các nhân tố kiểm soát và tiềm năng Gas Hydrates ở Trung Quốc

Accepted Manuscript
Geological features, controlling factors and potential prospects of the gas hydrate
occurrence in the east part of the Pearl River Mouth Basin, South China Sea
Guangxue Zhang, Jinqiang Liang, Jing’an Lu, Shengxiong Yang, Ming Zhang,
Melanie Holland, Peter Schultheiss, Xin Su, Zhibin Sha, Huaning Xu, Yuehua Gong,
Shaoying Fu, Lifeng Wang, Zenggui Kuang
PII:

S0264-8172(15)00170-1

DOI:

10.1016/j.marpetgeo.2015.05.021

Reference:

JMPG 2249

To appear in:

Marine and Petroleum Geology

Received Date: 24 September 2014
Revised Date:

8 May 2015

Accepted Date: 18 May 2015

Please cite this article as: Zhang, G., Liang, J., Lu, J.’a., Yang, S., Zhang, M., Holland, M., Schultheiss,
P., Su, X., Sha, Z., Xu, H., Gong, Y., Fu, S., Wang, L., Kuang, Z., Geological features, controlling factors

and potential prospects of the gas hydrate occurrence in the east part of the Pearl River Mouth Basin,
South China Sea, Marine and Petroleum Geology (2015), doi: 10.1016/j.marpetgeo.2015.05.021.
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Geological features, controlling factors and potential prospects of the gas hydrate occurrence
in the east part of the Pearl River Mouth Basin, South China Sea
Guangxue Zhanga, Jinqiang Lianga, Jing’an Lua, Shengxiong Yanga, Ming Zhanga, Melanie Hollandb, Peter

a

Guangzhou Marine Geological Survey, Guangzhou, 510075,China

b

Geotek Ltd, United Kingdom, Daventry, NN118PB, UK

c

China University of Geosciences , Beijing, 100083, China

Corresponding author:

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Tel:+86 20 82253873; fax: +86 20 82250265

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Schultheissb, Xin Suc,Zhibin Shaa, Huaning Xua, Yuehua Gonga, Shaoying Fua, Lifeng Wanga, Zenggui Kuanga

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E-mail address:

ABSTRACT: Logging-while-drilling (LWD) and wireline log (CWL) data were acquired
during China’s second gas hydrate drilling expedition (GMGS-2) in the east of Pearl River
Mouth Basin, South China Sea. Disseminated and massive gas hydrates deposits were found
at different sites. Gas hydrate-bearing lithologies identified from the sample coring included
the fine-grained sediments and coarse-grained sediments. LWD logs from Site GMGS2-08

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indicate significant gas hydrate in clay-bearing sediments including two layers with massive
gas hydrate with a bulk density near to 1.08 g/cm3. High electrical resistivities with a range of
2.5~2000.0 Ωm and high P-wave velocities are simultaneously observed in the hydratebearing sediments. The average gas hydrate saturation estimated from the pore water freshing

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analysis ranges from 45~55 % of the pore space. Buried carbonate layers above the massive
gas hydrate deposit discovered at Sites GMGS2-08 indicate that the formations are likely to

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have formed initially at the surface and then were buried. Significant high amplitude seismic
anomalies, discontinuous bottom simulating reflection (BSR) and blanking zone are detected
in the drilling zone. The hydrate-bearing sediments predominantly consist of silty clay and
limestone grains in which the gas hydrates are deposited primarily in the form of laminated,
massive, veins or nodule. The gas hydrates occurrences are subjected to the sediment lithology, new tectonic activities, migration of fluid and gas and also the factors such as heat flow,
salinity and time which affect the nucleation of gas hydrates. Its natural morphologies present
massive, laminated, nodular, nugget and disseminated, of which the former four often formed


