Origin of natural gases and associated gas hydrates in the Shenhu area, northern South China Sea: Results from the China gas hydrate drilling expeditions

Journal of Asian Earth Sciences 183 (2019) 103953

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Origin of natural gases and associated gas hydrates in the Shenhu area, northern South China Sea: Results from the China gas hydrate drilling expeditions

T

Wei Zhang, Jinqiang Liang , Jiangong Wei , Pibo Su, Lin Lin, Wei Huang ⁎

⁎

MLR Key Laboratory of Marine Mineral Resources, Guangzhou Marine Geological Survey, Ministry of Natural Resources, Guangzhou, Guangdong 510075, China

ARTICLE INFO

ABSTRACT

Keywords: Gas hydrate Genetic origin Gas fractionation Shenhu area South China Sea GMGS3 and GMGS4 expeditions

Geochemical data for hydrate gases acquired from the GMGS3 and GMGS4 gas hydrate drilling expeditions conducted by the Guangzhou Marine Geological Survey (GMGS) are used to explore the origin of hydrate gases and their relationship to deep hydrocarbon reservoirs, and to evaluate the contribution of different genetic types of gases to the formation and accumulation of gas hydrates in the Shenhu area of the northern part of the South China Sea (SCS). Compositionally, methane is the dominant gas (> 90%) in the void gas and pressure core gas. In addition, as much as ~3% of the gas is composed of C2+ hydrocarbons, including ethane, propane, iso-butane, butane, iso-pentane, and n-pentane. The δ13C-CH4 and δD-CH4 values indicate a mixed biogenic-thermogenic origin for the hydrate-forming gas. The methane isotope correlation indicates that the source of the hydrate gas is closely related to the deep conventional gas reservoirs discovered in the Baiyun Sag-Panyu Low Uplift area. Both the hydrate gases and the deep reservoir gases are sourced from the hydrocarbon kitchens in the Baiyun Sag, revealing a paragenetic relationship within the same petroleum system. The composition of the hydrocarbons and the isotopic variation of methane with depth suggest that the thermogenic gas was likely affected by compositional and isotopic fractionation during the long-distance migration from the deep source rocks to the shallow gas hydrate stability zone (GHSZ). The impact of biodegradation on a solely thermogenic gas could also affect the final composition of the hydrate-forming gas. Analysis of the GHSZ based on gas hydrate compositions suggests that the occurrence of thermogenic gas could also indicate the coexistence of structure I (SI) and structure II (SII) gas hydrates in the Shenhu area, with the SII hydrates accumulating in or below the lower part of the SI GHSZ. The confirmed presence of SII hydrates in the Shenhu area relocated the base of the GHSZ deeper than was indicated by the bottom simulating reflector, which warrants further study in future explorations for gas hydrates in the Shenhu area.

1. Introduction Gas hydrates are solid ice-like compounds consisting of gas (mostly methane) trapped inside water molecules that form under well-defined temperature and pressure conditions (Kvenvolden, 1993). They mainly accumulate within the permafrost zone or in deep water sediments at depths greater than 300–600 m (Buffett and Archer, 2004; Dickens et al., 1995; Kvenvolden, 1988, 1993, 1995; Sloan, 1998; Wei et al., 2015). The formation and stability of gas hydrates depends on a number of factors including temperature and pressure, gas composition and saturation, and the chemical characteristics of the interstitial water. In addition, the mineral composition, grain size, and physical properties of the reservoir constrain the crystallization and growth of gas hydrates

(Clennell, 1999). Most importantly, gas hydrate formation and accumulation require a sufficient hydrocarbon supply, which also determines the gas hydrate type, distribution, and resource potential. Thus, the hydrocarbon supply is of great significance to hydrate resource evaluation, as well as exploration and development (Boswell and Collett, 2006; Walsh et al., 2009; Wu et al., 2018; Boswell and Collett, 2011; Su et al., 2018). Most of the gas hydrates that have been discovered worldwide are biotic, and the composition of the hydrate-bound gas is either biogenic, thermogenic, or a biogenic-thermogenic mixture (Limonov et al., 1997; Ginsburg and Soloviev, 1997; Lykousis et al., 2009; Vaular et al., 2010). Biogenic gas hydrates are thought to account for the majority of gases worldwide, occurring in areas such as the Blake Ridge in the western

⁎ Corresponding authors at: Guangzhou Marine Geological Survey, Ministry of Natural Resources, No. 188, Guanghai Road, Huangpu District, Guangzhou, Guangdong 510075, China. E-mail addresses: [email protected] (J. Liang), [email protected] (J. Wei).

https://doi.org/10.1016/j.jseaes.2019.103953 Received 15 February 2019; Received in revised form 5 August 2019; Accepted 5 August 2019 Available online 05 August 2019 1367-9120/ © 2019 Elsevier Ltd. All rights reserved.

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Fig. 1. Geologic setting and locations of the gas hydrate drilling and coring sites in the Shenhu area. (A) The Shenhu gas hydrate drilling area within the Baiyun Sag in the Pearl River Mouth Basin, which developed in the middle of the northern slope of the South China Sea. The Baiyun Sag is located at the transition between the continental shelf and the continental slope. (B) A series of conventional oil and gas fields have been discovered in the Baiyun Sag-Panyu Low Uplift area in the vicinity of the Shenhu gas hydrate drilling area (Dai, 2014; Sun et al., 2014). (C) Locations of the GMGS3 and GMGS4 gas hydrate drilling and coring sites (Yang et al., 2017).

Pacific, the northern California sea area, the Oregon sea area, the Nankai Trough, the Ulleung Basin, the Okhotsk Sea, and the Black Sea (Brooks et al., 1991; Choi et al., 2013; Ginsburg and Soloviev, 1997; Waseda and Uchida, 2004a, 2004b). Thermogenic gas hydrates have also been recovered in many places, including the Gulf of Mexico (Sassen et al., 1999, 2001a, 2001b), the Svalbard (Smith et al., 2014), the west African province (Cunningham and Lindholm, 2000; Serie' et al., 2016; De Prunele et al., 2017), the Caspian Sea (Lüdmann and Woon, 2003), the Cascadia Margin (Pohlman et al., 2005), the NW Borneo region of the South China Sea (SCS) (Paganoni et al., 2016, 2018), and the western SCS (Ye et al., 2019). In addition, mixed biogenic-thermogenic gas has been found in gas hydrates retrieved from a few submarine settings, such as in the Norwegian Sea (Vaular et al., 2010) and Japan Sea (Waseda and Iwano, 2008). Previous studies have demonstrated that thermogenic gas emitted from oil and gas reservoirs also makes a significant contribution to the accumulation of gas hydrates (Kvenvolden, 1995; Sassen et al., 2001a, 2001b). Moreover, structure II (SII) or H (SH) gas hydrates may coexist with structure I (SI) gas hydrates in geological settings with an ample thermogenic gas supply (Kida et al., 2006; Lu et al., 2007; Klapp et al.,

2010; Paganoni et al., 2016). The source and migration mechanism of hydrate gases, and how these factors contribute to the formation of gas hydrates have been widely discussed in previous studies (Horozal et al., 2009; Choi et al., 2013; Wang et al., 2014; Kang et al., 2016; Su et al., 2016; Lorenson and Collett, 2018; Dai et al., 2017; Su et al., 2017; Su et al., 2018). In addition, many studies have investigated the relationship between hydrate gas sources and deep hydrocarbon reservoirs in specific geologic settings (Chen and Cathles, 2003; De Prunele et al., 2017; Ostanin et al., 2013; Paganoni et al., 2016; Sassen et al., 1999, 2001a, 2001b). In 2007, the Guangzhou Marine Geological Survey (GMGS) conducted the first gas hydrate drilling expedition in the Shenhu area of the northern SCS and successfully acquired gas hydrate samples (Yang et al., 2008). Geochemical analysis of these gas hydrates revealed that the hydrate gases are dominated by methane with trace levels of C2+ hydrocarbons (Huang et al., 2010; Wu et al., 2011). However, the origin of the hydrate gases is still debated. Many researchers have proposed that hydrate gas in the Shenhu area has primarily been sourced from biogenic gas generated through CO2 reduction (Fu and Lu, 2010; Wu et al., 2010, 2011; Zhu et al., 2013), and have concluded that 2

