Seismic evidence and formation mechanism of gas hydrates in the Zhongjiannan Basin, Western margin of the South China Sea

Accepted Manuscript Seismic evidence and formation mechanism of gas hydrates in the Zhongjiannan Basin, Western Margin of the South China Sea Yintao Lu, Xiwu Luan, Fuliang Lyu, Bin Wang, Zhili Yang, Taotao Yang, Genshun Yao PII:

S0264-8172(17)30136-8

DOI:

10.1016/j.marpetgeo.2017.04.005

Reference:

JMPG 2879

To appear in:

Marine and Petroleum Geology

Received Date: 27 July 2016 Revised Date:

6 April 2017

Accepted Date: 11 April 2017

Please cite this article as: Lu, Y., Luan, X., Fuliang, L., Wang, B., Yang, Z., Yang, T., Genshun, Y., Seismic evidence and formation mechanism of gas hydrates in the Zhongjiannan Basin, Western Margin of the South China Sea, Marine and Petroleum Geology (2017), doi: 10.1016/j.marpetgeo.2017.04.005. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

ACCEPTED MANUSCRIPT Seismic Evidence and Formation Mechanism of Gas Hydrates in the Zhongjiannan Basin, Western Margin of the South China Sea Lu Yintao1,2,3 Luan Xiwu4,5* Lyu Fuliang2 Wang Bin2 Yang Zhili2 Yang Taotao2 Yao Genshun2 (1. Institute of Oceanology, Chinese Academy of Sciences, Qingdao, 266071; 2. Petrochina Hangzhou Research Institute of Geology, Hangzhou, 310023; 3. University of Chinese Academy of Sciences, Beijing, 100049; 4. Key Laboratory of Marine Hydrocarbon Resources and Environmental Geology, Ministry of Land and Resources, Qingdao Institute of Marine Geology, Qingdao, 266071, China; 5. Evaluation and Detection Technology Laboratory of Marine Mineral Resources, Qingdao National Laboratory for Marine Science and

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Technology, Qingdao, 266071, China)

Abstract:

An analysis of 3D seismic data from the Zhongjiannan Basin in the western margin of the South China Sea (SCS) reveals seismic evidence of gas hydrates and associated gases, including pockmarks, a bottom simulating reflector (BSR), enhanced reflection

(ER), reverse polarity reflection (RPR), and a dim amplitude zone (DAZ). The BSR mainly surrounds Zhongjian Island, covering

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an area of 350 km2 in this 3D survey area. The BSR area and pockmark area do not match each other; where there is a pockmark developed, there is no BSR. The gas hydrate layer builds upward from the base of the stability zone with a thickness of less than 100 m. A mature pockmark usually consists of an outside trough, a middle ridge, and one or more central pits, with a diameter of

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several kilometers and a depth of several hundreds of meters. The process of pockmark creation entails methane consumption. Dense faults in the study area efficiently transport fluid from large depths to the shallow layer, supporting the formation of gas hydrate and ultimately the pockmark.

Key words:

Seismic evidence; Gas hydrate; Pockmarks; Zhongjiannan Basin; South China Sea

1.

Introduction

The occurrence of oceanic gas hydrate has been inferred from the observation of bottom-simulating reflectors (BSRs) (Shipley

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et al., 1979; Hyndman et al., 1992). The presence of gas hydrate in sediment pore space elevates the interval velocity in the gas hydrate stability zone (GHSZ) and may significantly reduce interstratal acoustic impedance contrasts, causing a marked decrease in the seismic amplitude above the BSR (Dillon et al., 1998) and resulting in a seismic anomaly known as amplitude blanking or a dim amplitude zone (DAZ). Gas hydrates, filling a portion of the pore space above the BSR, also effectively decrease the permeability of the sediment, thereby working as a trap for the underlying free gas (Hovland and Svensen, 2006). When the

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temperature and pressure of the free gas below the BSR are high enough, the BSR might elevate and intersect and coincide with the seafloor. At the same time, free gas might migrate upward through the GHSZ (Ginsburg and Soloviev, 1997; Wood et al., 2000; Liu and Flemings, 2006). Free gas migration is evidenced in numerous ways, for example, as negative relief pockmarks (e.g., Faure et al., 2006; Luan et al., 2005, 2006; Gay et al., 2012).

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Previous Gas hydrate research in the South China Sea (SCS) has mainly been confined to the Northern South China Sea margin, including the first BSR reported in 1999, massive 2D and 3D seismic acquisitions, 22 BSR areas that have been delineated upon, and gas hydrates samples recovered by 4 gas hydrate drilling cruises (GMGS-1 in the Shenhu area in 2007, GMGS-2 in the Dongsha area in 2013, GMGS-3 again in the Shenhu area in 2015, and GMGS-4 in the Qiongdongnan area in 2016) (Zhang et al., 2007; Wu et al., 2011; Wang et al., 2014; Wang et al., 2016), whereas the other parts of the South China Sea have not been studied as much. In this study, we report seismic evidence of gas hydrates based on 3D seismic data in the Zhongjiannan Basin (ZJNB) on the Western South China Sea margin (Fig. 1).

Figure 1. Location of major Cenozoic sedimentary basins, BSRs, gas hydrate drilling areas in the Northern and Western margins

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of the South China Sea. Seismic lines (grey from China oil company, white from foreign oil company) on the Western margin of the SCS show the investigation activities. (PRMB: PearlRiver Mouth Basin; QDNB: Qiongdongnan Basin; TXNB: Taixinan Basin; YGHB: Yinggehai Basin; ZJNB: Zhongjianna Basin; GU: Guangle Uplift; NS: Northern Sag; NU: Northern Uplift; MS: Middle Sag; SU: Southern Uplift; SS: Southern Sag; EVBF: East Vietnam Bounday Fault; GMGS1-4: gas hydrate drilling cruise areas of 2007, 2013, 2014 and 2016, respectively).

