WITHDRAWN: Numerical studies of gas hydrate evolution time in the Shenhu area in the northern South China Sea

WITHDRAWN: Numerical studies of gas hydrate evolution time in the Shenhu area in the northern South China Sea

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CNGEO-00001; No of Pages 6 China Geology xxx (xxxx) xxx

Contents lists available at ScienceDirect

China Geology journal homepage: http://www.keaipublishing.com/en/journals/china-geology/

Numerical studies of gas hydrate evolution time in the Shenhu area in the northern South China Sea Yun-xin Fang a,b, Jing-an Lu a,b, Jin-qiang Liang a,b, Zeng-gui Kuang a,b, Yun-cheng Cao c,⁎, Duo-fu Chen c a b c

Guangzhou Marine Geological Survey, China Geological Survey, Guangzhou 510075, China Gas Hydrate Engineering Technology Center, China Geological Survey, Guangzhou 510075, China Shanghai Engineering Research Center of Hadal Science and Technology, College of Marine Science, Shanghai Ocean University, Shanghai 201306, China

a r t i c l e Available online xxxx Keywords: Shenhu South China Sea Numerical simulation Formation rate

i n f o

a b s t r a c t Although the Shenhu sea area has been a topic and focus of intense research for the exploration and study of marine gas hydrate in China, the mechanism of gas hydrate accumulation in this region remains controversial. The formation rate and evolution time of gas hydrate are the critical basis for studying the gas hydrate formation of the Shenhu sea area. In this paper, based on the positive anomaly characteristics of chloride concentration that measured in the GMGS3-W19 drilling site is higher than the seawater value, we numerically simulated the gas hydrate formation time of GMGS3-W19 site. The simulation results show that the gas hydrate formation rate positively correlates with the chloride concentration when the hydrate reaches the measured saturation. The formation time of gas hydrate in the GMGS3-W19 site is approximately 30 ka. Moreover, the measured chloride concentration is consistent with the in-situ chloride concentration, indicating that the formation rate of gas hydrate at the GMGS3-W19 site is very fast with a relatively short evolution time. Copyright © 2019 Editorial Office of China Geology. Publishing services provided by Elsevier B.V. on behalf of KeAi. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).

1. Introduction Gas hydrate is an ice-like compound mainly composed of methane and water under the conditions of low temperature and high pressure, which is widely distributed in continental margins sediments and permafrost regions around the world (Kvenvolden, 1999). With tremendous potential for energy resources and as an important global environmental impact factor (Ullmann et al., 2015; Chong et al., 2016; Mestdagh et al., 2017; Maslin et al., 2010), gas hydrate has received a great deal of attention within the international research community during the past several decades. The Shenhu sea area on the northern slope of the South China Sea is located near the southeastern Shenhu Shoal and is structurally affiliated to the Zhu II depression of the Pearl River Mouth Basin. It is a key target area for the exploration and development of marine gas hydrate in China and a hot spot for the study of the hydrate reservoir forming mechanism. Because of the relatively fast sedimentation rate of Cenozoic strata, great hydrocarbon generation potential of the hydrocarbon source rock, active deep fluid, and fluid migration and aggregation primarily controlled by faults, the Shenhu area forms a unique hydrate accumulation system (Yang et al., 2017a, 2017b). Since 2007, the Guangzhou Marine Geological Survey conducted three voyage drillings of gas hydrate like GMGS1, GMGS3, and GMGS4 in the Shenhu area, and ⁎ Corresponding author. E-mail addresses: [email protected] (Y. Fang), [email protected] (Y. Cao).

recovered a large number of high-saturation gas hydrate samples (Zhang et al., 2017; Yang et al., 2015, 2017a, 2017b). In 2018, China Geological Survey carried out the production test of gas hydrate in Shenhu sea area, the South China Sea. It is the first trial on gas hydrate bearing within a silty clay formation in the world (Li et al., 2018; Ye et al., 2018). The mechanism and control factors of gas hydrate accumulation in the Shenhu area is complex, and deep thermogenic gas is an important gas source for gas hydrate accumulation in this sea area (Wang et al., 2014). The pore structure of Foraminifera provides space favorably for gas hydrate (Chen et al., 2011). There are gas migration channels developed below the hydrate stability zone, and fluids rich in methane rapidly vent into the gas hydrate stability zone through faults and fractures, which is the controlling factor of hydrate accumulation in the Shenhu sea area (Yang et al., 2017a, 2017b; Zhang et al., 2017; Zhang et al., 2018). The simulated hydrate formation process based on the GMGS1-SH2 site shows a high saturation hydrate might have been formed in the early time of the fracture system, but the rapid sedimentation and slow advection during the recent 1.5 Ma resulted in the gradual reduction or disappearance of shallow hydrate. Although the gradual realization that gas hydrate in the Shenhu area is methane rich, the mechanism of gas hydrate accumulation has not been fully understood. In particular, one of the important controlling factors is the time of the gas hydrate formation. The formation time of gas hydrate can be calculated according to the abnormality of the chloride concentration. On the one hand, the salt exclusion during the gas hydrate formation increases the chloride

