Lithological characteristics and hydrocarbon gas sources of gas hydrate-bearing sediments in the Shenhu area, South China Sea: Implications from the W01B and W02B sites

Marine Geology 408 (2019) 36–47

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Lithological characteristics and hydrocarbon gas sources of gas hydratebearing sediments in the Shenhu area, South China Sea: Implications from the W01B and W02B sites

T

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Jing Lia,b,c,d, Jing'an Lue, Dongju Kange, Fulong Ninga,c, , Hongfeng Lue, Zenggui Kuange, ⁎⁎ Dongdong Wanga, Changling Liub,c, Gaowei Hub,c, Jiasheng Wanga, Jinqiang Liange, a

Faculty of Engineering, China University of Geosciences, Wuhan 430074, China Key Laboratory of Gas Hydrate, Ministry of Natural Resources, Qingdao Institute of Marine Geology, Qingdao 266071, China Laboratory for Marine Mineral Resources, Qingdao National Laboratory for Marine Science and Technology, Qingdao 266071, China d Chinese Academy of Geological Sciences, Beijing 100037, China e MLR Key Laboratory of Marine Mineral Resources, Guangzhou Marine Geological Survey, Guangzhou 510075, China b c

A R T I C LE I N FO

A B S T R A C T

Editor: Shu Gao

The Shenhu area, located in the Pearl River Mouth Basin, South China Sea (SCS), is currently one of the most promising exploration areas for gas hydrates. In this study, the lithological characteristics, as well as the molecular and isotopic composition of hydrocarbon gases are systematically reported for the first time for core sediments obtained from two new drilling sites (referred to as W01B and W02B) in the southeast Shenhu area. Both gas hydrate-bearing and gas hydrate-free samples are characterized by fine-grained sediments (dominantly coarse silt and to a lesser extent, fine and medium silt) and similar median grain-sizes (5.94Φ to 6.49Φ), sorting (2.32 to 2.59), kurtosis (0.82 to 0.97), skewness (−0.12 to 0.03) and mineral compositions. Abundant authigenic pyrites (including euhedral and framboidal pyrites) also occur in these sediments. All these characteristics collectively indicate that the core sediments from the southeast Shenhu area formed in a relatively stable, lowenergy and anoxic sedimentary environment. Nevertheless, the hydrate-bearing layers in this study are characterized by lower sand contents and more foraminifera than gas hydrate-free layers. The molecular and isotopic composition of hydrocarbon gases hosted by secondary minerals in the studied sediments indicates that they are of thermogenic origin. Combined with previously published data, we suggest that the abundant foraminifera within the sediments of the Shenhu area play a significant role in controlling the formation of gas hydrates by increasing the porosities of sediments, and that the thermogenic gas is an important source for the hydrocarbon gases of hydrates in the SCS.

Keywords: Gas hydrate Grain-size distribution Foraminifera Shenhu South China Sea

1. Introduction Natural gas hydrate (NGH) is an unconventional and promising energy with potential reserves of 1.5 × 1015 m3 (Makogon et al., 2007). It is a crystalline clathrate compound comprising molecules of natural gases (e.g., methane, ethane, propane and CO2) in lattices of water molecules through hydrogen bonds (Sloan and Koh, 2007). It is widespread within submarine, high-latitude lakes, and permafrost environments where high-pressure and low-temperature conditions are commonly manifested (Egorov et al., 1999; Vanneste et al., 2001; Max, 2003). It has been suggested that the occurrence of NGH in sediments, also known as gas hydrate stability zones, is generally heterogeneous

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depending on the interaction between the supply of methane and the lithology of the host sediments (Kvenvolden, 1994; Paull et al., 1996; Paull and Dillon, 2001; Riedel et al., 2009; Bahk et al., 2011; and references therein). It has also been demonstrated that the characteristics of NGH, such as its morphology, distribution and aggregation, are closely related to the properties of gas hydrate-bearing sediments (sediment components, particle size, porosity, etc.) (Torres et al., 2008; Bahk et al., 2011; Lu et al., 2011; Wang et al., 2011; Rose et al., 2014; Winters et al., 2014). The South China Sea (SCS), covering an area of 350 × 104 km2, is known worldwide as an attractive, potential resource-rich area of gas hydrates (Yao et al., 1994; Wu et al., 2005; X.J. Wang et al., 2006;

Correspondence to: F. Ning, Faculty of Engineering, China University of Geosciences, Wuhan 430074, China. Corresponding author. E-mail addresses: nfl[email protected] (F. Ning), [email protected] (J. Liang).

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https://doi.org/10.1016/j.margeo.2018.10.013 Received 26 January 2018; Received in revised form 28 October 2018; Accepted 31 October 2018 Available online 07 November 2018 0025-3227/ © 2018 Elsevier B.V. All rights reserved.

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Fig. 1. (a) Geological map of the Shenhu area, SCS. (b) Locations of drilling sites cored during the GMGS4 in the Shenhu area, SCS. (Modified after Wu et al., 2009; Yang et al., 2017.)

A new gas hydrate drilling expedition (GMGS4) run by the Institute of Guangzhou Marine Geological Survey, China Geological Survey was initiated in the Shenhu area in 2016 and provided access to the gas hydrate sediments in this area. In this study, both gas hydrate-bearing and gas hydrate-free samples recovered from two new drilling sites (referred to as W01B and W02B, respectively) were investigated. For the first time their lithological characteristics including sediment composition and grain-size distribution, as well as the molecular and isotopic composition of hydrocarbon gases hosted by these sediments, were systematically examined. The purpose of this study is to identify the lithological features of sediment cores and the sources of hydrocarbon gas that were recovered during the GMGS4. By comparing gas hydrate-bearing and gas hydrate-free samples, the genetic correlation between the formation of gas hydrate and the lithological characteristics of sediments within the Shenhu area is therefore illustrated. This is of great significance for gas hydrate exploration in the SCS.

