Geological controls on the occurrence of recently formed highly concentrated gas hydrate accumulations in the Shenhu area, South China Sea

Journal Pre-proof Geological controls on the occurrence of recently formed highly concentrated gas hydrate accumulations in the Shenhu area, South China Sea Jiapeng Jin, Xiujuan Wang, Yiqun Guo, Jie Li, Yuanping Li, Xin Zhang, Jin Qian, Luyi Sun PII:

S0264-8172(20)30077-5

DOI:

https://doi.org/10.1016/j.marpetgeo.2020.104294

Reference:

JMPG 104294

To appear in:

Marine and Petroleum Geology

Received Date: 13 August 2019 Revised Date:

10 February 2020

Accepted Date: 10 February 2020

Please cite this article as: Jin, J., Wang, X., Guo, Y., Li, J., Li, Y., Zhang, X., Qian, J., Sun, L., Geological controls on the occurrence of recently formed highly concentrated gas hydrate accumulations in the Shenhu area, South China Sea, Marine and Petroleum Geology (2020), doi: https://doi.org/10.1016/ j.marpetgeo.2020.104294. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. 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. © 2020 Published by Elsevier Ltd.

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Geological controls on the occurrence of recently formed highly concentrated gas

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hydrate accumulations in the Shenhu area, South China Sea

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Jiapeng Jin a,b,c,d, Xiujuan Wang a,b,c,d,*, Yiqun Guo e, Jie Li f, Yuanping Li f, Xin Zhang a, d, Jin Qian a, d

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, Luyi Sun a,c

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a

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Academy of Sciences, Qingdao, 266071, China

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b

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Technology, Qingdao, 266071, China

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c

University of Chinese Academy of Sciences, Beijing, 100049 China

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d

Center for Ocean Mega-Science, Chinese Academy of Sciences, Qingdao, 266071, China

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e

Guangzhou Marine Geological Survey, MNR, Guangzhou, 510760, China

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f

Shenzhen Branch of CNOOC (China) Ltd, Shenzhen, 518054, China

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*Corresponding author. E-mail: [email protected] (X. Wang)

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Abstract

Institute of Oceanology and Key Laboratory of Marine Geology and Environment, Chinese

Laboratory for Marine Mineral Resources, Qingdao National Laboratory for Marine Science and

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The Shenhu area is an important gas hydrate exploration and production test

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zone in the northern slope of South China Sea. A sediment layer with high gas hydrate

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saturations and abnormally high pore-water chlorinity concentrations has been

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identified from recently acquired logging-while-drilling (LWD) data and core samples.

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Data from four sites (W19, W18, SC-01 and SC-02) indicates that most gas

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hydrate-bearing sediments (GHBSs) form preferentially in low gamma ray log

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inferred coarser-grained units, except at SC-02. At this site, a striking increase in

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pore-water chlorinity values, with a maximum value exceeding 816 mM at a depth of

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158 mbsf, indicates an active or recently-active system. A one-dimensional (1D)

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diffusion model is used to estimate the time when the gas hydrate formed based on the

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saturation, thickness, and porosity of gas hydrate-bearing units at each site. The

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results show that gas hydrates at sites SC-02 and W18 respectively formed 19-29

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thousand and 26-28 thousand years ago (assuming a closed system). To further

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investigate the apparently young age of these highly saturated gas hydrate

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accumulations, three-dimensional (3D) seismic variance and frequency attributes

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extracted along different sedimentary layers are shown to provide evidence for

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vertical and lateral gas migration along normal faults and gas chimneys. The high

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pore-water chlorinities and evidence for fluid migration from deeper sedimentary

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sections suggest that upward migration of methane gas into the hydrate stability zone

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may have contributed to the recent formation of the highly concentrated gas hydrate

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occurrences identified in the Shenhu area.

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Key word: High gas hydrate saturation; upward migration; high pore-water chlorinity;

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South China Sea

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1. Introduction

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Gas hydrate systems are commonly associated with gas venting features, which

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have attracted attention because of the potential relationship between global warming

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and methane leakage into overlying ocean waters (e.g., Hovland & Judd, 1988;

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Schmuck & Paull, 1993; Cranston 1994; MacDonald et al., 1994; Judd et al., 1997;

44

Chen & Cathles, 2003; Lu et al., 2007; Reitz et al., 2011; Ruppel, 2011; Römer et

45

al.,2012; Boetius & Wenzhoer, 2013; Wenau et al., 2015; Alexander et al., 2017;

46

Ruppel & Kessler, 2017; Zhao et al., 2017; Du et al., 2018). In such settings, gas

47

hydrates may be exposed at the seafloor as photographed by remotely operated

48

vehicles, such as in the South China Sea (Zhang et al., 2017b; Du et al., 2018; Wang

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et al., 2018) and the Gulf of Mexico (MacDonald et al., 1994; Soloviev & Ginsburg,

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1997). However, in the Gulf of Mexico Green Canyon Block 185, gas hydrate also

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occurs as “vein-filling” fractures in hemipelagic muds within the subsurface interval

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from near the base of gas hydrate stability zone (BGHSZ) to the seafloor (Sassen et al.,

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2001a, b; Chen et al., 2004). Previous studies of core samples have shown that

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anomalous high pore-water chlorinities are often found in near subsurface gas hydrate

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systems such as Hydrate Ridge at site 1249 (e.g. Torres et al., 2004; Tréhu et al., 2006)

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and the Cascadia margin at IODP site U1328 (Riedel et al., 2006, 2009). The vertical

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migration of gas along faults and chimney-like features is thought to contribute to the

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formation of these gas venting-associated gas hydrate systems (e.g. Torres et al., 2004;

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Liu and Flemings, 2006). However, high pore water chlorinities (high bottom water

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salinities) indicative of high hydrate saturations have also been reported near the

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BGHSZ, for examples in the Black sea (Riboulot et al., 2018), and Krishna-Godavari

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Basin, India (Solomon et al., 2014), and notably within the Shenhu area of the South

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China Sea (Yang et al., 2015). These relatively deep gas hydrate occurrences are

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different from those encountered in venting systems such as on the Cascadia margin

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(e.g. Torres et al., 2004; Tréhu et al., 2006; Riedel et al., 2006, 2009).