https://doi.org/10.1016/j.cngeo.2019.01.001 2096-5192/Copyright © 2019 Editorial Office of China Geology. Publishing services provided by Elsevier B.V. on behalf of KeAi. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).

Please cite this article as: Y. Fang, J. Lu, J. Liang, et al., Numerical studies of gas hydrate evolution time in the Shenhu area in the northern South China Sea, China Geology, https://doi.org/10.1016/j.cngeo.2019.01.001

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concentration, on the other hand, the ion diffusion transfers the high concentration chloride in layers that contain gas hydrate to other layers with a low chloride concentration, thus reducing the chloride concentration of gas hydrate layers. Based on the fact that the measured chloride concentration of ODP1249 is higher than the seawater value, Torres et al. (2004) concluded that the gas hydrate formation time of this site must be shorter than 1.8 ka. Otherwise, the chloride concentration of this site would be lower than the measured value. On September 1,

2015, Guangzhou Marine Geological Survey completed their third gas hydrate drilling expedition (GMGS3) in the South China Sea. Specifically, gas hydrate with a high saturation was obtained from 135 m to 157 m below the seafloor (mbsf) in the logging GMGS3-W19 site, and the relevant drilling sites are shown in Fig. 1. Gas hydrate saturation could reach 45% through resistivity logging in the GMGS3-W19 site. The chloride concentration test of pore water showed a significantly negative anomaly characteristic of chloride from 135 mbsf to

Fig. 1. Distribution of the GMGS3-W19 site in the Shenhu area (b) in the northern South China Sea (a, modified from Li et al., 2018).

Please cite this article as: Y. Fang, J. Lu, J. Liang, et al., Numerical studies of gas hydrate evolution time in the Shenhu area in the northern South China Sea, China Geology, https://doi.org/10.1016/j.cngeo.2019.01.001

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157 mbsf, as shown in Fig. 2. However, beyond the range of 135–160 mbsf, the chloride concentration exhibited a positive anomaly above seawater value, as shown in Fig. 2, indicating the salt exclusion during the formation of gas hydrate with high saturation. In this paper, the chloride concentration profile of the GMGS3-W19 (W19) site was used to simulate the formation time of gas hydrate with an expectation to provide support for the metallogenic mechanism study of high-saturation hydrate in the Shenhu area. 2. Geological and simulation parameters of the W19 site in the Shenhu area The gas hydrate pilot production area, with a water depth range of 1000–1300 m, is located in the middle of the continental slope of the southeast Shenhu area in the South China Sea, and tectonically belongs to the Baiyun sag of the Zhuer depression in the Pearl River Estuary Basin. The gas hydrate of the GMGS3 drilling sites were all disseminated and occur near the Bottom Simulating Reflector in the seismic profile. The simulation model used in this study is based on the publication by Cao et al. (2013) in which deep methane gas vents into the gas hydrate stability zone during the process of venting into the seabed. The rate of methane consumption due to hydrate crystallization is determined as follows: Rh ¼ kΔX exp

   E 1 1 ϕð1−Sh Þ − R T T þ 273:15

ð1Þ

where Rh [kg/(m3·a)] is the methane consumption mass which is precipitated as hydrate per unit volume sediment and per year, E/R = 10,000 K, and T⁎ = 273.15 K. Specifically, k [kg/m3·a·mol] and ΔX [mol/kg] refer to the pre-exponential factor and the driving force of the reaction respectively. T [K], ϕ, and Sh represent the temperature at time t [a] at the depth of z [mbsf] (for the convenience of description, temperature is referred to as degrees Celsius in this manuscript), porosity, and the saturation of the hydrate in the pore (accounting for volume fraction of the pore space filled with gas hydrate) respectively. A detailed description of the parameters such as temperature, chloride concentration, and methane gas flux for this model are listed in the publication by Cao et al. (2013).