Zhang et al., 2007; Liu et al., 2012). It belongs to a transitional continental margin that formed during the Middle Oligocene-Early Miocene opening of the Central Basin (Briais et al., 1993; Clift et al., 2002). It still retains many features of the continental geological structure because of the comparatively short expansion history (Lüdmann et al., 2001). The Shenhu area, located in the center of the north continental slope of the SCS, is characterized by complex topography and fault systems that are favorable for the hydrate formation (Wu et al., 2009). Great attention has been paid to this area since gas hydrates were recovered for the first time in the Shenhu area during the drilling campaign (GMGS1) implemented in 2007 (Liu et al., 2012). Unlike the areas where high-saturation gas hydrates are mainly recovered from coarse-grained reservoirs, such as the Mallik in Canada (Dallimore et al., 1999; Dallimore and Collett, 2002), the Nankai Trough offshore of Japan (Uchida and Tsuji, 2004), and the southern Hydrate Ridge offshore of Oregon (Tréhu et al., 2004), the gas hydrates recovered from the Shenhu area occur in fine-grained sediments with a maximum sand content of 4.24% and silt content ranging from 70% to 80% (Chen et al., 2011; Wang et al., 2013; Liang et al., 2017). Some studies also found that gas hydrates in the Shenhu area seemed to preferentially accumulate in sediments hosting abundance of calcareous fossils (such as foraminifera) (Chen et al., 2011; Wang et al., 2014), but the reasons were not well illustrated. A systematic study regarding the physical properties of sediments (e.g., sediment composition and grain-size distribution), as well as the hosted fossils, is therefore of the essence to get an insight into the accumulation mechanism of gas hydrates in the Shenhu area. Moreover, the origin of hydrocarbon gas in the Shenhu area is still controversial. Although most studies argued that hydrocarbon gas recovered from hydrate-bearing sediments was of microbial origin, recently hydrocarbon gas of thermogenic origin was also reported in the Shenhu area (Su et al., 2017; Wei et al., 2018).

2. Geological setting The SCS, formed by oceanic spreading during the Oligo-Miocene, is one of the largest marginal seas in the western Pacific (Wu et al., 2007). Tectonically, it is closely controlled by the relative motion of the Philippine Sea plate to the east and the ongoing collision with the IndoAustralian plate to the south (Wu et al., 2007; and references therein). The SCS is 1212 m deep on average with a maximum depth of 5377 m, and the thickness of sediments within this area varies between 500 m and 10 km (S.H. Wang et al., 2006). The northern SCS consists of a total area of ca. 2.3 × 105 km2 with a length of 900 km and a width of 143 to 342 km (Wu et al., 2007). Chronologically, the source rocks in this area are predominantly of Paleogene and Oligocene-Early Miocene ages. Sediments deposited since the Late Oligocene are characterized by fluvial, lacustrine, deltaic, shallow marine and bathyal facies. 37

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3. Materials and methods

10 mL of 10% hydrochloric acid was added at 70 °C to dislodge inorganic carbon. The disposed samples were then submerged into the deionized water for at least 8 h. The supernatant solution was decanted and 10 mL of 1% sodium diphosphate solution (Na4P2O7·10H2O) was added as a dispersing agent prior to grain-size analysis. Grain-size analysis was conducted using a Marvin Mastersizer 3000 laser-diffraction particle size analyzer at the School of Earth Science, China University of Geosciences, Wuhan. The measured size ranges from 10 nm to 3.5 mm. The results were expressed as volume percentages, and the reproducibility was better than ± 1% for the median grain-size. The grain-size data as well as the following geometric parameters, i.e., mean, sorting, kurtosis, skewness and median grain-size (D50), were then analyzed by the GRADISTAT software (Blott and Pye, 2001) according to Folk and Ward's (1957) method. The calculation equations of sorting, kurtosis and skewness are as follows:

3.1. Sampling

sorting =

The sites W01B and W02B in this study are located in the Shenhu area of the Pearl River Mouth Basin, the northern continental slope of the SCS (Fig. 1). This area is adjacent to several large oil and gas fields, such as the Pearl River Mouth basin and Beibu Bay basin. The great water depth (2000–3000 m), high sediment thickness and high organic matter contents, as well as the large sedimentation rate and low geothermal gradient (45–67.7 °C/km), collectively advocate a favorable environment for the generation of gas hydrates in this area. Analyses of the discontinuous core samples recovered from GMGS1 showed that gas hydrate-bearing sediments in the Shenhu area consisted of silt and clayey silt, and that these sediments contained abundant foraminifera and calcareous nannofossils as well as a few diatoms and radiolarians (Wang et al., 2011; and references therein).

Approximately 47 m of pressure sediment cores recovered between 140.86 and 188.20 m below the sea floor (mbsf) were acquired from the W01B and W02B drilling sites of GMGS4. Their geographic positions are close to each other (Fig. 1b), with the water depths being 1287 and 1272 m, respectively. Twelve representative sediment samples in this study were examined and ordered by depth, seven of which were obtained from the W01B drilling site (i.e., SH2, SH5, SH6, SH7, SH10, SH11, SH12; 147.88 to 188.20 mbsf) and the others were acquired from the W02B drilling site (140.86 to 176.55 mbsf) (Table 1). These samples comprise five gas hydrate-bearing sediment cores (i.e., SH1, SH3 to SH6) and seven hydrate-free cores (i.e., SH2, SH7 to SH12). Gas hydrate hosted in sediments had decomposed on board. The asymmetrical sample intervals, varying from 0.15 to 7.60 m, were adopted because of the limited access to the specimens. The thicknesses of samples are in the range of 0.20 to 0.30 m, with an average of 0.25 m. Lithologically, they are all coarse silt, except for samples SH1 (medium silt), SH2 (fine silt) and SH9 (medium silt). These samples were depressurized and stored at 4 °C immediately after measurement of the physical properties of these sediments.

kurtosis =

D84 − D16 D − D5 + 95 4 6.6

(1)

D84 − D16 2.44(D75 − D25 )

skewness =

(2)

D16 + D84 − 2D50 D + D95 − 2D50 + 5 2(D84 − D16 ) 2(D95 − D5 )

(3)

where D is the phi-scale grain size at the relevant cumulative curve and subscripts refer to the appropriate spot on the cumulative curve.