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Multi-channel seismic and well log data are widely used to delineate the

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architecture of gas hydrate-bearing reservoirs and gas-migration conduits in marine

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gas hydrate settings (e.g. Shipley et al., 1979; Lu et al., 2002; Ruppel et al., 2008;

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Kim et al., 2011; Boswell et al., 2012a; Chand et al., 2012; Cook et al., 2012; Frye et

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al., 2012; Lee & Collett 2012; Sun et al., 2012a, b; Fujii et al., 2015; Komatsu et al.,

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2015a, b; Hillman et al., 2017). In many areas, gas hydrate-bearing sediments

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(GHBSs) have been identified in sand-rich units characterized by high electrical

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resistivity, high P-wave velocity, and low gamma ray log, such as in the Gulf of

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Mexico, the Krishna-Godavari Basin, India, the Nankai Trough, Japan, and the

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Ulleung Basin off the eastern coast of Korea (e.g. Fujii et al., 2009; Bahk et al., 2011;

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Noguchi et al., 2011; Riedel et al., 2011; Boswell et al., 2012b; Collett et al., 2014; Ito

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et al., 2015; Haines et al., 2017; Collett et al., 2019). Most sand-rich reservoir units

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are associated with turbidite channels or channel-levee systems, such as in the Gulf of

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Mexico (McConnell, et al., 2010; Boswell et al., 2012b; Burwicz et al., 2017; Haines

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et al., 2017). On seismic reflection data, pore-filling gas hydrates in sand-rich

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reservoir units are typically characterized by high-amplitude reflections above bottom

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simulating reflectors (BSRs) that correspond to the BGHSZ, below which the

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presence of free gas may be recognized from enhanced reflections or localised bright

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spots (e.g. Shipley et al., 1979; Luheshi et al., 1996; Brown et al., 2010).

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Core samples from gas hydrate drilling expeditions in the Shenhu area, South

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China Sea (Fig. 1) indicate that an active or recently active gas hydrate system may

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exist at depth, with moderate to high concentrated gas hydrate occurrences just above

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the BGHSZ (Yang et al., 2015, 2017a, b). Moreover, Raman spectral analysis of core

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samples above the BSR at site SC-01 indicates the occurrence of structure I and II gas

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hydrates (Wei et al., 2018). These findings suggest that thermogenic gas contributes to

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the gas hydrate system in the Shenhu area (Qian et al., 2018; Wei et al., 2018). At sites

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SC-02, W19 and W18, the chlorinities are significantly higher than the overlying

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baseline value above the BGHSZ despite fresh water input from gas hydrate

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dissociation (Yang et al., 2017a; Zhang et al., 2020). However, what controls the

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anomalous chlorinity concentrations at these sites is unclear. In addition, gas hydrate

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formation time and geological controls on the occurrence of recently active highly

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concentrated gas hydrate accumulations are not well known.

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The aim of this paper is to present new information on the active gas hydrate

99

system in the Shenhu area of the South China Sea and on the recent formation of

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highly saturated gas hydrates near the base of gas hydrate stability zone (BGHSZ).

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We focus on the reservoir variations of high saturation gas hydrate using

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three-dimensional (3D) seismic data, correlated to recently logging-while-drilling

103

(LWD) data and core samples. The ages of gas hydrate formation are estimated using

104

a simplified one-dimensional (1D) decay model. Seismic evidence and geothermal

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anomalies are argued to provide evidence of upward fluid migration from deeper

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sources to form structure II gas hydrate. The results support a discussion of the

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processes that have led to the formation of highly saturated gas hydrate reservoirs

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near the BGHSZ in this area.

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2. Geological setting

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The Pearl River Mouth Basin (PRMB) is an important hydrocarbon producing

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sedimentary basin on the northern slope of the South China Sea (Fig. 1a). The tectonic

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evolution of the PRMB can be divided into two stages: a syn-rifting Paleogene stage

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and a post-rifting Neogene and Quaternary stage (e.g. Pang et al., 2006, 2008). The

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thickness of the Cenozoic sediments is over 12000 m in the Baiyun sag (Fig. 1a),

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which provides a thick syn-rifted source rock section for the occurrence of oil and gas

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resources as well as gas hydrates (e.g. Chen et al., 2006; Pang et al., 2006, 2008; Zhu

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et al., 2009; Cheng et al., 2013; Mi et al., 2016). The Shenhu area is located in the

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Baiyun Sag on a slope incised by seventeen submarine canyons (Fig. 1a) (e.g. Zhu et

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al., 2010; Zhou et al., 2015; Li et al., 2016). The submarine canyons are composed of

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a series of complex architectural elements including a basal erosion surface and a

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series of sand-rich strata associated with channel-levee complexes and canyon

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confined sediments fans (e.g. Mayall et al., 2006; Pang et al., 2006; Zhu et al., 2010;

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Palamenghi et al., 2015; Zhou et al., 2015). Gas hydrate drilling expeditions (GMGS1,

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GMGS3 and GMGS4) were conducted in the Shenhu area, and the drilling sites are

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located on a ridge between two canyons (Fig. 1; Canyon 1 and Canyon 2) that seismic

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data show to be characterized by a prominent erosional surface that lies close to the

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estimated depth of the BGHSZ. A buried trough shaped feature is present along the

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mapped erosional surface near sites SC-02, W19, and W18 (Fig. 2a and b, red arrows)

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(Jin et al., 2017). Normal faults are seismically imaged in the Shenhu area, and likely

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act as important pathways for fluid vertical migration from the deeply buried LW3-1

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gas field and the BY6-1 gas-bearing structure (Fig. 1; Hou et al., 2008; Shao et al.,

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2013; Zhang et al., 2016; Zhou et al., 2018).

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The occurrence of gas hydrate was identified from core samples and downhole

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wireline logging data acquired during GMGS1, in which gas hydrate was absent at

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site SH1 but present at SH2, SH3, SH4 and SH7 (Fig. 1b, rectangle) (e.g. Zhang et al.,

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2007; Wu et al., 2008, 2011; Wang et al., 2011, 2012, 2014a, b). Subsequent drilling

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expeditions (GMGS3 and GMGS4) revealed that gas hydrate saturations in different

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reservoirs range from 17% to 73% of the pore space with variable thickness, while

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concentrated gas hydrate layers occur at sites SC-01, SC-02, W18, W19, W11 and

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W17 (Fig. 1b) (e.g., Guo et al., 2017; Jin et al., 2017; Yang et al., 2015, 2017b; Zhang

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et al., 2017a; Li et al., 2019; Qian et al., 2018). Previous studies also showed that the

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occurrence and distribution of gas hydrates have a close relationship with the presence

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of foraminifera (e.g. Chen et al., 2009, 2016), and provided evidence of fluid

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migration pathways along gas chimneys, faults, slides and slumps from the deeper

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sediments in the Shenhu area (e.g. Wang et al., 2011, 2014b; Sun et al., 2012a, b; Yu

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et al., 2014; Yang et al., 2015, 2017a; Chen et al., 2016; Jin et al., 2017; Zhang et al.,

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2017b).

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3. Data and Methods

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3.1 Seismic data and attribute extraction

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The 3D seismic data have a bin spacing of 12.5 m and 25 m in the in-line and

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cross-line directions, respectively. The main frequency is about 50Hz and the

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sampling interval is 2 ms. 3D seismic attributes (low frequency and variance)

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extracted using Petrel E&P (e.g. Chen et al., 2016; Jin et al., 2017; Waage et al., 2018)

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were used to identify fluid migration pathways. Three horizons (H1-H3) and an

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erosional surface were identified and traced through the 3D data line by line to

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delineate the variations in depositional environment (Fig. 2b) and gas hydrate-bearing

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units (Fig. 3). The erosional surface near the BGHSZ in the study area truncates

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underlying high amplitude reflections and defines a trough-like feature that contains

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gas hydrates at sites W19 and W18 (Jin et al., 2017).

