Geological and geophysical features of and controls on occurrence and accumulation of gas hydrates in the first offshore gas-hydrate production test region in the Shenhu area, Northern South China Sea

Journal Pre-proof Geological and geophysical features of and controls on occurrence and accumulation of gas hydrates in the first offshore gas-hydrate production test region in the Shenhu area, Northern South China Sea Wei Zhang, Jinqiang Liang, Jiangong Wei, Jing'an Lu, Pibo Su, Lin Lin, Wei Huang, Yiqun Guo, Wei Deng, Xiaolu Yang, Zhifeng Wan PII:

S0264-8172(19)30645-2

DOI:

https://doi.org/10.1016/j.marpetgeo.2019.104191

Reference:

JMPG 104191

To appear in:

Marine and Petroleum Geology

Received Date: 15 May 2019 Revised Date:

13 December 2019

Accepted Date: 14 December 2019

Please cite this article as: Zhang, W., Liang, J., Wei, J., Lu, Jing'., Su, P., Lin, L., Huang, W., Guo, Y., Deng, W., Yang, X., Wan, Z., Geological and geophysical features of and controls on occurrence and accumulation of gas hydrates in the first offshore gas-hydrate production test region in the Shenhu area, Northern South China Sea, Marine and Petroleum Geology (2020), doi: https://doi.org/10.1016/ j.marpetgeo.2019.104191. 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. © 2019 Published by Elsevier Ltd.

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Geological and geophysical features of and controls on occurrence and accumulation of gas hydrates in the

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first offshore gas-hydrate production test region in the Shenhu area, northern South China Sea

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Wei Zhanga,b,c, Jinqiang Liang a,c, Jiangong Weia,c*, Jing’an Lua, Pibo Sua,c, Lin Lina, Wei Huanga,c, Yiqun Guoa, Wei

5

Denga, Xiaolu Yangb, Zhifeng Wanb*

6 7

a

MNR Key Laboratory of Marine Mineral Resources, Guangzhou Marine Geological Survey, Ministry of

Natural Resources, Guangzhou 510075, China

8

b

School of Marine Sciences, Sun Yat-sen University, Zhuhai, 519000, China

9

c

Southern Marine Science and Engineering Guangdong Laboratory, Guangzhou,511458, China

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Abstract: Integrated three-dimensional seismic, logging, sediment cores, and geochemical testing data were

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collected from Guangzhou Marine Geological Survey 3 and 4 hydrate drilling expeditions and used in this study for

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a comprehensive investigation of the geological and geophysical features and accumulation mechanism of hydrates

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in the first offshore gas-hydrate production test region (GHPTR) in the Shenhu area of the South China Sea.

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Seismic signatures indicative of disseminated‐hydrates and free gas include the bottom simulating reflector (BSR),

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gas chimney, and mud diapirs associated with enhanced seismic reflections, acoustic blanking, masking, and chaotic

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appearance have been observed. The acoustic travel-time responses, density, and compensated neutron three porosity

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log analysis, high-precision grid tomography inversion analysis, and constrained sparse spike inversion confirm the

*

Corresponding author, E-mail: [email protected], E-mail: [email protected]

Wei Zhang and Jinqiang Liang are both regarded as the first author. 1

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presence of free gas below the gas-hydrate-bearing zone (GHBZ). Free-gas-bearing zones have significantly different

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p-wave impedances and low-velocity anomalies than the overlying GHBZ and surrounding strata. These anomalous

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zones are controlled by the structural attitude of the reservoir strata, which are characterized as inter-bedded

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stratigraphic units. Variations in the type and geological characteristics of the hydrocarbon migration pathways were

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observed between sites W18 and W19 on the western ridge and sites W11 and W17 on the eastern ridge in the GMGS

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study area. The efficiency of gas migration in the western ridge may be higher than that in the eastern ridge, resulting

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in variations in hydrate gas types, thickness of the GHBZ, and gas migration flux and accumulation. Except for site

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W11, hydrates were recovered below the structure I inferred BSR at sites W17, W18, and W19. The gas-hydrate

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stability zone calculations reveal that the structure I hydrate stability zone differs from the BSR depth and is

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generally shallower than the base of the logging anomaly, indicating the coexistence of structure I and II hydrates.

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The BSR is not indicative of the BGHSZ; it is rather regarded as a transitional indicator of structure I and II gas

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hydrates in the GHPTR. The appearance of free gas and hydrates below the structure I inferred BSR indicates that

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the Shenhu area is characterized by a complex hydrate formation and accumulation system resulting from the

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supply of biogenic and thermogenic gases. Despite fine-grained host sediments predominating the GHPTR, the

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coupling of favorable conditions including efficient hydrocarbon generation, sufficient gas supply, multiple

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pathways for gas migration, and relatively high reservoir porosity have led to the development of highly saturated

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gas-hydrate accumulations within relatively thick sedimentary sections, which demonstrates a significant resource

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potential.

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Keywords: Gas hydrates; Logging while drilling; Velocity and impedance inversion; Free gas; Migration and accumulation mechanism; Gas-hydrate production; Shenhu area; northern South China Sea

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1. Introduction

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Gas hydrates are clathrate crystalline compounds composed of natural gas (mainly methane gas) and water

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molecules under appropriate temperature and pressure conditions (Kvenvolden, 1988). Gas hydrates are widely

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distributed in deep-water continental slopes with a water depth of more than 300 m and in the permafrost

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(Kvenvolden, 1993). They have attracted increasing attention around the world and have been considered as a

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potential clean energy source to supplement future energy supplies (Collett, 2002; Collett, 2004; Boswell and

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Collett, 2006). Because of their significant potential as an environment-friendly energy resource and positive

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impacts on the greenhouse effect and marine ecological systems, the governments of the United States, Canada,

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Japan, South Korea, and India as well as many research institutes have invested in the exploration and development

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of gas hydrates. Many gas-hydrate drillings and test productions have been undertaken on land in permafrost

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regions and continental slopes. These tests achieved significant breakthroughs (Tréhu et al., 2004a; Tréhu et al.,

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2004b ; Moridis et al., 2004; Dallimore et al., 2005; Dallimore and Collett, 2005; Collett et al., 2008; Anderson et

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al., 2011; Boswell et al., 2012; Collett et al., 2012; Ryu et al.,2013; Collett et al., 2014; Wang et al., 2014; Fujii et

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al., 2015; Zhang et al., 2007; Yang et al., 2008; Yang et al., 2015; Zhang et al., 2015; Zhang et al., 2017; Li et al.,

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2018), and the processes of the exploration and development of gas hydrates and utilization of these resources have

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been actively promoted.

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The northern continental slopes of the South China Sea (SCS) (Fig. 1A) possess good geological,

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geomorphological, and geochemical conditions for the formation and accumulation of gas hydrates (Zhang et al.,

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2002; Wu et al., 2005; Wang et al., 2006; Yang et al., 2008; Wu et al., 2009; Wu et al., 2010; Wu et al., 2011; Wang

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et al., 2014; Zhang et al., 2015; Sha et al., 2015; Yang et al., 2017a; Yang et al., 2017b; Yang et al., 2017c; Yang et 3

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al., 2017d; Zhang et al., 2017; Zhang et al., 2018a; Zhang et al., 2018b; Zhong et al., 2017; Wei et al., 2018; Ye et

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al., 2019; Wei et al., 2019; Zhang et al., 2019). Regional marine geological surveys and oil and gas exploration

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research indicate that there are abundant gas-hydrate resources in the northern continental slope of the SCS (Zhang

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et al., 2002; Wu et al., 2005; Wu et al., 2010; Wang et al., 2014; Zhang et al., 2015; Sha et al., 2015; Yang et al.,

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2015; Zhang et al., 2017; Wang et al., 2018; Su et al., 2018). The Guangzhou Marine Geological Survey (GMGS)

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undertook two gas-hydrate drilling expeditions in the Shenhu area of the northern continental slope of the SCS in

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2007 and 2015, viz., the GMGS1 and GMGS3 expeditions, to obtain core samples containing gas hydrates, confirm

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the existence of gas hydrates, explore the formation and accumulation mechanisms of hydrates, and evaluate the

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hydrate resources in the northern SCS (Zhang et al., 2007; Yang et al., 2008; Yang et al., 2015; Zhang et al., 2017).

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During these expeditions, eight sites were drilled in the 2007 GMGS1 expedition (Wang et al., 2014; Su et al., 2016),

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and hydrate samples were obtained at three sites: SH2, SH3, and SH7 (Fig. 1B). Post-drilling research showed that

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gas hydrates, which were invisible to the naked eye, are distributed within the pores of the clay-dominated sediments

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at approximately 200 mbsf (meters below seafloor); the morphologies of the gas hydrates are disseminated, and the

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methane concentration of the hydrate gas components is up to 99.7% (Zhang et al., 2007; Wang et al., 2014; Su et al.,

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2016). The thickness of the hydrate-bearing layer in the GMGS1 drilling zone is approximately 10-47 m. In addition,

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the average saturation of the hydrates acquired from the coring sites varies from 20% to 48% (Wang et al., 2014). The

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GMGS1 expedition was the first time that the GMGS obtained gas-hydrate samples in the SCS. Furthermore, the

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hydrate geology, geophysics, gas source genetic types, reservoir characteristics, and bioorganic geochemistry were

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extensively studied after drilling (Wu et al., 2010; Wu et al., 2011; Wang et al., 2014; Su et al., 2016; Wang et al.,

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2018; Guan et al., 2018; Guan et al., 2019; Fang et al., 2019; Xiong et al., 2019). However, because of the small

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quantity of drilling boreholes‐and limited areas covered by these boreholes in the GMGS1 expedition, few studies 4

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have focused on the distribution, factors controlling accumulation, and resource calculation and evaluation of

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hydrates throughout the Shenhu area (Wu et al., 2010; Wang et al., 2014; Su et al., 2016), which restricts further

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research on the accumulation mechanism of hydrates and resource evaluation.

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The GMGS implemented the GMGS3 expedition in the Shenhu area in 2015 to obtain high-quality geophysical

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logging data and more core samples of the hydrate-bearing layers, evaluate high-saturation hydrate accumulation

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characteristics and the amount of hydrate resources, and provide a basis for the optimization of the gas-hydrate

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production test region (GHPTR) in the SCS (Fig. 1B) (Yang et al., 2015). The GMGS3 expedition completed drilling

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at 19 sites, and 23 wells were drilled. Coring was conducted and hydrate samples were obtained at sites W11, W17,

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W18, and W19, where the logging-while-drilling (LWD) data indicates significant reserves of gas hydrates. The

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hydrate saturations at these four sites, which were calculated using the pore-water freshening analysis from chlorinity

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and mass balance calculations from the pressure cores, were as high as 76%, with an average value of ~20%–40%

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(Yang et al., 2015; Zhang et al., 2017; Guo et al., 2017; Qian et al., 2018). In addition, the thickness of the main

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gas-hydrate-bearing zone (GHBZ) was generally up to 10-80 m (Yang et al., 2017b; Yang et al., 2017d). Generally,

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the thickness and saturation of gas hydrates discovered during the GMGS3 expedition were significantly larger than

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those discovered during the GMGS1 expedition (Wang et al., 2014; Yang et al., 2015; Su et al., 2016; Guo et al.,

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2017; Zhang et al., 2017; Yang et al., 2017b).

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In 2016, the GMGS conducted gas-hydrate drilling (GMGS4) in the Shenhu area again (Fig. 1B). Four deep

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drilling and coring sites were finished in the vicinity of the coring sites of the GMGS3 expedition (Yang et al., 2017c;

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Yang et al., 2017d). In addition to recovering gas-hydrate samples with high saturations, hydrate deposits with

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predicted original in-place gas of more than 100 billion cubic meters were confirmed (China Geological Survey, 5

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Ministry of Land and Resources, 2016).

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The geochemical testing results indicate that the compositions of the hydrate gases obtained from the GMGS3

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and GMGS4 expeditions are methane dominated; its hydrocarbon gas content was generally greater than 93.5%.

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However, heavier hydrocarbons, viz., ethane, propane, and even butane and pentane, were also detected in several gas

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samples. These were different from the hydrate gas compositions in the GMGS1 drilling area (Huang et al., 2010; Su

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et al., 2016; Liu et al., 2017; Li et al., 2019a; Zhang et al., 2019) where the content of heavy hydrocarbons, such as

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ethane and propane, were extremely low and methane accounted for the vast majority of the total gas content (Huang

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et al., 2010; Wu et al., 2011; Zhu et al., 2013). Further analysis revealed that the ratio of C1 to C2 in the hydrate gases

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generally decreased with depth, and there were abrupt changes near the top interface of the hydrate layer, thereby

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indicating the increase in the contribution of the thermogenic gas with depth (Yang et al., 2015; Zhang et al., 2017;

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Cong et al., 2018; Zhang et al., 2019). The C1/(C2+C3) versus δ13C1 diagram reveals that most of the hydrate

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decomposition gas and void gas samples from the gas-hydrate layers at sites W17, W18, and W19 were in the range

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of mixed gases. The hydrate decomposition gases were primarily within the range of thermogenic gas (Zhang et al.,

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2019) on the δ13C1 versus δDCH4 genetic type diagram. In addition, the hydrate gas recovered from the first

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gas-hydrate production test well SHSC-4, which is located near site W17 within the GHPTR, also exhibits a “mixed

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origin” (Ye et al., 2018), which is similar to the hydrate gas origin of coring sites W11, W17, W18, and W19.

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Furthermore, the genetic type of hydrate gases obtained from other districts with gas-hydrate occurrences indicates

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that the origin of the hydrate gas within the GHPTR is different from the hydrate gas from the Krishna Godavari

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Basin (KG Basin) (Lorenson and Collett, 2018), Ulleung Basin (Choi et al., 2013), and Nankai Trough (Kida et al.,

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2015), which have an entirely microbial origin. In addition, the origins of the hydrate gas from sites W18 and W19

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are the same as the gas from the Gulf of Mexico (Sassen et al., 2001), i.e., thermogenic origin. In summary, the

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hydrate gas in the GHPTR has a mixed origin, and both biogenic and thermogenic gases significantly contribute to

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the formation and accumulation of these gas hydrates (Yang et al., 2015; Zhang et al., 2017; Liu et al., 2017; Cong et

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al., 2018; Ye et al., 2018; Zhang et al., 2019).

