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Multi-beam and seismic investigations of the active Haima cold seeps, northwestern South China Sea
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Bin Liu1, Jiangxin Chen2, 3, *, Li Yang1, Minliang Duan2, 4, Shengxuan Liu1, Yongxian Guan1, Pengcheng Shu4
Acta Oceanologica Sinica | 2021, 40(7) : 183 - 197
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Acta Oceanologica Sinica | 2021, 40(7): 183-197
• Marine Geology •
Multi-beam and seismic investigations of the active Haima cold seeps, northwestern South China Sea
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Bin Liu1, Jiangxin Chen2, 3, *, Li Yang1, Minliang Duan2, 4, Shengxuan Liu1, Yongxian Guan1, Pengcheng Shu4
Affiliations
  • 1 Key Laboratory of Marine Mineral Resources, Guangzhou Marine Geological Survey, Ministry of Natural Resources, Guangzhou 510760, China
  • 2 Key Laboratory of Gas Hydrate, Qingdao Institute of Marine Geology, Ministry of Natural Resources, Qingdao 266071, China
  • 3 Laboratory for Marine Mineral Resources, Pilot National Laboratory for Marine Science and Technology (Qingdao), Qingdao 266071, China
  • 4 Key Laboratory of Submarine Geosciences and Prospecting Techniques of Ministry of Education, Ocean University of China, Qingdao 266100, China
Published: 2021-07-25 doi: 10.1007/s13131-021-1721-6
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To confirm the seabed fluid flow at the Haima cold seeps, an integrated study of multi-beam and seismic data reveals the morphology and fate of four bubble plumes and investigates the detailed subsurface structure of the active seepage area. The shapes of bubble plumes are not constant and influenced by the northeastward bottom currents, but the water depth where these bubble plumes disappear (630–650 m below the sea level) (mbsl) is very close to the upper limit of the gas hydrate stability zone in the water column (620 m below the sea level), as calculated from the CTD data within the study area, supporting the “hydrate skin” hypothesis. Gas chimneys directly below the bottom simulating reflectors, found at most sites, are speculated as essential pathways for both thermogenic gas and biogenic gas migrating from deep formations to the gas hydrate stability zone. The fracture network on the top of the basement uplift may be heavily gas-charged, which accounts for the chimney with several kilometers in diameter (beneath Plumes B and C). The much smaller gas chimney (beneath Plume D) may stem from gas saturated localized strong permeability zone. High-resolution seismic profiles reveal pipe-like structures, characterized by stacked localized amplitude anomalies, just beneath all the plumes, which act as the fluid conduits conveying gas from the gas hydrate-bearing sediments to the seafloor, feeding the gas plumes. The differences between these pipe-like structures indicate the dynamic process of gas seepage, which may be controlled by the build-up and dissipation of pore pressure. The 3D seismic data show high saturated gas hydrates with high RMS amplitude tend to cluster on the periphery of the gas chimney. Understanding the fluid migration and hydrate accumulation pattern of the Haima cold seeps can aid in the further exploration and study on the dynamic gas hydrate system in the South China Sea.

fluid escape  /  cold seep  /  natural gas hydrate  /  bubble plume  /  Qiongdongnan Basin  /  South China Sea
Bin Liu, Jiangxin Chen, Li Yang, Minliang Duan, Shengxuan Liu, Yongxian Guan, Pengcheng Shu. Multi-beam and seismic investigations of the active Haima cold seeps, northwestern South China Sea[J]. Acta Oceanologica Sinica, 2021 , 40 (7) : 183 -197 . DOI: 10.1007/s13131-021-1721-6
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1 Introduction
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Cold seeps are areas where gases and fluids leak into the ocean water column from the seafloor sediments (Judd and Hovland, 2007; Foucher et al., 2009). The word “cold” is used to distinguish them from the hydrothermal vents. Unlike hydrothermal vents that mostly occur near the ocean ridges where a new crust is formed or derived from magmatism in the sedimentary basins, cold seeps are widespread globally (Judd and Hovland, 2007). Cold seeps have attracted considerable attention because of their potential role in the global methane budget (Judd, 2003; Etiope, 2012) and their association with natural gas hydrates and seafloor ecosystems (Foucher et al., 2009; Suess, 2014).
The cold seeps can be verified by the presence of gas bubble plumes that can be detected using high-frequency acoustic methods. Gas bubble plumes often manifest themselves as gas flares in acoustic images that have been observed in most of the oceans around the world, such as the Hydrate Ridge (Bangs et al., 2011), Barbados (Barnard et al., 2015), Costa Rica (Crutchley et al., 2014), Hikurangi margin (Klaucke et al., 2010), Gulf of Mexico (Solomon et al., 2009), Mediterranean (Prinzhofer and Deville, 2013), Black Sea (Greinert et al., 2006), U.S. Atlantic margin (Brothers et al., 2013, 2014; Skarke et al., 2014), sub-Antarctic island (Römer et al., 2014) and the Svalbard continental margin (Westbrook et al., 2009).