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the contribution of deep thermogenic gas to hydrate formation is limited. In contrast, Wang et al. (2014) concluded that the hydrate gas recovered from the Shenhu area has mostly originated from a deepseated thermogenic source. These differing viewpoints have resulted from the limited and varied data sets that are available for this area. Therefore, more hydrate gas samples and geochemical data are needed to better understand the origin of the hydrate gases and to discuss the role of thermogenic gas in fluid migration and hydrate accumulation in the Shenhu area. The GMGS carried out the GMGS3 and GMGS4 drilling expeditions in the Shenhu area of the Baiyun Sag in 2015 and 2016, respectively (Yang et al., 2015, 2017a, 2017b; Zhang et al., 2018). Both expeditions obtained large quantities of gas hydrates and hydrate gas samples, providing an excellent opportunity to study the origin of the hydrate gases and their relationship to the deep hydrocarbon reservoirs. In this study, for the first time, we present the systematic analyses for the hydrate gas samples acquired by the GMGS3 and GMGS4 drilling expeditions from seven coring sites in the Shenhu area. Based on the geochemical analysis of these hydrate gases and comparison to deep conventional gas fields in the Baiyun Sag-Panyu Low Uplift area, we discuss the origin of the hydrate gases and their correlation with the underlying gas reservoirs. The results of this study will aid in understanding the accumulation mechanism of gas hydrates in the Shenhu area of the SCS and determining their exploration potential.

formations mostly developed in the margin and slope areas of the Baiyun Sag, such as in the vicinity of the LW3-1 gas field and in the Panyu Low Uplift area. Some of these faults cut the T5 unconformity and extend into the Oligocene-Eocene strata (Qiao et al., 2014; Su et al., 2014; Cong et al., 2018). However, few large faults have been found in the center of the sag. Numerous small northeast trending step faults were active in the Pliocene-Quaternary due to neotectonic gliding along detachment surfaces in the unconsolidated sediments (Qiao et al., 2014; Su et al., 2014). In addition, active mud diapirs and gas chimneys are present in the center of the Baiyun Sag (Wang et al., 2006; Shi et al., 2009a; Su et al., 2016; Zhang et al., 2018). The gas chimneys, mud diapirs, and large deep faults connect with the source rocks and serve as vertical migration pathways for deep hydrocarbons. The Baiyun Sag and the surrounding area are hot spots for petroleum exploration in the PRMB (Pang et al., 2008; Shi et al., 2010; Zhu et al., 2012; Zhang et al., 2014; Lin et al., 2014). The Eocene mediumdeep lake mudstones and the Oligocene coal beds and marine-continental transition mudstones are the main source rocks (Fig. 2). The organic matter is mature-overmature and generates large amounts of oil and gas, resulting in the accumulation of conventional petroleum reservoirs (Shi et al., 2014; Li et al., 2015; Peng et al., 2017). The deeply buried mudstones of the Zhuhai Formation are another set of mature source rocks in this area (Gao et al., 2015). In contrast, the organic matter in the marine mudstones of the upper Zhujiang Formation and the overlaying strata are too immature to generate thermogenic hydrocarbons. However, they have the ability to produce large amounts of biogenic gas and supply hydrate accumulation in the Shenhu area (Fu et al., 2007; He et al., 2013; Su et al., 2018). The marine-continental transition mudstones and sandstones deposited in the Oligocene Enping and Zhuhai formations and the Miocene marine sandstones are the main reservoirs and exploration targets for conventional oil and gas in the Baiyun Sag (Pang et al., 2008; Liu et al., 2011; Zhu et al., 2012). In addition, a series of oil and gas fields, including the LW3-1, PY29-1, PY30-1, and PY34-1 fields, have been discovered in the Baiyun Sag and the Panyu Low Uplift area (Fig. 1B), demonstrating that the Baiyun Sag is a hydrocarbon-rich sag with a tremendous petroleum potential (Zhang et al., 2014; Dai, 2014).

2. Geologic setting and petroleum system The Shenhu gas hydrate drilling area is located within the Baiyun Sag in the Pearl River Mouth Basin (PRMB), which developed in the middle of the northern continental slope of the SCS. The Baiyun Sag is located between the continental shelf and the continental slope (Fig. 1A), covers more than 20,000 km2, has water depths of 200–2000 m, and has a crustal thickness of 18–28 km (Su et al., 1989; Yu, 1990). The gas hydrate drilling results revealed a geothermal gradient for the Shenhu area of 44 °C–67 °C/km (Zhang et al., 2018). The Shenhu area generally has temperature and pressure conditions favorable for gas hydrate formation and accumulation (Wu et al., 2010; Liang et al., 2014; Su et al., 2018). Cenozoic tectonic activity was strong in the PRMB. The structural evolution of the basin can be divided into three stages: a rifting stage in the Early Paleogene, a rifting-subsidence transition stage in the Late Paleogene, and a subsidence stage in the Neogene, which controlled the tectonic evolution and the pattern of sediment deposition in the PRMB (Shao et al., 2008; Xie et al., 2013; Shi et al., 2014; Pang et al., 2018). The PRMB stratigraphy is composed of two units (Paleogene and Neogene) separated by the T5 unconformity (Fig. 2), which marks the transition from active rifting to passive margin subsidence. The lower unit is composed of separated faulted depression sediments deposited from the Paleocene to the Lower Oligocene. From bottom to top, the lower unit includes the medium-deep lake sandstones and mudstones of the Shenhu and Wenchang formations and the lacustrine sandstones and mudstones as well as interbedded coal beds with sandstones and mudstones of the Enping Formation (Fig. 2) (Cui et al., 2009; He et al., 2009; Li et al., 2015; Mi et al., 2008; Shi et al., 2010). The upper unit is composed of regionally widespread marine deposits, including the Zhuhai, Zhujiang, Hanjiang, Yuehai, Wanshan, and Qionghai formations (Cui et al., 2009; He et al., 2012; Shi et al., 2014) (Fig. 2). The bottom of the upper unit is composed of large scale delta facies and offshore facies with overlying marine sandstones. In addition, reefs are present on the local basement highs (He et al., 2007; Liu et al., 2011). Three peaks in fault activity occurred in the Baiyun Sag during the Palaeocene-Early Oligocene, the Early Miocene-Middle Miocene, and the Late Miocene. Fault activity has been particularly intense since the Late Miocene. All of the faults are extensional or transtensional and may serve as migration pathways for gas-bearing fluids (Su et al., 2014, 2017). The large faults connecting the Paleogene and Neogene

3. Data and methods 3.1. Gas sampling During the GMGS3 and GMGS4 expeditions, the void gas (VG), pressure core gas (PG), and headspace gas (HG) of collected cores were sampled and tested. In this study, we analyzed the VG and PG samples from four GMGS3 coring sites (GMGS3-W11, GMGS3-W17, GMGS3W18, and GMGS3-W19) and three GMGS4 coring sites (GMGS4-SC1, GMGS4-SC2, and GMGS4-SC3). The distribution and locations of the coring sites are shown in Fig. 1C. The VG formed within the conventional (non-pressure) core liners after gas expansion during core retrieval. The core liners were punctured immediately upon recovery using a sampling probe equipped with a three-way valve, and the gas samples were collected into 140 mL plastic syringes (Choi et al., 2013; Lorenson and Collett, 2018). The VG samples from the cores were analyzed for light hydrocarbon gases. All of the pressure cores were depressurized in a controlled fashion using an autoclave degassing system (ADS) to quantify the total amount of hydrocarbon gas in all phases, including the gas hydrate phase (Wu et al., 2011). As the pressure in the ADS was slowly and incrementally decreased via the manifold, the gas and fluid were expelled. PG samples were collected throughout the depressurization process, and the PG was analyzed to determine both the total amount and type of gas present. The composition of the PG reflects the composition of the gas released from the pore water, the free gas, and the gas of the dissociated gas hydrate (Milkov et al., 2004). The selected VG and PG samples were stored in sample bags for 3