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Based on the exploration results of the GMGS 1, 3, and 4 expeditions, the GMGS constructed an optimization of

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the GHPTR in the Shenhu area in the SCS, promoting the success of the first offshore gas-hydrate production test in

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China in 2017 (Li et al., 2018; Ye et al., 2018) implemented after the offshore gas-hydrate production tests in the

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Nankai Trough in 2013 and 2017 (Konno et al., 2017; Yamamoto et al., 2019). The first successful offshore

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experimental production of hydrates in the Shenhu area indicates that the GHPTR discussed in this paper is likely

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to be a favorable target for large-scale development of gas hydrates in the SCS (Chen et al., 2018). However, prior

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to the GMGS drilling, the general consensus was that disseminated gas hydrates in fine-grained sediments were likely

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accumulated with very low saturations and limited thicknesses in the Shenhu area. However, the GMGS expeditions

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have confirmed that the accumulation does achieve unexpected saturation and thickness in the GHPTR. In addition,

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the gas hydrates have been discovered below the bottom simulating reflector (BSR) (Liang et al., 2017; Wei et al.,

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2018), and free gas has also been found to coexist with gas hydrates (Qian et al., 2018). However, the distribution of

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gas hydrates and associated free gases and their controlling factors are still unknown. Therefore, the factors in the

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entire gas-hydrate petroleum system need further investigation. Furthermore, although the geological and

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geophysical

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distribution characteristics of the Shenhu area have been studied by many previous researchers (Wang et al., 2014;

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Yang et al., 2015; Su et al., 2016; Guo et al., 2017; Zhang et al., 2017; Yang et al., 2017b; Qian et al., 2018; Wang

characteristics

of

the

hydrates,

geochemical

7

features

of

hydrate

gases,

and

hydrate

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et al., 2018; Li et al., 2019a; Zhang et al., 2019; Xiong et al., 2019), these studies were aimed at a certain scientific

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problem or a certain aspect of research. Therefore,‐the paucity of research on the geological and geophysical features

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of the indicators of gas-hydrate accumulation and hydrocarbon migration in the GHPTR and the lack of a

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systematic understanding of the accumulation mechanism of widespread gas hydrates with high saturations as well as

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associated free gas also pose difficulties to our understanding of the distribution and accumulation of gas hydrates

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and future optimization of hydrate production sites in the SCS.

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Therefore, we selected the GHPTR in the Shenhu area on the northern continental slope of the SCS as the study

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area and focused on the hydrate deep drilling and coring sites within this region. Based on the comprehensive

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interpretation and analyses of the cores, logging data, 3D seismic data, and geochemical testing data, this paper aims

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to: (1) identify the seismic and logging indicators of hydrocarbon migration and hydrate accumulation with high

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saturations and large thicknesses; (2) characterize the coexistence and distribution of gas hydrates and their

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associated free gases within difference regions; (3) discuss the inconsistent distribution between the BSR, base of

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gas-hydrate stability zone (BGHSZ), and gas hydrates; and (4) analyze the reservoir characteristics and their impact

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on the differential accumulation of gas hydrates with variable saturations and thicknesses. This paper can be a

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significant reference for the selection of favorable exploration areas and subsequent drilling sites in the SCS,

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evaluation of hydrate resource potential, and research on hydrate production. In addition, it can be of significance for

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further research on the hydrate accumulation mechanism and hydrate production in other areas with similar

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geological settings around the world.

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2. Regional geological background and petroleum system

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The study area of the GHPTR in the Shenhu area is within the southern part of the Pearl River Mouth Basin

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(PRMB) and is located on the vast continental shelf and the continental slope area between the South China continent,

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Hainan Island, and Taiwan Island (Fig. 1A). The PRMB is approximately 800 km long and 300 km wide, with a basin

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area of approximately 20 × 104 km2. The basin water depth is 50-2000 m. It is the largest petroliferous basin on the

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northern slope of the SCS.

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The PRMB possesses the characteristics of both passive and active continental margins, resulting in a complex

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regional geological background (Li and Rao, 1994; Dong et al., 2009). During the Cenozoic, many regional tectonic

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movements—including the Shenhu, Zhuqiong, SCS, Baiyun, and Dongsha events—occurred in succession (Li, 1993;

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Pang et al., 2008) (Fig. 2), resulting in three groups of normal faults with NE, NW, and NWW strikes controlling the

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distribution of the modern tectonic units in the PRMB (Pang et al., 2008; He et al., 2012; Zhong et al., 2014). The

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tectonic evolution of the Cenozoic basin in this area can be divided into two stages: an early rifting stage in the

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Paleogene and a late depression stage in the Neogene (Li and Rao, 1994; Zhong, 1994) (Fig. 2). In the Paleocene,

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Eocene, and Early Oligocene, there was a half graben-like or graben-shaped rift basin that deposited continental

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strata composed of the Paleocene Shenhu Formation, Eocene Wenchang Formation, and Lower Oligocene Enping

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Formation (Mi et al., 2008). The PRMB has gradually transitioned into the depression stage since the Upper

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Oligocene and has deposited Late Oligocene Zhuhai Formation neritic strata and Miocene-Pliocene neritic-abyssal

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strata. In addition, there is a prominent unconformity between the Lower Oligocene and Upper Oligocene, which are

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separated by the T6 interface (Fig. 2) (Shao et al., 2008; Xie et al., 2013; Shi et al., 2014).

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The Shenhu area is adjacent to the center of the southern deep-water Baiyun Sag in the PRMB (Fig. 1B). 9

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Petroleum exploration has confirmed the presence of sufficient natural gas resources in the Baiyun Sag area. A

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significant number of gas fields, including the LW-3-1, LH 19-1, and PY 35-1, have been discovered around the

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GMGS gas-hydrate drilling sites in this area (Shi et al., 2009a; He et al., 2009; Shi et al., 2010; He et al., 2012;

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Zhang et al., 2019). The Paleogene Eocene Wenchang Formation and Oligocene Enping Formation were identified

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as the main mature to highly mature source rocks, which generate abundant thermogenic hydrocarbons (Shi et al.,

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2014; Zhang et al., 2014; Li et al., 2015). Although the organic material deposited in the Zhujiang and Hanjiang

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formations was in the immature-low mature stage, the formations could act as source rocks and produce significant

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amounts of biogenic gases (Fu et al., 2007; He et al., 2013; Su et al., 2018). In addition, both thermogenic gas from

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the deep strata and biogenic gas from the shallow strata provided sufficient hydrocarbons for the formation of gas

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hydrates in the Shenhu area (Wang et al., 2014; Yang et al., 2017b; Zhang et al., 2017; Su et al., 2018; Zhang et al.,

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2019).

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Based on the geological characteristics of the hydrocarbon accumulation in the Baiyun Sag, the deep-seated

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thermogenic gas mostly migrated along continuous sand bodies extending into the Baiyun Sag and along regional

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unconformities, such as T7 and T6 (Fig. 2) (Yu et al., 2007; Hou et al., 2008; Shi et al., 2010; Lin and Shi, 2014). In

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addition, the normal faults, mud diapirs, and gas chimneys could also act as vertical pathways for oil and gas

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migrating from the deep strata to shallow strata (Shi et al., 2009a; Shi et al., 2009b; Liu et al., 2011). The

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Neotectonic movement induced plastic flow of the thick overpressure mud shale in the Paleogene strata, forming a

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large-scale mud diapir belt, which resulted in the wide distribution of diapirs and gas chimneys (2009b). The upward

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intrusion of diapirs started from the Early Neogene and is still continuing. This has resulted in overlying anticlines at

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different depths as well as high-angle faults and vertical fracture systems. These structures constitute important

10

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pathways for the migration of the deep and shallow gas-bearing fluids that provide sufficient natural gas for the

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formation of gas hydrates in the Shenhu area (Shi et al., 2009a; Shi et al 2009b; Su et al., 2016).

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The gas-hydrate drilling zones of the GMGS 1, 3, and 4 expeditions are located in the transition area from the

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continental slope to the abyssal plain, and the GMGS 3 and 4 drilling areas are adjacent to the eastern part of the

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GMGS1 drilling area (Fig. 1B). The GHPTR is located in the southwestern part of the GMGS 3 and 4 drilling areas,

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and the drilling and coring sites are located in the structural highs and marginal parts of the two seafloor ridges (the

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western and eastern ridges) (Figs. 1B, 3). The seafloor in this area is relatively flat, with an average slope of

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approximately 3°. The depth of the water deepens from ~500 m to more than 1,700 m, and the landforms in the

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northern part of the Shenhu area are relatively steep. From west to east, there are 17 submarine trenches striking

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nearly NS, which alternate with submarine ridges (Su et al., 2019). The southern terrain is flat and gradually

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transitions to an abyssal plains to the south. The research area mostly consists of landforms, such as seamounts,

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submarine canyons, scour channels, and erosion trenches (Wang et al., 2014; Su et al., 2017; Zhang et al., 2017).

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The seafloor temperature of the Shenhu area is between 3 and 6 °C, and the geothermal gradient is between

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~45 °C/km and ~67 °C/km, which satisfies the temperature conditions for hydrate formation. The depth of

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gas-hydrate distribution generally varies from ~150 to ~400 mbsf (Wu et al., 2010; Liang et al., 2014; Zhang et al.,

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2017).

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3 Data and methods

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3.1 Seismic and LWD data

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The GMGS3 and 4 drilling areas are located within the pseudo-3D seismic survey in the Shenhu area. The

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seismic volume, covering 17.1 km × 9.5 km, was shot by the GMGS from 2006 to 2009. The intervals of the inline 11

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and the crossline of the seismic data were 12.5 m and 25 m, respectively; and the sampling interval was 1 ms. The

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effective frequency band width of the seismic data is 5-85 Hz, and the main frequency is about 58 Hz. The seismic

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data has a high quality after it was processed by fidelity, amplitude-preservation, and pre-stacking depth migration.

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The interpretation of the seismic data was performed using Geoframe 4.5.

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Twenty-three boreholes were drilled spanning over nineteen drilling sites during the GMGS3 expedition (Yang

224

et al., 2015; Yang et al., 2017b). The caliper log, gamma ray log, drilling rate, resistivity, density, neutron log, and

225

acoustic log were monitored in real time during the LWD process. Four sites (W11, W17, W18, and W19) (Fig. 1B)

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were cored based on the logging response characteristics of the LWD data collected from the pilot hole (Wang et al.,

227

2014), which were targeted from the seafloor to 50–100 m below the BSR observed on the seismic profile. The other

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15 sites also showed indications for the presence of gas hydrate in the LWD; however, no cores were collected from

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these sites according to the coring plan. In addition to the LWD programs, drilling and coring activities were also

230

performed at four other sites (W07, SC01, SC02, and SC03) during the GMGS4 expedition (Yang et al., 2017c; Yang

231

et al., 2017d). The porosities and permeabilities of gas-hydrate reservoirs were calculated by the Schlumberger based

232

on the LWD data, the results are reported in Table 1. These results were used to explore the relationship between the

233

quality of the reservoirs and gas-hydrate accumulation. To further analyze the relationship between the gas-hydrate or

234

free gas distribution and the seismic reflection amplitude, polarity, and so on, synthetic seismograms were

235

constructed based on the LWD and seismic data acquired from the drilling and coring sites. Because sites SC01,

236

SC02, and SC03 were near sites W18, W19, and W11, respectively (Fig. 1B), the corresponding sites had similar

237

gas-hydrate geological occurrences and distribution features owing to the short distance between the sites (Yang et al.,

238

2017c; Yang et al., 2017d) (Figs. 1B, 3). Therefore, in this study, we primarily discuss sites W11, W17, W18, and

12

239

W19 within the GHPTR.

240 241

3.2 Core samples and grain size analysis

242

The drilling sites in GMGS 3 and 4 were conventionally cored using spaced out spot cores with pressure

243

coring tools, which were also used in the GMGS 1 and 2 gas-hydrate drilling expeditions (Wang et al., 2014; Sha et

244

al., 2015; Yang et al., 2017d). Owing to the limited time allotted for the GMGS 3 expedition, the boreholes could

245

not be cored continuously. The initial coring and testing plan for each site was generated by picking coring targets

246

from the LWD logs, which indicated the presence of gas hydrates. The temperature profile was determined from the

247

temperature measurements using the Wison EP temperature/cone penetrometer probe. The total footage of the

248

coring was about 975 m, and the coring recovery rate was 88.32%. In addition to the conventional cores

249

(non-pressured cores) recovered from above and below the GHBZ, 36 m of pressure core was recovered, with a

250

recovery rate of 64% of the pressure cored intervals.

251

The pressured and conventional cores were handled suitably with different methods in the field. The

252

conventional cores were sent to the onboard laboratory immediately upon recovery for infrared thermal imaging

253

using a forward looking infrared camera. If gas-hydrate-bearing segments were recognized on the infrared thermal

254

images, they were cut and preserved in liquid nitrogen tanks for further onshore analyses. The pressured cores were

255

sent to the onboard pressured core laboratory for depressurization experiments using the Automated Depressure

256

System (ADS) (Yang et al., 2017d). After the recovered cores were transported to the onshore GMGS lab, systematic

257

core observations and descriptions were undertaken and detailed sedimentology records were prepared. To

258

understand the characteristics of the lithologies of hydrate reservoirs, measurements of the grain sizes of the core 13

259

sediments were performed onshore using the Mastersizer 2000 Laser Grain Size Analyzer (SYE209) in the

260

Department of Testing in the GMGS. In addition, abundant foraminifera fossils were found within the core sediments

261

during the core observation and description, and foraminifera identification was done using the Zeiss Discovery,

262

V20/LEICA.MI165C Binocular stereo microscope. X-ray diffraction for the whole rock analysis was also performed

263

using the D/max-2500 18-kw high-power powder diffractometer in the GMGS.

