitated under the appropriate temperature and pressure conditions (Liu et al. 2006; Chen et al. 2010; Wang et al. 2018b). Notably, gas chimneys control hydrocarbon migration and gas hydrate formation and accumulation in the Dongsha area.

Fig. 2.5
3 contour plots and 3 photos. a has the western and eastern ridges that highlights the well location, seismic line, quaternary fault, and gas chimney. b and c highlight buried carbonate, B S R, pull down, and enhanced reflections, among others. d, e, and f are of the carbonate rocks recovered.

a Distribution of gas chimneys, faults, and gas hydrate drilling sites in the GMGS2 expedition drilling zone in the Dongsha area. The locations of the drilling sites are retrieved from Zhang et al. 2015. b and c Seismic profiles showing the gas hydrate accumulation features at the drilling and coring sites, where gas hydrates and buried carbonates were confirmed via logging and pressure coring (modified from Wang et al. 2018b). d and e Massive gas hydrate samples collected at site GMGS2-W08 (Sha et al. 2015b). f Carbonate rock recovered at site GMGS2-W08 (modified from Zhong et al. 2017)

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2.3.4 Taixinan Basin

Mud diapirs and mud volcanoes also developed in the deep-water TXNB offshore southwest of Taiwan (Schnürle et al. 2011). They exhibit a large range of vertical acoustic blanking and chaotic reflections in seismic profiles, and domes and cone-shaped structures are commonly observed in the seafloor. Continuous BSRs have been interpreted in the vicinity of mud volcanoes, mud diapirs, and gas chimneys, indicating the presence of gas hydrates (Fig. 2.6). Moreover, gas seepages and associated cold seeps have been confirmed via multibeam profiles and in situ ROV observations.

Fig. 2.6
3 two-way travel time contour plots and a photo. a highlights the mud volcano and B S R while graph. c highlights site F, fluid conduit, and multiple B S R. d displays the gas seepage, gas chimney, B S R, P R fault, and the unconformity. Photograph b displays the vents and hydrate pieces in the seafloor. e. A map highlights a detected gas plume.

a Seismic profile showing the BSRs observed in the vicinity of a mud volcano developed offshore of southwestern Taiwan (modified from Liu et al. 2006). b Cold seep vents and gas hydrates exposed on the seafloor at site F on the Formosa Ridge (Zhang et al. 2017b). c Seismic profile showing gas seepage and BSRs crossing Site F on the Formosa Ridge (Hsu et al. 2018). d Seismic profile showing gas seepage and BSRs on Pointer Ridge (Han et al. 2019). e Gas plume detected on Pointer Ridge (Han et al. 2019). Fault PR is the Pointer Ridge fault

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There are cold seeps with variable activity intensities and associated gas hydrate accumulations in the Dongsha area. The direct evidence obtained from geochemical analysis of available recovered hydrate gas samples indicates that the hydrate gas is microbial in origin and is unrelated to deep source rocks. No commercial conventional oil and gas reservoirs have been discovered in the Dongsha area, making it more difficult to prove the relationship between shallow gas hydrates and deep petroleum reservoirs. However, recent basin modeling and geochemical studies have shown that deep source rocks and possible oil and gas reservoirs may contribute to the formation of shallow gas hydrates (Gong et al. 2017; Li et al. 2021). In addition, thermogenic gas associated with mud volcanoes has been detected in the nearby central-eastern TXNB, suggesting a potential coupling relationship between shallow gas hydrates and potential deep petroleum reservoirs (Chen et al. 2017, 2020).

2.3.5 Beikang Basin in the Southern SCS

Based on high-resolution 2D seismic profiles and multibeam data, multiple types of deep-routed conduits—typically manifested as mud volcanoes, diapirs, fluid-escape pipes, and paleo-uplift-associated faults—were identified. These manifestations were generally accompanied by BSRs, blanking zones, locally enhanced reflections, pulled-up reflections, pockmarks, mounds, and possible gas plumes, which indicated fluid seepage and potential gas hydrate accumulation (Fig. 2.7). Arching deformation of deep mud source rock strata and accompanying gas-bearing fluid invasion yielded blanking zones, fuzzy zones, and enhanced reflections inside the mud volcanoes and diapirs. The sulfate methane transition zone (SMTZ) at the fluid seepage sites occurred at relatively shallow depths (325–660 cm below the seafloor (cmbsf)), suggesting a high methane flux and sufficient gas supply. The carbon isotope composition of methane (−92.6‰ < δ13C <  − 50.2‰) and the detection of ethane, propane, and n-butane, as well as multiple geological configuration biomarkers, all indicated that deep-sourced hydrocarbons and thermogenic gas migrated into the shallow layer through deep-routed conduits. Deep-routed fluid seepage and the accompanying hydrocarbon gas supply may have significantly influenced cold seeps and potential thermogenic gas hydrates (Huang et al. 2022).