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Related parameters such as initial and boundary conditions are primarily determined based on the drilling data. In particular, the porosity of the W19 site is assumed to decrease with increasing depth. The porosity of the W19 site fitted by the measured porosity is 0.4 + 0.4exp (−0.0095z). Three in situ temperature measurements are measured in W19 and the temperature linear fit is T = 5.5 + 0.057z. Considering the salt exclusion and heat released during the formation of hydrate may affect the salinity and temperature below the hydrate zone, the depth of the bottom boundary simulated in this paper is selected as 220 mbsf, which is far deeper than the bottom boundary of the stability zone, thus the temperature and the chloride concentration of the bottom boundary are not affected by the formation of hydrates. Since gas has a lower viscosity and density compared with pore water, methane gas moves relatively fast which may result in a low saturation of gas and an insignificant variation. Therefore, in this paper, we adopt the treatment of Torres et al. (2004), which only considers the changes of pore water saturation and hydrate saturation without considering the influence of the change of free gas saturation. The initial state of gas hydrate saturation and the pore water saturation are set to 0 and 1.0 respectively. The flux of pore water is referenced in the GMGS1-SH2 pore water flux of qw0 = 0.7 kg/m2·a calculated by Su et al. (2012a, 2012b). Both the boundary value and initial value of chloride concentration are set as sea water. The detailed parameter values are listed in Table 1. The gas flux control equation of the reference model is a first-order differential equation (Eq. (7) in the publication by Cao et al. (2013)), which requires a boundary condition. So, the methane gas flux of the bottom boundary of the gas hydrate system qg, z=220 is taken as this condition. Therefore, only the gas hydrate formation rate constant k and qg, z=220 are unknown. In this paper, these unknown parameters are solved by using the finite difference method with the time step of 1 year and the space step of 1 m. 3. Results According to Eq. (1), we can obtain characteristics such as gas hydrate saturation, chloride concentration, and temperature at different gas hydrate formation rates by varying the value of the constant rate k. As shown in Fig. 3, k is taken as 0.04 kg2/m3·a·mol and qg, z=220 is

Fig. 2. Fitted porosity and temperature of the W19 site.

Please cite this article as: Y. Fang, J. Lu, J. Liang, et al., Numerical studies of gas hydrate evolution time in the Shenhu area in the northern South China Sea, China Geology, https://doi.org/10.1016/j.cngeo.2019.01.001

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Table 1 Simulation parameters of GMGS3-W19 site. Parameters

W19

Water depth/m L, bottom depth/mbsf ϕ, porosity

1273.8 220 0.4 + 0.4exp (−0.0095z) 5.5 + 0.057z 0.54 5.5 18.04 0.54 0.54

Initial temperature/°C XCl,t=0, initial salinity value/(mol/kg) T(z = 0, t), seafloor temperature/°C T(z = 220, t), bottom boundary temperature/°C XCl(z = 0, t), seawater salinity value/(mol/kg) XCl(z = 220, t), bottom boundary salinity value/(mol/kg)

taken as −0.0065 kg/m2·a firstly, and t = 150 ka were utilized to simulate the formation of gas hydrate at site W19. As a result, the gas hydrate formed at 130 mbsf to 160 mbsf and exhibited a saturation up to 45%. Since the salt discharge occurs during the gas hydrate formation, the chloride concentration of the pore water increased up to 0.62 mol/kg. Although the chloride concentration of 110–170 mbsf is increased, the chloride concentration is still lower than the in situ chloride concentration, and the chloride concentration is higher than the in-situ chloride concentration above 110 mbsf. Given the measured chloride concentration of the hydrate layer is affected by the dilution, so it cannot represent the in situ concentration, we calculated the corresponding pore water chloride concentration after the complete dissociation of the hydrate (Fig. 3b). The result indicates that the diluted chloride concentration fell within the range of the measured chloride concentration of the hydrate layer. During the vent of natural gas, the gas gradually forms as gas hydrate with the depth decreasing. In the meantime, the flux of natural gas (negative value represents an upward direction) gradually decreases, dropping to zero at about 135 mbsf. The bottom supplied gas is completely consumed as gas hydrate with a bottom boundary gas flux of −0.0065 kg/m2·a and the gas hydrate formation rate per unit area is 0.0065 kg/m2·a under these conditions. Because of the relatively low rate of gas hydrate formation, the temperature of the gas hydrate system is hardly affected by the heat released from the hydrate formation. Thus, the temperature remains almost constant. The measured pore water chloride concentration of the hydrate layer is affected by the dissociation and dilution of gas hydrate, thus exhibiting a negative anomaly. However, the simulated chloride concentration of the gas hydrate layer and the nearby area (120–130 mbsf and 160–170 mbsf) are lower than the measured chloride concentration. Therefore, it can be concluded that the gas venting at the W19 site should be consumed as gas hydrate before 5 ka ago.