3.3. Mineral composition analysis The sediment samples (dry weight > 2 g) were dried for 24 h at 45 °C in a drying cabinet before analysis. Mineralogical and clay fraction data were measured with an X'Pert PRO DY2198 (PANalytical BV, Almelo, Netherlands) based on the X-ray diffraction (XRD) method at the State Key Laboratory of Geological Processes and Mineral Resources, China University of Geosciences, Wuhan. Analyses were performed with Cu-Kα radiation at 40 kV and 30 mA. The diffraction pattern was recorded from 3° to 65°. Semi-quantitative estimates of mineral concentration (wt%) were obtained by applying the correction factors determined by Cook et al. (1975) and Boski et al. (1998) to the measured intensities of reflections. The analytical precision was better than 5% relative standard deviation (RSD).

3.2. Grain-size measurements Weighed sediment samples (ca. 300–400 mg) were treated with 10 mL of 10% hydrogen peroxide (H2O2) for at least 2 h at 70 °C until the reaction was completed to remove organic matter. Subsequently,

Table 1 Grain-size compositions and geometric parameters of the core sediments in the Shenhu area, SCS. Sample no.

SH1a SH2b SH3a SH4a SH5a SH6a SH7b SH8b SH9b SH10b SH11b SH12b

Drilling site

Core depth (mbsf)

Thickness of sample (m)

W02B W01B W02B W02B W01B W01B W01B W02B W02B W01B W01B W01B

140.86–141.06 147.88–148.13 149.61–149.81 157.41–157.70 158.29–158.51 164.68–164.93 170.55–170.85 171.00–171.28 176.28–176.55 177.06–177.27 182.64–182.92 188.00–188.20

0.20 0.25 0.20 0.29 0.22 0.25 0.30 0.28 0.27 0.21 0.28 0.20

Lithology

Medium silt Fine silt Coarse silt Coarse silt Coarse silt Coarse silt Coarse silt Coarse silt Medium silt Coarse silt Coarse silt Coarse silt

Sand (%)

Clay (%)

Silt (%)

Mean (Φ)

Sorting

Skewness

Kurtosis

D50 (Φ)

1.69 2.30 1.68 2.67 0.60 1.56 1.89 2.01 1.53 4.09 4.12 3.96

0.89 1.03 0.71 0.70 0.77 0.69 0.88 0.75 0.99 0.75 0.74 0.74

97.42 96.67 97.61 96.63 98.62 97.75 97.22 97.23 97.48 95.15 95.11 95.30

6.43 6.47 6.30 6.08 6.36 6.25 6.19 6.20 6.48 6.04 6.10 6.05

2.44 2.59 2.40 2.35 2.32 2.37 2.38 2.34 2.36 2.49 2.49 2.47

−0.04 0.03 −0.09 −0.12 −0.12 −0.11 −0.12 −0.10 −0.03 −0.11 −0.08 −0.11

0.94 0.94 0.95 0.97 0.90 0.90 0.94 0.95 0.92 0.82 0.87 0.89

6.38 6.49 6.23 5.99 6.27 6.16 6.10 6.13 6.45 5.94 6.04 5.96

a

Capillary pressurec (MPa) 0.66 – 0.57 0.89 1.10 0.60 – – – – – –

Refers to gas hydrate-bearing samples. Refers to gas hydrate-free samples. c The capillary pressure of five hydrate-bearing sediment cores was calculated by using the capillary pressure model as follows (Van Genuchten, 1980): Pc = −P0[(S*)−1/λ − 1]−λ, S* = (SA − SirA)/(SmxA − SirA), where Pc is capillary pressure (in pascals), P0 (105 Pa for clay and mud) is injection pressure (in pascals), λ (0.15 for clay and mud) is the slope of capillary pressure curve, S* is effective degree of saturation, SirA (=1) is limit degree of gas hydrate saturation, SmxA (=0) is maximum degree of gas hydrate saturation, SA is degree of gas hydrate saturation. b

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evidenced by the published logging-while-drilling data (gamma-ray and resistivity log data; Fig. 2a). Thus, in this study the sediment cores from two drilling sites were not described separately. On the basis of sedimentary structures, sediment composition and degree of bioturbation, the core sediments from two sites should belong to a similar sedimentary facies, which is characterized by dark-olive colored silt (predominantly coarse silt) containing a high proportion of calcareous complete or fragmented microfossils. This facies is nearly stable in the two drilling sites (Fig. 2b), with weak disruption by pervasive cracks or mousse-like structures. A significant variation in grain-size composition between hydratebearing and hydrate-free sediments is not observed in the Shepard's diagram (Fig. 3) and all the studied samples are predominantly composed of silt (95.11% to 98.62%) (Table 1). This is also the case for the mean grain-size, sorting, skewness, kurtosis, and median grain-size, which range from 6.04Φ to 6.48Φ, 2.32 to 2.59, −0.12 to 0.03, 0.82 to 0.97 and 5.94Φ to 6.49Φ, respectively (Table 1). The calculated capillary pressures (Pc) of five hydrate-bearing sediments range from 0.57 to 1.10 MPa. Vertically, a weak variation with regard to sediment components is observed, with samples from 141.0–176.4 mbsf displaying higher sand content (3.96–4.12%) and lower silt content (95.11–95.30%) than those from 177.2–188.1 mbsf (0.60–2.67% and 96.63–98.62%, respectively) (Table 1 and Fig. 2b). Correspondingly, distinctions of geometric parameters are also shown between these two groups (Fig. 2b). Samples from 141.0–176.4 mbsf are characterized by high mean grain-size (mostly 6.19Φ to 6.48Φ), high kurtosis (0.90 to 0.97), and low sorting (mostly 2.32 to 2.44), whereas those from 117.2–188.1 mbsf are characterized by slightly lower mean grain-size (6.04Φ to 6.10Φ), lower kurtosis (0.82 to 0.89), and higher sorting (2.47 to 2.49).