In the South China Sea (SCS), numerous cold seeps have been discovered since the discovery of the “Jiu Long Methane Reef” in 2004 (Chen et al., 2005; Han et al., 2008). Cold seep-related seafloor features, e.g., mud volcanoes, gas seepages and pockmarks, have been identified in the Taixinan Basin (TXNB) (Chen et al., 2014), the Zhujiang River Mouth Basin (ZRMB) (Sun et al., 2012), the Yinggehai Basin (YGHB) (Huang et al., 2009), the Qiongdongnan Basin (QDNB) (Wang et al., 2018a; Zhang et al., 2019) and the Zhongjiannan Basin (ZJNB) (Chen et al., 2015a, b, 2018). An overview of the discovered cold seep systems in the SCS from the aspects of seabed manifestation, geochemical indicators and ecology system has been summarized (Feng et al., 2018). Most of the cold seeps in the northern SCS margin are currently inactive, at least since 11.5 ka BP (Tong et al., 2013; Han et al., 2014). Only two active cold-seep sites have been reported, which are Site F in the Dongsha area (Zhang et al., 2017) and the Haima cold seeps discovered during the dives of the Remote Operated Vehicle (ROV) “Haima” (Liang et al., 2017). Numerous geochemical studies on the Haima cold seeps have been conducted (Liang et al., 2017; Guan et al., 2018; Wang et al., 2018a; Feng et al., 2019; Hu et al., 2019; Fang et al., 2019), however, the geophysical and geological studies of this active seepage are limited. One 2D seismic line across Site ROV1 and Site ROV2 described the subsurface structures beneath the Haima cold seeps (Wang et al., 2018b). They thought magmatic activities probably make a big contribution to the Haima cold seeps (Wang et al., 2018b), which means that the Haima cold seeps may represent a unique active cold seep and natural gas hydrate system in the SCS (Liu and Liu, 2017; Yang et al., 2018), till now. Seismic data and samples geochemical analysis show that magmatism in the QDNB and Xisha massif was active during Middle Miocene and Pliocene (Zhao et al., 2016; Wang et al., 2019). Liang et al. (2017) and Wei et al. (2020) have given evidence of the existence of long-term gas venting (several thousands of years) at the Haima cold seeps, however, whether the Haima cold seeps were active over the Neogene is still unknown. More geophysical data and geological analysis should be conducted to study the subsurface structures beneath gas bubble plumes.
More recently, a systematic study based on the seafloor observation data and seismic data has been conducted, which presents a broader cold seep ecosystem and a potential fluid migration mechanism (Wei et al., 2020). However, the detailed subsurface structure and gas hydrate accumulation mechanism responsible for this active seepage environment, as well as the characteristic of gas plumes are still unclear. Therefore, the aims of this work are: (1) to study the behaviors of gas bubble plumes within the water column, the associated features on the seafloor and the subsurface structures using multi-beam and seismic data, (2) to characterize the fluid escape pathways and natural gas hydrate formation using high-resolution 3D seismic dataset and 2D seismic lines, and (3) to analyze and gain new insights to the unique fluid escape system. Our study may help to better understand the fluid flow and gas hydrate accumulation at the Haima cold seeps, having an implication for further gas hydrate research and exploration in the SCS.
2 Geological background and fluid flow system
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The study area is located in the southwest of the QDNB, northwestern SCS, and neighbors Hainan Island to the north (Fig. 1a). The QDNB is a NE-trended Cenozoic sedimentary basin. Its evolution is closely related to the opening of the SCS (Yuan et al., 2009; Huang et al., 2016). This basin has experienced two-phase evolution, the rifting stage during Palaeogene and the post-rifting thermal subsidence stage since Neogene (Ru and Pigott, 1986; Lüdmann and Wong, 1999; Hu et al., 2013). The QDNB can be divided into five main tectonic zones: the northern depression belt, northern (central) uplift belt, central depression belt, southern depression belt, and southern uplift belt (Shi et al., 2013). Average rates of accumulation in QDNB are fluctuating and extremely high, i.e., 86 m/Ma in 45–36 Ma, 200–300 m/Ma in 36–15.5 Ma, 90–130 m/Ma in 15.5–5.5 Ma and 150–630 m/Ma since 5.5 Ma (Zhao et al., 2015). The rapid sedimentation rates cause overpressure and lead under-compaction at depths of greater than 3 300 m beneath the sea surface throughout the central QDNB, especially in deep water areas (Zhu et al., 2009; Huang et al., 2016). In addition, gas hydrate occurrence in the QDNB has been confirmed by the hydrate samples recovered at the Haima cold seeps and during the fifth gas hydrate drilling expedition conducted by the Guangzhou Marine Geological Survey (Liang et al., 2017; Wei et al., 2019; Ye et al., 2019; Liang et al., 2019). The Haima cold seeps lie in the Lingnan low uplift, belonging to the southern uplift belt (Fig. 1b). The present-day geothermal regime of the Haima cold seeps is characterized by a high heat flow (~85–105 mW/m2; Shi et al., 2017).