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Fig. 2. Generalized stratigraphic and tectonic framework of the Baiyun Sag in the Pearl River Mouth Basin (modified from Pang et al., 2008).

shore-based analyses.

and distribution of gas hydrates in the Shenhu area, we used the CSMHYD program to model and calculate the gas hydrate stability zone (GHSZ) (Sloan, 1998) based on the data obtained during drilling and sampling. In this study, we present the modeling results for the GMGS4SC1 site (Fig. 8), which is a representative site for the recovery of SII gas hydrates in the Shenhu area (Wei et al., 2018). Water depth at the GMGS4-SC1 site is ~1286.5 m and the seafloor temperature is 3.76 °C. A linear geothermal gradient of 63 °C/km was used to model this site. A pore-water salinity of 3.5% and different gas compositions (100% CH4, 99.5% CH4 + 0.5% C2H6, 99% CH4 + 1% C2H6, 98.5% CH4 + 1.5% C2H6, 98% CH4 + 2% C2H6, 97.5% CH4 + 2.5% C2H6) were used as model input parameters to calculate the hydrate stability curve. In addition, the base of the gas hydrate stability zone (BGHSZ) was obtained by determining the intersection of the hydrate stability curve with the geothermal gradient. Finally, the depth of the BGHSZ was compared with the depth of the bottom simulating reflector (BSR) obtained through the interpretation of seismic results that were calibrated using well data.

3.2. Gas analyses The gas compositions of the VG and PG samples were analyzed offshore at the drilling sites. For accurate determinations, the effects of air contamination resulting from sampling were subtracted from the gas totals. The gas compositions were determined using an INFICON Micro GC Fusion gas chromatograph with a molecular sieve, PLOT Q column, and thermal conductivity detector (Wu et al., 2011; Wei et al., 2018). The concentrations of oxygen, nitrogen, methane, ethane, propane, butane, isobutane, isopentane, and n-pentane were measured with a detection limit for all gases of 10 ppm and a quantification limit of 30 ppm. Concentrations ranging between 10 and 30 ppm have been labeled as “trace” in Tables 1 and 2. The carbon and hydrogen isotopes of the methane in both the PG and VG samples were analyzed onshore to determine the genetic type and origin of the hydrate gas. Isotopes were measured using gas chromatography-isotope ratio-mass spectrometry (GC-IR-MS) at the GMGS (Wei et al., 2018). All of the isotopic values are reported as parts permill (‰). The δ13C values are reported relative to the Vienna Peedee Belemnite standard (V-PDB), and the δD values are reported relative to standard mean ocean water (SMOW).

4. Results 4.1. Gas composition Methane, ethane, propane, butane, iso-butane, iso-pentane, and npentane were detected in the gas samples. The concentrations of hydrocarbon gases collected at different coring sites varied with depth (Tables 1 and 2; Figs. 3 and 4).

3.3. Gas hydrate stability zone calculation To discuss the impact of the different gas types on the accumulation 4

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Table 1 Molecular and isotopic properties of void gas, pressure gas from the GMGS3 coring sites. Site/Hole

Core

Type

Depth (mbsf)

Methane (ppm)

Ethane (ppm)

Propane (ppm)

Butane (ppm)

Isobutane (ppm)

Pentane (ppm)

C1:C2

δ13CCH4 (‰)

δDCH4 (‰)

GMGS3-W11

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21

VG VG VG VG VG PG VG VG VG VG PG PG PG VG VG PG VG VG PG VG VG

101.0 103.4 106.4 109.8 110.8 146.0 149.5 150.2 152.1 152.6 153.5 153.5 171.0 173.3 173.8 183.0 191.0 192.7 206.0 208.6 209.5

987,646 989,667 990,439 956,667 943,070 982,898 994,652 993,770 989,168 992,413 902,531 896,134 961,102 984,057 983,177 986,542 990,850 989,296 887,340 988,368 983,461

327 314 314 348 355 3578 3975 3910 4069 4210 3141 3165 3978 4724 4779 4577 5131 5259 2960 3523 3256

trace b.d b.d b.d b.d 70 65 67 78 84 407 370 114 88 80 109 111 124 496 579 505

b.d b.d b.d b.d b.d b.d. b.d b.d b.d b.d trace trace b.d. b.d b.d b.d. b.d b.d 77 trace trace

b.d b.d b.d b.d b.d b.d. b.d b.d b.d b.d 237 225 b.d. b.d b.d b.d. b.d b.d 74 trace trace

b.d b.d b.d b.d b.d b.d. b.d b.d b.d b.d b.d. b.d. b.d. b.d b.d b.d. b.d b.d trace b.d b.d

3019 3156 3158 2751 2659 271 250 254 243 236 285 280 239 208 206 212 193 188 296 281 302

−65.0

−170.4

−63.9

−180.7 −169.3

−65.0 −59.5 −64.9

−174.1 −178.0 −180.3

−63.5

−175.3

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26

VG VG VG VG VG VG VG VG VG VG VG VG VG VG VG VG VG VG VG PG VG VG VG VG PG PG

51.6 56.2 62.1 63.1 87.5 88.1 125.6 157.0 158.6 161.2 189.0 190.7 193.0 199.9 201.2 207.2 207.9 217.2 218.9 222.0 225.9 226.2 238.5 239.6 241.0 263.0

934,761 942,442 953,486 954,743 974,313 974,326 979,091 981,608 981,963 982,715 982,145 985,029 985,573 974,924 967,853 980,562 981,117 988,429 988,885 969,780 993,770 988,698 945,140 967,535 972,194 979,623

1509 1513 1566 1598 1637 1664 1751 1787 1796 1830 1862 1845 1859 3201 4209 8735 8709 7806 7819 5587 5458 5463 4605 6247 4331 4096

590 598 n.a. 656 700 705 786 817 836 835 953 932 936 8863 16,181 6872 6870 1144 800 204 151 451 6614 11,853 585 1225

trace 115 135 133 175 183 179 206 222 229 258 252 249 652 928 274 269 54 48 trace trace 49 262 515 58 62

b.d b.d trace trace trace 33 trace 34 35 39 43 52 47 680 1355 399 403 83 49 trace b.d trace 401 843 56 137

b.d b.d b.d b.d trace b.d trace trace trace trace trace trace trace 51 53 trace b.d b.d b.d trace b.d b.d b.d trace trace trace

619 623 609 597 595 585 559 549 547 537 527 534 530 305 230 112 113 127 126 171 182 181 205 155 222 236

−73.8

−144.1

−63.7

−142.0

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22

VG VG PG VG VG VG VG VG VG VG VG VG PG PG PG VG VG VG VG VG VG PG

58.2 59.3 117.0 123.0 124.0 126.1 130.4 131.8 134.9 137.9 142.0 142.1 146.0 149.0 159.5 163.6 165.0 167.0 167.6 170.7 171.2 171.5

968,581 944,477 857,970 950,543 951,001 950,176 946,098 946,747 946,527 954,750 935,882 937,943 972,761 963,563 980,119 992,558 992,931 988,155 989,171 989,048 988,826 911,974

310 310 382 303 306 301 175 159 143 279 58 57 610 720 1495 4027 2972 7486 7533 7085 7328 5572

b.d trace 372 441 468 470 558 562 606 2755 1954 2297 9212 843 2939 1408 2149 3220 2850 3023 3659 1620

b.d b.d b.d n.a. n.a. n.a. n.a. n.a. n.a. n.a. n.a. n.a. 67 trace trace n.a. n.a. 31 36 40 41 trace

b.d b.d b.d n.a. n.a. n.a. n.a. n.a. n.a. n.a. n.a. n.a. 268 trace 87 n.a. n.a. 82 78 103 98 48

b.d b.d b.d n.a. n.a. n.a. n.a. n.a. n.a. n.a. n.a. n.a. trace b.d b.d n.a. n.a. b.d b.d b.d b.d b.d