264 265

3.3 Free-gas identification method

266

Free gases are generally distributed under the GHBZ and accumulate within reservoirs with relatively high

267

porosities. Four methods were used to identify the distribution of free gases based on the geophysical

268

characteristics of the GHPTR.

269

(1) Because of the distinct difference in the impedance values of the gas reservoir and surrounding strata,

270

enhanced reflections are usually seen below the BSR in seismic profiles (Vanneste et al., 2001; Kumar et al., 2018).

271

Therefore, the distribution of the free-gas layers can be determined by tracing the strong reflection characteristics

272

below the hydrate layers. In addition, there is a strong difference in impedance values between the GHBZ and

273

underlying free gas. Using the constrained sparse spike inversion (CSSI) method in the software Jason, under the

274

control of the GHBZ and BSR, the entire logging data for the hydrate orebody in the study area was used to

275

complete the post-stacking CSSI processing and obtain the p-wave impedance, which revealed the distinct

276

distribution of the GHBZ and underlying free gas at the four coring sites.

277 278

(2) The acoustic travel-time difference, density, and compensation neutron three porosity log intersection method were used to identify and interpret the GHBZ and the free-gas-bearing zones (FGBZ) in the GHPTR. 14

279

(3) Because the FGBZ usually has the electrical characteristics of "excavation effect" with a low density and

280

high acoustic time difference, it generally exhibits a remarkable low-velocity feature, which is significantly

281

different from the background velocity of the surrounding strata without hydrocarbon accumulation. Thus, the

282

extent of the free gas under the GHBZ can be determined by tracing the low-velocity anomalies (Fohrmann and

283

Pecher, 2012; Cordero et al., 2016). Based on the pre-stacking depth migration processing of the seismic data,

284

accurate shallow strata velocity volumes can be obtained using high-precision grid tomography inversion (Xue et

285

al., 2017; Li et al., 2019b). Subsequently, the tracing of the distribution of free gas can be implemented.

286

(4) Because of the strong attenuation of the high-frequency components of seismic waves in gas-bearing strata,

287

the seismic attributes are usually characterized by a low instantaneous frequency. Therefore, the entrapment area of

288

the free gas below the BSR can be roughly judged by extracting and analyzing the seismic instantaneous frequency

289

attributes (Coren et al., 2001; Satyavani et al., 2005; Shankar et al., 2014; Kumar et al., 2018).

290 291

3.4 Gas-hydrate stability zone modeling

292

Based on the in situ measured temperature (Table 2) and geochemical data for the hydrate gases acquired from

293

sites W11, W17, W18, and W19 (Zhang et al., 2019), the gas-hydrate stability curves of each site were calculated

294

using the CSMHYD program (Sloan, 1998). The geothermal gradient of each site was calculated using the in situ

295

measured temperature (Table 2). Subsequently, the BGHSZ was obtained by identifying the intersection of the

296

gas-hydrate stability curves with the geothermal gradient (Liang et al., 2017). In addition, to verify the possible

297

presence of gas hydrates below the BSR, the BGHSZ was calculated using different gas compositions (100% C1, 99%

298

C1 + 0.5% C2 + 0.5% C3, and 95% C1 + 2.5% C2 + 2.5% C3) and was compared with the BSR, logging anomaly

15

299

distribution intervals, and pore-water anomalies tested within the GHPTR (Cong et al., 2018; Guo et al., 2017;

300

Yang et al., 2017c; Zhang et al., 2017). Structure I (SI) gas hydrates contain gas, with 100% methane gas; whereas

301

the presence of C2+ hydrocarbons in the hydrate cage results in the formation of structure II (SII) gas hydrates.

302

Therefore, the BGHSZ calculated with 100% methane corresponds to the BGHSZ of SI hydrates, and the BGHSZ

303

calculated with the other gas compositions used in this study corresponds to the BGHSZ of SII hydrates. If the

304

logging curves and core sediments show indicators of gas hydrates and pore-water anomalies occur below the BSR

305

or calculated SI gas-hydrate BGHSZ, gas hydrates will be accumulated below the BSR and SI and SII hydrates may

306

coexist at the site (Paganoni et al., 2016; Liang et al., 2017; Zhang et al., 2019; Wei et al., 2019).

307

308

4. Results

309

4.1 Geological occurrence of gas hydrates

310

In situ measurements and coring were performed at sites W11, W17, W18, and W19 (Figs. 3–7), and samples of

311

gas hydrates, which mainly occur in fine-grained clayey silt, were obtained. They were mostly disseminated hydrates,

312

which were not visible to the naked eye. Some of the hydrates were of the fracture-filled type (Yang et al., 2015; Yang

313

et al., 2017b, d). The occurrence characteristics of the drilling and coring hydrates at each site are given in Table 1

314

and Figures 8–11.

315

4.1.1 Site W11

316

Site W11 is located on the lower part of the eastern submarine ridge in the southeastern corner of the drilling area

317

(Figs. 1B, 3, 4 . The water depth was ~1310 m; the actual drilling depth was ~222 mbsf; and the geothermal gradient 16

318

was 5.46 °C/100 m. The pilot hole LWD resistivity curves exhibit four sections of anomalously high values between

319

112 mbsf and 201.6 mbsf, and the layer thickness is approximately 90 m (Fig. 8A). The actual coring indicates that

320

the depth of the gas-hydrate occurrences is consistent with resistivity anomalies (Fig. 9A). The core data from this

321

site reveals that gas hydrates occur over a 72 m interval from 116.5 mbsf to 192.5 mbsf. The gas hydrates

322

are dispersed within the pores of the sediments and are not visible to the naked eye. One distinct characteristic

323

observed is that the hydrates in the cores obtained from the strata at 116–117 mbsf and 120–121 mbsf are decomposed,

324

which resulted in inflation and creation of cavities in cores (Figs. 11A, 11B). Because of the dissociation of the

325

hydrates, the sediments exhibit porridge-like characteristics at depths of 139–142 mbsf (Fig. 11C). In addition, the

326

infrared thermal imaging of the cores obtained from site W11 exhibits significant negative thermal anomalies in the

327

GHBZ (Fig. 9A), indicating the presence of gas hydrates.

328

4.1.2 Site W17

329

Site W17 is located at the tip of the eastern submarine ridge in the southeastern corner of the drilling area,

330

adjacent to site W11 (Figs. 1B, 3, 5), which has a water depth of 1259 m. The actual drilling depth was ~315 mbsf,

331

and the geothermal gradient was 4.43 °C/100 m. The pilot hole LWD logging curves at site W17 exhibit an anomaly

332

at 210-270 mbsf. The resistivity increases significantly, and the acoustic travel-time difference decreases, suggesting

333

the presence of an approximately 60 m thick GHBZ (Fig. 8B). The actual coring results confirm that the depth of the

334

gas hydrates is well correlated with the logging anomaly layer. The core data from this site reveals that the gas

335

hydrates mainly occur over a 56 m interval from 206 mbsf to 265 mbsf. In addition, indications of gas hydrates are

336

also observed in several sections above 190 mbsf (Fig. 9B). Gas hydrates are dispersed in the pores of the sediments

337

and are not visible to the naked eye. During the drilling process, a significant number of massive hydrates was

17

338

observed, using a camera placed on the seabed frame, near the wellhead (Fig. 11D). In addition, the core infrared

339

scanning results exhibit significant negative thermal anomalies, which are presumably caused by hydrates (Fig. 9B).

340

The core obtained from 238 mbsf to 241 mbsf contains sediments with porridge-like characteristics, suggesting that

341

the hydrates have decomposed (Fig. 11E).

342

4.1.3 Site W18

343

Site W18 is located on the lower part of the western submarine ridge (Figs. 1B, 3, 6). The water depth was ~1285

344

m, and the actual drilling depth was ~234 mbsf. The geothermal gradient was 6.26 °C/100 m. The pilot hole logging

345

curves for this site contains an anomaly between 144 and 174 mbsf. The resistivity increases and the acoustic

346

travel-time difference decreases significantly, indicating that there are hydrates within this layer (Fig. 8C). The actual

347

coring indicates that the depth of the gas hydrates corresponds well with the anomalous layer in the logging (Fig.

348

10A). The core data from this site reveal that the gas hydrates occur over a 25 m interval from 147 mbsf to 172 mbsf.

349

In addition, gas hydrates with low saturations may occur at 137–147 mbsf because gas inflation cracks were observed

350

within the cores at these depths (Fig. 10A). The gas hydrates are disseminated within the pores of the sediments and

351

are not visible to the naked eye. Hydrate cores were obtained at site W18 at depths of 152 mbsf to 155 mbsf. By the

352

time the cores reached the deck, the hydrates had decomposed. The cores had a honeycomb appearance, and some of

353

the sediments exhibited porridge-like characteristics (Fig. 11F). The core sediments obtained at the depths of

354

155-158 mbsf exhibit distinct negative thermal anomalies on the infrared scanning image (Fig. 11G), indicating the

355

presence of hydrates within this layer. In addition, the cavities created by the decomposition of the hydrates were also

356

observed from 165.5 mbsf to 168.5 mbsf (Fig. 11H).

18

357

4.1.4 Site W19

358

Site W19 is also located on the lower part of the western submarine ridge, close to site W18 (Figs. 1B, 3, 7), with

359

a water depth of ~1274 m. The actual drilling depth was ~240 mbsf, and the geothermal gradient was 5.56 °C/100 m.

360

The pilot hole logging curves for this site reveal that the resistivity curves vary from 134 mbsf to 202 mbsf, and the

361

total thickness of the abnormal layer is approximately 68 m (Zhang et al., 2017) (Fig. 8D). The acoustic travel-time

362

differences within this layer decrease significantly and are the mirror image of the resistivity curve, indicating the

363

presence of gas hydrates. The actual coring demonstrated that the depth of the gas hydrates corresponds well with the

364

anomalous logging layer (Fig. 10B). The core data from this site revealed that gas hydrates are present over a 28 m

365

interval from 135 mbsf to 163.5 mbsf. The gas hydrates are disseminated within the pores of the sediments and are

366

not visible to the naked eye. Similar to site W17, a significant number of massive hydrates were observed near the

367

wellhead during the drilling process (Yang et al., 2015; Zhang et al., 2017). The cores obtained from depths of

368

134–137 mbsf and 140–142 mbsf show distinct indications of gas hydrates, such as cavities and a strong inflating

369

phenomenon from the dissociation of gas hydrates (Fig. 11I). Some of the hydrates in the sediments had decomposed

370

and turned mushy (Fig. 11J). In addition, the infrared images of the core sediments also show significant negative

371

anomalies (Fig. 10B), which fully demonstrate that these layers contain gas hydrates (Zhang et al., 2017).

372

373

4.2 Seismic reflection characteristics of drilling sites

374

Recognizing and identifying the submarine BSR is an effective method of exploring for gas-hydrates (Zillmer et

375

al., 2005; Majumdar et al., 2016).The distribution of hydrates and associated free gas below the GHSZ can be

376

roughly characterized by studying the seismic reflection features of the BSR and nearby strata (Hyndman and Davis, 19

377

1992; Haacke, et al., 2007; Crutchley et al., 2015).

378

The BSR in the GHPTR in the Shenhu area is widespread with prominent characteristics (Figs. 3–7), exhibiting

379

strong to relatively strong amplitude reflections generally crosscutting the nearby strata and roughly parallel to the

380

seafloor but with opposite polarities. Most of the identified BSR is distributed along the slope area and exhibits

381

medium to high continuous characteristics; however, the BSR is discontinuous in some locations. Based on the

382

seismic interpretation from the drilling sites within the GHPTR, the BSR is mainly located between ~170 mbsf and

383

~400 mbsf, and the length of a single BSR is ~6 km. The acoustic blanking reflections occur above the BSR in some

384

sites, such as sites W11 and W17 (Figs. 3–5). However, in some areas, the reflections directly above the BSR also

385

show a high amplitude obliquely adjacent to or connected to the BSR (Figs. 3–5). In addition, acoustic blanking,

386

masking, and chaotic reflections with significant vertical extensions are observed below the BSR (Figs. 3–7). The

387

drilling results confirm that the coring sites of the GMGS3 and 4 expedition-obtained hydrates correspond well to the

388

position of the BSR with a strong amplitude and good continuity. There are often also amplitude abnormalities above

389

and below the BSR (Guo et al., 2017; Zhang et al., 2017; Zhang et al., 2018b; Qian et al., 2018).

390

Based on the interpretation of the seismic data, the two-way seismic travel time of the BSR near site W11 is

391

1965 ms, and the depth of the BSR is approximately 200 mbsf (Guo et al., 2017). The BSR distinctly crosscuts the

392

ambient bedding with good continuity and strong amplitude. In addition, a pronounced amplitude-blanking zone

393

occurs above the strong reflections obliquely connecting to the BSR, indicating the accumulation of gas hydrates or

394

free gas (Fig. 4). Several high-angle faults occur below site W11. Enhanced reflections (ERs) were observed near

395

these faults. The two-way seismic travel time of the BSR near site W17 is 1968 ms, and the depth of the BSR is

396

approximately 230 mbsf. The BSR is characterized by good continuity and strong amplitude. It extended for 20

397

approximately 2.5 km and obliquely crosscut the surrounding strata. Similar to site W11, the amplitude-blanking

398

zone above the BSR is also prominent at site W17, and the seismic events below the BSR exhibit pull-down features

399

and have an abnormal velocity-amplitude structure, indicating the accumulation of gas hydrates above the BSR and

400

free gas below the BSR (Fig. 5). Several near-vertical faults are interpreted to exist in the lower left part of site W17,

401

and ERs are also present in the vicinity of these faults. Based on the interpretation of the seismic data across site W18,

402

a distinct BSR with good continuity and strong amplitude was identified on the seismic profile. The two-way seismic

403

travel time of the identified BSR is 1898 ms, and the depth of the BSR is approximately 171 mbsf. The BSR crosscuts

404

the surrounding bedding. No amplitude-blanking zones are present above the BSR, although acoustic masking is

405

prominent below the BSR (Fig. 6). The BSR is clearly divided into two sections with strong amplitude features near

406

site W19, as in the seismic profile (Fig. 7). The two-way seismic travel time of the BSR is 1888 ms, and the depth of

407

the BSR is approximately 170 mbsf. In addition, the BSR distinctly and obliquely crosscuts the sedimentary strata.