Many cold seep-related seafloor features in the SCS have been reported, including mud volcanoes, pockmarks, and pipes (Bai et al., 2014; Chen et al., 2015a). During the dives of the ROV “Haima” in 2015, an active cold seep area was surprisingly discovered (Liang et al., 2017). Massive gas hydrates were recovered at Sites ROV1 and ROV2 (Fig. 1c). The oxygen isotopic results indicate probable destabilization of gas hydrates in the past and the carbon isotopic ages suggest a major episodic of carbonate precipitation between 6.1 ka and 5.1 ka BP (Liang et al., 2017). Moreover, dead bivalves are common at both Site ROV1 and Site ROV2 , suggesting a decline of seepage activity in recent time. The variation of seepage intensities is also indicated by lipid biomarker analysis (Guan et al., 2018) and the compositions of sediments (Wang et al., 2018a). Pore fluid analysis showed lateral migration of methane-rich fluids from the Site ROV1 to the nearby sites (QDN-14A and QDN-14B) which contributes to the enhanced methane flux at the nearby sites (Hu et al., 2019). The geochemical analysis finds that the gas hydrate samples from the Haima cold seeps are typical structure I hydrates and all the hydrate-bound gases are mixtures of biogenic and thermogenic gas (Fang et al., 2019). Moreover, in the latest study, three new gas plumes are discovered and a year-level spatiotemporal variation of cold seeps is confirmed by comparing different multibeam datasets obtained in 2016 and 2018 (Wei et al., 2020).
3 Datasets and methods
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Multi-beam data comprise bathymetric, backscatter and water column data that were acquired in 2016 with a Kongsberg EM122 system, operating at 11.25–12.60 kHz. The water column data and depth data were simultaneously recorded in a grid pattern with an acquisition line spacing of 700 m. The speed of the vessel was about 4 m/s, and the interval of shooting was ca. 2.5 s. The raw data are processed in the Caris HIPS and SIPS 8.1 software by the following sequence: navigation filtering, parameters calibration, correction for transducer draft, correction for sound velocity, data filtering and interpolation. The cell size of the raster grids is 20 m×20 m and the vertical resolution is ca. 3‰ of the water depth. Water column images capturing the bubble plumes from the Haima cold seeps are derived from the same software.
A 3D seismic survey was conducted by the Guangzhou Marine Geological Survey (GMGS) to assess the potential of gas hydrates in the cold seep area, in 2013. The 3D seismic survey is achieved by acquiring densely-spaced 2D lines and processing them with 3D method. The shot line spacing is 100 m and the shot spacing is 12.5 m. The streamer has 768 channels and the interval is 3.125 m. The 3D seismic dataset was processed on the basis of standard industry protocol. The data were high-pass filtered with the low-frequency limit of 3−6 Hz. A frequency-dependent method was used to attenuate the noise, and the multiples were attenuated in a 3D way. The bin size is 3.125 m×50 m. The routine normal moveout (NMO) velocity analysis was used to produce the velocity model. Finally, the 3D prestack Kirchhoff time migration was used to obtain the images of subsurface structures.
The high-resolution multichannel seismic (MCS) data were acquired by GMGS in 2007 with the aim to investigate the nature and distribution of gas hydrates. The seismic source is a GI gun array with a total volume of 160 cu.in (1 cu.in=16.387 cm3). The shot spacing is 25 m. A streamer of 240 channels with a spacing of 12.5 m was used to record the seismic data. This dataset has been processed commercially, and the post-stack migration method was used to obtain the subsurface image. Data were high-pass filtered with the low-frequency limit of 10 Hz. The FK filter method was used to attenuate the strong linear noise. The multiples were attenuated by jointly applying the high-resolution radon transform and the near-offset trace muting. The velocity model was produced based on the routine normal move-out (NMO) velocity analysis. Finally, the prestack Kirchhoff time migration was used to obtain the images of the subsurface structure. The final migration data has a frequency band of 10–120 Hz and a dominant frequency of ~60 Hz. The vertical resolution of the data is about 6.25 m in the shallow part, the velocity provided is 1 500 m/s and the vertical resolution is 1/4 wavelength.