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Highly variable and low frequency seismic responses together with amplitude

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blanking, and frequency absorption can indicate the accumulation and migration of

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free gas (e.g. Tingdahl et al., 2001; Meldahl et al., 2001; Aminzadeh et al., 2002). In

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addition, negative polarity and enhanced reflections have been interpreted to indicate

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the occurrence of free gas beneath the BGHSZ in various basins (e.g. Anderson et al.,

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1980; Luheshi et al., 1996; Orange et al., 2005; Dai et al., 2008; Wang et al., 2010; Ito

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et al., 2015; Haines et al., 2017). In this study, seismic attributes (variance and

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frequency) are extracted from 3D seismic data using Petrel software to show the fluid

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flow migration and accumulation below the BGHSZ for methane (I-BGHSZ). Based

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on the theory of error analysis and similarity between adjacent seismic traces, the

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variance attribute was used to describe the lateral amplitude discontinuities and to

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enhance faults and fractures. Variance slices along layer H3 and at 2450 ms two-way

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travel time were extracted to highlight the faults and lateral amplitude anomalies. The

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frequency attribute, which is sensitive to attenuation by gas absorption, is overlain on

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seismic sections to show the potential vertical fluid migration pathways.

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3.2 Well log data

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The LWD data acquired during GMGS3 and GMGS4 at sites W19, W18, SC-02

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and SC-01, including natural gamma-ray (GR), ring resistivity (RES), bulk density

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and P-wave velocity (Vp), are used to detect the occurrence of gas hydrate (Fig. 3)

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(Yang et al., 2015, 2017a, b). For this study, anomalously high ring resistivity and

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P-wave velocity log responses were combined to infer the occurrence of gas hydrate.

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Gas hydrate saturations at these four sites were calculated from ring resistivity log

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data using the Archie equation (Archie 1942) and compared to pore water freshening

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to infer the occurrence of gas hydrate. The Archie empirical constants (a and m) were

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obtained from crossplots between formation factor and density porosity log values

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(e.g. Malinverno et al., 2008; Lee & Collett, 2011, 2012; Shankar & Riedel, 2011,

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2014; Wang et al., 2011, 2014a) at each site (Table 1). Density porosity (Φ) was

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calculated from bulk density (ρb), water density (ρw) and matrix density (ρg) with the

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equation:

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Φ=(ρg-ρb)/(ρg-ρw)

(1)

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Here, bulk density is from well log, ρw and ρg equal to 1.03 and 2.65 g/cm3 (Wang et

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al., 2011).

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Gamma ray logs were used to infer grain size changes (e.g. Cant 1992; Chow et

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al., 2005; Komatsu et al., 2015a, b; Nazeer et al., 2016; Zhong et al., 2017). Upward

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decreasing gamma ray values generally indicate a coarsening-upward succession,

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typically associated with a progradational environment such as submarine fans, deltas

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and levee deposits (e.g. Selley 1978; Nazeer et al., 2016). Here, sedimentary sections

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with highly variable gamma ray (e.g., between horizons H1 and H2, Fig. 3) often

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indicate inter-bedded clays and silts, and environments with relatively constant

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sedimentation characteristics with only slight changes in the depocenter related to the

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lateral inclined sediment packages in canyon sediments.

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3.3 Calculating the base of gas hydrate stability zone

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Core samples from sites W19, W18, SC-02 and SC-01 revealed various gases

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including ethane, propane, butane, isobutene, oxygen, carbon dioxide (e. g. Yang et al.,

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2015, 2017a; Zhang et al., 2017a; Wei et al., 2018). The occurrence of structure II gas

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hydrates inferred at site SC-01 from core samples (Wei et al., 2018) and the

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significant concentrations of ethane, propane and butane indicate the presence of

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thermogenic gas at these four sites (Table 2). Parameters of water depth, inferred

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seafloor temperature, geothermal gradient and hydrocarbon gas composition (Tables 1

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and 2) were used to calculate the depths of I-BGHSZ (i.e. methane hydrate) and

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structure II hydrate (II-BGHSZ) at four sites using the CSMHYD program (Sloan,

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1998).

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3.4 Modelling gas hydrate formation time

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In a closed system, anomalous chlorinity values shown at sites SC-02 and W18

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(Fig. 4) can be caused by salt exclusion during in situ gas hydrate formation which

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have not been removed by diffusive or advective process over the time since the

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formation of the gas hydrate (Ussler and Paull, 1995, 2001; Matsumoto et al., 2005).

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Therefore, the formation times (t) of gas hydrate, can be calculated using 1D decay

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model (Crank, 1975), which assumes the transport of dissolved ions by diffusion:

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C=

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Where C is the ion concentration filling a percentage of the pore volume; Ci is the

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initial ion concentration of the pore fluid, which is near to a constant value as 542.5

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mM calculated from non-hydrate-bearing sediments; Vh is the average gas hydrate

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saturation of GHBS; x is the depth below seafloor; h is a half width of gas hydrate

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thickness; D is the effective sediment diffusion coefficient, which can be computed

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using the equation:

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D=D0*Φ2

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Where Φ is the average density porosity within the GHBS. D0 is the free solution

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diffusion coefficient (Li and Gregory, 1974), which can be written as an experimental

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equation:

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D0= 0.3797*T+9.6015 with unit of *10-6 cm2s-1

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Where T is the temperature of GHBS section calculated from the geothermal gradient

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and seafloor temperature. The input parameters (Table 3) include gas hydrate

1 Ci  h−x h+x  + erf erf  2 1 - Vh  2 D * t 2 D* t 

(2)

(3)

(4)

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saturation, porosity, thickness of gas hydrate-bearing layer and other parameters,

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which will influence the calculated formation time of gas hydrate. The saturations and

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thickness of gas hydrate-bearing units obtained from resistivities are slightly different

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from those obtained from pore-water freshening, which will affect the calculated

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formation time. We therefore performed separate calculations of the formation time

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based on gas hydrate saturation and thickness obtained from resistivity and chlorinity,

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respectively.

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4. Results

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4.1 Gas hydrate identified from well log and core data

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The calculated depths for I-BGHSZ (black line) at sites SC-02, W19, W18 and

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SC-01 are 172, 171, 172, and 175 mbsf, respectively, while those of II-BGHSZ

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(broken line) using the gas compositions in Table 2 are 199, 197, 192 and 195 mbsf,

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respectively (Fig. 3, Table 1). High resistivity and high P-wave velocity above the

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I-BGHSZ indicate the occurrence of gas hydrate with variable thickness and

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saturations at each site (Fig. 3, Table 1). Gas hydrate saturations calculated from LWD

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data have similar trends to those from chlorinity values above the I-BGHSZ, with

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different saturations at sites SC-02, SC-01, W19, and W18. At sites W18 and W19,

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the maximum saturations from LWD are 58.9% and 72.1% of the pore space,

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respectively. The average gas hydrate saturations are 30.9% for a 28 m-thick layer and

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37.1% for an 18 m-thick layer above I-BGHSZ at sites SC-02 and W18, respectively

259

(Table 3). The average gas hydrate saturations from chlorinities are about 29.8% for a

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29.2 m-thick layer and 39.5% for a 19.5 m-thick layer at sites SC-02 and W18,

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respectively (Table 3).