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in shallow fault, inter-layer’s weak cementation zone and on the seabed. The “buried” gas
hydrates with high saturation are good zones for gas hydrate exploitation.
Key words: Massive gas hydrate; carbonate, fine-grained, South China Sea
1. Introduction

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Gas hydrates are ice-like crystalline compounds that can be formed by gas (mostly methane)
and water under suitable temperatures and pressure conditions. Gas hydrates usually occur in the
sediments of continental slopes and continental rises within the gas hydrate stability zone (GHSZ),
and are controlled by several factors of gas hydrate petroleum system, such as geothermal gradi-

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ent, seafloor temperature, pressure, gas composition, pore water salinity, gas source, gas migration

and reservoir (Collett et al., 2009). The dissociation of the gas hydrates and subsequent destabili-

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zation of the sediments possibly lead to the serious submarine slide in the marine environment and
on the other hand, submarine slide also influences the formation and deformation of gas hydrates.
The BSR represents the base of the GHSZ and often assumes presence of gas hydrates and free
gas in the marine sediments. Since 1999, several gas hydrate investigations have been carried out
in the northern slope of South China Sea. Bottom simulating reflectors (BSRs) have been identified from the two-dimensional (2D) and pseudo-three dimensional (3D) multi-channel seismic

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data in the Xisha Trough, the Qiongdongnan Basin, the Pearl River Mouth Basin and the deep
slope of the east of Pearl River Mouth Basin (Zhang et al., 2002; Wu et al., 2005; Liu et al., 2006;
Wang et al., 2006; Zhang et al., 2007; Yang et al., 2008; Wang et al., 2010). In addition, a number
of geological and geochemical surveys also revealed the occurrence of gas hydrate in the northern

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slope of South China Sea (Lu et al., 2011; Han et al., 2008). Two Guangzhou Marine Geological
Survey gas hydrate expeditions (GMGS 1&2) have been performed the Pearl River Mouth Basin,

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SCS, in 2007 and 2013 respectively (Zhang et al., 2007; Yang et al., 2008; Zhang et al., 2014).
The characteristic, distributions and geological controls on the occurrences of gas hydrate in

GMGS-1 have been well discussed by many people (Zhang et al., 2007; Yang et al., 2008; Lu et
al., 2008; Wang et al., 2011a,b; Wang et al., 2012; Wu et al., 2011; Wu et al., 2009a,b; Gong et al.,
2009; Sun et al., 2012; Wang et al., 2014). Gas hydrate was found to be present above the base of
gas hydrate stability zone as pore-filling morphology in the sediments (Zhang et al., 2007; Yang et
al., 2008; Wang et al., 2011b). The GMGS-2 was conducted by the Chinese Geological Survey
(CGS) incorporating with Fugro and Geotek from June to September in 2013 (Zhang et al., 2014;
Yang et al., 2014). The primary objective of GMGS-2 expedition was to accurately quantify gas


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hydrate in the sediment cores and to determine the nature and distribution of gas hydrate within the
sedimentary sequence in this basin. Thirteen sites were drilled which include 10 logging-whiledrilling (LWD) and 3 downhole wireline logging (DWL) pilot holes on the rises of the east of the
Pearl River Mouth Basin in the water depth from 664 to 1420 m (Figure 1 and 2). In addition, five
of the sites were cored for further analysis. Sites GMGS2-05, GMGS2-08, GMGS2-09 and

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GMGS2-16 were cored during GMGS-2 Leg 2 (Fig. 2, red points), while Sites GMGS-2-07 and
GMGS2-16 were sampled during GMGS-2 Leg 3. Cores were acquired with the Fugro Hydraulic
Piston Corer (FHPC), which takes a non-pressured sample about 8 m long from soft to stiff clays
and silty clays and Fugro Corer (FC) conventional wireline core systems. Due to limited time for

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GMGS-2, boreholes could not be cored continuously. The core plan at each site was first developed from existing seismic data and was refined with the analysis of the LWD and DWL data col-

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lected at each site.