Geological Setting

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ZJNB, occupying most of the western continental margin of the SCS offshore Vietnam, is bordered by the Yinggehai Basin (YGHB), Qiongdongnan Basin (QDNB) and Xisha Uplift (XSU) to the north; Wan'an Basin to the south; Southwest Sub-basin, which is of true oceanic crust, to the east; and East Vietnam Boundary Fault (EVBF) and a narrow shelf and the Indochina

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Peninsula to the West. It formed when continental crust ruptured along the East-Vietnam Boundary Fault (EVBF) at the western edge of the SCS and during the rifting of the SCS (Fig. 1). The formation and evolution of this basin seem to follow several phases, including a diffuse stretching phase when the crust and mantle were decoupled, a delocalized thinning phase during which the lower crust was thinned and formed boudins complementary to the crustal block in the upper crust, a localized exhumation phase when faults in the upper crust were mechanically coupled with a serpentinized detachment in the mantle, a spreading phase of SCS and, finally, a post-rift phase during which the basin thermally subsided (Savva et al., 2013) (Fig. 2). In response to the stretching phase, there was an early rifting stage during the Late Cretaceous/Eocene-Oligocene (32 Ma), with a north-south direction of extension creating east-west trending grabens. These grabens were predominantly filled with continental fluvio-lacustrine sediments (Fyhn et al., 2009). The thinning phase, with a different NW-SE direction of extension, coevals with the spreading of the SCS documented by Briais et al. (1993) and Barckhausen and Roeser (2004). During the thinning phase, the upper crust is severely thinned and forms tilted fault blocks with a characteristic width between 4 and 20 km,

ACCEPTED MANUSCRIPT creating vast accommodation space for the following non-marine alluvial, fan delta to marine transgression deposits (Fyhn et al., 2009; Lee and Watkins, 1998). During the exhumation phase, the lower crust is extended thinly at some places. The upper crust rests directly on the upper mantle, and the two become mechanically coupled with each other (Péron-Pinvidic and Manatschal, 2009). The detachment exhumed the mantle and put in contact with synrift sediments and the upper mantle along an axis parallel to the propagator (Lavier and Manatschal, 2006; Péron-Pinvidic and Manatschal, 2009). Thermal relaxation and associated subsidence has occurred from the Late Miocene (12-10.5 Ma) to the present, with heat released by conduction and convection through volcanism and fluid escape (Savva et al., 2013) (Fig. 2).

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Figure 2. Schematic evolution of the Zhongjiannan Basin integrated at the scale of the SCS (Modified from Cullen et al., 2010). 1: First direction of extension along the N-S axis with the opening of the E-W graben during the Late Cretaceous (?) to Oligocene. 2: Spreading of the oceanic crust to the SW and extension along the second axis, NW-SE, with NE-SW grabens. 3: End of spreading of the oceanic crust, but with the extension continuing in the Basin until the Late Miocene following the

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NW-SE axis. 4: Thermal subsidence as the last evolution stage with an unconformity as the base of the post-rift layer. Due to little exploration, especially limited well penetration, the stratigraphy of the ZJNB has not yet been finalized. Of its seismic unconformities and stratigraphic sequences, different authors have different understandings (Fig. 3). In this study, we summarized and recognized 12 seismic boundaries marked as T10, T20, T30, T40, T51, T50, T61, T60, T70, T80, T90, and

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T100 and 10 seismic sequences bounded by the reflection boundary named the Renjun, Lingtou, Yacheng, Lingshui, Sanya, Meishan, Huangliu, Yinggehai, Guanya and Ledong formations. T80 marks the starting of the South China Sea spreading, and T40 marks the final surface of the Wan'an movement. Before T40, the sediments all deformed and faulted due to Wan'an tectonic movement. After T40, the sequences had high unification and continuity; compared with lower sequences, only slight deformations can be found within sequences after T40. For the gas hydrate study, we divided the Yinggehai formation into T30, T31, T32, T33, T34, T35, and T36; the Huangliu formation into T40, T41, T42, T43, T44, and T45; and the Meishan formation into T50, T51, T52.

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Figure 3. Schematic stratigraphic division of the Zhongjiannan basin. The seismic horizon division is summarized here. F is Fyhn et al., 2009; G is Gao et al., 2006; S is Savva et al., 2013, and L is this paper. The sea level change of the ZJNB, QDNB and global eustatic were adopted from Zhong et al., 2005; Wei et al. (2001); and Haq et al. (1987), respectively. The seismic profile is from Savva et al., 2013; for the location of this profile, see Fig. 1. The names of the stratigraphy formation are temporarily adopted from QDNB following Xie et al., 2006; Zhu et al., 2009; and Sun et al., 2010. H-T uplift is the Himalaya-Tibet uplift (Hu et al., 2016), and Taiwan A-C Collision is the Taiwan Arc-Continent Collision (Nagel et al., 2013; Lüdmann et al., 1999; Lüdmann et al., 2001). The sedimentary fill of the rift duration in ZJNB was mainly confined within tiled grabens in the Middle Sag, Northern Sag and Southern Sag separated by uplift, roughly in a NE-SW direction. The thickness of the sediment column in those sags reaches 2.3 seconds in the two-way travel time (TWT) seismic sections. The draped sediments after the Wan’an movement covered the whole ZJNB almost at the same thickness, 0.7s in TWT. The sediment sequence in between was deformed and of various thicknesses, indicating mild tectonic activity and intermittent deposition during the time of the SCS spreading due to sea level