408

Two gliding faults nearly connect with the BSR and extend to the seafloor at W19 site. No distinct seismic-blanking

409

zones occur above the BSR at site W19, and the seismic reflections below the BSR are fuzzy and chaotic (Fig. 7).

410

411

4.3 Characteristics of hydrate logging curves

412

Coring sites W11, W17, W18, and W19 in the GHPTR showed distinct indications of gas hydrates during the

413

LWD process, and the GHBZ corresponded well with the well logging curve anomalies, which have a high apparent

414

resistivity, high acoustic velocity, low acoustic travel-time difference, relatively low density, and low natural gamma

415

ray value (Fig. 8) (Yang et al., 2017b; Yang et al., 2017c; Yang et al., 2017d; Zhang et al., 2017; Qian et al., 2018).

416

The resistivity logging curve for site W11 reveals a general high-value anomaly at 112–201.6 mbsf, with a 21

417

thickness of approximately 90 m. There are four high-value abnormal layers (Fig. 8A) located at 112–130 mbsf,

418

132–138 mbsf, 141–153 mbsf, and 158–201 mbsf. The maximum abnormal value of resistivity is approximately 5.6

419

Ω.m. The natural gamma curve value is between 43.75 and 98.75 API; mean natural gamma between the seafloor and

420

112 mbsf is 61.25 API; and natural gamma below 112 mbsf is approximately 72.5 API, with little change. The

421

acoustic travel-time difference curve mirrors the resistivity curve for the abnormal layers. The acoustic travel-time

422

difference value decreases significantly when the resistivity value increases. The minimum value of the acoustic

423

travel-time difference is approximately 132.5 us/ft (approximately 2300 m/s) at a depth of 170 mbsf. The density

424

curve shows a high anomaly of 2.13 g/cm3 at 115.38 mbsf, and the corresponding neutron curve decreases, which is

425

most likely due to changes in lithology. The density at 127.4 mbsf is significantly lower, with a value of

426

approximately 1.37 g/cm3; whereas, the corresponding neutron porosity increases significantly, exhibiting anomalous

427

characteristics typical of the GHBZ. In summary, the hydrates are inferred to exist at depths of 112–130 mbsf,

428

132–138 mbsf, 141–153 mbsf, and 158–201 mbsf. Samples of gas hydrates were recovered from the four

429

corresponding layers during the coring (Figs. 9A, 11A–C).

430

The logging curves for site W19 (Fig. 8D), which are described by Zhang et al. (2017) and Yang et al. (2015,

431

2017c, 2017d), reveal that the resistivity curve increases sharply from 1 Ω.m to 8.67 Ω.m above 134 mbsf, then

432

decreases slowly, and finally significantly decreases at 156 mbsf. The resistivity curve starts to fluctuate between 156

433

mbsf and 202 mbsf. The corresponding acoustic travel-time difference curve also decreases significantly. In addition,

434

the shape of the acoustic travel-time difference curve is the mirror image of the resistivity curve. Thus, we speculate

435

that the layer between 134 and 202 mbsf is the GHBZ. In addition, the natural gamma curve for this layer exhibits an

436

increasing trend, and the minimum acoustic travel-time difference of this layer is 119.2 us/ft (2557 m/s). In addition,

22

437

the density curve does not exhibit a decreasing trend; however, the neutron porosity exhibits a decreasing trend.

438

The characteristics of the logging curves for the GHBZ at sites W17 (Fig. 8B) and W18 (Fig. 8C) are similar to

439

those of the curve for site W19, i.e., high resistivity values and low acoustic travel-time differences. Generally, the

440

density and compensated neutron curves have no distinct response characteristics within the possible GHBZ at any

441

site. However, the logging values in the GHBZ at each site are different. The resistivity values of sites W18 and W19

442

are significantly higher than those at sites W11 and W17. The resistivity values of sites W18 and W19 decrease

443

with depth within the abnormal logging zone, exhibiting an anti-rhythm feature. However, resistivity logs of sites

444

W11 and W17 exhibit fluctuation characters. This indicates that the hydrate saturation gradually decreases with depth

445

within the GHBZ at sites W18 and W19; whereas, the saturation of the gas hydrates may fluctuate within the

446

abnormal logging sections at sites W11 and W17.

447

448

4.4 Features of gas-hydrate host sediments

449

The lithology and grain size analysis of the core sediments in the GHPTR show that the gas-hydrate host

450

sediments in this area are mainly fine grained. Four major types of lithology (Zhang et al., 2017; Kang et al., 2018;

451

Li et al., 2018; Li et al., 2019a), viz., calcareous clayey silt, calcareous silt, calcareous and siliceous clayey silt, and

452

siliceous and calcareous clayey silt, were identified in the coring sites (Fig. 12).

453

The sediments recovered from coring sites W11 and W17 have low clay content and relatively high sand content

454

in the upper part of the core; however, the lower part of the core has the opposite composition. The total core clay

455

content, silt content, sand content, and median grain size (Mz) of site W11 range from 14.2% to 38.7%, 61.1% to

23

456

83.0%, 0.07% to 8.51%, and 6.12 to 7.91 Ф, respectively (Fig. 12A). The total core clay content, silt content, sand

457

content, and median grain size of site W17 range from 16.1% to 37.7%, 46.1% to 82.0%, 0.09% to 18.70%, and 6.17

458

to 7.88 Ф, respectively (Fig. 12B). At a depth of 50.1 m in the core, the sediment consists of 37.4% clay, 56.1% silt,

459

and 16.5% sand, which may be due to sliding of the upper continental slope or depositional filling of channels.

460

In contrast to the drilling sites W11 and W17, the results of the grain size analysis show that the clay content of

461

site W19 gradually decreases with depth, ranging from 17.2% to 44.2%; the silt content ranges from 55.6% to 80.1%;

462

and the sand content ranges from 0.04% to 10.9%. The median grain size ranges from 6.14 to 7.97 Ф (Fig. 12D).

463

The grain size of the sediments recovered from site W18 is similar to that of site W19. The total core clay, silt,

464

and sand contents range from 17.5% to 33.9%, 54.2% to 80.0%, and 1.11%‐to 23.8%, respectively. The grain size

465

ranges from 5.82 to 7.58 Ф (Fig. 12C). In addition, commonly distributed foraminifera fossils were found at the

466

coring sites in the GHPTR, particularly sites W18 and W19 where abundant foraminifera fossils were found at

467

~100-170 mbsf (Figs. 11K, 13). Foraminifera fossils are the main source of the carbonate composition of the

468

sediments and are positively correlated with the concentration of sand, indicating that the foraminifera fossil shells

469

are a major component of the sand-scale sediments. In addition, it is worth noting that the total abundance of the

470

foraminifera in the GHSZ has a good correspondence with the concentration of CaCO3 and the variation in the

471

reservoir porosities.

472

Based on the results of the well-logging interpretation (Table 1), the hydrate reservoirs are generally

473

characterized by a relatively low permeability and high porosity. The average effective porosities of the hydrate

474

layers at site W11, W17, W18, and W19 were 34.5%, 32.2%, 56.7%, and 48.3%, respectively. The corresponding

475

average permeabilities were 0.22 mD, 0.32 mD, 100 mD, and 5.5 mD.

24

476

477

4.5 Velocity structures and acoustic impedance inversion of drilling sites

478

The high-precision velocity profile across sites W11 and W17 (Fig. 14A) shows that the layer above the BSR is

479

characterized by a high velocity of 1800-2000 m/s. This high-velocity anomaly coincides with the strong reflection

480

amplitude observed above the BSR, and the acoustic travel-time difference and resistivity curve are characterized by

481

decreasing and increasing trends, respectively, indicating that high velocity is associated with the presence of gas

482

hydrates. Above the high-velocity anomaly zones, the velocity is 1600-1700 m/s, i.e., normal strata velocity

483

characteristics, and the acoustic travel-time difference and resistivity curves are not significantly changed, indicating

484

the absence of gas hydrates in sediments. In the layer below the BSR, a low velocity anomaly with 1400-1600 m/s is

485

observed, which is significantly lower than the velocity of the surrounding strata, strongly indicating the

486

accumulation of free gas in the strata.

487

High-velocity anomalies with 1700-1900 m/s also occur in the strata above the BSR at site W19. However, the

488

strata velocity anomaly above the BSR at site W18 is not distinct, and the velocity is 1600-1700 m/s, which is only

489

slightly higher than the velocity of the surrounding strata (Fig. 14B). Corresponding to the abnormal velocity region,

490

the acoustic wave time difference and resistivity exhibit decreasing and increasing trends, respectively. This

491

anomalous velocity region coincides with the high-amplitude reflection above the BSR, indicating the presence of

492

gas hydrates in the sediments. The strata over the high-velocity anomaly area generally exhibit relatively

493

low-velocity characteristics; therefore, there may be free gas accumulation. In addition, a local, relatively

494

high-velocity anomaly (~1700 m/s) occurs near the seafloor between sites W19 and W18. In the strata below the BSR,

495

the low-velocity anomaly zones are distinct, and the velocity of the strata is 1400-1500 m/s, which also strongly 25

496

indicate the entrapment of free gas. In addition, the low-velocity anomaly region exhibits a large scrotiform structural

497

feature on the velocity profile, extending from 1900 ms down to at least 2500 ms.

498

Based on the seismic profile and p-wave impedance inversion profile analysis (Fig. 15), the inversion profile

499

resolution was improved compared with that of the seismic profile. The seismic inversion wave impedance is

500

generally consistent with the variation trend in the borehole p-wave impedance curve, and the lateral variation in the

501

wave impedance is consistent with the seismic reflection characteristics. The red region above ~2000 ms in Figure 15

502

represents the relatively higher p-wave impedance (

503

variation in the blue region with a relatively lower impedance ( ~3000) generally reflects the lateral heterogeneity

504

of the free-gas reservoir. In addition, the GHBZ exhibits a layered distribution on the inversion profile, and the FGBZ

505

and the GHBZ exhibit a distinct wave impedance difference. Except for site W11, the FGBZ was identified below the

506

GHBZ at sites W17, W18, and W19, which is consistent with the distribution of the free gas and gas hydrates

507

reflected by the above-mentioned velocity inversion profiles (Fig. 15).

3800), corresponding to the GHBZ; whereas the lateral

508

509

4.6 Modeling results of gas-hydrate stability zone

510

The calculated BGHSZ of SI (100% CH4) gas hydrates at site W11 is ~194 mbsf (~1504 mbsl), which is close

511

to the BSR (~200 mbsf) and base of the LWD electrical resistivity anomaly (~202 mbsf). The calculated BGHSZ of

512

the SII gas hydrates (99% C1 + 0.5% C2 + 0.5% C3) is 220 mbsf (~1530 mbsl). Considering the calculation error,

513

the base of the SI gas hydrates should be located at around 200 mbsf at site W11 (Fig. 16A). The calculated

514

BGHSZ of the SI (100% CH4) gas hydrates at site W17 is ~239 mbsf (~1498 mbsl), which is close to the BSR

515

(~230 mbsf) but significantly shallower than the base of the LWD electrical resistivity anomaly (~270 mbsf). In 26

516

addition, the BGHSZ of the SII gas hydrates calculated using 99% CH4 + 0.5% C2H6 +0.5% C3H8 is ~270 mbsf

517

(1529 mbsl), which is consistent with the base of the LWD electrical resistivity anomaly (Fig. 16B). The calculated

518

BGHSZ of the SI (100% CH4) gas hydrates at site W18 is ~161 mbsf (~1446 mbsl), which is close to the BSR

519

(~170 mbsf) but shallower than the base of the LWD electrical resistivity anomaly (~174 mbsf). In addition, the

520

BGHSZ of the SII gas hydrates calculated using 99% CH4 + 0.5% C2H6 + 0.5% C3H8 is ~189 mbsf (~1474 mbsl),

521

which is deeper than the base of the LWD electrical resistivity anomaly (Fig. 16C). The calculated BGHSZ of the

522

SI (100% CH4) gas hydrates at site W19 is ~165 mbsf (~1439 mbsl), which is close to the BSR (~170 mbsf) but

523

significantly shallower than the base of the LWD electrical resistivity anomaly (~185 mbsf). In addition, the

524

BGHSZ of the SII gas hydrates calculated using 99% CH4 + 0.5% C2H6 +0.5% C3H8 is ~192 mbsf (~1466 mbsl),

525

which is very close to the base of the LWD electrical resistivity anomaly (Fig. 16D). In addition, at each site, the

526

BGHSZ calculated using 95% CH4 + 2.5% C2H6 + 2.5% C3H8 is generally deeper than the BSR and BGHSZ

527

calculated using 99% CH4 + 0.5% C2H6 + 0.5% C3H8 (Fig. 16, Table 1).

528

529

5. Discussion

530

As mentioned already, gas hydrates with a high saturation of up to 76% were recovered in the fine-grained host

531

sediments of the GHPTR in the Shenhu area, which revealed the widespread distribution of distinct BSR and

532

amplitude anomalies including ERs, acoustic blanking, masking, and chaotic. All the gas hydrates encountered were

533

concluded to be of a fine, disseminated nature and were hosted in the clayey silt dominated sediments. The

534

gas-hydrate accumulations sampled were relatively thick and uniform compared with those of other sites worldwide.