Fig. 2.7
4 profiles plot two-time travel time. a and b highlight the mud volcanoes, blanking zones, diapirs, fuzzy zones, and enhance reflections. c highlights the gas seepage, shallow gas, and B S Rs. d plots the fluid escape pipe, pull-up reflection, and fracture among the 4 layers of T 0 to T 3.

Seismic profile showing geophysical characteristics of the a mud volcanoes and b mud diapirs. Seismic profiles showing geological features of the fluid escape pipes. c Fluid escape pipes originating at the top of the shallow gas zone. d Fluid escape pipes originating in the depressions with their roots connected to the fault

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2.4 Gas and Fluid Migration Paths and Seafloor Seepage

Although the majority of the gas seeps discovered in the SCS are related to preexisting pathways composed of faults, gas chimneys, and mud diapirs, there are gas seeps that formed due to the behavior of the gases themselves. Many high-resolution seismic profiles exhibit indicators of the presence of shallow gas seeps in Quaternary sediments in the SCS, such as pipe structures, bright spots, seabed mounds, and pockmarks. These subsurface gas seepage/leakage areas are probably caused by gas hydrate dissociation and exhibit a close relationship with fluid pathways and deep petroleum systems.

The drilling sites of the GMGS1, GMGS3, and GMGS4 expeditions in the Shenhu area were located atop gas chimneys or mud diapirs, which provided pathways by which deep gas-bearing fluids could migrate into shallow strata and transport thermogenic gas generated by Paleogene source rocks and shallow biogenic gas in the Baiyun Sag to the GHSZ to form gas hydrates (Wang et al. 2014; Su et al. 2017). At the drilling sites where high-saturation gas hydrates (~60%) were recovered through coring, such as sites W11, W17, W18 and W19 (Fig. 2.8), gas-bearing fluids migrated upward from the deep Paleogene strata along large-scale mud diapirs and gas chimneys, resulting in large fuzzy seismic zones in the seismic profiles (Zhang et al. 2019b, 2020b). In plan view, the development range of the diapirs and gas chimney attained a very obvious relationship with the distribution of the BSR and gas hydrate accumulations, which are mainly distributed along the submarine ridge (Fig. 2.3). This demonstrates that the migration pathways of these gas-bearing fluids controlled gas hydrate accumulation in the Shenhu area (Wang et al. 2014; Su et al. 2017; Cheng et al. 2020). Based on high-resolution 3D seismic data (Fig. 2.8), it was found that there are also large, deep faults in the Shenhu area that connect the Paleogene source rocks and the shallow GHSZ, which also served as vertical pathways for fluid migration, especially thermogenic gas derived from deep source rocks (Wang et al. 2014; Cheng et al. 2020; Jin et al. 2020). For example, the deep thermogenic gas in gas field LW3-1 was transported to the GHSZ via large faults and mud diapirs to form high-saturation gas hydrates (Fig. 2.3), which was confirmed based on the isotopic compositions of the hydrate gas and conventional gas in the Baiyun Sag (Li et al. 2019; Zhang et al. 2019a; Jin et al. 2020). Within the GHSZ, due to sediment gliding and/or slope failure, some of the gas hydrate-bearing strata formed slump faults, which connect the base of the GHSZ with the seafloor, constituting pathways along which gas entering the GHSZ could move further upward, and some of the gas may leak from the sea floor after gas hydrate dissociation, producing upward fluid migration (Fig. 2.8c). Additionally, gas-bearing fluids commonly migrate along the preexisting slump faults developed above the BGHSZ, and this fluid escape may in turn cause seafloor collapse and gas hydrate dissociation.

Fig. 2.8
3 profiles of time travel time. a and c highlight the mud volcanoes, blanking zones, pull-downs, pull-ups, mud diapir, E R s and B S Rs for the former and faults, B S Rs, blanking zones and acoustic chaos for the latter. b displays the B S R and the low instantaneous frequency from the bottom.