The thirteen sites established during GMGS-2 were located along the crest of two prominent
seafloor ridges as shown in Figure 2. Gas hydrates have been identified from the LWD and DWL
data and cores acquired at Sites GMGS2-01, GMGS2-04, GMGS2-05, GMGS2-07, GMGS2-08,
GMGS2-09, GMGS2-11, GMGS2-12 and GMGS2-16 (Zhang et al., 2014). The gas hydrate samples containing > 99 % methane were successfully recovered in 5 sites with various types of gas

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hydrate morphologies such as laminated, massive, veins or nodular in the sediments, which marks
the milestone in the gas hydrate researches and developments offshore China. The drilling project
displays various types of gas hydrates and overlying authigenic carbonates, which sufficiently confirm that the drilling area has favorable geological conditions for the occurrence of gas hydrate. In

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this paper, we use the LWD and DWL data, gas compositions from core samples, lithology data
from core analysis, and seismic data to identify the occurrence of gas hydrate, and its relationship

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with the above carbonates, the evolution of gas hydrates and its control factors.

2. Geological setting

Under the influences of Eurasian Plate, Pacific Plate and Indo-Australian Plate interactively
colliding with each other, the northern slope of South China Sea was divided into three geological
domains: the passive continental margin in the west including Qiongdongnan Basin; quasi-passive

continental margin in the center including the Pearl River Mouth Basin; active continental margin
in the east including the east of Pearl River Mouth Basin. The east of Pearl River Mouth Basin is
located in the northeastern part of South China Sea, resting on the geological structures which ex-


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perienced two stages of significant extensions ranging from the end of lower Cretaceous to Oligocene-Early Miocene (32 - 17 Ma) with thick littoral-neritic sand deposits. During the third extending (Middle Miocene - Pliocene), the seabed subsidence widely took place with thick marine deposit covering along the northeastern slope of South China Sea (Zhu et al., 2009).
In the east of Pearl River Mouth Basin there are many normal faults that can be classified into

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two distinct groups with the NW - NWW and NNE - NEE striking respectively (Fig. 3). The former faults cut the subsequent ones, we can infer that the former ones are younger than the latter
and the age is estimated about 15 Ma. The sedimentary processes in Quaternary are controlled by
both groups of fault systems. The shallow soft soils derived from quaternary are considered to be

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in an environment of high pressure and low extension caused by the tectonic activities of underlain
NW trending buried faults. In Luzon which is located to the east of drilling area, the well-

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developed accretionary prisms lead to a series of thrust faults and westward-dipping imbricated
folds (Lin et al., 2009; Liu et al. 1997). During R/V SONNE 177 cruise co-operated by Chinese
and German scientific team in 2007, widely distributed cold vents and carbonate crust cracks
called Jiulong Methane Reef (JMR) were found in the northern shallow water area (Suess et al.,
2005; Han et al., 2008), also revealing the possible presence of gas hydrate.

The occurrences of BSR have been reported in many studies from the multichannel seismic da-

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ta (Wang et al., 2006; Li et al., 2013; Wang et al., 2009). The drilling area located in the central rift
of the east of Pearl River Mouth Basin. The morphology is featured by plenty of troughs, canyons,
seamounts, escarpments, slopes, scour channels and sea knolls. In particular, the submarine channels mostly parallel to NW trending faults, including a 110km long canyon, are quite well devel-

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oped, ploughing the northern slopes of South China Sea with series of deposit transportation systems left in the deep seafloor. The central rift of the east of Pearl River Mouth Basin is bounded by

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a number of secondary depressions of surrounding basins on the north and south flanks. Those
depressions have multiple depositional centers occurring respectively during lower Cretaceous,
Paleogene (Paleocene, Eocene and Oligocene), Neogene (Miocene, Pliocene) and Quaternary (Fig.
4). The corresponding source rocks are almost made up of type III kerogen in a high stage of maturity (Yi et al., 2007). The gas hydrates are predicted to exist in the sediments deposited from
Miocene to Pliocene.
The calcareous nannofossils and foraminifera biostratigraphy in 5 sites (GMGS2-05
GMGS2-07