4 Results
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4.1 Surface expressions of the Haima cold seeps
The Haima cold seeps are discovered in 2015 and cover a wide area (over 400 km2) in the central QDNB. Based on newly acquired multibeam bathymetry data, four massive gas venting plumes, named A, B, C, and D (in order of their discovery time), are confirmed in the water column images (Fig. 2). Gas bubbles were observed to tilt up in the same direction, manifesting themselves as flares. These plumes are 735 m, 750 m, 770 m, and 823 m in height, respectively (Figs 2a–d). All of them disappear at water depths of ~640 m. The diameters of these plumes are 30–50 m. The Plume C, closest to Site ROV2, has been sampled by three different multibeam lines and shown in a 3D view (Fig. 2e). The shape of Plume C varies within three hours, while the height is almost constant.
The bathymetric data from the study area show a gentle slope (ca. 0.26°) from southwest to northeast. The water depth ranges from 1 350 m to 1 450 m. Deepwater channels are observed in the upper slopes of the basin (Fig. 3a). However, the backscatter pattern indicates the northeast direction of bottom current flow (Fig. 3b), corresponding to the direction indicated by Plume C.
4.2 Subsurface structures of active Haima cold seeps
4.2.1 Bubble Plume B and subsurface structures
Seismic profile traversing Plume B show an irregular basement topography consisting of three basement uplifts, labeled as Uplifts 1 to 3, from SE to NW, respectively (Fig. 4a). For the convenience of description, the subsurface structure beneath Plume B can be divided into three parts, i.e., Part 1 (from CDP 3000 to the southeast), Part 2 (from CDP 3000 to 6500), and Part 3 (from CDP 6500 to the northwest). In Part 2, seismic reflections above Uplifts 1 and 2 are characterized by dimmed amplitude and discontinuity, indicating two distinct large gas chimneys (GC1 and GC2) (Fig. 4a). Reflection events are dimmed in the interior of the gas chimneys, with some discontinuous subparallel reflections visible. Instead, the sediments at the same depth above Uplift 3 are stratified but highly faulted. Moreover, at about 2.6 s two-way traveltime (TWT), a bright spot is found on the top of the highly-faulted zone above the crest of Uplift 3. A minor gas chimney (GC3) with a diameter of ca. 300 m is located directly above this bright spot (Fig. 4a).
GC1 and GC2 are topped by the bottom simulating reflectors (BSRs) with reverse polarity at ~2.1 s TWT, above which a distinct acoustic blanking zone exists (Fig. 4a). Minor fluid expulsion anomalies are observed between the seafloor and the acoustic blanking zone (Fig. 4b). A small-scale vertical amplitude anomaly manifested as stacked high amplitude reflection extends from the BSRs through the blanking zone to the seafloor, and connects to Plume B. Moreover, a minor depression occurs at the flat seafloor at the site of Plume B.
4.2.2 Bubble Plume C and subsurface structures
A flat, prominent basement uplift reaching over 10 km in length is observed beneath Plume C (Fig. 5a). A giant gas chimney, approximately 11 km in diameter, manifested in the almost absence of internal reflections, develops between the basement uplift and the overlying BSRs. The likely pull-up feature is observed on the inner sides of the gas chimney (Fig. 5b). The BSRs with poor continuity lie at ~2.1 s TWT and are crosscut by substantial fractures. Two acoustic blanking zones exist above the BSRs, separated by a chaotic, stair-step, high amplitude reflection zone. Interestingly, the BSRs shoal sequentially along NE direction, and the sites of pockmarks and Plume C coincide with the lateral change points of the BSRs. Three vertical amplitude anomalies, corresponding to the three seafloor anomalies (PM1, PM2 and Plume C), share comparable features with the vertical amplitude anomalies below Plume B (Fig. 4b), but the difference is that they do not obviously extend to the underlying blanking zone. The vertical continuities of such amplitude anomalies below PM2 and Plume C are more distinct than that below PM1 (Figs 5c and d). In addition, the morphology of PM2 is significantly different from that of PM1. PM1 is typical of a distinct depression on the seafloor, whereas a small mound lies in the center of PM2 (Fig. 5c).
4.2.3 Bubble Plume D and subsurface structures
Relative to other sites, the gas chimney beneath Plume D is defined by much smaller size and is located above the slope area of the basement (Fig. 6a). The gas chimney, approximately 1 km in diameter, from CDP 4000 to CDP 4500, disturbs the BSRs and terminates below a V-shaped lump of enhanced amplitudes with chaotic and heterogeneous facies (Fig. 6b). The BSRs characterize less disturbance than the BSRs beneath Plume C, with only partial fluctuations and break. Several acoustic blanking zones are interbedded in the strata above the BSRs. Different from the vertical amplitude anomalies observed beneath Plumes B and C (Figs 4b and 5b), a deflecting amplitude anomaly can be identified just beneath Plume D. There are some enhanced reflections on the flanks of the gas chimney. On the SE side, a dome-shaped anomaly with high amplitude occurs, above which the reflections exhibit the same upward-bending feature. On the NW side, the enhanced reflections are more extensive and exhibit a stronger continuity.