3.4. FE-SEM/EDS analysis Approximately 15 mL of sediment samples was extracted and dried completely in a drying cabinet at 60 °C for at least 24 h. Subsequently, the dried samples were soaked in ultrapure water for 24 h, rinsed gently with ultrapure water in a sieve (≥65 μm), re-dried in an air-dry oven, and then stored after weighing. Using a high-power binocular microscope, foraminifera and authigenic pyrites were identified and handpicked. Foraminiferal concentrations (individuals/g) were calculated as relative concentrations to host sediments. Typical foraminifera and authigenic pyrites were further observed by a field emission scanning electron microscopy (FE-SEM, Quanta 450 FEG) combined with energy dispersive spectroscopy (EDS, SDD Inca X-Max 50) analysis at the State Key Laboratory of Geological Processes and Mineral Resources, China University of Geosciences, Wuhan. 3.5. Acidolysis hydrocarbon analysis The hydrocarbons in the sediments can occur as (1) free hydrocarbons within sediment voids, (2) weakly absorbed hydrocarbons on the surface of sediments, (3) dissolved hydrocarbons in terms of microbubbles within the pore water of sediments, (4) hydrocarbons entrapped by clay minerals and organisms within the sediments, and (5) hydrocarbons entrapped by secondary minerals (e.g., carbonate minerals) and their cements within sediments (Zhu et al., 2008; and references therein). The last hydrocarbon assemblages can be extracted by the acidolysis method (commonly known as acidolysis hydrocarbons), which was adopted by this study. The effectiveness of the acidolysis hydrocarbon index in the oil-gas geochemical exploration in China has been widely demonstrated (e.g. Chen et al., 2014; Sun et al., 2014). In fact, the acidolysis hydrocarbons are more stable than other types of hydrocarbons (e.g., the headspace hydrocarbons) since they are insusceptible to microbial interference (Sun et al., 2012). A total of 10 g of air-dried, powdered sediment specimens were used for the acidolysis hydrocarbon analysis. Samples were placed in a sealed glass bottle, evacuated to approximately −0.1 MPa in a 40 °C water bath and then exposed to 1:6 hydrochloric acid (V/V). The gases released from the samples were absorbed by 300 g/L potassium hydroxide solution to remove CO2, and then the light hydrocarbon components (C1–C5) of the desorbed gases were measured using a 7890A gas chromatography (Agilent Technologies, U.S.), which was equipped with a flame ionization detector and an HP-AL/S column (50 m × 0.530 mm × 15.0 μm) at the Sinopec Petroleum Exploration and Production Research Institute, Anhui, China. Values were calculated using an external standard method. The results were given in μL/ kg, and the detection limit is 0.1 × 10−6 μL/kg. Detailed analysis procedures were similar to Sun et al. (2012). Carbon and hydrogen isotopes of methane were measured at the Sinopec Petroleum Exploration and Production Research Institute, Jiangsu, China, using a GV-IsoPrism High Performance instrument with a continuous flow Gas Chromatography-Isotope Ratio mass spectrometer. The analytical precision was better than ± 0.3‰. The carbon isotopic composition was reported in per mil (‰) relative to the PeeDee Belemnite (PDB), and the hydrogen isotopic composition was reported in per mil (‰) relative to the Vienna standard mean ocean water (V-SMOW).

4.2. Mineral composition characteristics Mineral composition of core sediments from the Shenhu area is summarized in Table 2. The powder XRD analysis reveals that the studied samples are mainly composed of quartz, illite, calcite, dolomite, chlorite, albite, Kfeldspar, tremolite and pyrite (Fig. 4). The predominant minerals are illite and quartz and to a lesser extent, chlorite and calcium carbonate (i.e., calcite and dolomite), which range from 22.8 to 32.6 wt%, 23.3 to 34.44 wt%, 12.5 to 17.2 wt%, and 12.3 to 24.1 wt%, respectively (Table 2). These minerals do not significantly vary between gas hydrate-bearing and gas hydrate-free samples, except for calcium carbonate (Fig. 5). Calcium carbonate from gas hydrate-bearing samples is generally higher than that from gas hydrate-free samples (16.7–21.0 wt % vs. 12.3–14.4 wt%). Concentrations of other minerals (i.e., albite, Kfeldspar, pyrite, and tremolite) are low (mostly < 10 wt%) and generally scattered between samples. Nevertheless, a slight variation in the pyrite content between gas hydrate-bearing and gas hydrate-free sediments is observed (Fig. 5), with the former primarily characterized by higher pyrite contents (average: 0.5 wt%). 4.3. Morphology analysis 4.3.1. Foraminiferal morphology Foraminifera within the studied samples are generally surrounded by authigenic minerals (e.g., carbonates and pyrites) and coccolith plates (Fig. 6a). They are generally 0.065–1 mm in size (Fig. 6) and thus visible to the naked eyes. Foraminiferal assemblages dominantly occur within hydrate-bearing sediments, with the abundance being 2041 to 4257 ind/g (individuals/g) (average: 3300 ind/g) (Table 2). Foraminiferal assemblages within hydrate-free samples are relatively low, ranging from 1109 to 2693 ind/g (average: 1979 ind/g). Foraminiferal shells can be either smooth or spinous; the thickness of shells ranges from 4 to 113 μm, as shown by the broken ones (Fig. 6b, c). The energy dispersive spectroscopy (EDS) analysis indicates that the

4. Results 4.1. Lithological characteristics The lithological characteristics and calculated geometric parameters, including the mean, sorting, kurtosis, skewness and median grain-size (D50) of the studied samples are given in Table 1 and Fig. 2. It should be noted that the two drilling sites (W01B and W02B) are geographically close to each other and the sediment composition and locations of hydrate-bearing layers in two sites are highly consistent, as 39

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Fig. 2. (a) Lithological characteristics of the sediment cores from the W01B and W02B sites. The gamma ray and resistivity results recorded in the SH-W18 and SHW19 sites are also shown. Note that the two drilling sites in this study (W01B and W02B) were re-drilled in the locations close to SH-W18 and SH-W19, respectively (Yang et al., 2017). (b) Depth profiles of grain size distribution, foraminiferal abundance and acidolysis hydrocarbon in core samples. Note that the core depth of stratigraphic column is asymmetrical and drawn referring to the scale.