GMGS2-08

GMGS2-09 and GMGS2-16) from the drilling area have been studied

(Chen et al., 2015). A total of 3 nannofossils events and 2 foraminifera events from middle Pleis-

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tocene to Holocene were recognized. The oldest sediments recovered are in an age of middle Pleistocene, younger than 0.50Ma, the age assigned for gas hydrate occurrence zone is middle Pleistocene to Holocene at sites GMGS2-05

GMGS2-07

GMGS2-08 and GMGS2-16 (Table 1). Sed-

imentation rates varied from 36.9 cm/k.y. to 73.3 cm/k.y., and reaching the highest average value

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54.2cm/ k.y. since 0.12Ma in the GMGS2 drilling area. The mean sedimentation rate has been
47.2cm/ k.y. since 0.44 Ma.

3. Results
3.1 BSR occurrence from seismic data

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Bottom simulating reflectors (BSRs) were identified from the high-resolution seismic data in
this area. On seismic profiles, the BSR is sub-parallel to the seafloor, cross-cuts strata, and is char-

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acterized by high amplitudes, and reversed polarity compared to the seafloor reflections. The
depths of BSR are in a range of 160 - 220 mbsf with an average depth of 180 mbsf in the drilling

area where the water depth is in a range of 700 - 900 mbsf with a maximum 1127 mbsf. Seen from
the seismic sections across Site GMGS2-08 (Fig. 5), high interval velocity appears above the BSR
while low interval velocity under the BSR comparing with the surrounding formations. The BSR
exhibit a distinguished velocity boundary with gas hydrate deposits upwards and free gas down-

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wards. The velocity patterns, contrary to the normal expected velocity profiles, are quite compatible with the well-known model of gas hydrate overlying free gas. The lateral variations of P-wave
velocities in the sediments show the distributions of gas hydrate in this area. High-amplitude re-

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flections with a prominent high P-wave velocity above the BSR indicate the presence of massive
or layered gas hydrate deposits.

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3.2 Gas hydrate occurrence from well Log data
Site GMGS2-08 was drilled to a depth of 138 meter below seafloor (mbsf) with a water depth
of 798 m. The caliper log shows smooth through the whole site indicating the good borehole condition (Fig. 6). The LWD data (Fig. 6) showed high P-wave velocities and electrical resistivities in
two distinct intervals at Sites GMGS2-08 and GMGS2-16, respectively. The data showed significant gas hydrate occurrence with high resistivities 2.5, 4.0, 15.0, 40.0 and 200.0 Ωm and high
acoustic velocities detected in those anomalous intervals. At Site GMGS2-08, the upper gas hydrate horizon occurred in the depth from 9 to 23 mbsf with a maximum resistivity of 17.5Ωm. The
acoustic velocity at this interval slightly increased to a maximum of 1662.2 m/s at the depth of 9


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mbsf (Fig. 6). The lower gas hydrate horizon occurred in the depth from 66.4 to 98.0 mbsf with a
thickness of 33 m. The acoustic velocity and resistivity showed significant increases. The resistivity shows a maximum value of 20,000 Ωm and the acoustic velocity is obviously increased to a

maximum value of 2746.0 m/s. The gamma-ray and density logs showed distinct decrease at this
interval (Fig. 6). At the depth of 71.5 mbsf, the density log decreases to the lowest value of 1.08

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g/cc similar to that identified at Site NGHP01-10B (Lee and Collett, 2009), where gas hydrate is in
massive form. In the depth from 58 to 62 mbsf, this layer had high resistivity, high acoustic velocity and high density (with a maximum value of 2.3 g/cc), which was interpreted as carbonate

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(Zhang et al., 2014).

