1. Introduction
In 2017, the first offshore natural gas hydrate (NGH) production test in the South China Sea (SCS) was conducted by the China Geological Survey (CGS). The production test site, SHSC-4, is located in the middle of the continental slope of southeast Shenhu area, northern SCS, about 300 km southeastward away from Hong Kong (
Figure 1). The water depth of SHSC-4 is about 1263.5 m, according to the ROV (Remote Operated Vehicle) in situ survey. The production test lasted for 60 days, from 10 May to 9 July. The cumulative gas volume produced during the sixty-day test was approximately 309,000 m
3 (under the atmospheric pressure). The gas production rate was approximately 5454 m
3/day, with a maximum of 35,000 m
3/day. The gas hydrate production test in SCS holds the new record for the longest production time and the highest cumulative volume in the world’s offshore gas hydrate production test, which is essential to the future global energy supply.
The core data from site SHSC-4 showed that gas hydrate occurred over a 50-meter interval from 201 mbsf (meter blow the seafloor) to 251 mbsf [
1]. The characteristics of gas hydrate-bearing sediment is clayey silt, which accounts for more than 90% of the global NGH [
2]. The gas hydrate saturation from both the pore-water freshening and pressure core mass balance is variable throughout the interval, with mean values of 35%. The mean effective porosity of the gas hydrate reservoir is about 34%, and the mean permeability is 2.9 mD [
1].
In this document, we report on the shore-based analyses of the produced gas component and isotopic composition from the SHSC-4 site. The objective of this report is to present the gas characteristics of China’s first gas hydrate production test in SCS, based on the molecular and isotopic analysis.
2. Samples and Analytical Methods
All of the gas samples were collected from the flow line on the rig, which was extracted from the gas hydrate interval between 201 mbsf and 251 mbsf with depressurization. All of the samples had no hydrogen sulfide smell. They were sealed in sample bags and were taken back for onshore analysis immediately. Three hundred gas samples were examined for geochemistry by gas chromatography (GC) and isotope ratio mass spectrometer.
An Agilent 7890A GC equipped with a 30 m × 0.53 mm HP-PLOT/Q column was used to measure the hydrocarbon gases from C1 to C5. The Agilent 7890 A is configured with a 1-mL, valve-actuated, sample loop for injection, and a thermal conductivity detector (TCD) for gas detection. The samples were introduced by syringes under atmospheric pressure, and a minimum of 10 mL of gas was used to flush the injection loop. The experiments were initially performed under 50 °C, with an increasing temperature rate of 20 °C/min to 200 °C. Helium was used as the carrier gas at a constant flow rate of 5 mL/min. The TCD temperature was kept under 200 °C. The GC was calibrated using the hydrocarbon gas standards of known concentrations.
Isotopic ratios of carbon and hydrogen in the hydrocarbon gases were measured with a Thermo Fisher Trace GC ultra-gas chromatography coupled to a MAT-253 isotope ratio mass spectrometer via a GC-isolink interface, at Guangzhou Marine Geological Survey Laboratory. The gas components were separated on a 30 m × 0.3 mm ID HP-PLOT/Q column using helium as the carrier gas. The flow rate was 1.5 mL/min. The GC oven was set under 50 °C for 2 min, then programmed to increase to 140 °C at a rate of 30 °C/min, and was maintained under 140 °C for 4 min. The GC injection port was set under 100 °C and then the split ratio was 50:1. The combustion oven temperature was 1000 °C in order to convert all of the hydrocarbons to CO2, and the high temperature cracked oven temperature was 1420 °C in order to determine the hydrogen isotope. The δ13C values were reported in the unit of per mil (‰), relative to the VPDB (Vienna Pee Dee Belemnite) standard. The carbon isotope data are more accurate for the hydrocarbon gases (accuracy within ±0.15‰). The δD values were also reported in the unit of ‰ (accuracy within ±1.0‰), relative to VSMOW (Vienna Standard Mean Ocean Water). The carbon and hydrogen isotopic ratio for both methane and ethane were measured, and other hydrocarbon gases were not detected because of the instrument limit.