Plume D is the only one within a 3D seismic survey zone and its subsurface structures can be further analyzed using the available 3D seismic dataset. From the time slice of the 3D seismic volume at 2 300 ms TWT (Fig. 7a), the gas chimney in the horizontal plane appears to be an oval shape with low amplitude inside and high amplitude in the surrounding. In almost the same location, a low RMS amplitude zone is depicted from the slice of RMS amplitude attribute along the BSRs, with an extremely high RMS amplitude zone to the NW (Fig. 7b). High RMS amplitude zones are mainly distributed in the NW and SE of the attribute slice. In the NW, the high-value anomalies are in the form of clumps and granules, while the high-value anomalies are principally diffuse in the SE. Note that there is a medium-high RMS amplitude zone in the north of the attribute slice.
5 Discussion
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5.1 Fate of gas bubble plumes
Methane is a more potent greenhouse gas than carbon dioxide and may contribute to global warming. It is still a debate whether the methane released from the seabed can reach the atmosphere (Archer, 2007; Isaksen et al., 2011; Myhre et al., 2016; Ruppel and Kessler, 2017). To reach the atmosphere, methane should not be oxidized during the ascending process in the water column. Hydrate-skin is a mechanism that is proposed to explain why the formation of hydrates in the exterior of venting underwater gases prevents gas bubbles from being oxidized in the water (Rehder et al., 2002; Sauter et al., 2006; Warzinski et al., 2014; Li and Huang, 2016). Thus, gas bubble plumes may disappear near the upper limit of the stability zone of the gas hydrates.
Gas hydrate stability zone (GHSZ) in the ocean is calculated using the CSMHYD program (Sloan, 1998). Shipboard analysis of the head-space gas showed the absolute advantage of methane (Yang et al., 2018). Therefore, pure methane is assumed. Based on the temperature data obtained from the CTD within the study area, GHSZ within the water column is displayed in Fig. 8. All four plumes extend from ~1 400 m below the sea level and disappear at 630–650 m below the sea level, regardless of the water depth. The upper limit of GHSZ calculated from the CTD data is about 620 m. Therefore, gas bubble plumes disappear before they reach the upper limit of the GHSZ in the water. The depths where gas plumes disappear are very close to the upper limit of GHSZ that is calculated from the CTD data (Table 1). The minor discrepancy (about 10–30 m) between them can be caused by many factors, such as the gas composition, inaccurate CTD data, and the various hydrate decomposition rate. Therefore, our observations support the “hydrate skin” hypothesis. Heeschen et al. (2003) have a similar conclusion based on the coincidence of the depths at which gas flares disappear and the predicted depth of the GHSZ boundary.
Water column data acquired at three hours interval show that the shape of the plumes has great variations at the scale of ~3 h, which may indicate a short-scale temporal variation in the methane flux. If methane flux of the Haima cold seeps is to be estimated in the future, the short-scale temporal flux variation must be considered and continuous observations for a long time are necessary, otherwise serious deviations may occur just on the basis of the interpolation and extrapolation of limited observations.
5.2 Mechanisms of gas migration, gas hydrate accumulations, and seepage
5.2.1 Gas chimneys below the BSRs
Gas chimneys, topped by BSRs with high amplitude, are observed beneath all plumes, which suggests that gas chimneys are essential pathways for gas migration from deep gas reservoirs to shallow sediments at the Haima cold seeps. Gas chimneys beneath Plumes B and C, with several kilometers in diameter and dimmed internal structure, develop above the basement uplift (Figs 4 and 5), where faults and fractures form easily in the vicinity due to disequilibrium compaction and probable denudation (Liang et al., 2019). Numerous subvertical faults within the gas chimney beneath Plume C have been imaged (Wei et al., 2020). Although the faults and fractures within GC1 and GC2 are hard to discern due to the presence of gas, a highly faulted zone is depicted above the uplift in Part 3. In addition, in the QDNB, an uplift area is often deemed as a favorable zone of hydrocarbon migration from the sag center to structural high through lateral permeable layers and the slope of the uplift (Zhu et al., 2009). Thus, gas migrates from the top of uplift to the shallow depths through the fracture networks, forming these gas chimneys, as documented by the bright spot and the minor gas chimney (GC3) directly above the highly faulted zone (Fig. 4a). In contrast, the gas chimney beneath Plume D is defined by a much smaller size and is located above the slope area of the basement (Fig. 6). We speculate that it may derive from in situ generation and accumulation of biogenic gas or the transition of gas from lateral advection along the underlying inclined stratigraphic layers to vertical advection through the localized highly fractured zone.