Fig. 3. Plot of grain-size composition on the Shepard's (1954) ternary diagram. Data sources: Eastern Nankai Trough, Japan (Ito et al., 2015); Blake Ridge (Ginsburg et al., 2000); Northern Slope of SCS (Zhang et al., 2015); Southern SCS (Tamburini et al., 2003); Northern SCS (Tamburini et al., 2003).

chambers (Fig. 6f, g). The micro-pores from foraminifera with noticeably thick shells are commonly filled with recrystallized minerals; as a result, authigenic pyrite is less developed within these foraminiferal chambers.

shells are predominantly calcareous (Fig. 6d). In addition, foraminifera in this study are characterized by massive micro-pores, which range broadly from 0.6 to 17 μm (mostly 3–15 μm) (Fig. 6b, c, g). These micro-pores provide favorable pathways for the energy exchanges between foraminiferal chambers and external environments, as evidenced by large numbers of authigenic pyrite within the foraminiferal

40

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Table 2 Mineral concentrations (wt%) and foraminiferal abundance of the core sediments in the Shenhu area, SCS. Sample no. a

SH1 SH2b SH3a SH4a SH5a SH6a SH7b SH8b SH9b SH10b SH11b SH12b a b

Drilling site

Chlorite

Quartz

Calcite

Dolomite

Illite

Albite

K-feldspar

Pyrite

Tremolite

Halite

Foraminiferal abundance (ind/g)

W02B W01B W02B W02B W01B W01B W01B W02B W02B W01B W01B W01B

15.1 15.2 13.3 13.9 13.6 12.5 15.8 13.6 17.2 16.1 15.3 12.9

26.0 23.3 24.0 30.6 26.4 30.8 34.4 32.4 33.3 28.1 33.6 30.8

15.0 21.7 17.0 14.2 13.0 15.7 10.7 12.9 12.9 11.2 11.1 11.8

3.6 2.4 2.7 2.5 7.2 5.3 3.2 1.5 1.3 1.8 1.2 2.3

30.7 30.0 31.3 27.9 28.8 22.8 23.2 27.4 25.9 32.6 26.0 26.7

7.5 6.8 9.0 8.1 7.6 8.9 9.7 7.2 8.0 7.7 9.2 9.3

1.1 – 2.3 2.0 3.1 1.6 2.5 2.9 1.5 2.3 1.3 1.6

0.8 0.7 0.5 0.4 0.4 0.6 0.5 0.0 0.0 0.3 0.0 0.0

– – – – – 2.0 – 2.0 – – 2.3 4.6

0.4 – – 0.5 – – – – – – – –

3077 2534 4257 3268 2041 3859 2693 2232 1334 1801 1109 2148

Refers to gas hydrate-bearing samples. Refers to gas hydrate-free samples.

pyrite microcrystals. Clustered framboids are much more common than isolated ones within the studied samples. Euhedral pyrite crystals (mainly octahedra) ranging from 10 to 30 μm in diameter are also observed. Generally, they are observed as overgrowths along the foraminiferal chambers (Fig. 6e–f). It is noted that in the studied samples detrital pyrites, which are characterized by high psephicity, are also observed but quantitatively less abundant than authigenic pyrites with euhedral morphology. Since detrital pyrites are minor and normally exotic, they were not examined in this study.

4.3.2. Authigenic pyrite morphology The authigenic pyrite aggregates from the sediments of the Shenhu area can be divided into three types according to their morphology: (1) irregular aggregations, which are the most common type; (2) rod-like pyrite aggregates, which vary from 1 to 5 mm in length and 0.3 to 0.8 mm in diameter (Fig. 6h); and (3) organism-filling aggregates, which are characterized by pyrite filling the foraminiferal chambers and tests (Fig. 6e–g). Locally, rod-like pyrite aggregates may represent the dominant pyrite morphology, although in most cases they are less common than organism-filling and irregular pyrite aggregations. These pyrite aggregates are predominantly composed of framboids (Fig. 6c, d, g–i). Individual framboids range broadly from 5 to 50 μm in diameter and are generally composed of uniform-sized, octahedral

Fig. 4. XRD charts for the sediment cores. (a) Hydrate-bearing sample SH1 (W02B, ~141 mbsf); (b) hydrate-free sample SH11 (W01B, ~182 mbsf). 41

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Fig. 5. Stratigraphic variation of main minerals in the studied sediments. Note that the shaded areas refer to the gas hydrate-bearing sediment cores. Purple and green symbols refer to samples from the W01B and W02B sites, respectively. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

Fig. 6. SEM images of foraminifera and authigenic minerals from the sediment cores of the Shenhu area. (a) Foraminifera occurring with abundant coccolith plates (W02B, ~149 mbsf); (b) foraminifera chambers surrounded by authigenic carbonates (W01B, ~148 mbsf); (c) foraminifera aggregation cemented mainly by pyrites and a few clay minerals (W01B, ~148 mbsf); (d) close-up view of assemblages of authigenic carbonate and pyrite in (c); (e) a foraminifer chamber filled with pyrites (W01B, ~158 mbsf); (f) close-up view of authigenic pyrites in (e); (g) pyrite framboids in a foraminifer chamber (W01B, ~158 mbsf); (h) irregular pyrite aggregates (W02B, ~141 mbsf); (i) close-up view of (h). 42

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Table 3 Molecular (μL/kg) and isotopic composition of hydrocarbon gases in core sediments from the Shenhu area, SCS. Sample no.