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At site SC-02 gas hydrates are also identified from core samples with low

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chlorinity values below the I-BGHSZ and the estimated saturations from pore-water

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freshening range from 3% to 15.6% (Fig. 3, purple zone). At depths of 180 and 187

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mbsf, resistivity and P-wave velocity are slightly increased below the I-BGHSZ at site

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SC-02, indicating the possible presence of gas hydrate and the estimated saturation is

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approximately 20% from resistivity log (Fig. 3, purple zone). Similarly, resistivity,

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P-wave velocity and chlorinity anomalies are also found at site SC-01 below the

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I-BGHSZ. Gas hydrate saturation estimated from pore-water freshening is about 10%,

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while calculated from resistivity is higher with a peak value of 20% of the pore space

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(Fig. 3). However, at sites W18 and W19, no core samples were collected below the

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I-BGHSZ. The resistivity and P-wave velocity between I-BGHSZ and II-BGHSZ are

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slightly higher than the water-saturated sediments especially at site W19 (Fig. 3

274

purple zone), which is likely caused by the occurrence of structure II gas hydrates (SII

275

hydrates) as found at site W17 (Qian et al., 2018) and site SC-01 (Wei et al., 2018) in

276

the same area (Fig. 1).

277

The gas hydrate saturations calculated from the Archie equation match well with

278

the coarsening upward sedimentary sections at sites W19, W18, and SC-01, except for

279

SC-02 where the gamma-ray log is more uniform in appearance. The gamma ray

280

values indicate that gas hydrate saturations is only partially related to inferred

281

sediment grain sizes.

282

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4.2 Gas hydrate and free gas identified from seismic data

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3D seismic data provide evidence of BSRs that cross-cut strata and are of

285

reversed polarity compared to the seafloor reflection (Figs. 5-8). High amplitude

286

reflections are observed both above and below the BSR at sites SC-02, W19 and W18

287

(Figs. 5b and 6b). A synthetic seismogram generated from P-wave velocity and

288

density logs is used to compare the well sites with seismic data. High amplitude

289

reflections above the BSRs match well with high P-wave velocities and high

290

resistivities at sites SC-02, W19, and W18, consistent with the presence of gas

291

hydrates (Figs. 3 and 5-7). High amplitude reflections related to slightly increased

292

resistivity and P-wave velocity below the BSRs at sites SC-02, W19, and W18 are

293

interpreted to indicate the potential occurrence of structure II hydrates (Figs. 3, 5 and

294

6c). Enhanced reflections of negative polarity together with chaotic signatures and

295

pull-down below the I-BGHSZ are identified at sites SC-02, W19 and W18 (Fig. 5),

296

suggesting the presence of free gas. The P-wave velocity below I-BGHSZ shows high

297

to low fluctuation at site W19, which supports the seismic anomalies. However, the

298

anomalous amplitude reflections are laterally discontinuous, which is not observed at

299

W14 (Fig. 5a).

300

The occurrence of enhanced reflections in the study area corresponds with

301

abundant normal faults identified on seismic sections and along variance seismic

302

slices (Fig. 8a). The low-frequency (yellow shadow) and high variance anomalies

303

(yellows-reds-blues) rooted on deeper layer H3 are interpreted to indicate the

304

presence of free gas (Fig. 8b). These chaotic zones coincide with apparent pull-down

305

reflections, with a clear negative relief on horizon H3 (Fig. 5a). Normal faults

306

terminate at various depths below the BSR and pass through the gas chimneys at sites

307

W19 and W18 (Fig. 8). The faults and gas chimneys provide potential fluid migration

308

pathways from deeper layers to the I-BGHSZ.

309

310

4.3 Reservoir characteristics in the buried trough sediments

311

A buried trough is identified from an erosional surface traced from the 3D

312

seismic data (Fig. 2) based on reflection truncations and onlaps, as in Figures. 5c and

313

7b. The depths of the erosional surface at sites SC-02, W19, W18 and SC-01 are 147,

314

157, 166 and 167 mbsf, respectively (Table 1). The gas hydrate-bearing interval

315

occurs below the erosional surface at SC-02, and above it at W18, but is traversed by

316

the erosional surface at sites SC-01 and W19 (Figs. 3 and 6c, Table 1). BSRs

317

interpreted from 3D seismic data are located at the trough head and levees, but are

318

absent in the trough axis (Figs. 5b and 6b).

319

The gamma ray log at sites SC-02, W19, W18 and SC-01shows upward

320

decreasing values consistent with a coarsening-upward succession. In the low gamma

321

ray intervals, the presence of calcite was determined from analyses of downhole

322

elemental capture spectroscopy data and core samples at site W18 (Kang et al., 2018).

323

The average calcite content is about 23%, which increases to 49% in the low-gamma

324

intervals at depths from 110 to 157 mbsf (Fig. 9) (Kang et al., 2018). Abundant

325

foraminifera were also identified from core samples at sites SC-01 and SC-02 (Li et al.

326

2019), which can provide a granular pore volume for the nucleation and growth of gas

327

hydrates (Fig. 9).

328

High and moderate amplitude reflections are identified at the buried head of the

329

sediment trough (sites SC-02 and W19) and at the local paleo-uplift of its western

330

levee (site W18) (Figs. 6a and 7). Low gamma ray values above the mapped erosional

331

surface reflect the occurrence of levee deposits, supplying a reservoir for GHBSs. The

332

sediment trough exhibits a NW-SE negative topography, 3.7 km long and 0.75 km

333

wide in plane view (Figs. 2 and 6a). Horizons H1 and H2 lie above the erosional

334

surface, where gamma ray values suggest uniform grain sizes, overlain by shallow

335

canyon sediments with a thickness of about 150 m (Figs. 3, 5a, 6c and 7).

336

337

4.4 Gas hydrate formation time

338

The pore-water chlorinity values at sites SC-02 and W18 display complex

339

variations compared to background values were found at sites W11, SH1, SH7 and

340

SH2 (Fig. 4). At site SH1, in which gas hydrate was found, pore water chlorinity

341

remains constant and similar to seawater (green squares Fig. 4), and similar values are

342

recorded at SH2, SH7, and W11 in intervals lacking GHBSs. All sites (except

343

non-hydrate bearing SH1) include chlorinity values lower than background due to

344

fresh water input from gas hydrate dissociation (Fig. 4). Despite fresh water input,

345

striking increases in chlorinity are also observed with maximum values of 816.6 mM

346

and 659.1 mM at sites SC-02 and W18, respectively (Fig. 4). The anomalously high

347

chlorinity values may have been produced by the formation of gas hydrate in a closed

348

system (Ussler and Paull 1995; Matsumoto et al., 2005).