ACCEPTED MANUSCRIPT changes and a shortness of sediment supply. Lacustrine facies mudstones in the rifting sequences, neritic mudstones and coastal plain coal-bearing strata in the spreading sequences and post rifting sequences were all very well developed and are believed to be high-quality source rocks in the ZJNB (Zhang & Huang, 1991; Huang et al., 2003; Xie et al., 2006; Zhu et al., 2009). Carbonate platforms first developed on top of the tilted basement blocks at the beginning of SCS spreading and then on the Xisha Uplift and Guangle Uplift at the time of the early Miocene (Fyhn et al., 2009; Ma et al., 2011; Sun et al., 2011). The development of carbonate platforms gradually terminated in the Xisha Uplift at the time of the Pliocene (Ma et al., 2011) and in the Guangle Uplift at the time of the late Miocene (Sattler et al., 2004, 2009; Sun et al., 2011) in response to the rapid rising of

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the South China Sea level. However, Carbonate platforms have continued to develop at structural highs until the present day,

forming large atoll reefs in the Xisha Uplift (Ma et al., 2011). The Huangliu Formation, Yinggehai Formation, Guanya Formation and Ledong Formation accumulated in a hemipelagic-pelagic environment characterized by high sedimentation rates (up to 1.2

mm/yr) and together act as an excellent regional seal for hydrocarbon fluids (Xie et al., 2006; Sun et al.,2010; Zhu et al., 2009). In general, tectonic movements were weak in the post-rifting stage. However, neotectonic movement has continued since the

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late Miocene. The events, such as the collision of Taiwan with the Chinese continent (Lüdmann et al., 1999), changing of the

movement direction of the Red River strike-slip fault (Morley et al., 2002; Hu et al., 2016) and Dongsha Movement (Lüdmann et al., 2001) at approximately 5.5 Ma affect the upper layer faulting and deformation. The neotectonic movement resulted in a

migration in the ZJNB.

3.

Data Acquirement and Method

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number of new uplifts (Lüdmann et al., 1999), widespread volcanism, and slope instability and may have also triggered fluid

The 3D marine seismic data used by this study were acquired using a seismic vessel of WesternGeco Geophysical Company in 2011. Eight parallel streamers were towed behind the vessel, each 6000 m long, with a total of 960 channels in a group interval of 6.25 m. The streamers were towed 7-9 m below the sea surface, covering a width of 700 m of each line in a cable distance equally of 100 m. Two 5085 in3-air-gun arrays were deployed 6 m beneath the sea surface directly ahead of steamers and fired

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crosswise in a shot interval of 50 m. The minimum offset (distance between shot and first receiver) is 150 m. the 3D survey covers an area of approximately 37 km by 50 km (Fig. 4).

Data processing with a bin spacing of 12.5 m and 25 m, in the inline and crossline directions, respectively, included (1) tidal correction, (2) true amplitude recovery with exponential gain and spherical divergence corrections, (3) an f-k velocity filter to suppress low velocity backscatter including wave energy and mud roll energy, (4) zero-phase deconvolution based on the

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modelled source signature, (5) random de-multiples, (6) velocity analysis, (7) gapped deconvolution after stacking to further reduce multiple energy, (8) 3D pre-stack migration, and (9) stacking. The sampling interval was 2 milliseconds. The dominant frequency in the interval of interest (shallower than 2 seconds TWT of this study) is 40Hz.

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Figure 4. Pockmarks showing a seafloor morphology map of the 3D survey area (for location see Fig. 1) derived from the 3D seismic data set by using a water velocity of 1500 m/s with an inline resolution of 5.2 m and crossline resolution of 20 m. The

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blank blocks on the SW corner and NE corner are due to missing data. Morphology scaled in ms in TWT. PM1 is pockmark number 1 and the same for PM2-PM20.

For the final 3D seismic data, each shot will have 960 water depth points along each streamer, and have 700 shots, 672000 water depth points for one streamer along the inline, creating a resolution of 5.2 m in the inline direction and 20 m in the crossline

4.

Results

4.1 Pockmarks

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direction. After tidal correction, we have a seafloor morphology map of the 3D seismic survey area (Fig. 4).

The prominent structure on the morphology map is Zhongjian Island in the middle of the northern boundary of the 3D survey area. In addition to Zhongjian Island, the seafloor morphology of the 3D survey area is a two-step terrain. For the NW area, the

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seafloor is gentle and flat within 1200-1400 ms TWT. For the eastern area, the seafloor is also gentle and flat, but within 1800 ms TWT. In the middle part, eastern Zhongjian Island is a step belt. The water depth within this belt drops from 1400 ms to 1800 ms TWT across a distance of approximately 20 km. Another prominent structure on the morphology map is pockmarks. Due to the limited coverage of our 3D survey area, it is difficult to finalize the distribution pattern of the pockmarks on the seafloor but, generally, the pockmarks shown on the morphology map are large and grouped. First, the pockmarks are clearly grouped along the step belt eastern of or circle Zhongjian Island from 1400 ms to 1800 ms in a TWT span with a distance of 20 km. The other pockmarks lie on the west boundary of our 3D survey area and also join the pockmarks on the step belt circling Zhongjian Island. There are also pockmarks (or submarine gullies or trough) located around the foot of Zhongjian Island. The last group of pockmarks situated on the eastern boundary of the 3D survey area is at a water depth of 1800 ms in TWT. Most pockmarks are distributed on the step belt, such as 7, 8, 10, 11, 12, 13, 14, 15, and 17, with various shapes and sizes. In plan view, they are mostly circular in shape, such as pockmark numbers 2, 3, 4, 5, 6, 7, 8, 9, and 10. Some others have an

ACCEPTED MANUSCRIPT elongated shape, such as pockmarks number 11, 12, 15, 16, and 17. Some are elliptical in shape or have a crescent shape, such as pockmark numbers 18, 19, and 20. The size of the pockmarks is quite large, mostly comparable with the dimension of Zhongjian Island. The diameters of pockmarks 4, 7, and 6 are approximately 6000 m, 5500 m and 8000 m, respectively. The diameters of pockmarks 2 and 3 are smaller than those of 4 and 6, but still reach 2000 m. The elongated pockmarks 11, 12, 15, 16, and 17 are 2000 m in width and 5000 m to 8000 m in length. The elliptical pockmarks 18, 19, and 20 are approximately 3000 m long in the major axis and 1000 m wide in the minor axis. According to a circular pockmark in this paper, we mean that the external boundary of the pockmark takes a circular shape.