3124 3047 2229 3141 3113 3157 5393 5940 6628 3425 16,212 16,511 1561 1318 643 247 334 132 131 140 135 162

GMGS3-W17

GMGS3-W18

−140.8

−146.3

−55.7

−149.5

−60.9 −62.4

−180.5 −179.9

−38.4 −38.4 −34.9

−154.0 −137.7 −156.6

−41.1

−130.0

(continued on next page)

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Table 1 (continued) Site/Hole

Core

Type

Depth (mbsf)

Methane (ppm)

Ethane (ppm)

Propane (ppm)

Butane (ppm)

Isobutane (ppm)

Pentane (ppm)

C1:C2

GMGS3-W19

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15

VG VG VG VG VG VG VG PG PG VG VG PG VG VG VG

52.5 105.6 108.7 115.6 119.6 126.3 129.2 134.0 137.0 142.1 149.0 154.5 162.1 165.0 167.4

975,269 986,937 979,456 985,910 986,655 984,290 988,143 974,744 975,509 974,561 972,035 983,319 991,902 990,294 984,741

164 749 741 535 461 266 208 498 1499 2890 7504 5662 4663 5690 7819

b.d 166 161 418 431 601 778 11,028 9033 5477 1084 4599 3044 3139 3628

b.d b.d b.d b.d b.d b.d b.d 234 693 trace b.d 135 53 68 67

b.d b.d b.d b.d b.d b.d b.d 891 2287 trace b.d 375 213 356 256

b.d b.d b.d b.d b.d b.d b.d b.d b.d b.d b.d b.d b.d b.d b.d

5951 1318 1322 1843 2138 3702 4753 1922 637 337 130 170 213 174 126

δ13CCH4 (‰)

δDCH4 (‰)

−143.8

−45.4 −45.5

−170.0 −165.4

−45.8

−173.5

b.d.: below detection limit (near 10 ppm); trace: below quantification limit (near 30 ppm); n.a.: not available; mbsf: meters below seafloor.

In all of the gas samples, methane was the dominant hydrocarbon gas; however, there were elevated concentrations of other heavier hydrocarbons (C2+) (Tables 1 and 2). The methane content in gas samples from all coring sites ranged from 857,970 ppm to 999,150 ppm. With the exception of PG samples collected from 153.5 mbsf (meters below seafloor) and 206 mbsf at site GMGS3-W11 and from 117 mbsf at site GMGS3-W18, all of the PG and VG samples had > 90% (gas volume percent) methane with a maximum value of 99.9%. The ethane content ranged from 24 ppm to 25,251 ppm. Although propane gas was not detected in some of the gas samples collected from above the gas hydrate-bearing zone (GHBZ) (Tables 1 and 2), propane levels generally increased with increasing depth, with a maximum measured value of 16,181 ppm. Both the ethane and propane contents were higher within the GHBZ than above the GHBZ. In addition, both the ethane and propane contents of the VG and PG samples increased within the GHBZ at all of the coring sites, except site GMGS3-W17. Trace amounts of iso-butane, n-butane, iso-pentane, and n-pentane were detected in gas samples recovered from the coring sites. The VG and PG samples from sites GMGS4-SC1 and GMGS4-SC2 had relatively higher iso-butane and n-butane contents with maximum values of 2287 ppm and 5380 ppm, respectively. In addition, iso-pentane contents of up to 8082 ppm and 6357 ppm were detected in the GMGS4SC1 and GMGS4-SC2 site samples, respectively. However, the iso-butane and pentane concentrations in samples from the GMGS3 and GMGS4-SC3 sites were below the detection limit. In general, although some differences in hydrocarbon concentrations were noted between gas samples from the GMGS3 and GMGS4 coring sites, there was no noticeable difference with respect to the gas composition. The C1/C2 ratio of the gas samples generally decreased with increasing depth but changed abruptly at the top of the GHBZ (Figs. 3 and 4). The C1/C2 ratios within and below the GHBZ were less than 300, while the C1/C2 ratios above the GHBZ exceeded 800–1000 (Figs. 3 and 4).

gas samples obtained from site GMGS3-W18 was analyzed for stable carbon isotopes. These four samples had δ13C-CH4 values of −41.1‰ to −34.9‰ with an average of −38.2‰. The δD-CH4 values of site GMGS3-W18 ranged from −156.6‰ to −130.0‰ with an average of −144.6‰. Unfortunately, only three gas samples from site GMGS3W19 had detectable isotope ratios. Their δ13C-CH4 values ranged from −45.8‰ to −45.5‰ with an average of −45.6‰ while the δD-CH4 values ranged from −173.5‰ to −143.8‰ with an average of −163.2‰. Isotopic data were also obtained for methane from the GMGS4 samples. The δ13C-CH4 for site GMGS4-SC1 ranged from −63.3‰ to −46.4‰ with a mean of −49.5‰ while the δD-CH4 ranged from −177.7‰ to −138.3‰ with a mean of −162.7‰. The VG and PG samples obtained from site GMGS4-SC2 had methane isotopic values similar to those from site GMGS-SC1. The δ13C-CH4 values of site GMGS4-SC2 ranged from −67.7‰ to −44.4‰ with an average of −49.9‰ while the δD-CH4 ranged from −185.0‰ to −144.0‰ with a mean of −167.8‰. Finally, samples from site GMGS4-SC3 had δ13CCH4 values of −66.4‰ to −64.6‰ with an average of −65.2‰ and δD-CH4 values of −189.5‰ to −162.2‰ with an average of −181.7‰. 5. Discussion 5.1. Origin of the natural gas The molecular and isotopic composition of a recovered gas sample allows us to determine the type and origin of the gas (Benard et al., 1977; Schoell, 1983; Clayton, 1991; Paull et al., 1993; Whiticar, 1999; Matsumoto et al., 2000). Geochemical analysis of the VG and PG samples showed methane to be the dominant hydrocarbon gas in all of the gas samples acquired from the GMGS3 and GMGS4 drilling and coring sites. In addition, ethane, propane, butane, and pentane were also detected. In general, the C2+ contents of both the VG and PG samples increased gradually with increasing depth (Figs. 3 and 4), indicating the presence of a mixed source below the GHSZ where the gas hydrates accumulated. Further analysis showed that the C1/C2 ratios of the VG and PG samples decreased with increasing depth. The C1/C2 ratios generally exceeded 1000 above the GHBZ, but they decreased significantly within and below the GHBZ (Figs. 3 and 4), indicating that the concentration of C2 increased with increasing depth. The variation in C1/C2 within the GHBZ may simply be caused by the preferential incorporation of C2+ gas within the hydrate cage, making the deeper hydrates richer in C2+ compared with the shallower hydrates. In addition, we speculate that the sources of hydrate gases at the coring sites are composed of both biogenic and thermogenic gas.

4.2. Isotopic compositions of the gases The carbon and hydrogen isotopic compositions of methane recovered from the four GMGS3 coring sites and the three GMGS4 coring sites are summarized in Tables 1 and 2, respectively. The stable carbon isotope ratios of methane (δ13C-CH4) from site GMGS3-W11 ranged from −65‰ to −59.5‰ with an average of −63.6‰ while the stable hydrogen isotope ratios (δD-CH4) ranged from −180.7‰ to −169.3‰ with an average of −175.4‰. The δ13CCH4 values from site GMGS3-W17 ranged from −73.8‰ to −55.7‰ with an average of −63.3‰, and the δD-CH4 values from −180.5‰ to −140.8‰ with an average of −154.7‰. The methane from only four 6

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Table 2 Molecular and isotopic properties of void gas, pressure gas from the GMGS4 coring sites. Site/Hole

Core

Type

Depth (mbsf)

Methane (ppm)

Ethane (ppm)

Propane (ppm)

Isobutane (ppm)

Butane (ppm)

Isopentane (ppm)

Pentane (ppm)