3.3 Sample features

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All five of the cored sites contain gas hydrate with various morphologies deposited in the sediments of silty clay or clast in GMGS2 (Fig. 7). The coring plan for Site GMGS2-08 was to collect and preserve visible gas hydrate. A total of 4 the Fugo Hydraulic Piston corer (FHPC) cores, 5
Fugro Corer (FC) cores, 10 (Fugro Marine Core Barrel (FMCB) cores, 4 Fugro Pressure Corer
(FPC) cores, and 4 the Aumann Pressure Core Toll with Ball valve (PCTB) cores were taken from
Sites GMGS2-08. Core GMGS2-08F-5A-1a6, 74.5 mbsf, contained gas hydrate veins and thick

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layers of massive hydrate (Zhang et al., 2014). Gas hydrate saturations derived from the porewater
freshening based on chlorinity are approximately in the range of 30 - 50 % (Fig. 8). Gas hydrate
saturation from the pressure core degassing of GMGS2-08C-4P, 17 mbsf, using methane mass

balance analysis ranges from 10 to 14%. Gas hydrate saturations from core GMGS2-08F-5A (73.5

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mbsf) and GMGS2-08F-10A (87.5mbsf) are 33% and 17-30%, respectively. The methane content

ture.

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is 99% with small amount of ethane and propane so that the gas hydrate sample is the type I struc-

3.4 Gas hydrate distributions
3.4.1 Vertical distribution

Gas hydrate samples at Site GMGS2-08 were collected from 5 holes (denoted by B, C, E, F and
G) based on the LWD data (Fig. 9). The core samples were recovered to thoroughly understand the
mechanism of gas hydrate occurrence in the drilling area. The samples of holes B and G were
cored according to sulfate methane transition (SMT) zone. The samples of hole C test the shallower sediments. Holes E and G focus on the middle carbonate rocks and deeper sediments around the


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GHSZ (Fig. 9). We summarized the gas hydrate samples from the profiles of core-derived lithology at Site GMGS2-08 as the following (1) In the shallow part (9.0 - 23.0 mbsf), the interval is
dominated by gray-green silty clay with some light-yellow authigenic carbonates partially dispersed. A roughly 15 m thick gas hydrates, widely laterally distributed in the form of massive
blocks, nodules and veins, occur in a range of 8.0 - 23.0 mbsf (Fig. 7). (2) In the middle part (58.0-

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63.0 mbsf), the interval was consisted of a suite of clastic deposits, light-gray breccia carbonates
and porous breccia limestones. The volume ratio of breccia in the sediments is > 70 % soaking
within the matrix of fine light-gray authigenic carbonates and calcites. The minerals of carbonate
are partially scattered with plenty of pores which are filled with crystalline calcite layers or veins.

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(3) In the deep part (66.0 - 94.0 mbsf), the interval is dominated by silty clay with some muddy
carbonates partially dispersed. A 30-m-thick gas hydrate layer, tightly laterally distributed with the

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massive form, is found in a range of 66.0 - 94.0 mbsf. At the sample F (94 mbsf), we discovered a
0.5 m-thick layer of gray-purple clastic rocks uncomformity underlying gray silty clay which may
be caused by turbidity currents.

3.4.2 Lateral distribution

Based on the seismic data and interpretations of well log data at all if the drilled sites, the esti-

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mated potential extension of gas hydrates probably covers an area of about 55.0 km2 (Fig. 10). The
drilling area has two distinct blocks divided by one of trenches. Seven sites (Sites GMGS2-08, -09,
-07, -05, -04, -01 and -12 in chronological order) were drilled in the western part with the water
depths range from 644 to 1127 mbsf. Double gas hydrate layers are detected at Sites GMGS2-08

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and GMGS2-16, while the other sites only have one gas hydrate-bearing layer identified from the
well log data. The extent of potential gas hydrate occurrence in the western part is large stretching

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from northwest to southeast similar to that of the dividing trench (Fig. 10). However, the extent of
potential gas hydrate occurrence in the eastern part cannot be estimated accurately because only
Site GMGS2-16exists in this crest (Fig. 10).