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However, in plan view, the structures of the pockmarks are quit complex. Take pockmark number 4 as an example (Fig. 5).

There is a trough circle outside of the pockmark. In particular, the outside trough is curved but does not joint at the northwestern corner. Inside the trough, there is an annular ridge, and inside the ridge, there is a pit in the center of the pockmark. Although we say a circular pockmark, the shape of the central pit is slightly irregular.

For the other pockmarks, the outside trough is commonly developed, but none of them completely joint to form an annular

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trough. They mostly only initially developed, leaving a small part of trough embedded on the outside edge of pockmark, such as pockmarks 7, 8, 10, 11, and 12, whereas the middle ridge inside the trough is not common or clearly seen. There is not necessarily only one pit in the central of the pockmark. As for pockmark 7 (Fig. 5), there are three pits developed at

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the center and are probably 4 in pockmark 9. Pits within the same pockmarks can be different in size and shape. The seafloor outside the pockmarks is flat, approximately 1350 ms in TWT around pockmark 4 (approximately 1000 m in depth). Compared to the background seafloor, the depth of the outside trough of pockmark 4 is approximately 190 m, the depth of its central pit is approximately 230 m, and its middle ridge altitude from the bottom of the central pit is approximately 200 m. The morphology of the pockmarks is uneven and rugged. The slope gradient of the trough and the central pit is approximately 7°or less. Note that Fig. 4, Fig. 5 and Fig. 6 are all exaggerated in the vertical direction.

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Figure 5. Plan view structure of pockmarks 4 and 7. A pockmark is composed of an outside trough, a middle ridge, and a central pit. Each of them may not be wholly developed and may have more than one pit in the center. The isobath is in TWT (ms). For the location, see Fig. 4. The arrows highlight the zig-zag character of the outside boundary. Seismic profile A-A´cutting through pockmarks 1 to 4 and profile B-B´ cutting through pockmarks 4 and 7 used here to show the pockmark structure and trough, ridge, pit morphology and deformation of the seafloor and near surface sedimentary stratum (Fig. 6). For our data, pockmarks 4 and 7 are almost fully developed with an outside trough, a middle ridge and a central pit. We suggest a pockmark is systematically organized by those three components. Others are not fully developed with the three standard components, or one component of them may not be fully developed. However, each part is developed and is right in position, so the whole pockmark, whether fully developed or not, can be outlined by extrapolation based on the existing parts.

Figure 6. Morphology of pockmarks shown in the seismic profile A-A´(6a)4, and profile B-B´(6b)-see Fig. 4 for the location.

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PM1 is pockmark 1 and the same for PM2-13. Ot is the outside trough, mr is the middle ridge, cp is the central pit, RPR is the reverse polarity reflection, and BSR is the bottom simulation reflection. The vertical scale bar is only for the water column. The vertical direction is approximately 7 fold exaggerated. The red dotted lines are faults within the pockmark sediment, red arrow shows the direction of sediment sliding along the collapse fault, and circle marks the pull down or pull up area. As to the area of the South China Sea, the tectonics after the Wan’an Movement were generally mild. The faulting and

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deformation of the sediment layer before and after the Wan’an Movement differ greatly. the sediment layers above T40 are generally flat, parallel, and continuous, with almost no faulting and deformation. However, these characteristics are true only outside the pockmark area. Under pockmark area, the sediment layer is still severely deformed. Two faults are clearly developed, oppositely tilted down and jointed beneath pockmark 4. From the outside pockmark boundary, each fault concaved down,

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showing the form of a listric fault within the curved section. The decollement parts of each of the fault are not developed. The deformed sediments are confined within the two faults area forming a deformed cone. Above the fault, sediments slide down and roll over. Differing from a typical listric fault, sediments within the pockmarks roll over not only along the fault but also at the central pit. The sediment slide and deform along the fault not only because of sediment stretching and extension but because of collapse. In this paper, we name this type of fault a collapse fault. On profile A-A´, opposite collapse faults developed only under pockmark 4. Under other pockmarks, only one collapse fault developed. Mini-up-throw faults can also be distinguished within the sediments of pockmarks. Each of them goes up through the seafloor and forms a distinguishable steep slope or uneven seafloor. Due to the existence of free gas, pull down and pull up as well as reverse polarity reflection are observed in the seismic profile under the area of pockmarks. 4.2 BSRs

ACCEPTED MANUSCRIPT The amplitude of the reflection of collapse faults can be as high as that of the seafloor. Some of them, such as those under pockmarks 1 and 2, even have reverse polarity reflection (RPR) compared to that of the seafloor. Another RPR was also found surrounding a mound body under pockmark 4. Under step belt on profile B-B´, bottom simulating reflectors (BSRs) clearly exist by jointing collapse faults under pockmarks 7, 10 and 13. They mimic the seafloor both in the area occupied by a pockmark and outside the pockmark on the normal seafloor, crosscutting sediment layers, showing a phase reversal compared to that of the seafloor, and with abnormally high amplitude. Reflections below and above the BSR are weak in amplitude and uniform in appearance, continuity and deformation, making the

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BSR easy to identify.

The BSRs show a strong characteristic of reflection. The reflection coefficients are large, up to 50%-60% that of the seafloor.

This is the same or even slightly larger than that of typical BSRs reported elsewhere in the world (for example Hyndman et al.,

1992). A detailed examination of the wave form panel (Figs. 7b, 8b, 9b, 10b) shows that, on average, the reversed waveform for the BSR is almost identical to that of the seafloor reflection, which is generally a single symmetrical pulse.