C1:C2

GMGS4-SC1

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38

VG VG VG VG VG VG PG PG PG PG VG VG PG PG PG VG PG PG PG PG VG VG PG PG PG PG PG PG PG VG VG PG VG VG VG PG PG VG

57.0 108.3 119.4 126.2 129.7 132.5 135.7 136.0 136.1 137.3 137.9 142.0 147.4 147.6 148.2 150.2 155.2 155.5 155.6 155.8 156.7 157.0 158.2 158.9 159.7 160.1 161.6 162.2 162.9 162.9 169.2 170.0 175.9 178.1 180.9 182.6 185.3 189.9

944,104 951,701 941,081 931,487 927,935 922,611 932,267 935,487 906,166 905,425 955,365 942,379 986,871 998,274 999,150 994,921 979,753 975,222 996,136 997,637 994,190 993,896 996,329 993,931 982,311 991,410 982,679 988,661 992,397 984,980 985,799 988,932 991,152 988,165 964,941 987,937 984,325 984,029

233 380 283 150 126 59 96 106 57 52 70 168 44 51 128 24 888 1043 644 1051 908 886 1436 2401 4144 7382 4550 7417 4842 10,271 11,140 7766 6467 8426 8558 9508 11,217 10,628

b.d. 261 385 504 539 713 1411 2113 2035 2049 2200 15,733 1086 1142 485 1060 1436 2917 2084 837 3503 3667 1238 2489 900 713 1440 2704 1411 3437 1495 1976 1399 2110 1955 1746 3082 3498

b.d. trace 40 43 45 79 319 436 464 406 533 2182 148 180 60 167 220 244 443 141 671 729 261 529 184 121 268 474 242 656 292 360 238 473 443 378 673 866

2483 111 575 1038 1196 1997 4843 4913 7563 5735 5168 3048 176 118 trace 69 103 273 290 151 300 277 305 235 164 73 483 280 443 181 278 238 126 240 214 187 212 365

b.d. trace 261 720 914 2119 6565 6428 12,041 6124 8082 1680 209 125 trace 75 124 246 317 198 82 70 265 184 159 79 626 288 608 b.d. b.d. 263 127 252 249 189 148 283

b.d. b.d. b.d. b.d. trace 65 182 163 360 136 262 trace b.d. b.d. b.d. b.d. b.d b.d trace b.d b.d. b.d. b.d. b.d. b.d. b.d. trace b.d. trace b.d. b.d. trace b.d. b.d. b.d. b.d. b.d. 63

4049 2506 3324 6195 7354 15,528 9674 8841 15,856 17,524 13,627 5624 22,178 19,509 7827 42,043 1104 935 1548 949 1095 1121 694 414 237 134 216 133 205 96 88 127 153 117 113 104 88 93

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32

VG VG VG VG VG VG PG PG PG VG PG PG PG PG PG PG PG PG VG PG PG VG VG PG PG VG VG VG VG PG PG VG

94.7 95.3 100.4 109.2 115.8 129.3 132.6 133.4 134.2 136.4 142.0 142.8 143.1 143.3 143.5 143.9 149.3 149.6 153.7 157.4 158.0 160.5 165.9 169.7 170.3 173.1 174.3 177.0 178.4 179.2 180.0 188.4

976,262 962,810 953,422 988,286 937,067 943,528 970,835 970,503 969,197 967,313 987,353 978,603 988,031 994,384 998,389 997,341 986,639 992,340 983,682 985,037 982,832 977,756 980,679 979,627 983,523 985,695 985,256 983,193 984,164 978,389 971,955 983,007

426 360 343 288 206 127 170 116 112 29 36 40 356 407 695 1339 5332 7038 11,759 7935 11,161 17,760 18,187 17,385 13,149 10,564 10,562 12,686 11,894 18,610 25,251 13,643

225 146 206 372 482 817 536 543 2028 717 1445 14,131 849 4363 656 1041 686 371 3827 5281 4638 3129 847 2288 2505 2239 2640 2647 2249 2379 2224 2140

70 b.d. trace 71 84 182 137 112 376 75 232 1577 93 532 64 117 79 40 444 826 692 577 110 328 423 354 429 391 322 327 319 283

90 b.d. 206 1721 2889 5380 3219 2178 1924 1199 606 990 100 150 55 35 171 60 60 357 253 216 trace 130 181 202 219 188 160 140 117 154

69 b.d. trace 968 2217 6357 3582 2224 1548 727 428 339 95 57 64 trace 218 81 trace 402 293 115 trace 122 174 234 263 178 178 93 77 123

b.d. b.d. b.d. b.d. 41 153 119 78 53 trace trace b.d. b.d. b.d. b.d. b.d. b.d. b.d. b.d. trace trace 58 b.d. b.d. b.d. trace trace b.d. b.d. b.d. b.d. b.d.

2289 2676 2777 3429 4550 7405 5724 8331 8690 33,613 27,625 24,515 2776 2442 1436 745 185 141 84 124 88 55 54 56 75 93 93 78 83 53 38 72

GMGS4-SC2

δ13CCH4 (‰)

δDCH4 (‰)

−63.3 −57.0 −52.9

−158.3 −151.6 −145.0

−46.6 −50.1

−170.3 −148.9

−48.1 −47.1 −49.0 −50.7 −46.8 −46.5

−174.1 −177.7 −147.3 −138.3 −171.4 −165.9

−47.4

−172.6

−46.4 −47.6 −47.8

−164.7 −170.0 −173.2

−47.3

−167.1

−46.7

−169.4

−67.7

−161.8

−62.6 −58.8 −52.9

−159.5 −157.8 −152.0

−49.3

−144.0

−46.9 −44.4 −46.6

−174.3 −145.3 −174.1

−46.4 −47.3 −46.2 −47.2

−174.5 −177.4 −185.0 −171.3

−45.8

−170.1

−46.8 −47.0

−182.5 −177.8

−46.5

−166.5

−46.4

−178.0

(continued on next page)

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Table 2 (continued) Site/Hole

Core

Type

Depth (mbsf)

Methane (ppm)

Ethane (ppm)

Propane (ppm)

Isobutane (ppm)

Butane (ppm)

Isopentane (ppm)

Pentane (ppm)

C1:C2

GMGS4-SC3

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33

VG VG PG PG PG PG PG PG VG VG VG PG VG PG VG VG PG PG PG VG VG VG VG VG VG VG VG VG VG VG PG PG PG

45.3 94.6 135.3 135.7 136.6 136.7 138.1 138.4 149.3 153.7 154.4 161.1 161.2 161.7 170.3 174.4 177.4 177.7 179.1 181.1 188.1 193.9 200.0 202.6 203.5 208.1 209.7 212.4 216.3 216.8 219.6 220.4 221.2

982,446 997,213 997,271 996,969 997,551 997,778 996,668 996,973 996,218 993,281 994,051 995,571 995,739 995,219 994,613 994,047 993,733 994,628 993,765 993,791 994,543 994,436 994,683 994,152 993,872 993,730 993,328 991,590 996,292 996,223 995,700 995,866 995,465

225 278 2488 2852 2350 2072 2893 2783 3113 3365 3374 4243 3884 4547 5012 5479 6092 5200 6021 5453 5162 5063 5005 5165 5209 5834 5798 7310 2576 2676 3386 3332 3599

b.d. b.d. trace 39 40 40 68 40 90 1784 1282 74 72 80 88 88 82 98 88 111 115 137 132 235 253 169 216 200 438 433 556 484 578

b.d. b.d. b.d. b.d. b.d. b.d. b.d. b.d. trace 1045 735 b.d. b.d. b.d. b.d. b.d. b.d. b.d. b.d. b.d. b.d. b.d. b.d. trace trace b.d. trace b.d. 42 39 53 42 55

b.d. b.d. b.d. b.d. b.d. b.d. b.d. b.d. 64 60 45 b.d. b.d. b.d. b.d. b.d. b.d. b.d. b.d. b.d. b.d. trace b.d. 43 51 trace 33 trace 77 78 93 80 91

b.d. b.d. b.d. b.d. b.d. b.d. trace b.d. 67 106 76 b.d. b.d. b.d. b.d. b.d. b.d. b.d. b.d. b.d. b.d. trace b.d. 46 58 trace 42 trace 64 63 54 40 37

b.d. b.d. b.d. b.d. b.d. b.d. b.d. b.d. trace b.d. b.d. b.d. b.d. b.d. b.d. b.d. b.d. b.d. b.d. b.d. b.d. b.d. b.d. b.d. trace b.d. b.d. b.d. trace trace trace b.d. trace

4357 3591 401 350 425 481 344 358 320 295 295 235 256 219 198 181 163 191 165 182 193 196 199 192 191 170 171 136 387 372 294 299 277

δ13CCH4 (‰)

δDCH4 (‰)

−66.2 −66.4

−179.1 −189.5

−64.9 −65.2

−185.4 −185.0

−65.4 −64.7

−181.3 −185.6

−65.4 −64.6 −64.6

−162.2 −184.6 −185.4

−64.6

−181.2

−65.2

−179.3

b.d.: below detection limit (near 10 ppm); trace: below quantification limit (near 30 ppm); mbsf: meters below seafloor.