535

For example, site W11 contains a remarkable zone of continuous gas hydrates over 90 m thick. In addition, the FGBZ 27

536

was identified below the GHSZ, indicating that the Shenhu area contains a gas-hydrate petroleum system with an

537

abundant hydrocarbon supply. Free gas was also present at some of the sites, and gas-hydrate formation was observed

538

on and around the seabed frame during drilling near the base of the resistivity anomalies (Yang et al., 2015). In

539

addition, the increased pore-water salinity above the top of the GHBZ observed at sites W18 and W19 (Figs. 16C, D)

540

indicates that these adjacent locations are part of a currently active system. Gas hydrates may be forming in these

541

sediments, with the resulting exclusion of saline fluids. In addition, both biogenic and thermogenic gases were

542

discovered in the hydrate gases. The depth of the calculated BGHSZ differs from the depth of the BSR and logging

543

anomalous sections, indicating the presence of gas hydrates below the BSR and even the coexistence of SI and SII gas

544

hydrates in the GHPTR. In summary, the GHPTR in the Shenhu area exhibits some fascinating and apparently unique

545

characteristics, which are of significance to investigating the accumulation mechanism of gas hydrates in the SCS.

546 547

5.1 Indications for gas-hydrate accumulation and hydrocarbon migration

548

As discussed above, seismic reflection anomalies, including high-amplitude reflections, acoustic blanking,

549

masking, and chaotic reflections occur above and below the BSR within the GHPTR. However, using the seismic

550

reflection to determine the geological implications of these reflection anomalies and identifying and distinguishing

551

hydrate layers and free gas are indeed challenging. However, relying solely on identifying the BSR to identify gas

552

hydrates may not be accurate. A recent development in gas-hydrate exploration is to use the event amplitude as well

553

as polarity (Boswell et al., 2016). Therefore, we identified and distinguished the hydrate distribution by making

554

synthetic seismograms to determine the top and bottom of the GHBZ (Figs. 17, 18). Based on the multi-well

555

calibration results, the GHBZ has a high resistivity, low acoustic travel-time difference, high impedance on the log

28

556

curves, and strong reflection characteristics on the seismic profiles. In addition, the strong amplitude reflection

557

characteristics of the two marking layers at the mud line of the seabed and at the top of the GHBZ are distinct (Figs.

558

17, 18); however, the reflection characteristics of the bottom interface of the GHBZ are different at each site. Some of

559

the sites exhibit a wave trough with strong amplitude characteristics, such as site W11 (Fig. 17). Other sites exhibit a

560

wave trough with low-amplitude characteristics or zero phase, such as site W18 (Fig. 18). These different reflection

561

characteristics are related to the thickness of the gas hydrates and physical properties of the host sediments.

562

The GHBZ exhibits two kinds of characteristics on the seismic profile. The first is a well with a thick single

563

layer (~10 m), which is characterized by obvious strong reflection characteristics. The top boundary of the

564

gas-hydrate layer is a strong reflection wave crest, and the bottom boundary is a strong reflection wave trough. In

565

addition, the inside of the GHBZ is characterized by strong amplitude, which is in sharp contrast to the relatively

566

weak amplitude outside the GHBZ. Site W11 on the eastern ridge is the most typical. The second is a well with a

567

single thin gas-hydrate layer. The GHBZ has no obvious reflection characteristics on the seismic section, and the

568

main performance is a weak wave crest reflection, which is typical of site W18 on the western ridge. The bottom

569

surface of the GHBZ, i.e., the BSR, is a strong wave trough reflection with distinct strata cross-cutting features,

570

which is clear throughout the GHPTR, and is consistent with the bottom surface of the GHBZ calibrated using the

571

drilling wells. Therefore, combined with the drilling data analysis, the gas-hydrate distribution within the GHPTR

572

can be qualitatively determined through the analysis of the polarity and reflection intensity of seismic reflections.

573

Significant acoustic blanking reflections often occur within the GHSZ. These are generally interpreted as the

574

presence of gas hydrates, accumulation of free gas, activity of fluid, and so on (Wu et al., 2005; Fraser et al., 2016;

575

Fraser, 2017; Wang and Pan, 2017; Behboudi et al., 2018). Distinct acoustic blanking zones were also identified

29

576

overlying the strong seismic reflections of the GHBZ at sites W11 and W17. The corresponding logging results show

577

that the resistivity of the blanking zone is very flat, there is no increase or decrease with depth, and the acoustic

578

travel-time difference log only decreases slightly (Fig. 14A). In addition, the seismic velocity profile shows that the

579

acoustic velocity of the blanking zone is approximately 1600-1800 m/s. This is not significantly different from the

580

normal sediment velocity; however, it is significantly different from the velocity of the strata containing an

581

accumulation of high saturation gas hydrates (Fig. 14A). Therefore, it is concluded that the acoustic blanking zones

582

overlying the strong reflections of the GHBZ at sites W11 and W17 do not indicate the accumulation of gas hydrates

583

or free gas; they rather indicate normal lithologically uniform fine-grained deposits.

584

Inclined strong, continuous events just below and connecting to the BSR were identified at sites W11, W17,

585

W18, and W19. These high amplitude reflections are interpreted to indicate the presence of free gas (Wang et al.,

586

2014; Li et al., 2018) (Figs. 3–7). Weak amplitude widespread acoustic blanking or chaotic reflections were found

587

below these strong reflections, indicating active gas-bearing fluid activity below the BSR. Generally, the impedance

588

contrast of free gas differs significantly from that of the overlying hydrate layers and the surrounding sedimentary

589

strata (Fig. 15). The accumulation of free gas is also controlled by the distribution characteristics of the strata;

590

therefore, free gas accumulation usually presents inclined multi-layered reflections with high-amplitude

591

characteristics. From the seismic reflection characteristics of the free gas in the GHPTR, the free-gas reservoirs of

592

all of the coring sites show strong reflection characteristics under the BSR (Figs. 3–7). The strong reflections can

593

be divided into two categories. The first is the strong reflection characteristics represented by sites W11 and W17,

594

which have extremely high seismic amplitudes and prominent high-frequency layered reflections under the BSR. In

595

addition, the top and bottom interfaces of the probable free-gas reservoirs are also clear (Fig. 14A). The second is

596

the strong reflection features represented by sites W18 and W19, which show that the adjacent reflections under the 30

597

BSR have high seismic amplitudes, but the amplitude gradually decreases downward and even changes into chaotic

598

and blanking reflections, indicating low saturation free gas within the reservoirs (Fig. 14B).

599

Sites W11 and W17 are located in the same geological structural position in the GHPTR. The free-gas

600

reservoirs under the BSR both exhibit strong reflections (Fig. 14A). However, the strong reflection has a finite

601

extent. Gas reservoirs cause multiple strong reflection wave groups, which have the same tendency as the strata but

602

distinctly crosscut the BSR and are different from those around sites W18 and W19 (Figs. 6, 7). In addition, the

603

strong reflection of the free-gas reservoir at site W17 rapidly weakens and terminates downward, forming a

604

prominent obliquely cross-cutting reflection feature, which is interpreted to be the bottom of the free-gas reservoir

605

(Fig. 14A). Coring sites W18 and W19 are located in similar geological structures. The free-gas reservoirs are well

606

developed below the hydrate layers, and their seismic reflection characteristics are basically the same. The top of

607

the free-gas reservoir exhibits strong reflection wave groups, whereas the strong reflection gradually transforms

608

into a weak reflection downward, which is more chaotic and similar to the reflection characteristics of the

609

surrounding strata (Fig. 14B). The strong reflections of the free-gas reservoir at site W19 are more prominent than

610

those at site W18, which is presumably related to the relatively higher structural position of site W19 and more free

611

gas accumulation.

612

5.2 Coexistence of gas hydrates and associated free gas

613

5.2.1 Low seismic velocity and p-wave impedance of free gas

614

Free gas is generally characterized by low-velocity anomalies on seismic velocity profiles (Fohrmann and

615

Pecher, 2012; Cordero et al., 2016). The low-medium velocity zones coincide with the strong reflection amplitude

616

below the GHBZ and exhibit a wide extension at different depths in the GHPTR (Fig. 14). As can be seen from 31

617

Figure 14B, there are low-velocity anomalies (blue zone) under the BSR at sites W18 and W19 with scrotiform

618

features, which are large scale and transversely continuous. The low-velocity zones are mainly concentrated in the

619

high part of the structure, and the top of the anomalous velocity area coincides with the strong seismic reflections.

620

In addition, they generally extend to a significant depth and exhibit a string-of-beads shape in the longitudinal

621

direction. We suggest that the deep part is related to the low-velocity anomaly formed by ongoing gas migrating

622

and charging, which is closely related to hydrocarbons transported from the deep source rocks. The low-velocity

623

anomaly at site W18 is more prominent, indicating a more developed free-gas reservoir. In addition, the bottom of

624

the low-velocity anomalous areas at sites W18 and W19 is relatively clear, which most likely indicates the bottom

625

of the free-gas reservoir.

626

There is no distinct low-velocity anomaly below the BSR at site W11 (Fig. 14A). The strata velocity is similar

627

to that of the surrounding sediments; therefore, there is no free-gas reservoir just below the GHSZ. However, a

628

local low-velocity anomaly is present to the south of site W11; however, the extent and thickness of the anomaly

629

are smaller than those at sites W18 and W19, and the scale of the free-gas reservoir is inferred to be limited. A

630

low-velocity anomaly area is also present beneath the BSR at site W17 (Fig. 14A), exhibiting a nearly horizontal

631

scrotiform distribution. The degree and extent of the low-velocity anomaly is also weaker and smaller than those at

632

sites W18 and W19. In addition, the low-velocity anomaly is confined to the vicinity of site W17 and generally

633

coincides with the strong seismic reflection. The bottom of the low-velocity zone is clear, which basically

634

corresponds with the bottom of the free-gas layer on seismic reflection. In addition, there is no low-velocity

635

anomaly in the deep part of sites W11 and W17, indicating that the distributions and scales of the free-gas

636

reservoirs are limited. The contribution of the hydrocarbons that migrated from the deep Paleogene source rocks to

32

637

the free-gas reservoirs below the GHBZ may be very limited or may even have ceased, and the accumulation of

638

free gas is inferred to be the result of the dissociation of gas hydrates at sites W11 and W17. Further study is needed

639

to verify this speculation.

640

The p-wave impedance inversion profile (Fig. 15) also clearly shows the distribution of the gas hydrates and

641

the underlying FGBZ in both submarine ridges. Consistent with the velocity inversion results, the distribution of

642

free gas in the lower part of the western ridge hydrates is significantly larger than that in the eastern ridge. However,

643

the thickness of the GHBZ in the western ridge is significantly smaller than that in the eastern ridge. Therefore, it is

644

speculated that, in addition to the supply of sufficient hydrate gas, other controlling factors such as the thickness

645

and physical properties of the reservoirs may control the thickness and distribution of the hydrates in the GHPTR.

646

5.2.2 Down-hole logging indications of gas hydrates and free gas

647

Using the acoustic travel-time difference, density, and compensation neutron three porosity log intersection

648

method, the GHBZ and FGBZ in the GHPTR were identified (Fig. 19). From the analysis results, a consistent feature

649

in each well is that the density and compensated neutron logs have no obvious corresponding features in the GHBZ.

650

However, the GHBZ is present in each well even when the adjacent wells have different characteristics on the

651

acoustic travel-time difference log, reflecting the complexity of the hydrate distribution in the GHPTR.

652

Except for site W11, FGBZs were identified below the GHBZ at sites W17, W18, and W19 (Fig. 19). The

653

free-gas layers exhibit different logging response characteristics in the different wells. The thickness of the FGBZ in

654

well W17 is small, but the electrical response is significant, thereby exhibiting relatively higher resistivity values and

655

rapidly decreasing acoustic travel-time differences, density, and neutrons. The FGBZs of sites W18 and W19 are

656

relatively thicker, especially at site W18 (~50 m thick); however, the electrical response is relatively less significant. 33

657

Compared with the upper and lower surrounding rocks, the resistivity log value slightly increases; whereas the three

658

porosity log decreases slightly.

659

By combining the seismic and velocity superimposed profiles with the impedance inversion profiles of the

660

coring sites within the GHPTR (Figs. 14, 15), we conclude that gas hydrates and free gas coexist in the GHPTR (Qian

661

et al., 2018), and the free gas reservoirs in the eastern ridge are most developed at site W17 and its northern part, and

662

rapidly decrease to site W11 until they disappear. In addition, compared with the western ridge area where sites W18

663

and W19 are located, the extent of the low-velocity anomaly in the eastern ridge is limited and the thickness of the

664

free gas reservoirs is significantly smaller. Therefore, the hydrate gas supply in the western ridge is more abundant

665

than that in the eastern ridge, and the contribution of deep thermogenic gas to gas-hydrate formation in the western

666

ridge may be greater than that to the eastern ridge, which is reflected by the hydrate gas compositions and isotopic

667

data (Zhang et al., 2019).

668

669

5.3 Hydrocarbon migration difference between eastern and western ridges

670

The formation of gas hydrates requires sufficient natural gases. In addition to in situ biogenic gas, thermogenic

671

gas, which is transported from the deep source kitchens to the shallow GHSZ along different migration pathways, is

672

also important. The types, distribution, and development characteristics of the hydrocarbon migration pathways

673

play a vital role in the accumulation of hydrates (Gorman et al., 2002; Wang et al., 2014; Satyavani et al., 2015).

674

Previous studies of the GMGS1 expedition demonstrated that the gas migration pathways are primarily gas

675

chimneys, which play an important role in the migration of hydrocarbons and accumulation of gas hydrate in the

676

drilling area (Su et al., 2014a; Qiao et al., 2014; Su et al., 2017; Zhang et al., 2018a). Based on seismic 34

677

interpretation, multiple hydrocarbon migration pathways, including mud diapirs, gas chimneys, high-angle faults,

678

and gliding faults, were identified within the GHPTR (Zhang et al., 2017; Zhang et al., 2018b). However, the types

679

and geological characteristics of the hydrocarbon migration pathways vary within the GHPTR, resulting in different

680

hydrocarbon migration efficiencies for the eastern and western ridges.