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The stratigraphy sequences above T40 are generally of neritic, hemipelagic and abysmal deposits, showing parallel, continuous and high-resolution characteristics in the seismic profiles. Though slightly deformed and faulted, they are all clear, finely organized and easy to identify. BSRs are dissonance features above T40 with strong amplitude and crosscutting the normal

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stratigraphy sequences. Normally, the BSRs are between T33 and T34, or sometime between T34 and T35, but the BSRs frequently crosscut them, probably due to their undulation feature in this area (Figs. 7d, 8d, 9d, 10d).

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Figure 7. BSR zoom to show the reverse polarity and crosscutting with the sediment layer. For the location of the profile C-C´, see Fig. 4. (b), which shows a detailed display of a 5 trace section –see (a) for the location illustrating the BSR and seafloor waveforms. (d) A detailed display of BSR crosscutting the sediment layer. PM2 for pockmark number 2 and the same for PM12,

ACCEPTED MANUSCRIPT 15, 17. Ot is the outside trough, mr is the middle ridge, cp is the central pit, RPR is the reverse polarity reflection, BSR is the bottom simulation reflection, DAZ is the dim amplitude zone, and ER is the enhanced reflection. The vertical scale bar is only for the water column. The vertical direction is approximately 14 times exaggerated. The red dotted lines are faults, and the red arrow shows the direction of sediment sliding along the collapse fault.⓪-⑥are sites for the BSR depth calculation (see table 1). We display the seismic profile using the traditional landmark display mode, with the seafloor showing in a red-blue-red pattern. All of the reflections following the seafloor are in the same red-blue-red pattern, but have relatively small amplitudes compared

amplitude and reverse polarity displayed in a blue-red-blue pattern.

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to that of the seafloor. The BSR is a prominent feature on the seismic profiles below the seafloor with a remarkably strong

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Figure 8. BSR zoom to show the reverse polarity and crosscutting with the sediment layer. For the location of the profile D-D´, see Fig. 4. (b) for a detailed display of the 5 traces section –see (a) for the location illustrating the BSR and seafloor waveforms. (d) A detailed display of the BSR crosscutting the sediment layer. PM3 is pockmark 3 and the same for PM11 and 14. Ot is the outside trough, mr is the middle ridge, cp is the central pit, RPR is the reverse polarity reflection, BSR is the bottom simulation reflection, DAZ is the dim amplitude zone, and ER is the enhanced reflection. The vertical scale bar is only for the water column. The vertical direction is approximately 14 times exaggerated. The red dotted lines are faults, red arrow shows the direction of sediment sliding along the collapse fault, and five stars mark the BSR location for the temperature and pressure calculations.

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Figure 9. BSR zoom to show the reverse polarity and crosscutting with the sediment layer. For the location of the profile E-E´, see Fig. 4. (b), which shows a detailed display of a 5 trace section –see (a) for the location illustrating the BSR and seafloor waveforms. (d) A detailed display of BSR crosscutting the sediment layer. PM7 is pockmark 7 and the same for PM8. Ot is the outside trough, mr is the middle ridge, cp is the central pit, RPR is the reverse polarity reflection, BSR is the bottom simulation

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reflection, DAZ is the dim amplitude zone, and ER is the enhanced reflection. The vertical scale bar is only for the water column. The vertical direction is approximately 14 times exaggerated. The red dotted lines are faults, red arrow shows the direction of sediment sliding along the collapse fault, and red and blue five stars mark the location for the temperature and pressure calculations.

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Figure 10. BSR zoom to show the reverse polarity and crosscutting with the sediment layer. For the location of the profile F-F´, see Fig. 4. (b), which shows an enlarged wiggle display from the chosen box. See (a) for the location illustrating the BSR and

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seafloor waveforms. (d) A detailed display of BSR crosscutting the sediment layer. PM3 is pockmark 3 and the same for PM11 and 14. Ot is the outside trough, mr is the middle ridge, cp is the central pit, RPR is the reverse polarity reflection, BSR is bottom simulation reflection, DAZ is the dim amplitude zone, and ER is the enhanced reflection. The vertical scale bar is only for the water column. The vertical direction is approximately 14 times exaggerated. The red dotted lines are faults, red arrow shows the

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direction of sediment sliding along the collapse fault, and five stars mark the location for the temperature and pressure calculations. Note the BSR crosscut with BSR0. Dim amplitude zones, as a frequently discussed gas hydrate-related phenomenon, are found throughout the data (Plaza-Faverola et al., 2012). The DAZ as an abnormal phenomenon is characterized by two features. One is its weak amplitude and chaotic reflection pattern. Normal reflections on the seismic profile with normal amplitudes are parallel with each other and clearly distinguishable. In the DAZ, the reflections are weak, with a chaotic pattern, at some places even wiped out and almost changing to acoustic blank zones (Fig. 8). The second one is its scope. Along or within the same sedimentary sequence, the reflection amplitude dims without fault blocking or sedimentary phase changes. The DAZ is not confined in the same layer vertically. Sometime it will crosscut the sedimentary layers vertically like a flare, as shown in profile D-D´ at the northeastern end between T40 and T33 (Fig. 8). The DAZ mainly appears above, below and along the BSRs laterally (Figs. 7-10). Above the BSR, the DAZ is mainly confined within the space between BSR and T36 spanning approximately 50 ms in TWT vertically. Below the BSR, it is mainly confined within the space between BSR and T30 spanning approximately 100 ms in TWTt vertically. The

ACCEPTED MANUSCRIPT thickness of the DAZ both above the BSR and below the BSR changed from an upper level terrace to a step belt and to the lower level terrace. The DAZ also appears in other places on the seismic profile, for example between T10 and T21, and the space under the pockmarks confined by opposing symmetric collapse faults.

5.