4. Discussions

The occurrence of gas hydrate has been shown to be controlled by many geologic parameters
including sediment properties, tectonic history, fluid migration, reservoir, time, and other factors
(Bahk et al., 2011; Boswell et al., 2011; 2012; Büne et al., 2003; Büne and Mienert, 2004; Collett et
al., 2009; Herbozo et al., 2013; Holbrook et al., 2002; Hornbach et al., 2008). The integrated anal-


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ysis of the seismic data, LWD and core data collected during GMGS-2, the occurrence of gas hydrate in the east part of the Pearl River Mouth Basin (PRMB) appears to be controlled by several
geologic parameters.

4. 1 Gas source

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The molecular ratios of methane to ethane ratios (C1/C2) that exceed 1,000 and stable carbon

values (δ13CCH14) less than -50‰ generally indicate a gas from a microbial source (Claypool and
Kvenvolden, 1983; Milkov et al., 2005). However, a gas with a C1/C2 ratio less than 1,000 and a
stable carbon compositions (δ13CCH14) of greater than -50‰ indicate the contribution of a gas from

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a thermogenic source. The gas compositions (methane, ethane, propane, isobutene, and butane)
were analyzed by gas chromatography. The C1/C2 at Site GMGS2-08 was shown in Figure 8,

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along with the gas hydrate saturation calculated from core-derived interstitial water-chlorinity
freshening trends. The C1/C2 ratios are high (>1000) throughout the entire cored section at Site
GMGS2-08 (Fig. 8), indicating the presence of microbial origin. The C1/C2 ratios from void gas
samples and pressure core gas at Sites GMGS2-05, 07, 09, and 16 are also greater than 1000
showing a contribution of microbial origin. However, the C/1/C2 ratios at Site GMGS2-16 showed

2014).
4. 2 Gas migration

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a sharp decrease at 202 mbsf indicating a possible external source of fluid at this depth (Yang et al.,

The formation of gas hydrate requires substantial volumes of gas, which can be derived from

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microbial and/or thermogenic sources. The gases forming gas hydrates migrated within a sedimentary section by three processes: (1) diffusion, (2) fluid flux, or (3) as bubbles (Collett et al., 2009).

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The drilling area of GMGS2 is near to the deformation front of SW Taiwan, which is a major
structural trap that may host a significant amount of gas (Lin et al., 2013). Faults, gas chimneys,
and diapiric structures have been identified from the seismic data in this area (Fuh et al., 2009;
Gong et al., 2008; Yan et al., 2006; Wang et al., 2006; Wang et al., 2009; Wu et al., 2007). Various types of complex morphological structures are found such as large slopes, canyons and trenches in the water depth of 300 - 2000 m. The submarine plateaus located on the flanks of trenches are
generally considered to be the desirable place for the occurrence of gas hydrates. The seismic data
obtained showed that during late Cretaceous and early Tertiary the large fault zones are actively
developed, extending to or even piercing the underlain basement within the extension environ-


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ment. Since the second extension in the late Oligocene, the activities of faults were increased,
dominating the behaviors of the first and second geological units in this area. From late Miocene,
many newly developed faults are driven upwards to piece the overlying sediments reaching to the
seafloor (Gong et al., 2008). Along those faults, the gas from more deeply buried organic sediments and biogenic methane produced during Pliocene and Pleistocene are migrating upwards into

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the GHSZ. In addition, the mud diapirs are developed widespread, indicating that gas is being fed
continuously into the GHSZ as the evidence of high correlation between the gas hydrate occurrence and faults-diapirs alignments. In the active tectonic structures, the gases generated in the
deeper sediments continuously migrate upward along faults or diapirs, and mix with the in situ

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gas hydrates in the silty clay and/or limestone deposits.