Otherwise, the gas hydrate system does not have sufficient time to diffuse (conduct) the chloride discharged, and the heat released during the formation of gas hydrate. As shown in Fig. 4, the simulation result indicates that the gas hydrate saturation could reach up to 45% by applying k = 1.0 kg2/m3·a·mol, the gas flux of qg, z=220 = −0.15 kg/m2·a, and the simulation time of 5 ka at the W19 site. Gas hydrate is mainly distributed at 124–150 mbsf. Although the thickness of the gas hydrate layer is similar to the actual hydrate layer thickness, the depth of the layers with hydrate developed is shallower than the depth of the actual hydrate distribution. It is attributed to the rapid formation of gas hydrate that releases a large amount of heat, which raises the temperature up to 0.57 °C and causes the gas hydrate stability zone to rise about 10 m (Fig. 4d). In addition, the rapid formation of gas hydrate significantly increases chloride concentration up to 0.9 mol/kg, which is much higher than the measured undiluted chloride concentration. The increase in the chloride concentration is mainly concentrated in 110–170 mbsf, especially the hydrate development layers (120–147 mbsf), in which the chloride concentration is significantly higher than the in-situ chloride concentration. If the gas hydrate is completely dissociated, the diluted chloride concentration is very close to the seawater value though it is diluted (dashed line in Fig. 4b) and is significantly higher than the measured chloride concentration of the hydrate layer. The simulation characteristics of the gas hydrate accumulation of two gas hydrate formation rates by employing rate constants k = 0.04 kg2/m3·a·mol and k = 1 kg2/m3·a·mol indicate that the simulated chloride concentration is lower than the in-situ dilution chloride concentration when the gas hydrate formation rate is too slow. However, the gas hydrate will be distributed at a shallower depth with a fast formation rate. As a result, the chloride concentration will be higher than the in situ measured chloride concentration. With different rate constants k applying to the simulation of gas hydrate saturation and chloride distribution, it turns out that the gas hydrate saturation and chloride are in agreement with the measured values in the case of k = 0.15 kg2/m3·a·mol, qg, z=220 = −0.015 kg/m2·a, and the simulation time of about 30 ka, as shown in Fig. 5. In particular, the gas hydrate saturation reaches about 45%, and it mainly distributes at 135–160 mbsf, which is consistent with the depth of the gas hydrate development. At the same time, the salt discharge effect of gas hydrate increases the chloride concentration. As a result, the chloride concentration near the hydrate development layer (z b 130 mbsf and z N 160 mbsf) is in agreement with the measured undiluted chloride concentration (Fig. 5b). Therefore, it is speculated that the gas hydrate at the W19 site may have undergone an evolution time of about 30 ka. And the gas hydrate formation rate is up to 0.015 kg/m2·a due to the complete consumption of methane gas. In

Fig. 3. Simulated gas hydrate saturation (a), chlorine concentration (b), methane gas flux (c) and temperature (d) under the model conditions of k = 0.04 kg2/(m3·a·mol), qg, z=220 = −0.0065 kg/(m2·a), and t = 150 ka. The dotted line of b is the chloride concentration that has been diluted after the gas hydrate is dissociated.