As can be seen from the plot of δ13C-CH4 versus C1/(C2 + C3) (Fig. 5A), all of the VG and PG samples from sites GMGS3-W17, -W18, and -W19 plotted within the mixed biogenic and thermogenic gas field. Except for one VG sample from site GMGS3-W11, which plotted in the biogenic gas field, all of the other PG samples fell within the mixed gas field. Similarly, the majority of the gas samples from site GMGS4-SC1 plotted within the mixed gas field, except for one VG sample that plotted in the biogenic gas field. The gas samples from site GMGS4-SC2 were predominantly mixed gas, but two of the VG samples plotted within the biogenic gas field. The gas samples from site GMGS4-SC3 were also predominately mixed gas, but one VG gas sample plotted within the biogenic gas field. On the plot of δ13C-CH4 versus δD-CH4 (Fig. 5B), the VG and PG samples from sites GMGS3-W11 and -W17 plotted between the microbial CO2 reduction to CH4 and mixed gas fields, while the gas samples from sites GMGS3-W18 and -W19 plotted within the thermogenic CH4 field. The majority of the gas samples from site GMGS4-SC1 plotted within the thermogenic CH4 and mixed gas fields. The gas samples from site GMGS4-SC2 were predominantly thermogenic CH4, but several samples plotted within the microbial CO2 reduction to CH4 or mixed gas fields. The gas samples from site GMGS4-SC3 all plotted within the microbial CH4 production via CO2 reduction field. Based on the comprehensive analysis detailed above, we conclude that the gas sources of the GMGS3 and GMGS4 gas hydrate drilling areas are mixed biogenic–thermogenic gas with thermogenic gas sources dominating. Although the concentrations of the C2+ hydrocarbons are much lower than the concentration of methane, the C2+ hydrocarbons still make an important contribution to the formation and accumulation of the gas hydrates (Zhang et al., 2017; Cong et al., 2018). Based on conventional petroleum exploration results, the Baiyun Sag possesses source rocks capable of generating both biogenic gas and thermogenic gas. Therefore, we suggest that the thermogenic hydrate

gas is closely related to the deep reservoirs in the Baiyun Sag (Su et al., 2016, 2017; Zhang et al., 2017; Cong et al., 2018; Wei et al., 2018). A series of oil and gas fields have been discovered in the Baiyun Sag and its surroundings, including the LW3-1 large gas field in the southeastern part of the sag and the LH19-1, PY30-1, and PY34-1 gas fields in the Panyu Low Uplift area (Zhu et al., 2008, 2012; Shi et al., 2010; Dai, 2014; Zhang et al., 2014) (Fig. 1B), indicating that the Baiyun Sag has abundant hydrocarbon resources. Previous hydrocarbon-source correlation studies have demonstrated that the oil and gas is derived from source rocks deposited in the Baiyun Sag (Zhu et al., 2008; He et al., 2009, 2012). The Baiyun Sag contains two sets of mature to overmature source rocks: the delta, shallow lake-swamp facies coal measure source rocks of the Oligocene Enping Formation and the medium-deep lake facies mudstone source rocks of the Eocene Wenchang Formation (Cui et al., 2009; Shi et al., 2010; Li et al., 2015) (Fig. 2). The Wenchang Formation source rocks are characterized by a relatively high abundance of organic matter (1.0%–2.5%) and they mainly contain type I-IIa kerogen (Fu et al., 2007; Dai, 2014). At present, some of the Wenchang Formation source rocks are highly mature to overmature, and others are mature to highly mature (Fig. 6); therefore, a large amount of gas is generated via thermal cracking of the organic matter (Gao et al., 2015). Because of the coal bed deposition (He et al., 2013), the source rocks in the Enping Formation are also characterized by a high abundance of organic matter (1.1%–56.2%) and they mainly contain type IIb-III kerogen (Fu et al., 2007; Dai, 2014). The source rocks in the Enping Formation are mature to highly mature (Fig. 6), so they generate a large amount of hydrocarbons (Gao et al., 2015). Consequently, both the Wenchang Formation and the Enping Formation are able to provide a sufficient gas supply to the oil and gas fields in the Baiyun Sag-Panyu Low Uplift area. In addition, they are also favorable source rocks for providing thermogenic gas for gas hydrate formation (Su et al., 2016; Zhang et al., 2017; Cong et al., 8

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(caption on next page) 9

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Fig. 3. Plot of methane, ethane, and propane concentrations and C1/C2 ratios of samples from the GMGS3 gas hydrate coring sites; mbsf: meters below seafloor. LWD:Logging while drilling. (A) site GMGS3-W11, (B) site GMGS3-W17, (C) site GMGS3-W18, and (D) site GMGS3-W19.

2018). Biogenic gas has been discovered in the upper Miocene-Quaternary sandstone reservoirs in the Baiyun Sag-Panyu Low Uplift area, such as in the PY29-1, PY30-1, and PY34-1 fields. The biogenic gas most likely originated from immature-low maturity source rocks in the upper Zhujiang Formation and its overlying strata (Fig. 6) (He et al., 2009, 2013). The general depth of the biogenic gas varies from 480 to 2300 mbsf, while the burial depth of the thermogenic gas is up to ~3900 mbsf (Zhu et al., 2008; He et al., 2009). However, thermogenic gas could also be present in the < 1000 mbsf sediments, e.g., the gas obtained from the LH19-1 and LH19-3 oil and gas fields.