349

The time to produce the chloride anomalies at sites SC-02 and W18 is estimated

350

using a 1D diffusion model, and input parameters for gas hydrate saturation and layers

351

thickness obtained either from resistivity logs or from the chloride anomalies. At site

352

SC-02, for an average gas hydrate saturation of 30.9% over a 28 m-thick layer from

353

the resistivity log and a porosity of 58.4%, the calculated time is 19,000 years with a

354

correlation coefficient of 78% (Fig. 4a, red line). For an average saturation of 29.8%

355

and a thickness of 29.2 m from the chlorinity anomaly, the gas hydrate formation time

356

is 29,000 years (Fig. 4a, blue line). At site W18, the calculated time to produce the

357

chloride anomaly is 26,000 years with 84% correlation coefficient (Fig. 4b, black

358

line), assuming an average gas hydrate saturation of 37.1% for an 18 m-thick layer

359

from resistivity log and porosity of 63.4% (Fig. 4b, red line). The calculated time is

360

28,000 years with an average gas hydrate saturation of 39.5% and a thickness of 19.5

361

m from the anomalous chlorinity (Fig. 4b, blue line). The calculated baselines are

362

nearly symmetrical and unimodal distributions (Fig. 4), consistent with diffusion of

363

the high chloride anomalies into the surrounding non-hydrate bearing sediments.

364

However, at site SC-02, two points in the measured chlorinity are significantly higher

365

than the calculated baseline (Fig. 4a), which indicates the gas hydrate system may be

366

younger than the calculated time. Therefore, the results indicate the gas hydrate

367

system is relatively young, and chloride enrichments have not yet diffused away at

368

sites SC-02 and W18.

369

370

5. Discussion

371

5.1 Partially reservoir controlled gas hydrate occurrences

372

The gas hydrate-bearing sediments in this study are characterized by moderate to

373

high concentrations of gas hydrate, and are buried by a thick sedimentary section

374

(>150 m) that lacks obvious gas conduits through the gas hydrate stability zone to the

375

seafloor. Gas hydrates occur at higher saturations at sites SC-01, W19 and W18 within

376

a seismically imaged trough-like feature. The concentrated gas hydrate layer

377

corresponds to low gamma ray values (Figs. 2b and 3), which are attributed to the

378

presence of foraminifera (Li et al. 2019) and elevated concentration of carbonaceous

379

grains and skeletal material at site W18 (Fig. 9). The foraminifera are 0.065-1.0 mm

380

i.e. sand-sized, thus increasing the average grain-size of the host sediments (Li et al.

381

2019). In addition, foraminifera have micro-pores which provide additional granular

382

space for gas hydrate (Chen et al., 2009; Li et al., 2019). The increase of calcite

383

content and the abundance of foraminifera together with quartz content likely

384

contribute to the low gamma ray values at these four sites.

385

The gas hydrate-bearing layer at site SC-02 coincides with high gamma ray

386

values indicating clay-rich sediments below the mapped erosional surface. No gas

387

hydrate was found in the low gamma ray log section above the erosion surface, in

388

contrast to sites SC-01, W19 and W18. Moreover, at sites W19 and W18 resistivity

389

and P-wave velocity logs indicate a vertically abrupt transition from GHBSs to

390

water-bearing sediments in an interval where gamma ray data show no apparent loss

391

of reservoir quality (low gamma ray layers thicker than GHBSs). This observation

392

suggests that the occurrence of gas hydrate at these four sites is only partially related

393

to the occurrence of sand-rich reservoir section.

394

From the log inferred gas hydrate occurrences and the calculated BGHSZ at the

395

above four sites (Figs. 3 and 4), highest concentrations of gas hydrate occur in close

396

proximity to the BGHSZ. A possible interpretation is that the upward migration of gas

397

(either dissolved or free gas) from deeper sediments into the gas hydrate stability zone

398

(as shown in Fig. 8) accounts for the occurrence of gas hydrate immediately above the

399

BGHSZ. It is likely that the initial formation of gas hydrate above the BGHSZ blocks

400

the pores and decreases the permeability of sediments, thus trapping free gas below

401

and not allowing gas to move higher into the gas hydrate stability zone. That may be

402

the reason why some of the lower gamma ray units occurring above the BGHSZ do

403

not contain gas hydrates. In this study, our gas hydrate formation modeling results can

404

help explain the time-dependent dynamic processes responsible for the formation of

405

highly saturated gas hydrate accumulations at four sites associated with the migration

406

gas into the GHSZ from underlying sources.

407

408

5.2 Controls on recent gas migration and formation of gas hydrate

409

The gas hydrate occurrence at site SC-02 is characterized by a significant

410

increase in pore-water chlorinities, comparable to anomalies found near seafloor in

411

cold seep settings found at IODP site U1328 and ODP sites U1249 and U1250 (e.g.

412

Riedel et al., 2006, 2009, 2010; Tréhu et al., 2004 2006; Torres et al., 2004, 2008; Liu

413

and Flemings, 2006; Cao et al., 2013). At IODP site U1325, salinity and chlorinity

414

profiles showed values higher than seawater of ~36‰ and ~600 mM respectively. The

415

elevated salt-concentrations are not caused by gas hydrate formation, but by

416

low-temperature diagenetic reactions, probably ash to zeolith transformations (Riedel

417

et al., 2006; 2010). In contrast to these near-seafloor vent systems, the high chlorinity

418

values at sites SC-02 and W18 are observed just above the I-BGHSZ. The mineral

419

contents of core samples at site W18 showed that the sediments mainly record pelagic

420

or turbidite sedimentation (Kang et al., 2018). The increasing pore fluid chlorinity is

421

coincident with LWD inferred gas hydrate occurrence indicating that these anomalies

422

are not caused by diagenetic reactions. The integrated analyses of LWD, pore-water

423

samples and seismic amplitude reflections show that the gas hydrate system is also

424

different from that reported in the adjacent canyon (e.g. Zhang et al., 2007a; Wu et al.,

425

2009, 2011; Wang et al., 2011, 2014a, b, 2016; Yu et al., 2014).

426

A thermal anomaly was also documented at sites SC-02, W19, W18 and SC-01.

427

The geothermal gradient is over ~61

/km at all four sites, which is higher than that

428

at sites SH1, SH2 and SH7 (less than 50

429

W11 and W17 (average value of 45

/km; Wang et al., 2014b) , and at sites

/km; Guo et al., 2017; Qian et al., 2018). We

430

propose that this anomaly is caused by hot fluids migrating from deeper sediments

431

along gas chimneys and faults (Figs. 5 and 10), and the thermal system has not yet

432

reached equilibrium. Mathematical modelling of methane venting through the hydrate

433

stability zone at southern Hydrate Ridge indicates that increased salinity may occur

434

above the BGHSZ when gas rapidly enters the system (Liu and Flemings, 2006). Our

435

1D modelling of chloride anomalies at sites SC-02 and W18 indicate that the gas

436

hydrate system formed within the last 29,000 years, assuming diffusion. The

437

calculated time is approximate because we assume diffusion, and use the average gas

438

hydrate saturation and consider the gas hydrate-bearing unit as one layer in a closed

439

system. The high-salinity residual waters occur in clayey silt sediments, in which

440

pore-water diffusion may be hampered by high concentration gas hydrate (Fig. 3), as

441

well as by the fine-grained lithology. These results demonstrate that there are various

442

stages in the gas hydrate formation history in the complex canyon environments of the

443

Shenhu area. In our 1D diffusion model, gas hydrates form with the introduction of

444

migrating methane, no added methane gas input or reoccurrence of ion exclusion after

445

the initial formation of gas hydrate are assumed (e.g. Torres et al., 2004; Cao et al.,

446

2013), which may increase the estimated time since the original formation of gas

447

hydrate. On the other hand, advective flow would allow for considerably younger

448

ages of formation, and may account for the two very high chlorinity values at site

449

SC-02 (Fig. 4). Nonetheless, the calculated results confirm that this system is likely a

450

product of recent gas hydrate formation when compared to previously discovered gas

451

hydrate occurrences, and that the gas hydrate system in the Shenhu area has not

452

reached equilibrium.