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biogenic gas to approach near the sediments close to seafloor, and therefore again form the visible

4. 3 Reservoir

Comparing to the pore-filling gas hydrate in GMGS-1, the lithologies of gas hydrate-bearing
sediments in GMGS-2 included complex reservoirs. Disseminated gas hydrate in deeply-buried,
fine-grained sediments is very common in the drilling zone, which is similar to that of GMGS1with gas hydrate saturation ranging from 20 to 55% of the pore volume. However, in GMGS-2,

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the dense, thin veins gas hydrates in near surface, were formed in fine-grained sediments with
moderate gas hydrate saturation. For example, at Site GMGS2-08, two layers gas hydrate were
present in the upper and lower sediments, which are mainly composed of fine-grained silty clay.
The samples are in the forms of thin layers, massive and veins that can be easily visualized by na-

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ked eyes. They are probably formed when the gases migrate upwards along something like conduits filling fissures and cracks under favorite condition.

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Pore-filling gas hydrate was also found in the coarse-grained sediments above the base GHSZ

at Site GMGS2-16. The gamma ray shows no great change of lithology, however, a shelly, foraminifera-rich sand layer close to the base of gas hydrate stability was occurred (Yang et al. 2014).
At Site GMGS2-08, a 3-m-thick authigenic carbonates deposited was found just above the top
of lower hydrate-bearing sediments. We infer that they have been experienced multi-phases of
chaotic cementation evolutions. Those angular breakups of early-stage carbonates are cemented by
crystalline calcites and quite possibly formed locally because they are poorly sorted and rounded.
In addition, a wide variety of scattered skeletal shells and small vents which are almost full of tiny
crystalline calcites.


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4. 4 Gas hydrate formulating time
In the drilling area, the absolute age of gas hydrate should be logically restricted between that
of authigenic carbonates as young limit and that of matrix as early limit. The borehole samples
suggest that the gas hydrates mainly occur in the intervals of middle Pleistocene - Holocene sedi-

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ments. The depositional rate of Neogene is about 1.4 - 5.8 cm/ka with a maximum of 11.4 cm/ka
(Huang et al., 2009; Wu et al., 2003). The recent analyses show that the depositional rates of Pleistocene range from 36.9 to 73.3 cm/ka (Table 1), which indicates that gas hydrate identified in
GMGS2 exists in a high-deposition-rate sediment zone. The shallow occurrence of gas hydrates is

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in the sediments of 8 - 30 mbsf which are deposited during upper Pleistocene (about 0.12 Ma). The
lower occurrence of gas hydrates is in the sediments of 68 - 226 mbsf which are deposited during

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lower Pleistocene (about 0.44 Ma). The two corresponding authigenic carbonates are assumed to
be relative to cold venting which are formed due to the decomposition of underlying gas hydrates.
The presumed start time of carbonates deposits in our study is close to the cold venting formation
in the South China Sea is likely to begin in 0.33 - 0.06 Ma in the middle Pleistocene, which indicates the formation time of gas hydrate in GMGS2 drilling zone is younger than the Lower Pleis-

5. Conclusions:

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tocene (0.44Ma).

The bottom simulating reflections are widely distributed in the drilling zone identified from the
seismic profiles, with the reversed polarity corresponding to the seafloor and crosscutting the stra-

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tigraphy. In addition, high amplitude reflections above the BSR are possibly related to the massive
and/or layer gas hydrates with higher gas hydrate saturations. Gas hydrates identified in fine-