SH1a SH2b SH3a SH4a SH5a SH6a SH7b SH8b SH9b SH10b SH11b SH12b

Drilling site

W02B W01B W02B W02B W01B W01B W01B W02B W02B W01B W01B W01B

C1

C2

C2H4

C3

C3H6

iC4

nC4

iC5

nC5

C1/(C2 + C3)

δ13CCH4 (‰, VPDB)

δDCH4 (‰, VSMOW)

449.02 155.26 909.93 165.74 72.19 559.52 225.94 656.65 370.89 992.73 396.54 627.11

40.98 11.10 71.94 12.31 5.90 43.10 13.37 37.86 17.64 63.61 23.18 40.15

1.21 0.41 0.68 0.15 0.11 0.56 0.42 0.41 0.32 1.03 0.40 0.56

16.77 5.09 29.88 4.85 2.49 17.41 5.27 14.77 6.97 24.58 8.81 15.88

0.67 0.16 0.52 0.11 0.05 0.44 0.16 0.22 0.23 0.44 0.15 0.25

9.39 2.87 17.41 2.78 1.50 10.09 2.87 8.68 3.55 13.85 5.11 9.53

6.78 1.81 10.34 1.69 0.92 5.97 1.89 4.96 2.26 8.25 3.05 5.38

13.38 3.93 26.15 4.20 2.04 14.50 4.31 12.59 5.09 19.64 7.65 13.70

0.94 0.24 1.76 0.28 0.10 1.05 0.32 0.88 0.37 1.36 0.52 0.88

7.78 9.59 8.94 9.66 8.61 9.25 12.12 12.48 15.07 11.26 12.40 11.19

– – −45.3 – – – – −43.2 – −48.6 – −43.7

– – −195 – – – – −202 – −221 – −156

Abbreviations: methane-C1, ethane-C2, propane-C3, N‑butane-nC4, iso‑butane-iC4, N‑pentane-nC5, iso‑pentane-iC5. a Refers to gas hydrate-bearing samples. b Refers to gas hydrate-free samples.

(~141–~188 mbsf) (Fig. 2). All these evidence indicates that the studied samples formed in a similar sedimentary environment. According to Blott and Pye (2001), the mean grain-size of 5–6Φ represents medium-energy deposition, whereas that of 6–7Φ and > 7Φ corresponds to relatively low-energy deposition and low-energy deposition, respectively. Thus, the mean grain-size of sediments in this study, which ranges from 6.04Φ to 6.48Φ, corresponds to relatively low-energy deposition. In addition, the sorting values of the studied sediments display a narrow range of 2.32–2.59, indicating the poorly sorted feature of the studied core sediments. Moreover, frequency curves of most samples from this study are unimodal (Fig. 7) and negatively skewed, with peak values varying over a broad range of 5.64–6.64Φ. Collectively, these characteristics further suggest a lowenergy sedimentary environment. In addition, numerous studies have suggested that growth of authigenic pyrite is sensitive to the depositional environment (Chen et al., 2006; Bond and Wignall, 2010; Wang et al., 2015). The authigenic pyrites in marine sediments, which are important products of the sulfate reduction process (Jørgensen, 1982; Hinrichs et al., 1999; Boetius et al., 2000), are commonly regarded as a proxy of anoxic environments (Wignall and Newton, 1998; Shen et al., 2007; Bond and Wignall, 2010). In this study, an anoxic environment is therefore manifested by

4.4. Molecular and isotopic composition of hydrocarbon gases in sediments The molecular composition of hydrocarbon gases and isotope composition of methane, which are hosted by secondary minerals in the studied sediments, are shown in Table 3. Gas hydrate-bearing samples do not always show a higher hydrocarbon concentration than gas hydrate-free samples, which is partly because that acid-extracted hydrocarbon hosted by secondary minerals reflects an accumulative concentration (Sun et al., 2014) and can change over time. Hydrocarbon gases from the studied sediments are predominantly composed of methane (C1), ethane (C2) and propane (C3), which range broadly from 72.19–992.73 μL/kg, 5.9–71.94 μL/kg, and 5.09–29.88 μL/kg, respectively. N‑butane (nC4), iso‑butane (iC4) and n‑pentane (nC5) are also high and range from 1.50–17.74 μL/kg, 0.92–10.34 μL/kg and 2.04–26.15 μL/kg, respectively. Other hydrocarbon gases including iso‑pentane (iC5) and alkenes (C2H4 and C3H6) are low (generally below 1%). Only 4 of 12 samples have meaningful carbon and hydrogen isotopic values of methane (i.e., δ13CCH4 and δDCH4), which strictly range from −48.6‰ to −43.2‰ and −221‰ to −156‰, respectively.

5. Discussion 5.1. Sedimentary environment Grain-size distribution is an important property of sediment particles that can reflect their entrainment, transport and deposition. Geometric parameters of sediments such as the median grain-size, sorting, kurtosis and skewness can quantitatively represent the characteristics of grain-size distribution. Therefore, these geometric parameters could provide significant clues to the sediment provenance, transport history, hydrodynamic conditions and depositional environments (Visher, 1969; Bui et al., 1989; Blott and Pye, 2001). As mentioned before, the sediment cores from the W01B and W02B sites belong to a similar sedimentary facies, which is characterized by darkolive colored silt (predominantly coarse silt). The grain-size values of sediments in all layers are relatively low and constrained within an extremely narrow range (mean grain-size: 6.04–6.48Φ) (Table 1). The distinctions of other geometric parameters (e.g., sorting, skewness, kurtosis) within two sites, as well as the main minerals, are also not evident and mostly negligible (Figs. 2–5). For example, skewness of the studied samples mainly ranges from −0.12 to −0.03 and kurtosis values strictly range from 0.82–0.97 (platykurtic to mesokurtic) with no significant stratigraphic variation. In addition, the downhole recorded gamma-ray log data from the two sites exhibit coherent and gentle curves, especially within the depth of the samples in this study