Discussions

5.1 Temperature and Pressure Currently, there are no seafloor temperature and geothermal gradient data available for our study area. For the temperature and pressure calculations, we reference reported data from the Shenhu gas hydrate drilling area on the North South China Slope,

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which has similar tectonic background and water depth. In simulation study, we used a seafloor temperature of 3.92°C,

geothermal gradient of 47°C/km (Wang et al., 2014; Yu et al., 2014), and layer velocity above BSR of 1750 m/s (Wang et al.,

2014). Taking the site red star on profile C-C´ as an example, the depth of the seafloor is 1312 ms in TWT, and at the BSR 1579 ms in TWT, the depth is approximately 984 m and 1251 m, respectively. The BSR is 267 m below the seafloor. Therefore, the temperature at the BSR is approximately 16.7 , and the pressure at the BSR is approximately 15.2 MPa. In Fig. 7c, the above

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temperature and pressure marked by the red star is right on the boundary line. Therefore, our BSRs serve as the bottom of the gas hydrate stability zone (BGHSZ). The blue star in profile E-E´ is used to examine another reflection at the BSR in a blue-red-blue pattern. For convenience, here, we named it BSR0.The two-way-travel times of the seafloor at the blue star and at the BSR0 are

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1222 ms and 1548 ms in TWT, with depths of approximately 917 m and 1243 m, respectively. The BSR0 is 326 m below the seafloor. Therefore, the temperature at the BSR0 is approximately 18.6 , and the pressure at BSR0 is approximately 15.7 MPa. In Fig. 9c, the above temperature and pressure marked by the blue star are above the boundary line. Therefore, the BSR0 is outside of the GHSZ.

5.2 Variation of the Gas Hydrate Stability Zone (GHSZ)

BSRs clearly show the characteristics of mimicking the seafloor and crosscutting the sedimentary sequence, mainly T23 (Figs. 7, 8, 9, 10). Therefore, the BSRs are neither parallel to the sedimentary sequence T23 nor to the seafloor. Because the BSR is not parallel to the seafloor, the BSR does not keep the same depth from the seafloor. On profile C-C´(Fig. 7), in the time domain, the

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seafloor slightly increases in depth from NW to NE, whereas the BSR mimicking the seafloor increases in depth, but the depth from the seafloor to the BSR changes from 263 m (at site 2) to 184 m (at site 6). That is, as the seafloor declines, the BSR becomes relatively shallower (table 1).

Table 1 variation of the BSR depth beneath the seafloor at chosen site on the profile C-C´(see Fig. 7 for the location of site1-6) 0

1

2

3

4

5

6

Water depth

941

902

924

944

974

1011

1057

m

Depth from sea surface to BSR

1287

1164

1225

1253

1254

1271

1268

m

Thickness of the GHSZ

346

229

263

270

245

228

184

m

Temperature at BSR

20.5

14.9

16.6

16.9

15.7

14.8

12.7

Pressure at BSR

16.3

14.3

14.6

14.3

14.3

13.8

10.6

297 298 299 300 301 302 303 304 305

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Site

Unit

MPa

In the gas hydrate temperature and pressure stability panel (Fig. 7c), the temperatures and pressure of sites 0-6 are not all distributed right on to the methane hydrate stability boundary line. Some of them, especially sites 1, 5, and 6, are slightly far under the boundary line. It is acceptable that we use the same seafloor temperature for sites 0 to 6 because they are all outside of the pockmarks on the normal flat seafloor and not very far from each other (50 km for the total length of the line C-C´). To correct the points of sites 0-6 in panel Fig. 7c to approach the boundary line, we need different geothermal gradients for each of them. For example, sites 1, 5, and 6 need higher geothermal gradient values than those of the others. Judging from the depositional environment as well as the tectonic and stratigraphy sequence characteristics of the seismic profile, we would like to believe that the sedimentary phases are the same for sites 0 to 6, with the same materials for heat transfer from the BSR to the seafloor. Thus, different geothermal gradients for different sites indicate fluid flows may occur at higher geothermal gradient

ACCEPTED MANUSCRIPT sites. On profile C-C´, faults clearly exist below site 1. Therefore, fluid may migrate up through the faults. The geothermal gradient at site 1 was enhanced by fluid migration from deep layers to the seafloor. Sites 5 and 6 are within the step belt; collapse faults serve as the conduits for fluid migration. Thus, enhanced geothermal gradients for both site 5 and 6 are reasonable. A similar situation was reported in gas hydrate drilling site SH4 in the Shenhu area in the north South China Sea (Wang et al., 2014). 5.3 Distribution of BSRs Geographically, the seafloor of the 3D survey area is terraced. The upper level occupies the northwestern part, and the lower

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level occupies the southeastern part. Between these layers is the step belt that is in the northeast direction and spans

approximately 20 km from the upper level in the northwest down to 200 m in the lower level (Fig. 4). Each part roughly occupies 1/3 of the total 3D survey area. Pockmarks are mainly distributed in the western boundary (pockmark number 1-4), step belt area (pockmark number 5-17) and eastern boundary area (pockmark number 18-20). Most parts of the upper level and lower level

terraces are normal flat seafloor area. The BSR is mainly developed surrounding Zhongjian Island in the upper level terrace and

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step belt (yellow area in Fig. 11), covering an area of 350 km2 roughly. Other BSRs are sporadically distributed under the outside trough area of pockmarks, such as pockmarks 2, 3, 5 and 6. BSRs are seldom discerned far from Zhongjian Island in the area of the upper level terrace, in the lower level terrace or in the area occupied by the pockmarks.