5.2.2 Gas migration into the GHSZ and occurrence of gas hydrate
As indicated by the BSRs on the top of all gas chimneys, the ascending gas through gas chimneys has entered into the GHSZ and induced the formation of gas hydrate. The acoustic blanking zone overlying the BSRs is often indicative of the gas hydrate layer (Figs 4–6), as the results of the cementation of the stratal interfaces by the gas hydrate molecules (Hyndman and Spence, 1992; Diaconescu et al., 2001), or merely caused by lithologic homogeneity of sediments (Holbrook et al., 1996). Continuous gas influx into the GHSZ gives rise to the enrichment of gas hydrates and the formation of authigenic carbonates in the shallow subsurface, then the pore space will be clogged and the permeability of the fractures via which gas moves upward will decrease (Loher et al., 2018). The occurrence of massive gas hydrate accumulations may be inferred from the seismic reflection complexes with strong amplitudes above the BSRs (Figs 5b and 6b), because enhanced reflection amplitude in the GHSZ is often used as an indicator of gas hydrate (Yoo et al., 2013). Under the sealing effect exerted by the gas hydrate layer, vertical migration of gas is likely restricted and gas accumulates below the BSRs, thus favoring the build-up of over-pressure and the lateral advection of gas within permeable rocks. Pore fluid analysis between ROV1 site and the nearby sites has exemplified such diversion of migration paths at the Haima cold seeps (Hu et al., 2019). Since high amplitude reflections below the BSR appear to be ascribed to the presence of free gas within sediments (Holbrook et al., 1996; Bünz et al., 2003; Yoo et al., 2013), as displayed in Fig. 6b, the enhanced reflections on the sides of the gas chimney and the dome-shaped anomaly below the BSRs may represent gas charging local patches of permeable beds and localized gas accumulation, respectively. Noticeably, as indicated by the distortion of the reflectors immediately above, the overlying sediments have deformed invoked by the dome-shaped anomaly, that may arise from high pressure within the gas accumulation, or may partially be associated with the growth of gas hydrate lenses (Hovland and Svensen, 2006; Paull et al., 2008).
Gas hydrate occurrence below the BSR has been proven by the samples recovered offshore NW Borneo (Paganoni et al., 2016) and the logging-while-drilling data in the Shenhu area of the PRMB during the GMGS3 to GMGS6 expedition (Zhang et al., 2017; Qian et al., 2018; Jin et al., 2020; Qin et al., 2020). Moreover, conclusive evidence for gas hydrate below the BSR has been obtained during the GMGS5 expedition in the QDNB where the gas hydrate system is also closely connected with an underlying gas chimney, and both thermogenic gas and biogenic gas are responsible for the formation of gas hydrate below the BSR (Ye et al., 2019; Liang et al., 2019). Piston and gravity cores collected from the Haima cold seeps have demonstrated the contributions of thermogenic gas and biogenic gas (Fang et al., 2019; Feng et al., 2019). The base of gas hydrate stability zone (BGHSZ) may extend much deeper than suggested by the BSR in the thermogenic gas-dominated setting (Paganoni et al., 2016). Even if we have no direct seismic evidence, given that the Haima cold seeps have the similar geological features (e.g., gas hydrate system sourced by a gas chimney developing above the uplift) to those of the GMGS5 drilling area (Wei et al., 2019; Ye et al., 2019; Liang et al., 2019), as well as the petroleum geological conditions in the QDNB (Ma et al., 2008; Li et al., 2012; He et al., 2013, 2015; Huang et al., 2014, 2016; Zhang et al., 2016), we propose that it is plausible for gas hydrate occurrence below the BSR, which is also revealed by the pull-up feature on the inner sides of the gas chimneys (Fig. 5b). Nevertheless, further investigations, such as long cores penetrating through these gas chimneys and precise velocity analysis, are necessary to verify the occurrence and distribution of gas hydrate below the BSR in the study area.
Once the pore pressure exceeds a critical value, local hydro-fracturing of the overburden including the BGHSZ is invoked. Then free gas trapped by the gas hydrate layer migrates upward into the overlying gas hydrate-bearing sediments through hydro-fractures and tectonic fractures (Petersen et al., 2010). The BSRs, especially beneath Plume C, are featured by poor continuity, which may imply that extensive hydro-fracturing has occurred. Additionally, the seismic profiles show a pattern of the BSRs that the degree of discontinuity seems to be positively correlated with the size of gas chimney, highlighting the effect of gas supply on the gas hydrate system. Gas emission induces pore pressure decrease in the shallow sediments so that the fractures are clogged by gas hydrates again and cannot conduit fluids, which in turn leads to re-build-up of pressure and consequent gas emission (Wei et al., 2020).