Fig. 7. Frequency distribution curve of particle size of the core sediments from the Shenhu area. 43

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approximately from 0.065 to 1 mm, and therefore can significantly increase the grain-size in host sediments by acting as coarse particles. Moreover, when comparing gas hydrate-bearing samples with gas hydrate-free samples, gas hydrate-bearing samples are characterized by a much higher foraminiferal content than gas hydrate-free samples (3300 ind/g versus 1979 ind/g). In addition, most hydrate-bearing layers are characterized by much higher contents of pyrites and carbonate minerals which commonly adhere to foraminiferal shells (Fig. 5), results probably from anaerobic methane oxidation coupled with sulfate reduction (SR-AOM) (Kasten and Jørgensen, 2000; Jørgensen and Kasten, 2006; Zhang et al., 2014; Egawa et al., 2015). Thus, the appearance of authigenic minerals seems to be another evidence for the intimate connection between foraminifera and the occurrence of gas hydrates. Furthermore, the calculated capillary pressure (average: 0.68 MPa) of hydrate-bearing sediments with high foraminifera (> 3000 ind/g) is noticeably lower than that (i.e., 1.10 MPa) of sediments with lower foraminifera (< 3000 ind/g). This indicates that the presence of foraminiferal shells would decrease the capillary force of sediments, which is favorable for the formation of gas hydrates. Therefore, we suggest that the abundant foraminifera within the sediments of the SCS are also one of the key factors in controlling the formation of gas hydrates in the Shenhu area, in view that they can significantly increase the porosities of the fine-grained sediments and thus provide a favorable space for gas hydrate formation. Other microfossils (e.g., cricoid coccolith, Fig. 6a) in sediments might also be beneficial for the formation of gas hydrates (Wang et al., 2011) but are less vital compared to foraminifera considering their smaller size.

the widespread authigenic pyrites (including euhedral and framboidal pyrites). Noticeably, it has been demonstrated that there is a positive correlation between the size of the pyrite framboid and oxygen fugacity (Wignall and Newton, 1998; Bond and Wignall, 2010). The authigenic pyrites (especially framboidal pyrites) from this study show a broad range in diameter from 5 to 50 μm even within samples, therefore implying that oxygen fugacity may slightly fluctuate during the formation of these sediments. Overall, it can be concluded that the core sediments from the two drilling sites formed in a relatively stable, low-energy and anoxic sedimentary environment. 5.2. Gas hydrate occurrence in relation to lithological features in the Shenhu area It has been demonstrated that the properties of host sediments play a crucial role in controlling the heterogeneous accumulation of gas hydrates (Behseresht and Bryant, 2012; Chatterjee et al., 2014). There is a consensus that gas hydrate is preferentially enriched in coarsegrained sediments (Collett et al., 1988; Uchida and Tsuji, 2004; Lu et al., 2011; Winters et al., 2014; Ito et al., 2015; Haines et al., 2017), which can be explained by the favorable permeability or high water availability of coarse-grained sediments (Nimblett and Ruppel, 2003; Johnson et al., 2011; Lu et al., 2011; Winters et al., 2011). Therefore, gas hydrate saturation commonly has a close relationship with geometric parameters of sediments, e.g., median grain size and skewness (e.g., Rose et al., 2014; Ito et al., 2015). However, in this study, the hydrate-bearing layers are hosted by fine-grained sediments (mainly coarse silt) with low sand contents, ranging from 0.60% to 2.67% (average: 1.64%) (Table 1). In contrast, a slightly higher sand content of 1.53–4.12% (average: 2.84%) is observed within gas hydrate-free layers. In addition, when comparing gas hydrate-bearing samples with gas hydrate-free samples, their geometric parameters, e.g., median grain size (6.20Φ versus 6.16Φ), sorting (2.38 versus 2.45), skewness (−0.10 versus −0.08) and kurtosis (0.93 versus 0.91) and main mineral composition (except for calcium carbonate) do not show significant variations. What's more, during Expedition GMGS4, Yang et al. (2017) also found that in the Shenhu area the low gamma ray sediments with coarse grain size distribution did not control the occurrence of gas hydrates. These results seem to suggest that the relationship between the grain-size distribution of sediments and the formation of gas hydrates are much more complicated than, if not opposite to, that suggested by previous studies (Collett et al., 1988; Uchida et al., 2004; Lu et al., 2011; Winters et al., 2014). Actually, the gas hydrates hosted by fine-grained sediments were recently reported in many areas, e.g., the SCS, the Central Mahanadi Basin and the KG Basin in India (Winters et al., 2014; Liu et al., 2015). It is, therefore, reasonable to suggest that there should be other factors that control the formation of gas hydrates. Recent studies regarding gas hydrate occurrence within the SCS revealed that foraminifera in the sediments might play a great role in controlling the formation of gas hydrates (Wang et al., 2011; Chen et al., 2013; Li et al., 2016). This is because the presence of foraminiferal shells can increase the porosity of the sediments and expand the space for hydrate growth. In addition, many foraminifera are characterized by empty chambers that are connected to the outside pores via the foraminiferal mouth or micro-pores in the shells, and therefore can be further enlarged and create space for the formation of gas hydrates. This was demonstrated by Li et al. (2016) using X-ray computed tomography (CT) analysis to observe the formation process of gas hydrates, which revealed that the liquid and methane gas migrated towards the inner space of the foraminiferal shells via the mouth and the micro-pores in the shells. In this study, foraminifera are generally characterized by one or more effectively empty chambers and by shells with multi-pores (mostly in the range of 3–15 μm). Besides, they generally have larger sized particles than sand particles, varying