Please cite this article as: Y. Fang, J. Lu, J. Liang, et al., Numerical studies of gas hydrate evolution time in the Shenhu area in the northern South China Sea, China Geology, https://doi.org/10.1016/j.cngeo.2019.01.001

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Fig. 4. Simulated gas hydrate saturation (a), chloride concentration (b), methane gas flux (c) and temperature (d) under the conditions of constant k = 1.0 kg2/(m3·a·mol), natural gas flux qg, z=220 = −0.15 kg/(m2·a), and t = 5 ka. The dotted line of b is the chlorine concentration that has been diluted after the gas hydrate is dissociated, indicating that the negative chloride anomaly is insignificant. The dashed line in d represents the background temperature while the solid line refers to the model temperature.

addition, the temperature of the system does not change significantly because of the relatively slow gas hydrate formation rate, which results in an unchanged bottom boundary of the hydrate stability zone. 4. Discussion In this paper, the authors simulate the hydrate saturation, chloride concentration, natural gas flux and temperature characteristics of the three different gas hydrate formation rates at the W19 site. The timescales required for the formation of gas hydrates are 100 ka, 10 ka, and 1 ka respectively. The results show that when the gas hydrate formation is relatively slow, the concentration of chloride in the hydrate layer is insignificantly increased as it is transported outside by diffusion, though the chloride concentration could be increased due to the salt discharge during the gas hydrate formation. As a result, the chloride concentration of the hydrate layer is lower than the chloride concentration in situ. However, the discharged salt during the formation of hydrate remains in the hydrate layer when gas hydrate is formed rapidly. As a result, the chloride concentration rises very significantly, which is higher than the in situ measured chloride concentration. And when the hydrate is dissociated, and the chloride concentration is diluted, the chloride concentration shows a close value to the background value and without a significant negative anomaly. In addition, the rapid formation of hydrate also lightens the base of the gas hydrate stability zone, resulting in a shallower hydrate zone. Therefore, different hydrate formation rates lead to different hydrate distribution depths, chloride concentration profiles, and so on. The simulated chloride concentration is consistent with the measured values only in the case of the

appropriate hydrate formation rate. The simulated hydrate distribution and chloride concentration are in agreement with the measured values in the W19 site on the base of the gas hydrate formation time of nearly 30 ka. The high saturation gas hydrate just accumulated above the BSR at the W19 site, and the rate of gas hydrate formation per unit area is equal to the bottom methane supply amount of 0.015 kg CH4/m2·a. The gas hydrate areas with BSR are widely distributed in the continent margin. The Blake Ridge ODP997 site on the passive continental margin is a gas hydrate area with BSR depth of about 450 mbsf, in which the gas hydrate formation rate per unit area is 4.3 × 10−3 mol/m2·a (6.88 × 10−5 kg CH4/m2·a) (Egberg and Dickens, 1999). In addition, the Cascadia hydrate ridge ODP1247 site also develops a BSR depth of about 120 mbsf with a gas hydrate formation rate per unit area of about (2.6 × 10−4–3.4 × 10−4 kg CH4/m3·a). The gas hydrate occurs in 50–124 mbsf and its unit volume of natural gas is 2.9 × 10−8–3.8 × 10−8 m3/m3·a (3.50 × 10−6–4.56 × 10−6 kg CH4/m3·a) (Zheng et al., 2017). Through comparison of the gas hydrate formation rate in these three regions, we find that the formation rate of deep buried high-saturation gas hydrate at the W19 site is about 2–3 orders of magnitude faster than those of deepburied low-saturation gas hydrates at ODP997 and ODP1247 sites. This result could be attributed to fact that the methane in the 997 and 1247 sites is mainly transported in the dissolved state. It leads to a low flux of the natural gas supply, slow hydrate formation rates and a low saturation of hydrate. Due to the very optimal natural gas supply conditions and a sufficient supply of natural gas, the gas hydrate formation rate in the Shenhu area is much higher than those of the ODP997 and 1247 sites.

Fig. 5. Simulated gas hydrate saturation (a), chloride concentration (b), methane gas flux (c) and temperature (d) under the model conditions of rate constant k = 0.15 kg2/m3·a·mol, gas flux qg, z=220 = −0.015 kg/m2·a, and t = 30 ka. The dotted line in b is the chlorine concentration that has been diluted after the gas hydrate is dissociated, indicating that the negative chloride anomaly is insignificant.