5.2.3 Gas seeping into the water column
In addition to forming hydrates, sufficient and sustained gas entering into the GHSZ migrates upwards to the seafloor and escapes, further inducing the formation of gas plumes and pockmarks (Figs 4–6). Faults, fractures, sediment mobilizations, pipes, and seismic chimneys are common conduits for seabed fluid escape (Cartwright et al., 2007; Haacke et al., 2009; Løseth et al., 2009; Andresen, 2012; Karstens and Berndt, 2015). Wang et al. (2018b) and Wei et al. (2020) suggest that fractures and minor faults are the primary pathways within the shallow sediments for gas seeping into the water column at the Haima cold seeps. However, based on our seismic data, vertical amplitude anomalies just below all plumes and pockmarks, characterized by stacked high amplitude reflections or vertical discontinuity zone diffuse with depth, are clearly imaged and interpreted as fluid conduits transporting gas to the seafloor (Figs 4–6). Comparable with the observations from seismic pipe structures (Løseth et al., 2011; Gay et al., 2012; Maestrelli et al., 2017; Hustoft et al., 2007), we prefer to term these vertical amplitude anomalies herein as pipe-like structures.
The characteristic of these pipe-like structures appears to, at least to some extent, reflect the dynamic process of gas seepage. The strictly columnar pipe-like structure below Plume B may indicate a more focused, persistent seepage (Fig. 4b). Its high amplitude may be either ascribed to free gas inside or the accumulation of gas hydrate along this fluid conduit.
Conversely, the pipe-like structures below the pockmarks and Plume C are relatively indistinct and diffuse (Figs 5c and d). Pockmarks are often seen as remnants of gas emissions (Judd and Hovland, 2007), while the site of Plume C where mussels covered large area is suggested as a young active seep (Wei et al., 2020). Focused gas seepage, i.e. gas plumes, will terminate as the pore fluid pressure drops, and associated feeder pipe will be plugged, collapse, or alter in later geological processes (Løseth et al., 2011), which may account for the indistinct pipe-like structures below PM1 and PM2. Moreover, comparison of their positions with the underlying BSRs reveals a spatial coincidence between seepage sites and the points where the BSRs shoal sequentially along NE direction (Fig. 5b). Local shoaling of the BSR, opposite to the seabed topography, tend to be associated with enhanced gas flux (Schwalenberg et al., 2010; Wei et al., 2019). These observations, together with the BSRs with poor continuity, may imply that the focused gas expulsion is possibly determined by the episodically active, progressively enhanced, plumbing system including the pipe-like structure, the fracture network, as well as the subjacent gas chimney, which may be controlled by the build-up and dissipation of pressure as mentioned above.
However, we cannot determine the feeder pipe of Plume D although there is a deflecting amplitude anomaly just below. It is speculated that the formation of Plume D may be linked to the underlying gas hydrate accumulation due to the clear spatial superimposed relationship of Plume D with the deflecting amplitude anomaly and the gas hydrate accumulation (Fig. 6b).
5.3 Links between gas chimneys and gas hydrate accumulations
All seismic profiles reveal that the gas chimney acts as the crucial pathway for gas migrating into shallow stratum, leading to the formation and accumulation of gas hydrate. However, the control of gas chimney on gas hydrate accumulation is still equivocal. In the light of the detailed 3D seismic data, the links between gas chimneys and gas hydrate accumulations are evaluated by comparing their spatial distributions.
The cross-section of the gas chimney appears to be an oval shape with low amplitude (Fig. 7a), consistent with the gas chimney reported in ZRMB (Sun et al., 2012). The seismic profile shows that the top of this gas chimney has reached the BSRs (Fig. 6b). Gas hydrate accumulation is highlighted by high RMS amplitude (Plaza-Faverola et al., 2010). Thus, two zones of gas hydrate accumulation are identified, one around the gas chimney while the other in the southeast (Fig. 7b). Massive gas hydrates are observed concentrated on the periphery of the gas chimney and may be characterized by high saturation, as indicated by the maximum RMS amplitudes. In areas away from the chimney, gas hydrates are more dispersed and less saturated. The results suggest that gas chimney provides favorable conditions for the accumulation of gas hydrate with high saturation. The periphery of a gas chimney may be an ideal target for future gas hydrate exploration in the QDNB. Similar gas chimneys in relation to high hydrate saturation are confirmed in various settings, including the northern Cascadia Margin (Riedel et al., 2006), the Ulleung Basin (Chun et al., 2011), the Shenhu area in the ZRMB (Zhang et al., 2017) and the GMGS5 drilling area in the QDNB (Wei et al., 2019; Ye et al., 2019; Liang et al., 2019).
5.4 Model of gas hydrate system and seepage at the Haima cold seeps
According to the interpretations of seismic data, we hypothesize the following evolutionary model for the development of the gas hydrate system and the gas seepage in the study area (Fig. 9).
Thermogenic gas migrates from the source layers in the sags adjacent to both sides of the basement uplift, through lateral permeable stratigraphic layers and the slope of the uplift, to the top of the uplift. Thermogenic gas moves upwards by charging the faults and fractures in the overlying strata and mixes with the biogenic gas generated from the shallow formations, responsible for the formation of the major gas chimney (Fig. 9a). In some cases, minor gas chimney develops when gas charges into localized zones with strong permeability.