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grained sediment with the morphologies of vein, nodular, laminated show that the electrical resistivity is extensively high (mostly >2.5 - 200.0 Ωm). The acoustic velocity is high and the density
is lower than upper and lower layers. The samples of gas hydrates are found in the sediments (<
220 mbsf) within silty clay deposits; double gas hydrate intervals with saturation of 45 - 55 % are
detected in Site GMGS2-08, and the thickness ranges from 15 to 35 m respectively. The molecular
ratios of methane to ethane ratios indicate that gas hydrate at this site was a microbial source. Massive forms of visible gas hydrates are recovered in non-pressure core are thought to be formed at

the seafloor surface of the carbonate platforms and then were buried. A 20-cm-thick section of
pure gas hydrate was recovered. The disseminated gas hydrate was found in coarse-grained sedi-


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ments at Site GMGS2-16. This occurred in a shelly, foraminifera-rich sand layer near to the base
of gas hydrate stability zone. The age of authigenic carbonate was analyzed to show the formation
time of gas hydrates. Gas hydrate occurred in the drilling zone is formed from the Lower Pleistocene to Holocene.

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Acknowledgments

We thank the gas hydrate science team of the gas hydrate program expedition Guangzhou Marine
Geological Survey-1 (GMGS-2). This study was supported by the national project of Exploration

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and Test Production for Gas Hydrate (GZH201100305).

References

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Figure captions:
Figure 1 Map of northeastern part of South China Sea. The rectangle shows the location of drilling
area.

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Figure 2 Bathymetric map of gas hydrate drilling area and sites. The blue points denote loggingwhile-drilling (LWD) and downhole wireline logging (DWL) boreholes and the red points denote
sampling boreholes.
Figure 3 Geological map of northeastern part of South China Sea.

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Figure 4 The seismic profile and the interpreted geological section through the drilling area in
South China Sea.

Figure 5 The Seismic profile and the inverted P-wave velocity profile through site GMGS2-08.

are shown near the borehole site.

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The P-wave velocity (purple line) and the resistivity (green line) from logging-while-drilling data

Figure 6 Logging while drilling (LWD) data at Site GMGS2-08 from left to right: the caliper log,
density, P-wave velocity, resistivity, and gamma-ray.

Figure 7 Morphologies of gas hydrate in core samples recovered from GMGS2 drilling sites,
South China Sea. 1 and 2 (Site GMGS2-08F) massive; 3 and 4 (Site GMGS2-08E) laminated; 5

GMGS2-16D) disseminated.

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(Site GMGS2-08E) and 6 (Site GMGS2-08C) nodular; 7 (Site GMGS2-08E) vein; 8 (Site

Fig. 8 The gas hydrate saturations estimated from core-derived pore-water chlorinity freshening

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trends and methane to ethane ratios (C1/C2) of void-gas samples versus depths at Site GMGS2-08.
Figure 9 The lithological columns at holes G, E, F, C and B of Site GMGS2-08 and their locations.
Figure 10 The lateral distribution of gas hydrate identified from well log data and seismic data in

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the drilling area showing with the black lines.

Table 1 The depths of biological events and the sedimentation rate since 0.12Ma at the second gas
hydrate drilling expedition in the east area of the Pearl River Mouth Basin, South China Sea.

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0.09

No sample

T G.ruber

Foraminifera

0.12

56.27

B E. huxleyi

Calcareous
Nannofossils

0.29

B G.ruber

Foraminifera

0.40

No sample

No sample

No sample

44

44.30

67.90

88

61.50

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nannofossils

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B E. huxleyi Acme

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Table 1 The depths of biological events and the sedimentation rate since 0.12Ma at the second gas hydrate drilling expedition in the east area of
the Pearl River Mouth Basin, South China Sea.
Depth (mbsf)

Biological
Biological event
Age (Ma)
type
GMGS2-05
GMGS2-07 GMGS2-08 GMGS2-09
GMGS2-16

＞74.76

＞93.84

＞104.85

166.8

No sample

No sample

No sample

No sample

197.60

46.9

36.9

56.6

73.3

51.3

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Sedimentation rates since 0.12 Ma （cm/k.y.）

＞203.3


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