Please cite this article as: Y. Fang, J. Lu, J. Liang, et al., Numerical studies of gas hydrate evolution time in the Shenhu area in the northern South China Sea, China Geology, https://doi.org/10.1016/j.cngeo.2019.01.001

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The chloride concentration of drilling in the Shenhu area show that only the in situ chloride concentration of the hydrate development layer in some sites are higher than that of the seawater value while the in situ chloride concentration in most sites are almost identical to the background value, such as the GMGS1-SH2 and GMGS1-SH7 sites (Wu et al., 2009; Yang SX et al., 2015). It is speculated that the gas hydrate formation time of these sites such as SH2 and SH7 is earlier than that of the W19 site. According to the simulation of Su et al. (2012a, 2012b), the deep buried high-saturation gas hydrate at the SH2 site was formed in a fracture system before 1.5 Ma, but rapid deposition of the recent 1.5 Ma and slow fluid advection resulted in a gradual diminishing or issociation of the shallow hydrate. The difference in gas hydrate formation time for different drilling sites in the Shenhu area indicates that the gas hydrate formation time exhibits spatial heterogeneity in time with possible multiple batches of formation. 5. Conclusion According to the chloride concentration and gas hydrate development characteristics of the GMGS3-W19 site in the Shenhu drilling area in the South China sea, the simulation reveals that the gas hydrate in the GMGS3-W19 site exhibits a significant fast formation rate and a high saturation with a formation time of approximately 30 ka. The gas hydrate formation time of the GMGS3-W19 site is much later than that of the gas hydrate system in the GMGS1-SH2 site, indicating that the natural gas hydrate formation in the gas hydrate drilling area of the Shenhu area has experienced multiple formation stages or periods. Conflict of interest The authors are grateful for the crew of all voyages on the cruise GMGS3 for assistance in sampling and the Executive Editor-in-Chief Dr. Yang Yan for their valuable suggestions and comments on the manuscript. Acknowledgments The authors are grateful to anonymous reviewers and the Executive Editor-in-Chief Dr. Yang Yan for their valuable suggestions and comments on the manuscript. This work was co-funded by National Natural Science Foundation of China (41406068, 41776050, 41730528), China Geological Survey Project (DD20189310), Guangdong Special Fund for Economic Development (Marine Economy) (GDME-2018D001), and Key Laboratory of Marine Mineral Resources, Ministry of Land and Resources (KLMMR-2013-A-24). References Cao, Y.C., Su, Z., Chen, D., 2013. Influence of water flow on gas hydrate accumulation at cold vents. Sci. China Earth Sci. 56 (4), 568–578. Chen, F., Zhou, Y., Su, X., Liu, G., Lu, H.F., Wang, J.L., 2011. Gas hydrate saturation and its relationship with grain size of the hydrate-bearing sediment in the Shenhu area of