321 322 323 324 325 326 327 328 329

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Figure 11. Distribution of BSRs (yellow area) 5.4 RPR In addition to the reflection of the seafloor and BSRs, another very strong reflection below the BSR is commonly developed and is fairly well recognizable (Figs. 6-9). Its amplitude can be as high as that of the seafloor (Figs. 8b and 9b). However, oddly enough, according to the blue-red-blue pattern (Variable Density) on the seismic profile, it has reverse polarity reflection (RPR) compared with that of the seafloor. Therefore, except for a large density difference origin between the strata above and below the RPR, we believe that free gas is trapped within the strata below the RPR. Other free gas indicators, such as the pull down reflection and frequency absorption, are also recognizable in the area of RPR on the seismic profile (Fig. 6). Beneath pockmark 4

ACCEPTED MANUSCRIPT on seismic profiles A-A´ and B-B´, a mound structure outlined by RPR is clearly recognizable. Judging by the appearance, a sharp frequency difference can be distinguished inside the mound structure and outside the mound structure (Fig. 6). The loss of frequency below the RPR and within the mound structure is most likely caused by the low-frequency filtering of gas (Horozal et al., 2009). The free gas existence under RPR is further evidenced by a pull down reflection right under the mound structure (Fig. 6). Matched by a pull down phenomenon under the mound body, we suppose that free gas was confined under the RPR. As for the causes of reverse polarity of collapse fault reflections under pockmarks 2 and 1, they may also be caused by the existence of gas

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hydrates due to suitable pressure and temperature conditions for gas hydrate formation except for the existence of free gas underneath. Here, we use a RPR other than the BSR, mainly because they are not parallel to the seafloor.

Our 3D survey area covers the northern edge of the Zhongjiannan Basin (Figs. 1 and 3), with assemblies totaling more than

6500 m thick of sediment column (Fig. 6, and Xie et al., 2006; Zhu et al., 2009). Of its source rocks, thermal evolutions, source rock maturation, stages of petroleum generation, potential reservoirs, and traps have been discussed previously and given a high

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hydrocarbon potential evaluation. Cenozoic lacustrine mudstones, neritic mudstones and coastal plain coal-bearing mudstones with total thicknesses up to 4000 m, high TOCs of 300-700 mg/g and m sufficient maturation to generate gas are the main

high-quality source rocks in the basin (Zhang & Huang, 1991; Huang et al., 2003; Xie et al., 2006; Zhu et al., 2009, Fyhn et al.,

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2009), whereas the tectonic structure, lithologic reservoir, carbonate platforms, pinnacle reefs are important traps (Fyhn et al., 2009). RPR encircles the traps and also serves as the source of upward-migrating gas for RPR, DAZ, ER, BSR, and gas hydrate formation. 5.5 Dim amplitude zone (DAZ)

Enhanced (or relatively enhanced) reflections (ER) are distinguishable between T21 and T20 as well as between T30 and T45. The ER between T21 and T20 is quite stable, whereas that between T30 and T45 is not and will split or disappear at some locations due to being affected by fault or layer deformation (Figs. 7-10).

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DAZ has been previously attributed to the presence of gas (Løseth et al., 2009) or gas hydrates in sediments (Riedel et al., 2006). We believed that gas hydrate formed and filled in the pores throughout the DAZ above the BSR and reduced its inhomogeneity laterally and lithologic difference vertically. As the gas hydrate formed and filled, the gas hydrate zone physically became uniform. The explanation of the formation of the DAZ below the BSR is mainly the scattering and attenuation of energy by gas

5.6 Faults system

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(Judd and Hovland, 1992; Løseth et al., 2009).

The 3D survey area located on the middle of the northwestern South China Sea margin showed that its margin materials extend from land to the Southwest Sub Basin (Fig. 1). Extensional faults on the seismic profile are systematically distributed, with its fault plane curved and dipping to the northwest (Fig. 6a).

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A higher sedimentation rate (Xie et al., 2006; Sun et al.,2010; Zhu et al., 2009) since the late Miocene in the study area has caused overburden, compaction and sinking to the rift sediments in the Zhongjiannan basin. Thermal subsidence related faults can be distinguished at the basin center with an opposite dipping direction to that of the extensional fault (Fig. 6a). Diaper, volcanism and basement uplift were another mechanisms for generating faults in the basin. Basement uplifted faults are generally around the uplift, with all of the fault roots convergent to the uplift top (Fig. 6a). Though the extensional fault, the thermal subsidence fault and uplifting fault have relatively different orientations, combinations and distribution styles, but all of which have the same feature of extension and all grow from deep layers up to the GHSZ; therefore, all can serve as conduits for fluid flow from the source layer to the gas traps and further from gas traps to the DAZ and GHSZ, as the main reason for gas hydrate formation in the Shenhu area, northern South China Sea (Gong et al., 2009; Wu et al., 2009; Wang et al., 2011; Sun et al., 2012), and Ulleung Basin (Kim et al., 2013). From the zoomed seismic profiles (Figs. 7-10), dense extensional faults are apparently recognizable, especially above the uplifted surface T50. These faults will efficiently transport fluid from deep layers to the shallow part on the one hand and will

ACCEPTED MANUSCRIPT lead to the escaping and loss of fluid on the other hand. 5.7 Model of gas hydrate formation Gas hydrate first formed at the BGHSZ when migrated fluids reached into the GHSZ (Hyndman et al., 1992). With gas hydrate formation, he BSR might serve as a seal preventing the fluid from the deep source layer further ascending into the GHSZ. On the one hand, the gas hydrate layer builds upward from the base of the stability zone (Wang et al., 2014) and will have a sharp dense base and gradational top. In this way, gas hydrates might not fill up the whole space (upper part) of the GHSZ. On the other hand, free gas might be gathered and dispersed laterally underneath the BSR (stage 1 on Fig. 12). Once the saturation of free gas in the

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layer underneath the BSR was high enough, the DAZ merged both above the BSR and below the BSR on the seismic profile. A sharp and dense base of gas hydrate above and free gas below matches the single symmetrical pulse shown on the waveform panel (Figs. 6b-10b)(Hyndman et al., 1992).

As fluids continued to fill the layer underneath the BSR, more free gas and more heat energy gathered, and a higher thermal

gradient and overpressure may have formed. Ultimately, heat melts the BSR and overpressure breaks the seal, creating a new

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channel leading to the seafloor resulting in the pockmark formation and disappearance of the BSR (stage 2 on Fig. 12).