453

454

5.3 Gas hydrate system gas source

455

At site SC-02, both ethane and propane concentrations varied within the

456

calculated zone of gas hydrate stability. Ethane decreased in concentration to ~60 ppm

457

just above the depth of the BGHSZ (142 mbsf) and then increased downhole from 400

458

to 7000 ppm. Propane increased in concentration downhole to 3600 ppm, with a rapid

459

increase in propane concentration between 130-132 mbsf (550-600 ppm) and 138

460

mbsf (2700 ppm) (Yang et al., 2017a). The mole ratios of methane, ethane, and

461

propane are 0.991, 0.0066, and 0.0018, respectively, which indicate that the gas

462

hydrates in these sediments are predominantly structure II hydrates of variable

463

compositions. Moreover, the coexistence of structure II and structure I gas hydrate

464

was also identified in core samples from site SC-01B (about 155 mbsf) using Raman

465

spectra analysis (Wei et al., 2018), which is located at only 70 m away from site W18.

466

Between the I-BGHSZ and II-BGHSZ, structure II hydrate was inferred to occur at

467

sites SC-02, W19 and W18, and also above I-BGHSZ at site SC-01 in this study (Fig.

468

3). The heavy hydrocarbon gas compositions and occurrence of structure II gas

469

hydrate suggest that the thermogenic fluids have migrated from deeper sediments to

470

the system. The recent release and migration of fluids and associated gases proposed

471

in this study are likely to be associated with the nearby LW3-1 gas field (e.g. Zhu et

472

al., 2011; Lin and Shi, 2014).

473

Overpressured formations were found at depth within the main Baiyun sag near

474

site BY6-1 (e.g. Kong et al., 2018). The deep Enping and Wenchang Formations show

475

significant overpressure with the pressure coefficient (an index to describe the relative

476

pressures in fluid dynamics, with normal pressure coefficients ranging from 1.0 to 1.2)

477

reaching about 1.6, as documented by basin modeling and the measured pressure data

478

at site BY6-1 within the Baiyun sag (Kong et al., 2018). The release of overpressured

479

fluids is closely related to fault activity such as listric normal faults developed in the

480

study area (Fig. 1). The primary migration directions of oil and gas in the Baiyun sag

481

are northwest and southeast, and into the LW3-1 field area and the gas hydrate drilling

482

area in the east (e.g. Pang et al., 2008). It can also be assumed that long-range

483

migration of deep thermogenic gas along gas chimneys, normal faults and erosional

484

surfaces contributes to the occurrence of structure II hydrate (Figs. 1 and 10).

485

High amplitude reflections below the BSR (Figs. 5, 6 and 8) combined with the

486

potential occurrence of structure II hydrate at sites SC-02, SC-01, W19 and W18

487

indicate a complex hydrocarbon generation and migration history, including

488

accumulations within dipping strata, particularly in structurally high positions at these

489

four sites examined in this study. Large-scale gas chimneys and various types of

490

normal faults identified on seismic sections are inferred to have provided pathways

491

for vertical fluid migration from deeper sediments (Fig. 8). These results suggest that

492

thermogenic gas migration from deeper sediments has contributed to the occurrence

493

of highly concentrated gas hydrate occurrences above the I-BGHSZ.

494

495

6. Conclusions

496

Three-dimensional seismic data, combined with logging while drilling (LWD)

497

and core data acquired during the GMGS3&4 expeditions, provide an improved

498

understanding of the evolution of gas hydrate formation and accumulating processes

499

in Shenhu area of the South China Sea. The available evidence suggests that gas

500

migration, free-gas and gas hydrate distribution are controlled by a complex

501

combination of structural and stratigraphic features. A prominent regional erosional

502

surface is shown to include a buried trough near the level of the base of gas hydrate

503

stability zone. Abundant free-gas is trapped below highly concentrated gas

504

hydrate-bearing sedimentary layers, which may contain both structure I and structure

505

II gas hydrate below the I-BGHSZ under certain conditions. Anomalously high

506

geothermal gradients and heavy hydrocarbons (propane, isobutane and n-butane)

507

provide evidence of episodic releases upward migrating fluids and gases from deeper

508

sediments along chimneys and faults in the Shenhu area of the South China Sea.

509

Pore-water chloride concentration enrichments indicate the gas hydrate system

510

formed within the last 29,000 years, assuming diffusion, and advective flow would

511

allow for much younger ages. The gas hydrate system characterized by drilling results

512

represents the most recent stages of its formation, with elevated or high gas hydrate

513

saturations controlled by transient, focused fluid flow, and the availability of coarse

514

grained carbonaceous- and foraminifera-rich sediments. All observations in this study

515

indicate that the occurrence of gas hydrate is only partially controlled by sandy

516

reservoir conditions. Due to the complexities associated with gas generation and

517

possible episodic release of thermogenic gas from deeply buried sources into the

518

overlying gas hydrate stability zone, structure I and structure II gas hydrate may

519

coexist widely in nature.

520

521

Acknowledgments

522

We are grateful to Guangzhou Marine Geological Survey (GMGS) and the gas

523

hydrate science team for the logging data. We are grateful to Timothy Collett for

524

providing many ideas on the geological controls on the occurrence of gas hydrate at

525

different sites and the relationship between the apparent lack of inferred reservoir

526

control on the occurrence of gas hydrate, free gas migration and rapid hydrate

527

formation model and he also helped to improve the manuscript. We thank the

528

anonymous reviewers for their comments and suggestions during their reviews. We

529

would like to thank the associate editor Daniel Praeg for the constructive comments

530

and many grammatical improvements. Sponsorship is by the National Key R&D

531

Program of China (2017YFC0307601), National Natural Science Foundation of China

532

(41676041 and 41676040), CAS Interdisciplinary Innovation team (JCTD-2018-12)

533

and the National 863 Program (2013AA092601).

534 535

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Yang, S.X., Liang, J.Q., Lu, J.A., Qu, C.W., Liu, B., 2017b. New understanding on

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954

during rifting stage in Baiyun Sag, Pearl River Mouth Basin(Chinese in English

955

Abstract). Journal of Topical Oceanography, 35(4), 82-94.