Northern South China Sea. Mar. Geol. Quat. Geol. 31 (5), 95–100 (in Chinese with English abstract). Chong, Z.R., Yang, S.H.B., Babu, P., Linga, P., Li, X.S., 2016. Review of natural gas hydrates as an energy resource: prospects and challenges. Appl. Energy 162, 1633–1652. Egberg, P.K., Dickens, G.R., 1999. Thermodynamic and pore water halogen constraints on gas hydrate distribution at ODP Site 997 (Blake Ridge). Chem. Geol. 153, 53–79. https://doi.org/10.1016/S0009-2541(98)00152-1. Kvenvolden, K.A., 1999. Potential effects of gas hydrate on human welfare. Proc. Natl. Acad. Sci. U. S. A. 96 (7), 3420–3426. Li, J.F., Ye, J.L., Qin, X.W., Qiu, H.J., Wu, N.Y., Lu, H.L., Xie, W.W., Lu, J.A., Peng, F., Xu, Z., Lu, C., Kuang, Z.G., Wei, J.G., Liang, J.Q., Lu, H.F., Kou, B.B., 2018. The first offshore natural gas hydrate production test in South China Sea. China Geol. 1, 1–12. Maslin, M., Owen, M., Betts, R., Day, S., Jones, T.D., Ridgwell, A., 2010. Gas hydrate: past and future geohazard? Phil. Trans. R. Soc. A 2369–2393. Mestdagh, T., Poort, J., Batist, M.D., 2017. The sensitivity of gas hydrate reservoirs to climate change: perspectives from a new combined model for permafrost-related and marine settings. Earth Sci. Rev. 169, 104–131. Su, Z., Cao, Y.C., Wu, N.Y., Chen, D.F., Yang, S.X., Wang, H.B., 2012a. Numerical investigation on methane hydrate accumulation in Shenhu Area, northern continental slope of South China Sea. Mar. Pet. Geol. 38 (1), 158–165. https://doi.org/10.1016/j. marpetgeo.2012.06.005. Su, Z., Cao, Y.C., Yang, R., Wu, N.Y., Yang, S.X., Wang, H.B., 2012b. Analytical research on evolution of methane hydrate deposits at Shenhu area, northern South China Sea. Chin. J. Geophys. 55 (5), 1764–1774 (in Chinese with English abstract). Torres, M.E., Wallmann, K., Tréhu, A.M., Bohrmann, G., Borowski, S., Tomaru, H., 2004. Gas hydrate growth, methane transport, and chloride enrichment at the southern summit of Hydrate Ridge, Cascadia margin off Oregon. Earth Planet. Sci. Lett. 226, 225–241. Ullmann, L.S., D Neto, R.N., Teixeira, R.H.F., Nunes, A.V., Silva, R.C., Pereira-Richini, V.B., 2015. A global survey of gas hydrate development and reserves: specifically in the marine field. Renew. Sust. Energ. Rev. 41 (4), 884–900. Wang, X., Collett, T.S., Lee, M.W., Yang, S.X., Guo, Y.H., Wu, S., 2014. Geological controls on the occurrence of gas hydrate from core, downhole log, and seismic data in the shenhu area, South China Sea. Mar. Geol. 357, 272–292. https://doi.org/10.1016/j. margeo.2014.09.040. Wu, N.Y., Yang, S.X., Wang, H.B., Liang, J.Q., Gong, Y.H., Lu, Z.Q., Wu, D.D., Guang, H.X., 2009. Gas-bearing fluid influx sub-system for gas hydrate geological system in Shenhu area, northern South China Sea. Chin. J. Geophys. 52 (6), 1641–1650 (in Chinese with English abstract). Yang, S.X., Zhang, M., Liang, J.Q., Lu, J.A., Zhang, Z., Melanie, H., Peter, S., Fu, S.Y., Sha, Z.B., the GMGS3 Science Team, 2015. Preliminary results of China's third gas hydrate drilling expedition: a critical step from discovery to development in the South China Sea. Fire Ice 15 (2), 1–5. Yang, S.X., Liang, J.Q., Lei, Y., Gong, Y.H., Xu, H.N., Wang, H. Lu J.A., Melanie, H., Peter, S., Wei, J.G., the GMGS4 Science Team, 2017a. GMGS4 gas hydrate drilling expedition in the South China Sea. Fire Ice 17 (1), 7–11. Yang, S.X., Liang, J.Q., Lu, J.A., Qu, C.W., Liu, B., 2017b. New understandings on the characteristics and controlling factors of gas hydrate reservoirs in the Shenhu area on the northern slope of the South China Sea. Earth Sci. Front. 24 (4), 1–14 (in Chinese with English abstract). Ye, J.L., Qin, X.W., Qiu, H.J., Liang, J.Q., Dong, Y.F., Wei, J.G., Lu, H., Lu, J.A., Shi, Y.H., Zhong, C., Zhen, X., 2018. Preliminary results of environmental monitoring of the natural gas hydrate production test in the South China Sea. China Geol. 1, 1–12. Zhang, W., Liang, J.Q., Lu, J.A., Wei, J.G., Su, P.B., Fang, Y.X., Guo, Y.Q., Yang, S.X., Zhang, G.X., 2017. Accumulation features and mechanisms of high saturation natural gas hydrate in Shenhu Area, northern South China Sea. Pet. Explor. Dev. 44 (5), 670–680 (in Chinese with English abstract). Zhang, W., Liang, J.Q., Su, P.B., Wei, J.G., Sha, Z.B., Lin, L., Liang, J., Huang, W., 2018. Migrating pathways of hydrocarbons and their controlling effects associated with high saturation gas hydrate in Shenhu area, northern South China Sea. Geol. China 45 (1), 1–14 (in Chinese with English abstract). Zheng, Z.H., Cao, Y.C., Chen, D.F., 2017. Prediction of the methane supply and formation process of gas hydrate reservoir at ODP 1247, Hydrate Ridge. Chin. J. Geophys. 60 (8), 3167–3176 (in Chinese with English abstract).

Please cite this article as: Y. Fang, J. Lu, J. Liang, et al., Numerical studies of gas hydrate evolution time in the Shenhu area in the northern South China Sea, China Geology, https://doi.org/10.1016/j.cngeo.2019.01.001