681

Similar to the GMGS1 drilling area, mud diapirs and gas chimneys acting as favorable pathways for gas

682

migration from depth to the shallow GHSZ were identified below the drilling and coring sites in the GHPTR (Su et

683

al., 2016; Zhang et al., 2018a; Zhang et al., 2018b). On the eastern ridge, acoustic blanking and masking zones

684

occur below the BSR at sites W11 and W17 (Fig. 20A), and several seismic events on both sides seem to terminate

685

at the edge of that amplitude-anomaly zone. The interior seismic reflection is chaotic. In the lower part, the seismic

686

events show pull-up features, which are likely the result of the upward migration of mud during mud diapirism. In

687

the upper part, pull-down features are seen on the seismic profile, which is likely caused by the trapping of free gas

688

that migrated upwards from the deep source rocks through the pathways provided by the gas chimneys and mud

689

diapirs (Løseth et al., 2009; Hustoft et al., 2010; Nakajima et al., 2014; Cartwright and Santamarina, 2015). ERs

690

commonly occur on both sides and in the interior of the chaotic reflection zone, indicating the trapping of free gas

691

in the sediments (Judd and Hovland, 1992; Heggland et al., 1997; Andreassen et al., 2007). In addition, by mapping

692

the instantaneous frequency image, the chaotic reflection zones show distinct low-frequency characteristics

693

compared with those of the surrounding sediments and strata (Fig. 20B), further demonstrating the result of gas

694

migrating and filling in the upper low-pressure strata through the mud diapir and gas chimney (Zhang et al., 2018b).

695

Mud diapirs were also identified in the deep strata (Fig. 20C) at sites W18 and W19 on the western ridge. They

696

result in anticline structures and are speculated to be the result of the upward movement of mud. Furthermore,

35

697

large-scale low instantaneous frequency features appear on the instantaneous frequency profile (Fig. 20D),

698

indicating the intense migration and charging processes of hydrocarbons below sites W18 and W19. By comparing

699

the mud diapirs developed below the two ridges, we conclude that the scale of the mud diapir under the western

700

ridge is significantly larger than that under the eastern ridge (Fig. 20). In addition, by comparing the extent of the

701

chaotic reflections and the low instantaneous frequency between the coring sites, we conclude that the hydrocarbon

702

migration and charging activities at sites W18 and W19 on the western ridge are stronger than those at sites W11

703

and W17 on the eastern ridge. These observations correspond well with the distribution and enrichment features of

704

the free gas within the two ridges, which is reflected by the relatively low velocity and the low instantaneous

705

frequency zones (Figs. 14, 20).

706

As stated above, high-angle faults were identified below sites W11 and W17 in the GHPTR. Some of the

707

nearly vertical faults penetrate the deep Paleogene source rocks and extend upward to the BSR. Bright spots with

708

an enhanced amplitude are generally observed near these faults (Figs. 3–5), indicating that deep-seated gas has

709

migrated along the faults and was captured by the adjacent reservoirs (Su et al., 2016; Zhang et al., 2018b).

710

Therefore, these high-angle faults act as important migration pathways and supply the hydrate accumulation with

711

sufficient gases, particularly the thermogenic gas at sites W11 and W17. In contrast, no large-scale high-angle faults

712

connect the deep source rocks below sites W18 and W19. However, small faults and fractures, which are hard to

713

identify due to the chaotic and acoustic masking seismic features, may be present below these drilling sites (Figs. 3,

714

6, 7). There are gas hydrates sourced from the lateral migration of hydrocarbons at sites W18 and W19 (Su et al.,

715

2014a; Su et al., 2017; Zhang et al., 2018b). The most apparent are the gliding faults and sliding surfaces resulted

716

from submarine slumps, which constitute lateral migration pathways for shallow gases (Zhang et al., 2018b). The

36

717

distribution areas of the BSR generally coincide with the sliding surface at sites W18 and W19 (Figs. 6, 7), and

718

continuous reflections with strong amplitudes generally occur along these sliding surfaces, indicating that the gases

719

migrate laterally along the sliding surfaces toward the incline direction of the strata. From the seismic profile across

720

sites W18 and W19 (Figs. 3, 6, 7), the strong reflection features of the BSR show a clear cross-cutting relationship

721

with the strata. Several slump faults even crosscut the BSR and extend to the seafloor. We suggest that these faults

722

act as pathways for the migration of gas near the BSR and its further migration into the GHSZ, which would

723

increase the saturation and distribution range of gas hydrates (Su et al., 2014a; Su et al., 2017; Zhang et al., 2018b).

724

Unlike the western ridge, there was no lateral migration of hydrocarbons observed within the GHSZ at sites W11

725

and W17 on the eastern ridge. As a consequence, significantly more hydrocarbon can accumulate within the GHSZ

726

in the western ridge than in the eastern ridge. The distribution range of the gas hydrates in the western ridge is

727

larger than that in the eastern ridge. This is most likely the reason why the length of the BSR in the western ridge is

728

significantly longer than that in the eastern ridge (Figs. 3–7). Actually, the distribution area of the hydrate orebody

729

controlled by wells W18 and W19 (11.24 km2) is significantly larger than that controlled by wells W11 and W17

730

(6.42 km2) (Zhang et al., 2018c), which also suggests that the scope of the gas supply in the western ridge is larger

731

than that in the eastern ridge. Therefore, we conclude that there are differences in the natural gas migration and

732

accumulation between the eastern ridge and the western ridge due to the differential development of hydrocarbon

733

migration pathways. The efficiency of gas migration in the western ridge is higher than that in the eastern ridge,

734

resulting in differences in the gas migration flux and gas-hydrate accumulation.

735

37

736

5.4 Inconsistent distribution relationship among BSR, BGHSZ, and gas hydrates

737

In general, the BSR represents the distribution base of the SI gas hydrates, which is sourced by 100% methane.

738

However, as discussed above, there is a difference between the BSR depth identified from the seismic profile and the

739

BGHSZ calculated using the phase equilibrium curve for the GHPTR in the Shenhu area. The depths of the BSR and

740

the calculated BGHSZ are generally shallower than the base of the LWD electrical resistivity anomaly (Table 1, Fig.

741

16). It is proposed that this difference is closely related to the accumulation of thermogenic gas hydrates in the

742

GHPTR (Liang et al., 2017; Wei et al., 2018; Zhang et al., 2019). The evidence of the coexistence of SI and SII gas

743

hydrates has been reported in the Shenhu area by previous researchers. Liang et al. (2017) found gas hydrates below

744

the BSR and suggested that deep thermogenic fluid locally entrapped within the shallow-buried sediments may

745

reinforce the SII gas-hydrate accumulation. Based on the composition of hydrate gas, Zhang et al. (2017, 2019) also

746

suggested that both SI and SII gas hydrates are present in the Shenhu area due to a thermogenic gas supply for the

747

accumulation of the gas hydrates. Qian et al. (2018) provided log evidence for the coexistence of SII gas hydrates and

748

free gas below the BSR and calculated the distribution depth of the SII gas hydrates below the SI gas hydrates based

749

on the logging data at site W17 in the Shenhu area. Wei et al. (2018) discovered direct evidence for the occurrence of

750

SII gas hydrates using laser Raman spectroscopy and log data for the Shenhu area. In addition, the evidence of SII gas

751

hydrates below the BSR has recently been found in the eastern part of the Qiongdongnan Basin, which is adjacent to

752

the Shenhu area (Ye et al., 2019; Wei et al., 2019). SII gas hydrates located below the BSR have also been discovered

753

in other places around the world, such as the convergent margin offshore of NW Borneo (Paganoni et al., 2016).

754

The above-mentioned occurrence and distribution characteristics of gas hydrates indicate that they are not only

755

always distributed above the BSR but also present under the BSR based on seismic data. There are distinct logging 38

756

anomalies indicating the presence of gas hydrates below the BSR (Liang et al., 2017; Qian et al., 2018).

757

Furthermore, hydrate-derived cores and pore-water freshening have also been found below the BSR at sites W17,

758

W18, and W19 (Yang et al., 2017d; Guo et al., 2017) (Fig. 16), indicating that the BSR is not the BGHSZ and that

759

SI and SII gas hydrates likely coexist within the GHPTR in the Shenhu area.

760

The calculated BGHSZ of the SI (100% CH4) gas hydrates at sites W11 indicates that the base of the SI gas

761

hydrates should be around 200 mbsf, which is close to the BSR (~200 mbsf) and base of the LWD electrical

762

resistivity anomaly (~202 mbsf). There is no evidence of gas hydrates in the two cores below 200 mbsf, and there is

763

no pore-water freshening anomaly below the BSR (Fig. 16A). Therefore, no SII gas hydrates are present below the

764

BSR or base of SI GHSZ at site W11.

765

The calculated BGHSZs of the SI (100% CH4) gas hydrates at sites W17 and W18 are close to the BSR but are

766

significantly shallower than the base of the LWD electrical resistivity anomaly (Table 1, Figs. 16B, C). There is

767

clear evidence for gas hydrates in the core and the pore-water freshening and supporting evidence from the core

768

thermal anomalies below the BSR and the LWD electrical resistivity anomaly (Figs. 16B, C) (Yang et al., 2015).

769

Hence, there is strong evidence for the presence of SII gas hydrates below the BGHSZ of the SI gas hydrates at

770

sites W17 and W18.

771

The calculated BGHSZ of the SI (100% CH4) gas hydrates at site W19 is close to the BSR (~170 mbsf) but is

772

significantly shallower than the base of the LWD electrical resistivity anomaly (~185 mbsf) (Fig. 16D). The

773

calculated BGHSZ of the SII gas hydrates (99% CH4 + 0.5% C2H6 + 0.5% C3H8) is very close to the base of the

774

LWD electrical resistivity anomaly. It is speculated that SII gas hydrates are present below the BSR based on the

775

LWD anomaly, hydrate gas composition, and isotope information acquired from site W19 (Zhang et al., 2017, 39

776

Zhang et al., 2019). However, all the cores were shallower than 170 mbsf; therefore, no pore-water data is available

777

below this depth, and it is not known whether SII gas hydrates are actually present below the base of the SI gas

778

hydrates (Fig. 16D).

779

As discussed above, there is a dual supply of biogenic and thermogenic gases in the hydrate gas source in the

780

GHPTR in the Shenhu area. Owing to the supply of thermogenic gas, C2+ gas may enter the hydrate cages, which may

781

result in the coexistence of SI and SII gas hydrates. In addition, under the same temperature and pressure conditions,

782

the distribution base of the SII gas hydrates will expand downward, eventually causing the distribution of the SII gas

783

hydrates below the BSR (Zhang et al., 2019). The fact that the GHSZ calculated using 100% methane is inconsistent

784

with the depth of the BSR may be due to the presence of SII gas hydrates above the BSR, i.e., the SI gas hydrates

785

coexist with the SII gas hydrates in the GHPTR (Liang et al., 2017; Qian et al., 2018; Wei et al., 2018; Zhang et al.,

786

2019).

787

The discovery of SII gas hydrates in the GHPTR further confirms the contribution of deep thermogenic gas to

788

gas-hydrate accumulation, and it also confirms that the above described gas migration pathways can act as a bridge

789

between the natural gas from the Paleogene source rocks and the shallow GHSZ. In addition, the BSR is not

790

indicative of the BGHSZ but may instead be regarded as a transitional indicator of SI and SII gas hydrates in the

791

GHPTR. The distribution depth of the SII gas hydrates is deeper than the base of the SI gas hydrates, and the resource

792

amount of the gas hydrates is larger than the original estimation for the GHPTR. More deep drilling and coring below

793

the BSR are needed to characterize the gas-hydrate petroleum system and reevaluate the resource potential of the

794

Shenhu area.

795 40

796

5.5 Impact of reservoir characteristics on gas-hydrate accumulation

797

Based on the LWD data and gas-hydrate saturation calculation results (Figs. 8, 16, 19), there are variations in

798

the distribution and saturation of gas hydrates in the different layers of the same site. However, there are no

799

significant differences in the parameter characteristics of the lithologies of the gas-hydrate-bearing reservoirs

800

(Figure 12). The host sediments in the four cored holes are mainly calcareous clayey silt and calcareous silt, and the

801

hydrate reservoir is generally fine grained. The grain-size analysis results show that the grain sizes of the western

802

and eastern ridges differ, and the core sediments recovered from sites W18 and W19 have a relatively high clay

803

content; whereas, the core sediments obtained from sites W11 and W17 have a relatively high silt content (Fig. 12).

804

When comparing the sediment samples from the GHBZ with sediment samples from gas-hydrate-free zones, their

805

geometric parameters, viz., median grain size, sorting, skewness, and kurtosis, and main mineral compositions

806

(except for calcium carbonate) do not show significant variations in the GMGS4 coring sites (Li et al., 2019a).

807

However, gas hydrates with high saturation were recovered from all the coring sites during the GMGS 3 and 4

808

expeditions. Thus, the lithology may not be the factor controlling the differential plane accumulation of gas

809

hydrates in the different drilling and coring sites within the GHPTR (Zhang et al., 2017; Li et al., 2019a).