It is still difficult to finalize a sequence for which a part of the pockmark is created first. However, we believe that a mature pockmark will have an outside trough, middle ridge, and central pit. Therefore, a pockmark probably develops in the sequence

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from stage 3 to stage 4 and to a mature stage 5, as shown on Fig. 12.

Obviously, the process of pockmark creation is a process of methane and other fluids consumption. A matured pockmark may serve as an indicator of historical violent methane leakage, whereas a new born pockmark, for example, with only one small central pit, may serve as an indicator of the existence of a gas hydrate layer, an overpressure free gas layer, and even a petroleum reservoir underneath. This likely explains why the BSR area and pockmark area do not match each other (Fig. 11). We further infer that a stable BSR with a normal seafloor may serve as an indicator of no overpressure free gas and lower gas hydrate saturation in the GHSZ.

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373 374 375 376 377 378 379 380 381 382 383 384 385 386 387 388 389 390 391 392 393 394

395 396 397 398 399 400

Figure 12. Summary of the evolution of pockmarks and the mechanism of gas hydrate formation in the Zhongjiannan Basin, western margin of the South China Sea based on the seismic interpretation. cp is the central pit, ot is the outside trough, mr is the middle ridge, BSR is the bottom simulation reflection, BGHSZ is the base of the gas hydrate stability zone. Note the different morphologies of pockmarks at different evolution stages and the corresponding size of gas hydrate and its supporting system, including the source kitchen, faults, and petroleum traps.

ACCEPTED MANUSCRIPT The origin of methane for gas hydrate formation still remains a complex issue in the literature (Collett, 2002; Collett et al., 2009; Yang et al., 2015, Sassen et al., 2001; Khlystov et al., 2013, Yoshioka, et al., 2015). Based on the location of gas hydrates inferred from the BSRs and DAZ and the large amount of fluid such as methane and water that is required for the formation of gas hydrates, we would like to regard thermogenic methane mainly from the deep source kitchen as contributing the most to gas hydrate formation (Hyndman et al., 1992). However, microbial methane may occupy the upper DAZ between T11 and T10. First, seismic features indicate the existence of free gas and even gas hydrate formation in this layer. Second, the seismically enhanced layer between T10 and T21, as a high permeability layer, may prevent deep fluids from further flowing up into this layer. Third,

N, 2010; Klapp S A, 2010; Hu Yang, 2012).

6.

Conclusions

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microbial gas hydrates have been reported in shallow areas in the South China Sea and worldwide (Hachikubo A, 2010; Vaular E

The BSR mainly developed around Zhongjian Island in the upper level terrace and step belt, covering an area of 350 km2 in this

match each other; where there is a pockmark developed, there is no BSR.

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3D survey area. Far from Zhongjian Island, the BSR is only sporadically distributed. The BSR area and pockmark area do not

The gas hydrate layer builds upward from the base of the stability zone. It has a sharp base along the BSR and gradational top, occupying a thickness of less than 100 m from the BSR to T21. Only the base layer of the GHSZ is densely filled with gas

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hydrate. Up to the seismically enhanced layer between T10 and T21, there may be no gas hydrate. In the upper layer within the GHSZ between the seafloor and T10, microbial methane may constitute the main part of gas and microbial gas hydrate may also be found to be distributed evenly throughout the layer.

A matured pockmark usually consists of an outside trough, middle ridge, and one or more central pits, with a diameter of several kilometers and a depth of several hundreds of meters. A pockmark will likely develop in a sequence from a single central pit to a mature stage. The process of creating of pockmark entails methane consumption. A matured pockmark may serve as an indicator of violent historical methane leakage, whereas newly a born pockmark may serve as an indicator of the existence of a gas hydrate

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layer, overpressure free gas layer, and even a petroleum reservoir underneath. We further infer that a stable BSR with a normal seafloor may serve as an indicator of no overpressure free gas and lower gas hydrate saturation in the GHSZ. Faults from marginal extension, thermal subsidence, basement uplift and so on in the study area have the same feature of extension and grow from deep up to the GHSZ; therefore, all faults can serve as conduits for fluid flow from the source kitchen layer to the gas traps and further the DAZ and GHSZ. Dense faults will efficiently transport fluid from deep layers to the shallow

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layer, forming a free gas layer beneath the BSR, in the BSR, in the gas hydrate layer, and ultimately in a pockmark.

Acknowledgements:

The authors wish to thank the Hangzhou Research Institute of Geology’s research team, especially the South China Sea team for their contributions and the approval of this publication. This study is supported by the Special Fund for Land & Resources

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401 402 403 404 405 406 407 408 409 410 411 412 413 414 415 416 417 418 419 420 421 422 423 424 425 426 427 428 429 430 431 432 433 434 435 436 437 438 439 440 441 442 443

Scientific Research in the Public Interest (Grants No. 201511037).

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Bulletin 93, 741-761.

* Corresponding author. Qingdao Institute of Marine Geology, China Geological Survey, 62, Fuzhounan Road, Qingdao, 266071, China. Tel: +86 13884637952.

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E-mail address: [email protected] (X. Luan).

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Highlights: 1. High-resolution 3D seismic data reveal evidence of gas hydrates developed in the Zhongjiannan Basin. The distribution of gas hydrates was identified and mapped based on a precise seismic interpretation. 2. Some evidence on the seafloor was also identified in the seafloor morphology map, such as pockmarks. The pockmarks differ greatly in shape and size, but are generally large. 3. The internal architecture of BSRs and other elements associated with gas hydrates is described; furthermore, the temperature and pressure conditions are analyzed to discuss the reliability of the BSRs. 4. The architecture of pockmarks is illustrated, and a 2D model is built to analyze the formation of gas hydrates and pockmarks, demonstrating the relationship between pockmarks and gas hydrates.