3. Results
All of the 300 gas samples show similar GC curves, which exhibit a large C1 peak (Methane), but a weak peak of the other components (
Figure 2). The GC analysis shows that all of the gas samples are predominated by CH
4 (91.35–97.69%, average = 95.87%) with minor N
2 + O
2 (1.79–8.18%, average = 3.63%), and trace CO
2 (0.007–0.142%, average = 0.013%), C
2H
6 (0.294–0.521%) and C
3H
8 (0.062–0.108%). No H
2S was detected. Among all of the hydrocarbon gases, CH
4 consists 99.38~99.60%, with an average of 99.50% (
Figure 3).
The molecular {C
1/(C
2 + C
3)} compositions of the 300 gas samples collected from the production test range from 165 to 259 (ave. = 209). The δ
13C and δD in methane are within the range of −66.09–−63.14‰ (average = −64.88‰) and −195.8–−186.0‰ (average = −191.3‰), respectively, which are similar to those of the methane experimentally released from the natural gas hydrate samples collected from the Shenhu Area (δ
13C
1 = −64.38–−61.57‰, δD
CH4 = −220.00–−191.00‰ [
3]).
The relationship between the C
1/C
2+ ratios and the δ
13C values of methane is a good way to distinguish the origin of hydrocarbon. High C
1/C
2+ ratios (C
1/C
2+ ≥1000) and low methane δ
13C values (δ
13C ≤ −55‰) are characteristic of a microbial origin, while low C
1/C
2+ ratios (C
1/C
2+ ≤ 100) and high methane δ
13C values (δ
13C ≥ −50‰) are characteristic of a thermogenic origin [
4,
5,
6,
7,
8,
9,
10]. On the plot of C
1/(C
2 + C
3) − δ
13C
1, all of the gas samples from the production test fall into the “mixed origin” region close to the microbial origin (
Figure 4), which is different from the gas hydrate from Blake Ridge, Hydrate Ridge, Mexico gulf, and a Japan gas hydrate prodution test site in Nankai Trough, but close to the Ulleung basin of Korea. Moreover, the discrimination diagram of δ
13C
1 − δD
CH4 (
Figure 5) shows that the methane in all of the samples is of microbial origin and is derived from a CO
2 reduction, which is different from the nearby LW3-1-1 gas well with methane thermogenic origin in SCS, but is the same as Blake Ridge, Hydrate Ridge, and the Ulleung basin of Korea. The information from
Figure 4 and
Figure 5 indicate that the gas extracted from China’s first gas hydrate production test possibly originate from bacterial and thermogenic gas mixtures, but have more of a bacterial component.
4. Summary and Conclusions
The extracted gas from China’s first natural gas hydrate production test in SCS are predominantly by methane, which accounts for 99.38~99.60% of all of the hydrocarbon gases. The δ13C and δD in methane are within the range of −66.09–−63.14‰ and −195.8–−186.0‰, respectively. Different from those of the nearby LW3-1-1 gas well with a thermogenic origin, these values imply that the methane of the natural gas hydrate in the Shenhu area was mainly derived from the bacterial reduction of CO2. Nevertheless, the contribution of the thermogenic origin cannot be excluded in this study. It is possible that the bacterial gas is the main gas origin for gas hydrate formation in SHSC-4.
Author Contributions
J.Y. made the substantial contributions of research conception and design. X.Q. contributed to made the critical revision. H.Q. contributed to data collection. W.X. made the contribution of data interpretation. H.L. drafted the article and made final approval of the article. C.L., J.Z. and J.L. analysed the component of samples. T.Y. collected the gas samples. J.C. and R.S. made the isotope analysis.
Funding
This research was funded by the GH Programs of China Geological Survey grant number DD20189320 and DD20160221.
Acknowledgments
This study was financially supported by the GH Programs of China Geological Survey (No. DD20189320 and No. DD20160221). We thank all of the scientists and staff who participated in China’s natural gas hydrate production test in SCS. The authors are grateful to Kuang Zenggui, Zhang Ruwei, Li Zhanzhao, Wang Tinghui, Wei Jiangong, and Liang Qianyong for their sampling and measurements in the field. We also thank Jiang Xuexiao and Deng Yinan for helping the carbon and oxygen isotope analyses in the laboratory.
Conflicts of Interest
The authors declare no conflicts of interest.
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