810

Nevertheless, the vertical distribution of the gas hydrates in the same site may be impacted and controlled by the

811

physical properties of the host sediments. Based on the physical property parameters of the reservoirs in the coring

812

sites with high-saturation hydrates, the hydrate reservoirs have a relatively high porosity of 33.2-56.7%. It is

813

necessary to explain why reservoirs mainly consisting of fine-grained sediments have such a relatively high

814

porosity. It is proposed that the sediments of the hydrate reservoirs are shallowly buried, and some pore spaces are

815

retained owing to the structure of the skeleton particles because they are less affected by sedimentary compaction

41

816

(Zhang et al., 2017). There are also a significant number of foraminifera fossils in the sediments acquired from sites

817

W18 and W19 (Figs. 11J, 13). Both the grain size of the fine-grained sediments and the roundness of the sediment

818

particles is higher, providing more space for the formation and accumulation of hydrates, and ultimately, a higher

819

saturation of the hydrates formed (Chen et al., 2009; Kraemer et al., 2000; Chen et al., 2013; Zhang et al., 2017; Li

820

et al., 2019a). We found that the low gamma ray response recorded by the LWD at sites W18 and W19 was actually

821

caused by the high calcareous content and the abundance of sand-sized forams. This observation is consistent with

822

the findings from GMGS1 in 2007, which also found rich gas hydrates in the foram-rich silt clays of nearby

823

locations (Chen et al., 2013; Wang et al., 2014). The ventricular structure of the foraminiferal fossil shells

824

constitutes a rich micropore space in the sediments, which increases the porosity of the hydrate reservoir and

825

provides favorable conditions for the formation of hydrates. However, it must be pointed out that there is

826

necessarily no correlation between the high abundance of foraminifera and gas hydrates with high saturation. While

827

there is a significantly high large calcareous component in the GHBZ at sites W18 and W19, a high abundance of

828

foraminifera and large calcareous component are also present above the GHBZ (Fig. 13). There is no such

829

indication of high calcareous contents at sites W11 and W17 where gas hydrates with saturation of up to 76% have

830

been recovered, indicating that the presence of calcareous material is not a prerequisite for pore-filling gas-hydrate

831

occurrence in fine-grained sediments at these locations in the GHPTR. The reservoir permeability shows a

832

significant variation, ranging from 0.22-100 mD, with depth at coring sites W11, W17, W18, and W19 in the

833

GHPTR (Table 1). Therefore, it is concluded that the variations in the physical properties of the host sediments,

834

rather than their lithologies, may lead to variations in gas-hydrate saturation between the different layers of the

835

different drilling wells or coring sites.

42

836

In terms of gas-hydrate production, the gas hydrates that have been successfully exploited around the world

837

have relatively high saturations (generally exceeding 60%) and predominately accumulate in coarse-grained

838

reservoirs with high porosities and permeabilities. The properties of the hydrate reservoirs in the Shenhu area are

839

significantly different from those discovered in the United States, Canada, and Japan. The lithologies of the hydrate

840

reservoirs in Canada and the United States are a glutenite type (Dallimore and Collett, 2005; Schoderbek and

841

Boswell, 2011). The hydrate reservoirs in Japan are generally of the coarse sand type (Fujii et al., 2015). The

842

lithology of the hydrate reservoir in the Shenhu area is fine grained and is dominated by clayey silt. Furthermore,

843

the reservoir permeability of the coring sites of the GMGS3 expedition is generally very low and varies

844

significantly. The coring sites of the GMGS4 expedition have permeabilities of 2-40 mD inside and outside the

845

GHBZ (Yang et al., 2017d); therefore, the permeabilities of these reservoirs are worse than those of other hydrate

846

reservoirs across the world (Li et al., 2018; Zhang et al., 2018c). However, owing to the presence of foraminifera,

847

the hydrate reservoir space is larger than expected from the lithology, and the effective matching of various types of

848

gas migration pathways and sufficient gas supply in the drilling area has led to the formation of a wide distribution

849

of high saturation gas hydrates in a fine-grained reservoirs. Furthermore, a successful production test of gas-hydrate

850

production was performed in the Shenhu area. It lasted for two months, with a mean daily yield of 5151 m3 in the

851

GHPTR (Li et al., 2018). This demonstrates that the gas hydrates accumulated in the fine-grained clayey silt can be

852

exploited, and the poor permeabilities of the hydrate reservoirs may not be a limiting factor affecting gas

853

production compared with the hydrate production test reservoirs with relatively high permeabilities but low gas

854

yields (Chen et al., 2018; Yamamoto et al., 2019).

855

43

856

5.6 Optimization of gas-hydrate drilling and production sites

857

In addition to the high porosities and permeabilities of hydrate reservoirs, which are crucial for hydrate

858

production, gas hydrates with high saturations and large thicknesses are preferred for hydrate exploitation because

859

they have larger quantities of gas resources within a given area. Thus, a method for discovering hydrates with large

860

thicknesses and high saturations is of significance worldwide (Dallimore et al., 2005; Anderson et al., 2011; Ito et

861

al., 2015). The above-discussed geological and geophysical characteristics of the drilling sites and the related

862

hydrate geochemistry in the study area demonstrate that the distribution of gas hydrates varies across sites and with

863

depth at the same site. Hence, identification of factors controlling the differential distribution of hydrates is the key

864

to the future exploration of gas hydrates with large thicknesses and high saturations in the Shenhu area, and even

865

throughout the SCS in the future. The concept of a gas-hydrate petroleum system (Collett et al., 2009) is an

866

effective way of characterizing and understanding gas-hydrate accumulation and has successfully aided the

867

exploration of gas hydrates around the world, such as in the Gulf of Mexico and offshore of India (Collett et al.,

868

2012; Collett et al., 2014). Although the gas-hydrate accumulations in the GHPTR are different from those in the

869

main hydrate occurrence areas, the concept of a gas-hydrate petroleum system can be applied to the exploration of

870

gas hydrates with high saturations and large thicknesses in the fine-grained sediments of the GHPTR (Wang et al.,

871

2014; Su et al., 2016; Wang et al., 2018). As discussed above, the gas supply, hydrocarbon migration pathways, and

872

reservoir properties are the three key factors that control the migration and accumulation of gas hydrates in the

873

Shenhu area.

874

Conventional oil and gas exploration has demonstrated that the Baiyun Sag is a huge hydrocarbon-generating

875

sag (Zhu et al., 2008; Zhu et al., 2009; Zhu et al., 2012). Whether of deep thermogenic gas or shallow biogenic gas, 44

876

the gas source is sufficient and can provide sufficient supply for hydrate formation in the study area (Fu et al., 2010;

877

He et al., 2013; Liang et al., 2014; Su et al., 2014b; Zhang et al., 2014; Zhang et al., 2019). However, as shown at

878

the GMGS3 and 4 coring sites, a sufficient hydrocarbon supply and coexistence of gas hydrates and associated free

879

gas may be preferred to the accumulation of high saturation gas hydrates. The gas-bearing fluid activities around

880

the coring sites may result in variable distribution of hydrates in the GHPTR. The presence of free gas below the

881

BSR indicates that the GHPTR in the Shenhu area is a complex hydrate accumulation system with disseminated gas

882

hydrates and free gas, which may be in a state of dynamic equilibrium due to the dual supply of sufficient biogenic

883

and thermogenic gases.

884

Drilling and coring results have shown that the GMGS3 and 4 coring sites with high-saturation hydrates are

885

mainly located in the down-dip positions along the canyon ridges (Fig. 1B). Compared with other sites with a

886

relatively low saturation, the sites with high saturation are located at the inclined end of the submarine canyons and

887

are significantly closer to the center of the Baiyun Sag (Yang et al., 2017a; Yang et al., 2017c; Yang et al., 2017d).

888

The geochemical data for the above coring sites indicate that the hydrate gas is mainly mixed gas, and thermogenic

889

gas also makes an important contribution (Zhang et al., 2019). However, the hydrate saturation of most of the sites

890

located in the upper and middle sections of the submarine ridge is relatively low (Yang et al., 2017b). Therefore, the

891

variations in the hydrate saturations and distributions of the different sites may be related to their structural

892

positions in the depressions. The differential development of hydrocarbon migration pathways may be one of the

893

important factors controlling the variations in hydrate saturation and vertical distributions at different sites in the

894

GHPTR (Zhang et al., 2018b), which is reflected by sites W11 and W17 on the eastern ridge and sites W18 and

895

W19 on the western ridge. Therefore, the areas or structures where the gas migration pathways develop and have a

45

896

good spatial coupling correspondence with the GHSZ are advantageous targets for exploring and obtaining higher

897

saturation hydrates (Zhang et al., 2018b). When follow-up drilling sites are selected, in addition to confirming the

898

sufficiency of the gas source, it is necessary to assess the development characteristics of the regional gas-bearing

899

fluid migration pathways and their coupling relationships with the other conditions for gas-hydrate accumulation.

900

Future studies should pay attention to not only the large-scale vertical migration pathways (including high-angel

901

faults, gas chimneys, and mud diapirs developed under the GHSZ) but also to the relatively smaller migration

902

pathways (including the gliding faults or cracks within the GHSZ). These migration pathways, although small in

903

scale, are numerous and widely distributed within the GHSZ. Furthermore, they may have a significant impact on

904

hydrate saturation and thickness over a long period of time.

905

Based on the lithologies and physical properties of the hydrate reservoirs, the physical properties of reservoirs

906

may affect the distribution of gas hydrates, resulting in differences in hydrate saturation at the different layers and

907

sites. Therefore, the characterization of the sediment properties is an effective way of identifying and locating

908

relatively high porosity and permeability reservoirs for further exploration of higher saturation and large-scale

909

hydrate resources, which are favorable for the exploitation of hydrates in the SCS. In general, fine-grained hydrate

910

reservoirs may not be favorable for hydrate development; however, the gas hydrates accumulated in the GHPTR

911

are characterized by a wide distribution, high saturation, large reservoir thickness, and high reservoir porosity,

912

which satisfy the basic conditions for hydrate production.

913

914

915

6. Conclusions

(1) Seismic indications of disseminated‐hydrates with high saturations and associated free gas in the gas-hydrate 46

916

production test region (GHPTR) include (a) widespread distinct BSRs, (b) enhanced seismic reflections (ERs) within

917

the gas-hydrate stability zone (GHSZ), (c) ERs connecting the BSR below the GHSZ, and (d) ERs, acoustic blanking,

918

and masking associated with high-angle faults, gas chimneys, and mud diapirs.

919

(2) Two types of GHBZs were identified. One is a well with a thick single layer (~10 m), which is characterized

920

by distinct strong reflections. The top boundary of the gas-hydrate layer is characterized by a strong peak leading

921

seismic reflector, and the bottom boundary is a strong reflection wave trough. The other type of the gas-hydrate

922

concurrence is the well with a thin single layer of gas hydrates, which shows no obvious reflection characteristics on

923

the seismic section and appears as a weak peak seismic reflection. The bottom surface of the GHBZ is characterized

924

by trough reflectors with distinct strata cross-cutting features.

925

(3) The free-gas-bearing zones (FGBZ) at sites W18 and W19, which generally extend to a significant depth,

926

are related to the low-velocity anomaly formed by ongoing gas migration and charging, which are closely related to

927

the hydrocarbons transported from the deep Paleogene source rocks. In contrast, the contribution of hydrocarbons

928

migrated from the deep source rocks to the FGBZ below the GHBZ at sites W11 and W17 may be limited or may

929

even have ceased. The accumulation of free gas is inferred to be the result of the dissociation of gas hydrates.

930

(4) There are variations in the types and geological characteristics of the hydrocarbon migration pathways of

931

sites W18 and W19 on the western ridge and sites W11 and W17 on the eastern ridge in the GHPTR. The efficiency

932

of gas migration in the western ridge may be higher than that in the eastern ridge, resulting in variations in the type of

933

hydrate gas, thickness of the GHBZ, and the gas migration flux and accumulation.

934

(5) The dual supply of thermogenic and biogenic gases results in the inconsistent distribution relationship

935

among the BSR, BGHSZ, and gas hydrates. The BSR is not indicative of the BGHSZ; instead it may be regarded as 47

936

a transitional indicator of SI and SII gas hydrates in the GHPTR. ‐

937

(6)‐Despite the fine-grained host sediments that dominate the GHPTR, relatively coarse-grained sedimentary

938

reservoirs should be the key to high-saturation hydrate accumulation. However, the presence of foraminifera is not

939

a prerequisite for the emergence of high-saturation hydrates. Coupling and matching of the geological conditions,

940

including a nearby hydrocarbon generation center, sufficient gas supply, multiple types of favorable gas migration

941

pathways, and relatively high reservoir porosity promote the formation of thick, highly saturated, gas-hydrate

942

reservoirs in the fine-grained sediments in the GHPTR.‐

943

944

Acknowledgements

945

This work was supported by the National Natural Science foundation of China (Nos. 41806071, 41602149,

946

41776056), Key Special Project for Introduced Talents Team of Southern Marine Science and Engineering

947

Guangdong

948

(2018YFC0310000), China Postdoctoral Science Foundation (No. 2017M622655), and Open Found of Key

949

Laboratory of Marine Mineral Resources, Ministry of Natural Resources (No. KLMMR-2017-A-13). The authors

950

wish to thank all those who contributed to the success of the China National Gas Hydrate Program Expedition 3

951

and 4. We would like to thank all reviewers and editors for their helpful suggestions and constructive comments

952

that helped us improve this manuscript.

Laboratory

(Guangzhou)

(GML2019ZD0102),

953

954 48

National

Key

R&D

Program

of

China

955 956

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the northern South China Sea (SCS) (modified from Su et al., 2019). (B) Bathymetric map and

1334

distribution of the gas-hydrate drilling and coring sites (after Yang et al., 2017c, 2017d). The

1335

gas-hydrate drilling areas of the GMGS1 (purple dashed frame) and GMGS3 and 4 (blue dashed frame)

1336

expeditions are located in the submarine canyon systems in the northern slope of the Baiyun Sag (after

1337

Yang et al., 2017c; Su et al., 2019). The distribution of the main drilling and coring sites is primarily

1338

located at the structural highs or on the edge of the seafloor ridges. The gas-hydrate production test

1339

region (GHPTR) is located in the southwestern part of the GMGS3 and 4 drilling areas. Two ridges, the

1340

western and eastern ridges, cross the GHPTR. The drilling and coring sites studied in this paper are

1341

located at the top of these two ridges. (QDNB = Qiongdongnan Basin).