5.3. Implications for the origin of hydrocarbon gases Hydrocarbon gases can be of thermogenic, microbial, and mixed origin, which are commonly distinguished by their molecular composition and the isotopic values of methane (Kvenvolden, 1988; Wiese and Kvenvolden, 1993; Kvenvolden, 1995; Zhu et al., 2008). Hydrocarbon gases with low C1/(C2 + C3) (< 100) and high δ13CCH4 (> −55‰) are normally of thermogenic origin, whereas those with high C1/(C2 + C3) (> 1000) and low δ13CCH4 (< −55‰) are indicative of biogenic gas (Bernard et al., 1976; Wiese and Kvenvolden, 1993; Whiticar, 1999). In addition, the thermogenic gas is generally characterized by high δ13CCH4 (ranging from −50‰ to −20‰) and δDCH4 (> ca. −300‰), whereas the microbial gas is characterized by more depleted δ13CCH4 and δDCH4, ranging from −100‰ to −50‰ and −400‰ to −150‰, respectively. In this study, the acid-extracted hydrocarbon gases are characterized by the C1/(C2 + C3) ratio of 7.78–15.07 and δ13CCH4 and δDCH4 values of −48.6‰ to −43.2‰ and −221‰ to −156‰, respectively. Thus, in Figs. 8 and 9, all the samples in this study plot into the “thermogenic gas” area, which therefore demonstrates that the thermogenic gas is an important source for the hydrocarbon gases in the Shenhu area. In Figs. 8 and 9, the previously published molecular and isotopic results of the hydrate-bound gas recovered from the SCS and other sea regions of the world are also shown (Zhu et al., 2008; Liu et al., 2015). It can be seen that the origin of the hydrocarbon gases within the SCS is complex, with both thermogenic and microbial gases occurring in this area. This is different from the hydrocarbon gases from the Ulleung Basin (UB), Nankai Trough (NT), Hydrate Ridge (HR) and Krishnae Godavari Basin (KGB), which are all of microbial origin (Waseda and Uchida, 2002; Milkov, 2005; Kim et al., 2011; Stern and Lorenson, 2014). The SCS is also different from the Marmara Sea (MS), Barkley Canyon (BC) and Green Canyon (GC), in which the hydrocarbon gases are solely thermogenic in origin (Sassen et al., 2004; Pohlman et al., 2005; Bourry et al., 2009). Generally, hydrocarbon gases are generated by either the microbial reduction of CO2 or the thermal decomposition of organic matter in marine environments (Kim et al., 2011). However, the multiple sources of hydrocarbon gases in the SCS implies that both 44

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Fig. 8. δ13CCH4 versus C1/(C2 + C3) diagram of hydrocarbon gases in sediments. The molecular and isotope data from the Ulleung Basin (UB), Nankai Trough (NT), Hydrate Ridge (HR), Krishnae Godavari Basin (KGB), Marmara Sea (MS), Barkley Canyon (BC), Green Canyon (GC) are derived from Zhu et al. (2008), Liu et al. (2015) and references therein. Note that the data showed as the South China Sea (SCS) refer to two samples from the Pearl River Mouth basin (Liu et al., 2015), and 154 samples from the Northern and Western Slopes of the SCS, Taixinan basin, Central basin, Xisha trough, Nansha trough, ODP-1146 and ODP-1143 (Zhu et al., 2008).

the microbial reduction of CO2 and the thermal decomposition of organic matter occur widely in the SCS, which in turn are favorable for the formation of gas hydrates by supplying sufficient hydrocarbon gases.

become a useful tool for the gas hydrate exploration, but this still awaits confirmation from more studies.

5.4. Implications for the hydrate exploration

Both gas hydrate-bearing and gas hydrate-free sediments from the southeast Shenhu area are characterized by fine-grained sediments, similar grain-size distributions (e.g., median grain-size, sorting, kurtosis, and skewness) and mineral compositions. They are also characterized by many authigenic pyrites. All these characteristics collectively indicate that the core sediments in this study formed in a relatively stable, low-energy and anoxic sedimentary environment. Nevertheless, the hydrate-bearing samples in this study are characterized by lower sand content and much more foraminifera than gas hydrate-free samples. We suggest that the abundant foraminifera within the sediments act as “coarse grains” that increase the porosity and permeability of sediments, which is favorable for the formation of hydrates in the Shenhu area. In addition, the molecular and isotopic composition of hydrocarbon gases hosted by secondary minerals in the sediments indicates that they are of thermogenic origin. It is, therefore,

6. Conclusions

The accumulation mechanisms of the gas hydrate within the SCS, e.g., the relationships between sediment composition and formation of gas hydrate, as well as the relationships between sedimentary environments and preservation of gas hydrate, are still not well studied. Too much attention might have been paid to the sand content of the host sediments, although a positive correlation between the sand content and the occurrence of gas hydrate has indeed been demonstrated by many studies. In this work, gas hydrate-bearing sediments display slightly lower sand content than gas hydrate-free sediments. This indicates that the importance of the sand content might be overvalued during the hydrate exploration. The positive relationship between the abundance of foraminifera within the sediments and the occurrence of gas hydrate, as illustrated by this study, indicates the foraminifera may

Fig. 9. δD versus δ13C diagram of CH4. The symbols are as in Fig. 8. 45

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demonstrated that thermogenic gas is an important source of hydrocarbon gases for the gas hydrates in the SCS.

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Acknowledgement This work was supported by The National Key Research and Development Program of China (2017YFC0307600), National HighLevel Talent Special Support Plan, the National Natural Science Foundation of China (41672367, 41474119), China Geological Survey Project (DD20160221, DD20189320 & DD20189330), Qingdao National Laboratory for Marine Science and Technology Open Fund (QNLM2016ORP0203) and Fundamental Research Founds for National University, China University of Geosciences, Wuhan (1810491T05). Special thanks are given to the Editor-in-Chief Prof. Shu Gao and two anonymous reviewers for their critical and constructive reviews. References Bahk, J.J., Um, I.K., Holland, M., 2011. Core lithologies and their constraints on gashydrate occurrence in the East Sea, offshore Korea: results from the site UBGH1-9. Mar. Pet. Geol. 28 (10), 1943–1952. Behseresht, J., Bryant, S.L., 2012. Sedimentological control on saturation distribution in Arctic gas-hydrate-bearing sands. Earth Planet. 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