Zhang et al., 2018). Thus, the hydrate gases in the Shenhu area are closely related to the deep gas reservoirs, suggesting a paragenetic relationship within the same petroleum system in the Baiyun Sag. 5.3. Potential effects on the hydrate gas geochemistry 5.3.1. Possible fractionation effects The above discussions conclude that the gas in the conventional fields in the Baiyun Sag is related to the hydrate-forming gas. However, now that the deep-seated thermogenic gas has migrated upward into the GHSZ and formed gas hydrates, we must determine why the composition of the hydrate gas and the isotopes of the methane exhibit characteristics of both biogenic gas and mixed biogenic-thermogenic gas (Fig. 5). We propose that except for the contribution of in situ biogenic methane derived from the biogenic source rocks in the vicinity of the GHSZ, the thermogenic gas that transported from depth into the GHSZ was mixed with biogenic gas, resulting in the mixed isotopic composition of the hydrate gas in the Shenhu area. In addition, thermogenic gas with possible molecular and isotopic fractionation, resulting from the long-distance (thousands of meters) migration from the deep reservoirs to the GHSZ, may contribute to hydrate precipitation (Cong et al., 2018). Such hydrocarbon fractionation indicates that as the gas migrated from the deep strata into the shallow sediments, the methane content increased while the C2+ gas content decreased, and the δ13C value of the methane was overwritten because 12Ce12C bonds are easier to break than 12Ce13C bonds (James and Burns, 1984). As a consequence, the δ13C-CH4 decreased gradually, exhibiting the isotopic characteristics of biogenic gas. In addition, this hydrocarbon fractionation effect is more distinct over long geological time scales and for long-distance diffusion. In terms of the Shenhu area, the C1/C2 ratios of the majority of the hydrate gas above the GHBZ are greater than 800–1000, while the C1/ C2 ratios of the hydrate gas within and below the GHBZ are less than ~300 (Zhang et al., 2017; Cong et al., 2018) (Figs. 3 and 4), indicating either an upward flux of deep thermogenic gas or hydrocarbon fractionation during hydrate crystallization. In addition, the isotopic composition of methane also changes with depth, as was shown for sites GMGS4-SC1 and GMGS4-SC2 (Table 2, Fig. 7) where the isotopic composition of methane varied from ~−45‰ to ~−68‰. Thus, the isotopic compositions become lighter with decreasing depth, suggesting possible isotope fractionation effects. Molecular and isotopic fractionation during hydrate precipitation has been demonstrated and modeled for the Gulf of Mexico (Chen and Cathles, 2003; Sassen et al., 1999, 2001a, 2001b), Svalbard region (Smith et al., 2014), and NW Borneo (Paganoni et al., 2016). In the latter case, the gas hydrates occurred just above a major oil and gas discovery. Other examples of hydrate systems with hydrocarbon fractionation phenomena are associated with leaky oil and gas fields, for example, in the Barents Sea and Black Sea (Pape et al., 2010; Ostanin et al., 2013). In addition, laboratory studies have shown that the degree of methane isotope fractionation during diffusion is positively correlated with the total organic carbon (TOC) content of the source rocks, but is negatively correlated with the methane migration rate, permeability, and the temperature of the migration media (Zhang and Krooss, 2001). Similar geological and geochemical conditions, including high TOC content of source rocks, low methane migration rate, low permeability, and low temperature of sediments, occur in the Shenhu area, providing the conditions required for the molecular and isotopic fractionation of deep gases (Cong et al., 2018). Thus, the thermogenic gas derived from the Paleogene source rocks in the Shenhu area likely underwent molecular and isotopic fractionation during longdistance vertical migration, and the thermogenic hydrocarbons became dryer. In addition, the carbon isotopes of the methane are characteristic

5.2. Relationship between the hydrate gas and the deep reservoirs The VG and PG geochemical analyses reveal that both biogenic gas and thermogenic gas are present at the Shenhu gas hydrate drilling sites. Where did these gases originate from? Petroleum explorations in the LH19-1, PY30-1, PY34-1, PY35-1, and LW3-1 oil and gas fields near the Shenhu area (Fig. 1B) have shown that the oil and gas originated from the source rocks in the Eocene Wenchang Formation and the Oligocene Enping Formation and that large amounts of hydrocarbons derived from the Baiyun Sag have migrated to the northern slope of the Baiyun Sag-Panyu Low Uplift area (Zhu et al., 2005; Chen et al., 2006; Zhu et al., 2008; He et al., 2009; Shi et al., 2010; Dai, 2014). In addition, thermogenic gas has been detected in the shallow strata (the shallowest depth is approximately 650 mbsf) of several gas fields, i.e., the LH19-1, LH19-3, PY29-1, and PY30-1 gas fields (He et al., 2009, 2013). By comparing the hydrate gases from the GMGS3 and GMGS4 coring sites with the gas obtained from the nearby PY29-1, PY30-1, PY34-1, and LW3-1 conventional oil and gas fields in the Baiyun-Panyu Low Uplift area (Fig. 5A), we found that the genetic types of the majority of the hydrate gases, which are biogenic gas or mixed biogenicthermogenic gas, are similar to those of the gas (HG) obtained from the shallow strata between the Hanjiang and Qionghai formations (480–2300 mbsf) in the PY29-1, PY30-1, and PY34-1 gas fields. In addition, the thermogenic hydrate gases recovered from the GMGS3-W18 and GMGS4-SC2 coring sites are similar to the gas in the Hanjiang Formation and Zhujiang Formation reservoirs in the PY29-1 (2845–2912 mbsf) and PY30-1 (1907–3192 mbsf) gas fields and in the Zhujiang Formation reservoirs in the LW3-1 gas field (3070–3499 mbsf) (Fig. 5) (Zhu et al., 2008; He et al., 2009; Dai, 2014). Based on previous hydrocarbon-source correlation studies and the geological characteristics, thermal evolution, and hydrocarbon generation history of the Baiyun Sag (Guo et al., 2007; Zhu et al., 2008, 2012; Shi et al., 2010; Dai, 2014; Zhang et al., 2014; Gao et al., 2015), we conclude that the biogenic gas in the gas hydrates in the Shenhu area was derived from the immature to low mature source rocks of the Late Oligocene and their overlying strata, while the thermogenic gas in the gas hydrates originated from the highly mature to overmature source rocks in the coal measures of the Enping Formation and the medium-deep lake facies mudstones of the Wenchang Formation (Huang et al., 2010; Shi et al., 2010; Yang et al., 2017a, 2017b; Zhang et al., 2017). In addition, as determined through the conventional oil and gas boreholes, the burial depth of the biogenic gas source rocks is about 2300 mbsf, while the depth of the thermogenic gas source rocks may be deeper than 3900 mbsf. Finally, based on an analysis of the migration pathways in the Baiyun Sag and the gas hydrate drilling and coring sites ( Shi et al., 2009b; Qiao et al., 2014; Su et al., 2014, 2017; Zhang et al., 2018), we believe that the deep thermogenic gas and the shallow biogenic gas are able to migrate to the GHSZ through vertical pathways, including high angle faults, mud diapirs, and gas chimneys and possibly landslide surfaces and listric gravity faults (Su et al., 2017; 10

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Fig. 4. Plot of methane, ethane, and propane concentrations and C1/C2 ratios of samples from the GMGS4 gas hydrate coring sites; mbsf: meters below seafloor. LWD: Logging while drilling. (A) site GMGS4-SC1; (B) site GMGS4-SC2; and (C) site GMGS4-SC3.

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Fig. 5. (A) Plot of δ13C-CH4 versus C1/(C2 + C3) for methane and (B) plot of δ13C-CH4 versus δD-CH4 for methane samples from sites GMGS3 (sample GMGS3-W19 is referred to by Zhang et al., 2017) and GMGS4 and from conventional gas fields in the Baiyun Sag-Panyu Low Uplift area (Zhu et al., 2008; He et al., 2009; Dai, 2014). (Genetic fields are modified from Bernard et al., 1977; Whiticar, 1999).

5.3.2. Potential biodegradation effects As the thermogenic gas moves upward into the shallow sediments, it is affected by microbial metabolic processes (Pape et al., 2010; Milkov et al., 2011). The composition of the shallow hydrate gas above the GHBZ at several coring sites in the Shenhu area was likely altered by biodegradation. The distribution and variation in isotopic compositions of the hydrate gas (Fig. 5A) are very similar to those of the gas in the Carnarvon, San Joaquin (Milkov, 2011), and Songliao basins (Zhang et al., 2011) where the occurrence of hydrocarbon biodegradation has been confirmed. Thus, it is speculated that the hydrate gases from sites GMGS4-SC1, GMGS4-SC2, GMGS3-W18, and GMGS3-W19 likely underwent biological alteration by biodegradation, whereas the other gas compositions and isotopic values could be the result of the mixing of biogenic and thermogenic gases. In addition, secondary biogenic methane derived from the biodegradation of hydrocarbons during longdistance migration may also have contributed to gas hydrate precipitation (Pape et al., 2010; Gong et al., 2017). It has been demonstrated that the biodegradation of petroleum results in the formation of secondary microbial gas dominated by methane that exhibits the same molecular and isotopic features as the shallow in-situ biogenic gas in the Black Sea (Blinova et al., 2003; Jones et al., 2008; Milkov, 2011). This phenomenon also likely affected the hydrocarbons in the Shenhu area, although biogenic gas containing predominantly methane is already present. Hydrocarbon biodegradation affecting the final composition of hydrate-forming gas could provide an alternative explanation to biogenic-thermogenic mixing for the composition of hydrate gas observed

Fig. 6. The thermal burial evolution of the source rocks in the LW3-1 gas field in the Baiyun Sag (modified from Gao et al., 2015. mbsf: meters below seafloor).