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Zhang, W., Liang, J.Q., Lu, J.A., Wei, J.G., Su, P.B., Fang, Y.X., Guo, Y.Q., Yang,

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Zhao, J.F., Song, Y.C., Lim, X.L., Lam, W.H., 2017. Opportunities and challenges of

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Sustainable Energy Reviews, 70, 875-885. https://doi.org/10.1016/j.rser.2016.11.269

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core-log facies analysis and depositional model of the gas hydrate-bearing sediments

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in the northern continental slope, South China Sea. Marine and Petroleum Geology,

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86, 1159-1172. https://doi.org/10.1016/j.marpetgeo.2017.07.012

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Architecture, evolution history and controlling factors of the Baiyun submarine

979

canyon system from the middle Miocene to Quaternary in the Pearl Mouth Basin,

980

northern South China Sea. Marine and Petroleum Geology, 67, 389-407.

981

https://doi.org/10.1016/j.marpetgeo.2015.05.015

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983

Continentward-dipping detachment fault system and asymmetric rift structure of the

984

Baiyun Sag, northern South China Sea. Tectonophysics, 726, 121-136.

985

https://doi.org/10.1016/j.tecto.2018.02.002

986

Zhu, M.Z., Grahamb, S., Pang, X., McHargue, T., 2010. Characteristics of migrating

987

submarine canyons from the middle Miocene to present: Implications for

988

paleoceanographic circulation, northern South China Sea. Marine and Petroleum

989

Geology, 27, 307-319. https://doi.org/10.1016/j.marpetgeo.2009.05.005

990

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991

Geochemistry, origin, and deep-water exploration potential of natural gases in the

992

Pearl River Mouth and Qiongdongnan basins, South China Sea. AAPG Bulletin, 93(6),

993

741-761. https://doi.org/10.1306/02170908099

994

Figure captions

995

Fig.1. (a) Inset map showing the location of the Shenhu area in the Baiyun Sag of the

996

Pearl River Mouth Basin (PRMB), on the northern slope of South China Sea (black

997

box). The bathymetric map (based on 3D seismic data) shows the location of normal

998

faults, the LW3-1 gas field and the BY6-1 gas-bearing structure (orange circle). (b)

999

Detail of bathymetric map showing the canyonised study area (red rectangle, Figure

1000

2a) and the locations of drill sites (red stars: gas hydrate identified at GMGS4 sites

1001

SC-02 and SC-01, GMGS3 sites W19, W18 and GMGS1 sites SH2, SH3, SH4 and

1002

SH7; red rectangles: no gas hydrate identified at GMGS3 site W14 and GMGS1 site

1003

SH1). The locations of seismic profiles across the drill sites are shown as orange line

1004

(Figure 8c) and black dash line (Figure 10).

1005

1006

Fig.2. 3D seismic views of a buried canyon and tributary (location of area shown as a

1007

red box in Figure 1b). (a) Structural map (in TWT) of an erosional surface at the base

1008

of a buried canyon (see Figure 2b) showing sites SC-02 and W19 at the head of the

1009

trough and site W18 at a paleo-uplift along its western flank; (b) Seismic profile

1010

across site W18 showing the seafloor and horizons H1 and H2 as well as the buried

1011

erosional surface at the base of the canyons; note high amplitudes above the

1012

paleo-uplift.

1013

1014

Fig.3. Logging-while-drilling (LWD) data (Yang et al., 2015, 2017a) at sites SC-02,

1015

W19, W18 and SC-01 from left to right: gamma ray, bulk density, ring resistivity

1016

(black line) and resistivity of water-saturated sediment (red line), P-wave velocity and

1017

gas hydrate saturation calculated from ring resistivity using Archie equation and

1018

chloride concentrations (Yang et al., 2017a). The yellow-shaded zones show gas

1019

hydrate-bearing sediments (GHBSs) interpreted on the base of an anomalous increase

1020

of P-wave velocity and resistivity above the calculated I-BGHSZ. The pink-shaded

1021

zone shows potential structure II hydrate-bearing sediments between I-BGHSZ and

1022

II-BGHSZ. Black line and black dashed line show the base of methane (I-BGHSZ)

1023

and structure II hydrate stability zones (II-BGHSZ).

1024

1025

Fig.4. Chloride concentrations at GMGS drilling sites. In both figures, the profile

1026

from site SH1 (green squares) shows background chloride levels where no gas hydrate

1027

was identified from core samples or logging data (Wang et al., 2014). Chloride

1028

concentrations at sites SH2, SH7, W11, W18 and W19 (Guo et al., 2017; Wang et al.,

1029

2014) include significant negative deviations from background values. Note the

1030

striking increases of chloride concentrations at sites SC-02 and W18 from 120 to 180

1031

mbsf (Yang et al., 2017a). (a) Gas hydrate formation times at site SC-02 calculated

1032

using a one-dimensional decay model (Ussler and Paull, 2001) based on gas hydrate

1033

saturations (Figure 3) rang from 19,000 (red line) to 29,000 years (blue line). (b) Gas

1034

hydrate formation times at site W18 calculated used the same method as site SC-02

1035

range from 26,000 (blue line) to 28,000 years (red line).

1036

1037

Fig.5. Interpreted seismic profiles across drill sites (see Figure 2a for locations)

1038

showing a prominent erosional surface (ES, green dashed line) and a BSR (blue

1039

dashed line). (a) Interpreted seismic sections through sites W14, W19, and W18; high

1040

amplitude and continuous reflections above the erosion surface (ES) and BSR, and

1041

enhanced reflections, blanking and chaotic reflections below near sites W19 and W18

1042

correspond to the presence of gas hydrate and free gas (see Figure 1b for location);

1043

The interpreted horizon H3 (in TWT) was shown by the colored surface at the bottom.

1044

(b) Interpreted cross-line through sites SC-02 and W19, showing high amplitude and

1045

positive reflections above the BSR (blue broken line), cross-cut by ES between two

1046

sites, and pull down features and enhanced reflections below the BSR and ES; (c)

1047

Interpreted cross-line through W18, showing the erosional surface (ES) defined by

1048

truncated and onlapping reflections (yellow broken lines) and BSRs on the top of two

1049

levees.

1050

1051

Fig.6. 3D seismic views of the erosional surface (ES: green dashed line) and BSR. (a)

1052

Seismic profile and perspective view of ES showing that site W19 lies at the head of

1053

the buried trough where high amplitude reflections are present above the erosion

1054

surface, and site W18 lies at the paleo-uplift of the levee (see Figure 1b for location).

1055

(b) Structural map of the interpreted BSR in two-way travel time (TWT) and the

1056

boundary of the buried trough (red line); The BSR is present in the trough head and

1057

levees but absent in the trough axis. (c) Perspective view of arbitrary seismic section

1058

linking the buried trough with sites SC-02 and W18. GHBSs appear below ES and

1059

above I-BGHSZ (blue dashed line), potential structure II gas hydrates appear below

1060

I-BHGSZ at site SC-02.