Seismic characteristics of the various types of hydrocarbon migration pathways and their coupling relationships with gas hydrate stability zones in the Shenhu area. Acoustic blanking (BZ) and acoustic chaos reflections below the BSR. Seismic events in the upper part of the abnormal reflection zone exhibit pull-down characteristics. Pull-downs and enhanced reflections (ERs) suggest the presence of gas. Distinct low-frequency zones in the instantaneous frequency image indicate free gas charging and trapping in sediments. The dark blue and white curved arrows indicate possible gas-bearing fluid migration

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Medium-strong seafloor gas seepage occurs in the deep-water QDNB, and many seepage pathways related to gas hydrate formation and accumulation have developed, including mud diapirs, gas chimneys, and faults of variable scales (Zhang et al. 2019b, 2020a, c). Most significantly, the seismic reflections below the Haima Cold Seep discovered in 2015 are chaotic, creating a large fuzzy zone due to the migration of deep gases into shallow strata (Wang et al. 2018a). In addition, several faults and fractures have been identified within the cold seep area, constituting vertical pathways for deep fluid migration and seepage (Wei et al. 2020a). In 2018 and 2019, massive gas hydrate samples were successfully obtained within a large gas chimney development area in the eastern QDNB (Liang et al. 2019; Wei et al. 2019). Seismic interpretation and gas hydrate drilling demonstrated that this gas hydrate accumulation is closely related to the underlying gas chimneys and deeply buried low uplift. The large faults developed on both sides of the uplift directly connect the deep source kitchens with the vertical gas chimneys and GHSZ, functioning as thermogenic hydrocarbon migration pathways (Liang et al. 2019; Lai et al. 2021).

Since the late Miocene, neotectonic activity has occurred in the cold seep and gas hydrate accumulation area of the Dongsha area. Many gas chimneys, mud diapirs, and mud volcanoes, which function as gas-bearing fluid migration pathways, were developed in this area. Clusters of gas chimneys were identified in seismic profiles of the GMGS2 gas hydrate drilling zone (Kuang et al. 2018; Wang et al. 2018b; Wu et al. 2020). Another type of hydrocarbon migration pathway in the Dongsha drilling area entails faults, which are mainly inherited faults and small active faults at the top and/or flanks of the large gas chimneys, and some of the inherited faults extend downward into the deeply buried basement (Kuang et al. 2018). Fault development is usually accompanied by gas chimneys or directly occurs within the gas chimneys. These faults have remained active since the late Miocene and are mainly distributed along the western ridge in the drilling area. However, the faults developed along the eastern ridge are less abundant and are smaller in size (Fig. 2.5). Most of the identified faults are subvertical and steeply dipping faults and were developed from the late Miocene to the Quaternary. Some of these faults even reach the seafloor. Many of the faults developed at the top and/or flanks of the gas chimney are steep in occurrence, small in size, and variable in strike. They often cut the BSR and are associated with enhanced reflections, which are the result of upward hydrocarbon migration along gas chimneys and deep large-scale faults. When the gas flux is sufficient, gas hydrates can form and accumulate in the GHSZ. In addition, the gas hydrate accumulation area corresponds well with the strike of the faults and the extension direction of the gas chimneys (Fig. 2.5; Zhang et al. 2015). The gas hydrate drilling and coring sites in the Dongsha area are basically located near the fault at the top of a gas chimney.

In the eastern Dongsha area, the mud volcanoes and mud diapirs near the BSRs are well developed in the TXNB, indicating that they constitute the main pathways for the migration of deep gas-bearing fluids needed for gas hydrate formation (Liu et al. 2006). Gas chimneys, pipe structures, and unconformities, which constitute the pathways by which gas-bearing fluids can migrate into the GHSZ, were also observed in this area (Fig. 2.6). Based on the seismic profile crossing the Site F (Formosa ridge) cold seep in the Taixinan Basin, gas chimneys constituting the migration pathway by which deep gas leaks into the seabed and forms a cold seep were identified (Hsu et al. 2018). In addition, mud diapirs and gas chimneys that directly connect the BSR with the seabed and constitute gas migration and leakage pathways were identified along the Pointer Ridge in the TXNB, forming cold seeps and gas plumes in the seabed (Schnürle et al. 2011; Han et al. 2019).

2.5 Summary and Perspectives

Many cold seep systems at different stages of evolution have been discovered in the SCS. These cold seeps are mainly distributed in petroliferous basins, and their formation and evolution are closely related to the activities of deep gas-bearing fluids. It is reasonable to conclude that the deep petroleum system and gas-bearing fluid migration system jointly controlled the development of submarine cold seep systems and gas hydrate accumulation and enrichment in the SCS. How the high temperature and overpressure associated with deep gas-bearing fluid activity affect the dynamics of the submarine gas hydrate system and cold seep activity remains unclear and should be further studied. In addition to deep-sea drilling and sampling analysis, numerical and physical simulations provide the potential to yield new insights into the distribution and accumulation relationships between cold seeps and gas hydrates.