of biogenic gas (depleted δ13C-CH4). This “reconstructed” thermogenic gas migrated upwards and mixed with the in-situ biogenic gas in the medium-shallow strata. Then, after entering the GHSZ they formed gas hydrates with mixed biogenic-thermogenic gas. Thus, hydrocarbon fractionation is likely to be one of the causes of the biogenic-thermogenic mixed gas features characterizing the hydrate gases in the Shenhu area, which are mostly sourced from deep thermogenic gas. 12

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Although the hydrate gases from sites GMGS-SC1 and GMGS-SC2 are dominated by methane, they also contain increasing amounts of ethane, propane, butane, and pentane with depth, indicating a thermogenic gas supply. In addition, the variation in the C1/C2 ratios of the hydrocarbons suggests that the concentration of thermogenic gas below the GHSZ may be even higher than within the GHBZ. Moreover, indistinct elevated resistivity and P-wave velocity anomalies have also been observed just below the BSR (Fig. 8), indicating the existence of gas hydrates with relatively low saturation. As seen from Fig. 8, the modeled base of the SI gas hydrate stability zone (BSIGHSZ) (100% CH4) is ~6 m shallower than the BSR, the depth of which is consistent with the base of the calculated gas hydrate stability zone (1461 mbsl) (99.0% CH4, 1.0% C2H6). In addition, calculating the GHSZ base using the highest concentration of ethane (~2.5%) obtained from sites GMGS3 and GMGS4 indicates that the deepest depth at which gas hydrates can occur at site GMGS4-SC1 is ~1469 mbsl, which correlates with the base of the SII gas hydrate stability zone (BSIIGHSZ). The above described modeling results not only suggest that both biogenic and thermogenic gas contributed to the formation and accumulation of the gas hydrates, but that SI and SII gas hydrates may coexist in the Shenhu area and that the SII hydrates may accumulate below the SI hydrates (Zhang et al., 2017). The coexistence of SI and SII gas hydrates in the Shenhu area was recently confirmed through Raman spectral analysis of GMGS samples (Wei et al., 2018). Hence, we conclude that the migration of thermogenic gas through a plumbing system composed of mud diapirs, gas chimneys, and high-angle faults has played an important role in the crystallization and precipitation of gas hydrates in the Shenhu area. In addition, the presence of SII hydrates expands the GHSZ to much deeper depths than those suggested by the BSR (Paganoni et al., 2016). Thus, in future studies, it is of great importance to re-examine and update the accumulation mechanism and to re-evaluate the resource potential of gas hydrates in the Shenhu area and even throughout the entire SCS. 6. Conclusions (1) The molecular composition and isotopic signatures of the void and pressure core gases collected from the GMGS3 and GMGS4 gas hydrate drilling and coring sites in the Shenhu area reveal that all of the hydrate gases are predominantly composed of methane, but also include up to ~3% C2+ hydrocarbons. The following processes are likely responsible for the observed molecular and isotopic gas compositions: (A) the mixture of biogenic and thermogenic gas, (B) biodegradation below the bottom of the gas hydrate stability zone (GHSZ), and (C) molecular fractionation within the GHSZ. (2) The gas supply for hydrate crystallization and precipitation in the Shenhu area is closely related to the deep conventional reservoirs, which are supplied by the hydrocarbon kitchens in the Baiyun Sag, indicating a paragenetic relationship within the same petroleum system. The biogenic gas in the gas hydrates likely originated from the immature to low mature source rocks of the Late Oligocene and their overlying strata. The thermogenic gas in the gas hydrates was likely derived from the medium to deep lake facies source rocks in the Eocene Wenchang Formation and the marine-continental transitional coal measure source rocks in the Oligocene Enping Formation. (3) The thermogenic hydrocarbons may have been affected by compositional and isotopic changes caused by fractionation during long distance migration from the deep reservoirs to the shallow GHSZ. The biodegradation of a solely thermogenic source could also affect the final composition of the hydrate-forming gas. (4) Structure I (SI) and II (SII) gas hydrates coexist in the Shenhu area due to the supply of thermogenic gas. The confirmed presence of SII gas hydrates expands the GHSZ to much deeper depths than those suggested by the bottom simulating reflector. Further research should be conducted to re-examine the accumulation mechanism

Fig. 7. δ13C of methane vs. burial depth for the void gas and pressure gas samples form gas hydrate coring sites (A) GMGS3 and (B) GMGS4.

in the Shenhu area. In addition, hydrocarbon biodegradation also contributes to the observance of different geochemical features of hydrate gases from sites GMGS3 and GMGS4. Thus, mixing and biodegradation of solely thermogenic gas likely occurred in the Shenhu area. More hydrate gas and deep hydrocarbon samples, as well as geochemical data, are needed to test this hypothesis. 5.4. Gas hydrate stability zone and hydrate structure The GMGS generally estimated the BGHSZ based on analysis of the BSR in seismic profiles obtained prior to drilling in the Shenhu area. The BSR was automatically regarded as the base of the SI gas hydrate, implying that it would be supplied by 100% microbial methane. However, the presence of thermogenic gas would change the GHSZ, making it larger than if only biogenic gas was present at the same temperature and pressure conditions. In addition, the gas hydrate structures would be more complex with the inclusion of C2+ hydrocarbons in the hydrate cage (Paganoni et al., 2016). Taking site GMGSSC1 as an example (Fig. 8), the different hydrate gas compositions would result in variable depths for the BGHSZ. As the C2+ concentration increases, the depth of the BGHSZ also increases and the hydrate structure may change from SI to SII or SH (Lu et al., 2007). In fact, in addition to the high saturation gas hydrates that have been recovered over the BSR at the Shenhu gas hydrate drilling sites, lower saturation gas hydrates have also been recovered below the BSR (Liang et al., 2017). 13

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Fig. 8. The logging while drilling (LWD) features and gas hydrate stability zone (GHSZ) calculations for site GMGS-SC1 in the Shenhu area (Sh: Hydrate saturation). Based on the seismic profile interpretation, the depth of the bottom simulating reflector (BSR) at site GMGS-SC1 is ~1461 mbsl (meters below sea level) and ~174 mbsf (meters below seafloor). The GHSZ calculation is based on actual data collected at site GMGS-SC1 and shows that the base of the structure I (SI) gas hydrate stability zone (BSIGHSZ) (100% CH4) is ~6 m shallower than the depth of the BSR, which is in line with the 1461 mbsl BGHSZ (99.0% CH4, 1.0% C2H6). The 1469 mbsl BSIIGHSZ represents the deepest depth at which gas hydrates can occur at site GMGS4-SC1 based on the highest concentration of C2H6 (~2.5%) measured in samples from the GMGS 3&4 sampling sites.

and to re-evaluate the resource potential of gas hydrates in the Shenhu area and even throughout the entire SCS.

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Declaration of Competing Interest We declare that we have no financial and personal relationships with other people or organizations that can inappropriately influence our work, there is no professional or other personal interest of any nature or kind in any product, service and/or company that could be construed as influencing the position presented in, or the review of, the manuscript entitled, “Origin of natural gases and associated gas hydrates in the Shenhu area, northern South China Sea: Results from the China gas hydrate drilling expeditions”. Acknowledgements This work was supported by the National Key Research and Development Program of China (No. 2018YFC0310000), National Natural Science Foundation of China (No. 41806071, No. 41602149), the China National Hydrate Project (DD20160211), China Postdoctoral Science Foundation (No. 2017M622655), Open Fund of Key Laboratory of Natural Resources, Ministry of Natural Resources (No. KLMMR-2017A-13), and the Foundation of the Guangzhou Science and Technology Project (No. 201909010002). The authors wish to thank those that contributed to the success of the China National Gas Hydrate Drilling Expedition 3 and 4 (GMGS 3&4). We would like to thank all reviewers and editors for their helpful suggestions and constructive comments that greatly improved this manuscript. Appendix A. Supplementary material Supplementary data to this article can be found online at https:// doi.org/10.1016/j.jseaes.2019.103953. 14

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