1342

1343 1344

Fig. 2. Structural evolution characteristics and stratigraphic column of the Pearl River Mouth Basin (modified from Pang et al., 2008 and Zhang et al., 2019).

1345

1346

Fig. 3. Integrated seismic profiles of the drilling and coring sites in the Shenhu area. Green and

1347

black solid lines indicate the positions of drilling wells. Black dotted lines indicate faults. Position of

1348

the connecting-well seismic line is shown in Figure 1B. (GC = gas chimney, BZ = blanking zone, ERs

1349

= enhanced reflections).

69

1350

Fig. 4. Seismic reflection and interpretation profile of site W11 in the gas-hydrate production test

1351

region. (A) Distinct BSR showing strong amplitude reflections, and the inclined strong seismic

1352

reflections connecting the BSR. Acoustic blanking zones (BZs) are present both above and below the

1353

high amplitudes around the BSR. Several high-angle faults and accompanying enhanced reflections

1354

(ERs) and pull-down features were interpreted below the drilling site. (B) Enlarged section showing

1355

features of enhanced reflections distributed just below the BSR, indicating the presence of free gas

1356

(position of seismic line is shown in Figure 1B).

1357

1358

Fig. 5. Seismic reflection and interpretation profile of site W17 in the gas-hydrate production test

1359

region. (A) The BSR shows strong amplitude reflections crosscutting the strata. Its polarity is opposite

1360

to that of the seafloor. Acoustic blanking zones (BZs) appear both above and below high amplitudes

1361

around the BSR. Amplitude anomalies, including acoustic masking and chaotic features accompanied

1362

by enhanced reflections (ERs) and pull-down features, are also present below the drilling site. Several

1363

high-angle faults developed below the BSR are interpreted. (B) Enlarged section showing features of

1364

obliquely enhanced reflections distributed just below the BSR, indicating accumulation of free gas

1365

(position of seismic line is shown in Figure 1B).

1366

1367

Fig. 6. Seismic reflection and interpretation profile of site W18 in the gas-hydrate production test

1368

region. (A) The BSR shows strong amplitude reflections crosscutting the strata and parallel to the

1369

seafloor. Gliding faults extending to the seafloor are recognized in the up-dip direction of the BSR. A 70

1370

significant range of acoustic blanking (BZ), masking, and chaotic features are present below the BSR.

1371

(B) Enlarged section showing features of enhanced reflections (ERs) distributed just below the BSR,

1372

indicating accumulation of free gas (position of seismic line is shown in Figure 1B).

1373

1374

Fig. 7. Seismic reflection and interpretation profile of site W19 in the gas-hydrate production test

1375

region. (A) Seismic characteristics of site W19 are very similar to those of site W18, in which a distinct

1376

BSR and accompanying gliding faults are interpreted. Acoustic blanking (BZ), masking, and enhanced

1377

reflections (ERs) are also present in the drilling site (Zhang et al., 2017). (B) Enlarged section showing

1378

features of enhanced reflections, indicating accumulation of free gas distributed just below the BSR

1379

(position of seismic line is shown in Figure 1B).

1380

1381

Fig. 8. Characteristics of pilot hole with the logging-while-drilling (LWD) curves for drilling sites

1382

in the gas-hydrate production test region (after Guo et al., 2017; Yang et al., 2017d; Zhang et al., 2017).

1383

The gas–hydrate bearing zones (GHBZs) were cored and sampled based on logging anomalies, which

1384

have a high apparent resistivity, low acoustic travel-time difference, relatively low density, and low

1385

natural gamma ray value. (A) site W11, (B) site W17, (C) site W18, and (D) site W19.

1386

1387

Fig. 9. Core observation, lithologic description, and distribution of the gas hydrates confirmed

1388

using coring, thermal imaging, and logging data at (A) site W11 and (B) site W17. No core was 71

1389

available within some sections because drilling sites were conventionally cored with spaced out spot

1390

cores with pressure coring tools, and some of the core lines containing hydrates were cut off and

1391

preserved in liquid nitrogen. Gas inflation cracks and low thermal anomalies with purple or

1392

deep-purple layers indicate presence of gas hydrates. (mbsf= meters below seafloor)

1393

1394

Fig. 10. Core observation, lithologic description, and distribution of gas hydrates confirmed using

1395

coring, thermal imaging, and logging data at (A) site W18 and (B) site W19. No core is available

1396

within some sections because drilling sites were conventionally cored with spaced out spot cores with

1397

pressure coring tools, and some of the core lines containing hydrates were cut off and preserved in

1398

liquid nitrogen. Gas inflation cracks and low thermal anomalies with purple or deep-purple layers

1399

indicate presence of gas hydrates. Thermal image of site W19 is modified from Zhang et al., 2017.

1400

(mbsf= meters below seafloor)

1401

1402

Fig. 11. Characteristics of hydrate cores recovered from coring sites within gas-hydrate

1403

production test region. (A & B) Cavity (116–117 mbsf) and inflation phenomena (120–121 mbsf)

1404

caused by gas-hydrate decomposition at site W11. (C) Porridge-like features resulting from

1405

decomposition of hydrates (139–142 mbsf) at site W11. (D) Significant number of massive hydrates

1406

observed near wellhead at site W17 during drilling process. (E) Porridge-like features resulting from

1407

decomposition of hydrates (238–241 mbsf) at site W17 (after Zhang et al., 2018b). (F) Honeycomb

1408

and porridge-like features resulting from decomposition of hydrates (152–155 mbsf) at site W18. (G) 72

1409

Hydrates (155–158 mbsf) accumulated in clayey silt reservoirs with dispersed morphology and

1410

low-temperature characteristics at site W18. (H) Cavity phenomenon caused by decomposition of

1411

hydrates (165.5–168.5 mbsf) at site W18. (I & J) Strong inflation phenomenon and mushy features

1412

resulting from dissociation of gas hydrates (140–142 mbsf) at site W19 (after Zhang et al., 2017). (K)

1413

Foraminifera fossils recovered from gas-hydrate-bearing section of cores from site W19.

1414

1415

Fig. 12. Lithology, sediment composition, and grain size analysis results of gas-hydrate

1416

host-sediments from coring sites in gas-hydrate production test region in the Shenhu area. (A) site W11,

1417

(B) site W17, (C) site W18, and (D) site W19. Mz (Φ) is medium grain size. GHBZ = gas-hydrate

1418

bearing zone.

1419

1420

Fig. 13. Vertical variation in foraminifera, CaCO3, sand, and porosity of core sediments from

1421

coring sites (A) W18 and (B) W19 in gas-hydrate production test region in the Shenhu area. GHBZ =

1422

gas-hydrate bearing zone.

1423

1424

Fig. 14. Distribution characteristics of free gas with relatively low sedimentary velocities below

1425

gas-hydrate stability zone with relatively high sedimentary velocities in gas-hydrate production test

1426

region in the Shenhu area.

1427 73

1428

Fig. 15. p-wave impedance inversion profile showing distribution characteristics of gas hydrate

1429

and underlying free gas in gas-hydrate production test region in the Shenhu area. Generally, free gas

1430

has a relatively low p-wave impedance below gas-hydrate bearing zone, which has a high p-wave

1431

impedance. Inversion wave impedance is consistent with variation trend of borehole p-wave

1432

impedance curve.

1433

1434

Fig. 16. Calculation results of gas-hydrate stability zone conducted using CSMHYD program

1435

(Sloan, 1998) based on in situ temperature measurements (solid black dots) and different

1436

hydrate-bound gas compositions from the GMGS3 drilling and coring sites (Zhang et al., 2019).

1437

Gas-hydrate saturations calculated from pore-water analysis and pressure core degassing for coring

1438

sites in gas-hydrate production test region in the Shenhu area are from Yang et al. (2017d) and Guo et

1439

al. (2017). Orange, green, and blue dashed lines represent the top of the resistivity anomaly, base of

1440

methane hydrate stability, and base of resistivity anomaly, respectively. (A) site W11, (B) site W17, (C)

1441

site W18, and (D) site W19. (mbsl= meters below sea level; mbsf= meters below seafloor.)

1442

1443 1444

Fig. 17. Synthetic seismograms constructed based on LWD and seismic data acquired from site W11.

1445

1446

Fig. 18. Synthetic seismograms constructed based on LWD and seismic data acquired from site 74

1447

W18.

1448

1449

Fig. 19. Distribution of gas-hydrate-bearing zones (GHBZ) and free-gas-bearing zones (FGBZ)

1450

identified from acoustic travel-time difference, density, and compensated neutron-three porosity log

1451

intersection. (A) site W11, (B) site W17, (C) site W18, and (D) site W19.

1452

1453

Fig. 20. Seismic characteristics of various types of hydrocarbon migration pathways and their

1454

coupling relationships with gas-hydrate stability zones in gas-hydrate production test region. (A)

1455

Acoustic blanking (BZ) and masking reflections below the BSR at sites W11 and W17. Pull-up

1456

features are present in the lower part of this abnormal reflection zone, suggesting the upward piercing

1457

of a mud diapir. Seismic events in upper part of abnormal reflection zone show pull-down

1458

characteristics. A suspected gas chimney is also interpreted near mud diapir. Pull-downs and enhanced

1459

reflections (ERs) suggest presence of gas (Loseth et al., 2009). (B) Distinct low-frequency zone on

1460

instantaneous frequency image, indicating charging and trapping of free gas in sediments (Zhang et al.,

1461

2018b). (C) Large range of acoustic masking and chaotic reflections below sites W18 and W19.

1462

Pull-up features are present in lower part of this abnormal reflection zone, suggesting the upward

1463

piercing of mud diapir. (D) Prominent low-frequency characteristics are also present in instantaneous

1464

frequency image, suggesting intense activity of hydrocarbons transported by mud diapirs from the

1465

deep to shallow. White curved arrows represent possible gas-bearing fluid migration.

75

1466

Table 1. Characteristics of gas hydrates recovered from drilling and coring sites in gas-hydrate

1467

production test region in the Shenhu area. (mbsl= meters below sea level, mbsf=meters below seafloor,

1468

LWD= logging while drilling, BSR= bottom simulating reflector, ERs= enhanced reflections, BZ=

1469

blanking zone.)

1470

1471

Table 2. In situ temperature measurements and geothermal gradients of drilling sites in

1472

gas-hydrate production test region in the Shenhu area. (mbsl= meters below sea level; mbsf= meters

1473

below seafloor.)

76

Table 1. Characteristics of gas hydrates recovered from drilling and coring sites in gas-hydrate production test region in the Shenhu area. (mbsl= meters below sea level, mbsf=meters below seafloor, LWD= logging while drilling, BSR= bottom simulating reflector, ERs= enhanced reflections, BZ= blanking zone.) Site

Water

Total

Depth

Depth

Depth of

Base of

Depth of

Thickness

Maximum

Average

Reservoir

Reservoir

Gas source

Indicators of

depth

drilling

of

of the

the SII

the LWD

the

of gas

gas

gas

porosity

permeability

indicated

gas

depth

BSR

from C1/C2

migration

SI

hydrate

electrical

hydrate

hydrate

hydrate

hydrate

hydrate

(99%C1

resistivity

layer

layer

saturation

saturation

(100%

+0.5%C

anomaly

confirmed

C1)

2+0.5%

and hydrate accumulation

by cores

C3) W11

(mbsl)

(mbsf)

(mbsf)

(mbsf)

(mbsf)

(mbsf)

(mbsf)

(m)

(%)

(%)

(%)

(mD)

1309.75

~222

~200

194

220

202

116.5-192

76

53

34

34.5

0.22

.5

Biogenic

BSR, ERs,

and Mixed

BZ, and faults

W17

1259.00

~315

~230

239

270

270

209-265

56

76

33

33.2

0.32

Biogenic

BSR, ERs,

and Mixed

BZ, pull-down, and faults

W18

1285.41

~234

~171

161

189

174

147-172

25

63

25

56.7

100

Thermoge

BSR, ERs,

nic and

BZ, and

Mixed

acoustic masking

W19

1273.80

~240

~170

165

192

185

135-157.5

22.5

71

45

48.3

5.5

Thermoge

BSR, ERs,

nic and

BZ, and

Mixed

acoustic masking

Table 2. In situ temperature measurements and geothermal gradients of drilling sites in gas-hydrate production test region in the Shenhu area. (mbsl= meters below sea level; mbsf= meters below seafloor.)

Site W11

Water depth mbsl 1309.75

w17

1259.00

W18

1285.41

W19

1273.80

Measured depth mbsf 48.00 95.00 188.00 123.00 206.00 258.10 54.00 121.00 159.50 63.00 107.00 150.50 171.00

Temperature 7.46 10.07 15.11 10.19 13.98 16.17 8.01 12.56 14.61 9.14 11.88 14.25 15.20

Geothermal gradient /100m 5.46

4.43

6.26

5.56

Highlights











Gas hydrate petroleum system of the China first offshore hydrate production test region is investigated. Gas hydrate and underlying free gas are evaluated based on velocity and acoustic impedance inversion. The BSR represents a transitional indicator of structure I and structure II gas hydrate. Presence of foraminifera is not a prerequisite for the emergence of high-saturation hydrates.

Author Contribution Statement

Wei Zhang： Writing- Reviewing and Editing, Conceptualization,Visualization Jinqiang Liang： Investigation,Supervision,Project administration Jiangong Wei： Conceptualization,Investigation,Visualization Jing’an Lu： Investigation,Supervision,Project administration Pibo Su： Resources,Investigation Lin Lin： Software,Methodology Wei Huang： Formal analysis,Resources Yiqun Guo： Methodology,Investigation Wei Deng： Software Xiaolu Yang： Writing - Original Draft Zhifeng Wan： Supervision, Original Draft, Reviewing and Editing.

Declaration of interests ☒ The authors 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:

Wei Zhang, Jinqiang Liang, Jiangong Wei, Jing’an Lu, Pibo Su, Lin Lin, Wei Huang, Yiqun Guo, Wei Deng, Xiaolu Yang, Zhifeng Wan