The gas migrates into the GHSZ through the gas chimneys, then gas hydrates begin to form and fill the sediment pore space. With the gradual enrichment of gas hydrates, gas hydrates tend to accumulate on the periphery of the gas chimney and hinder the further upward-moving of the free gas. Continuous gas influx induces the accumulation of free gas below the gas hydrate layer, which is conducive to the build-up of overpressure and the lateral advection of gas within permeable beds (Fig. 9b). Hydro-fracturing is invoked when the pore pressure exceeds a critical value. Subsequently, substantial gas-rich fluids migrate into the overlying gas hydrate-bearing sediments via the generated hydro-fractures and/or the reactivation of pre-existing fractures. A fraction of gas eventually escapes into the water column driven by pressure through pipe-like structures.
However, the pore pressure drops with the gas release so that the permeability of the original fractures decreases and the gas entering the shallow sediments is restricted. On one hand, due to the lack of enough gas influx and pore pressure, the plugging, collapse, or alteration of the pipe-like structures are induced. Then the gas plumes terminate. On the other hand, the pressure will be re-built-up as free gas accumulates, facilitating consequent gas emission (Fig. 9c). The build-up and dissipation of pressure may account for the dynamic gas seepage at the Haima cold seeps.
6 Conclusions
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The shapes of plumes at the Haima cold seeps are variable and influenced by the northeastward bottom current, but the water depth where these gas plumes disappear is very close to the theoretical calculation of the upper limit of the gas hydrate stability zone, supporting the “hydrate skin” hypothesis.
Plume B and C develop above the basement uplift while Plume D is located above the slope area of the basement. The gas chimney acts as the dominating pathway for both thermogenic gas and biogenic gas migrating upward into the GHSZ, leading to the accumulation of gas hydrate in the shallow sediments, particularly on the periphery of the gas chimney. It is speculated that the gas hydrate clogs the pore space, and subsequent overpressure-driven hydrofracturing derived from the underlying free gas accumulation may account for the broken BSRs. The pipe-like structure is considered as the primary conduit conveying gas from the BSRs to the seafloor, feeding the gas plume in the study area. The differences between these pipe-like structures appear to be related to the dynamic process of gas seepage, which may be controlled by the build-up and dissipation of pore pressure. The strictly columnar pipe-like structure may indicate a more focused, persistent seepage while the indistinct pipe-like structure may indicate a terminative or young seepage.
Acknowledgements
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We thank the Guangzhou Marine Geological Survey for their permission to release the seismic data. Lijie Wang is thanked for his constructive comments to the manuscript.
Funding
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  • The Shandong Province “Taishan Scholar” Construction Project; the fund of the Laboratory for Marine Mineral Resources, Pilot National Laboratory for Marine Science and Technology (Qingdao) under contract No. MMRKF201810; the National Natural Science Foundation of China under contract No. 41606077; the National Key R&D Program of China under contract No. 2018YFC0310000.
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doi: 10.1007/s13131-021-1721-6
  • Receive Date：2020-10-10
  • Online Date：2026-03-03
  • Published：2021-07-25
Article Data
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History
  • Received：2020-10-10
  • Accepted：2020-12-11
Funding
The Shandong Province “Taishan Scholar” Construction Project; the fund of the Laboratory for Marine Mineral Resources, Pilot National Laboratory for Marine Science and Technology (Qingdao) under contract No. MMRKF201810; the National Natural Science Foundation of China under contract No. 41606077; the National Key R&D Program of China under contract No. 2018YFC0310000.
Affiliations
    1 Key Laboratory of Marine Mineral Resources, Guangzhou Marine Geological Survey, Ministry of Natural Resources, Guangzhou 510760, China
    2 Key Laboratory of Gas Hydrate, Qingdao Institute of Marine Geology, Ministry of Natural Resources, Qingdao 266071, China
    3 Laboratory for Marine Mineral Resources, Pilot National Laboratory for Marine Science and Technology (Qingdao), Qingdao 266071, China
    4 Key Laboratory of Submarine Geosciences and Prospecting Techniques of Ministry of Education, Ocean University of China, Qingdao 266100, China

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表12种不同金属材料的力学参数
科
Family		 属数
Number of
genus		 种数
Number of
species		 占总种数比例
Percentage of
total species (%)		 属
Genus		 种数
Number of
species		 占总种数比例
Percentage of total
species (%)
鹅膏菌科Amanitaceae 2 11 5.26 鹅膏菌属 Amanita 10 4.78
小菇科 Mycenaceae 2 12 5.74 丝盖伞属 Inocybe 5 2.39
多孔菌科 Polyporaceae 8 14 6.70 蜡蘑属 Laccaria 5 2.39
红菇科 Russulaceae 3 23 11.00 小皮伞属 Marasmius 6 2.87
小菇属 Mycena 11 5.26
光柄菇属 Pluteus 5 2.39
红菇属 Russula 17 8.13
栓菌属 Trametes 5 2.39
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