1061

1062

Fig.7. Seismic sections across sites W19 and W18 (location shown in Figure 6b); RES

1063

represents resistivity log; GR represents gamma ray log; GHBS represents gas

1064

hydrate-bearing sediments. (a) Seismic section across sites W19 and W18 showing

1065

the distribution of GHBSs. (b) Seismic section across site W18 showing the

1066

distribution of GHBSs in the paleo-uplift of buried trough. The GR and RES are

1067

projected to seismic sections using the time-depth relation generated by synthetic

1068

seismograms at each site. GHBSs are indicated by high resistivity, and variations in

1069

lithology are shown by decreasing of gamma ray values. GHBSs characterized by

1070

high amplitude continuous reflections are identified above the BSR.

1071

1072

Fig.8. 3D seismic data showing evidence of upward gas migration via chimneys and

1073

faults toward the BSR. (a) Seismic section across sites W19 and W18 and variance

1074

slice at 2450 ms two-travel-time showing the distribution of normal faults and

1075

chimneys. (b) Variance attribute extracted along layer H3 showing the depositional

1076

environment influenced by gas chimneys and low-frequency anomalies (yellow

1077

shadow in seismic section).

1078

1079

Fig.9. Analyses of core samples at site W18 showing mineral components and

1080

foraminifera (revised from Kang et al., 2018). Calcite content increases upward with

1081

high foraminifera abundance at the low gamma ray layer. Yellow shading shows the

1082

methane hydrate-bearing sediments (SI hydrates), the green line shows the depth of

1083

interpreted erosion surface.

1084

1085

Fig.10. (a) Interpreted seismic section across sites SH2, W19, W18 and LW3-1. Gas

1086

hydrate-bearing sediments (GHBSs) are identified above the BSR, while gas

1087

chimneys and enhanced reflections are observed below. The 1.8 Ma horizon (black

1088

dashed line) is traced through sites SH2, W19 and W18. Site LW3-1 encountered a

1089

thermogenic gas reservoir that is inferred to supply shallow gas hydrate

1090

accumulations. Gas chimneys and normal faults provides pathways for deep

1091

thermogenic gas migration to form gas hydrate. (b) The schematic model shows the

1092

occurrence of SI hydrate at site SH2 and SII hydrate at sites W19 and W18, and their

1093

relationships to the faults, gas chimneys and thermogenic gas source. SI hydrates and

1094

SII hydrates represent structure I gas hydrates and structure II gas hydrates,

1095

respectively; I-BGHSZ and II-BGHSZ represent the base of methane hydrate stability

1096

zone and the base of structure II gas hydrate stability zone; ES represents erosional

1097

surface.

1098

1099

Table 1. Summary of gas hydrate occurrences at sites SC-02, W19, W18 and SC-01.

1100

ES represents erosional surface. Maximum gas hydrate saturations were calculated

1101

from LWD data using the Archie equation. The base of I-BGHSZ and II-BGHSZ were

1102

calculated using CSMHYD (Sloan 1998) and gas compositions shown in Table 2.

1103

1104

Table 2. Average gas compositions from core samples at sites SC-02, W19, W18 and

1105

SC-01.

1106

1107

Table 3. Parameters from sites SC-02 and W18 used to calculate gas hydrate

1108

formation times using the one-dimensional decay model (Ussler and Paull, 2001).

Archie Site

values a

m

Maximum gas hydrate saturation

Inferred seafloor

Thermal

Water

I-BGHSZ

temperature(℃)

gradient(℃/km)

depth (m)

(mbsf)

II-BGHSZ Erosion surface (mbsf)

depth(mbsf)

GHBSs relationship with ES depth

Indicators for gas

Indicators for

hydrate

free gas

High resistivity and P-wave velocity; SC-02

2.006 1.704

49.9%

4.95

61.2

1285

172

199

147

Below

Enhanced

Low chlorinity; High reflections below amplitude above

BSR

BSR High resistivity and P-wave velocity; SH-W19 1.398 1.866

72.1%

4.95

62.3

1272

171

197

157

Above

Enhanced

Low chlorinity; High reflection below amplitude above

BSR

BSR High resistivity and P-wave velocity; SH-W18 1.721 1.711

58.9%

4.84

61.7

1288

172

192

166

Above

Enhanced

Low chlorinity; High reflection below amplitude above

BSR

BSR High resistivity and P-wave velocity; SC-01

1.576 1.745

61.9%

4.94

64.9

1288

175

195

166

Above

Enhanced

Low chlorinity; High reflection below amplitude above BSR

BSR

Site

SC-02 W19 W18 SC-01

Depth mbsf

169.7 154.5 159.5 160.1

Methane % 97.35% 97.88% 95.41% 96.28%

Ethane ppm 15478.58 4483.26 1338.20 6866.66

Propane ppm 1318.20 2193.17 506.14 616.01

Butane ppm 188.06 84.09 109.31

Isobutane ppm 102.97 185.90 56.56 71.79

Pentane ppm 132.17 79.41

Oxygen Nitrogen CO2 % % ppm 0.42% 0.44% 220.30 0.31% 1.02% 147.36 0.82% 3.46% 111.47 0.91% 1.95% 150.8834058

Symbol D0 D t x h φ T Ci

Vh

Vi

L

n s

Parameter

Value SC-02

W18

Free solution diffusion 1.47*10-5cm2s-1 1.48*10-5cm2s-1 coefficient Effective sediment 4.488*10-6cm2s-1 6.462*10-6cm2s-1 diffusion coefficient Gas hydrate integration time depth from seafloor 30~270mbsf Half-width of the concentration spike Average porosity 58.4% 63.4% 13.5℃ 14.1℃ Temperature of GHBSs Initial ion concentration of the pore fluid Average gas hydate saturation from resistivity Average gas hydate saturation from chlorinity Per section gas hydrate saturation Thickness of GHBSs from resistivity Thickness of GHBSs from chlorinity Number of all sections of GHBSs Sample interval

542.5mM

30.9%

37.1%

29.8%

39.5%

28.0m

18.0m

29.2m

19.5m

184

92 0.1524m

Highlights: Drilling results reveal a concentrated hydrate layer with abnormally high pore-water chlorinities, interpreted as a recently formed hydrate system. A 1D diffusion model suggests the gas hydrate system formed within the last 19, 000-29, 000 years ed. Structure II hydrates and heavy hydrocarbon gas are consistent with seismic evidence of fluid migration from deeper sedimentary successions. Elevated geothermal gradients at several drill sites support the upward migration of fluids.

Author contributions statement Jiapeng Jin: Writing - Original Draft, Review & Editing, Methodology, Software Xiujuan Wang: Writing - Review & Editing, Conceptualization, Formal analysis, Validation, Supervision Yiqun Guo: Formal analysis, Data curation Jie Li: Formal analysis, Software Yuanping Li: Formal analysis Xin Zhang: Formal analysis Jin Qian: Methodology, Formal analysis Luyi Sun: Formal analysis, Software

Declarations of interests: The author declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:

Jiapeng Jin, Xiujuan Wang, Yiqun Guo, Jie Li, Yuanping Li, Xin Zhang, Jin Qian, Luyi Sun