Data Report: Molecular and Isotopic Compositions of the Extracted Gas from China’s First Offshore Natural Gas Hydrate Production Test in South China Sea

Abstract

Three hundred gas samples recovered from SHSC-4 during China’s first gas hydrate production test in the South China Sea were examined for gas component and isotopic composition. According to the gas chromatography analysis, all the gas samples from SHSC-4 are predominated by CH₄, with minor N₂ + O₂, as well as trace amounts of CO₂, C₂H₆, and C₃H₈. No H₂S was detected. The molecular and isotopic data of the gas samples fall into the region of “mixed origin” on the plot of C₁/(C₂ + C₃) − δ¹³C₁, which is close to the microbial origin. The discrimination diagram of δ¹³C₁ − δD_CH4 shows that the methane in all of the samples is of microbial origin, and is derived from the CO₂ reduction.

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 m³ (under the atmospheric pressure). The gas production rate was approximately 5454 m³/day, with a maximum of 35,000 m³/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 CH₄ (91.35–97.69%, average = 95.87%) with minor N₂ + O₂ (1.79–8.18%, average = 3.63%), and trace CO₂ (0.007–0.142%, average = 0.013%), C₂H₆ (0.294–0.521%) and C₃H₈ (0.062–0.108%). No H₂S was detected. Among all of the hydrocarbon gases, CH₄ consists 99.38~99.60%, with an average of 99.50% (Figure 3).

The molecular {C₁/(C₂ + C₃)} compositions of the 300 gas samples collected from the production test range from 165 to 259 (ave. = 209). The δ¹³C 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 (δ¹³C₁ = −64.38–−61.57‰, δD_CH4 = −220.00–−191.00‰ [3]).

The relationship between the C₁/C₂₊ ratios and the δ¹³C values of methane is a good way to distinguish the origin of hydrocarbon. High C₁/C₂₊ ratios (C₁/C₂₊ ≥1000) and low methane δ¹³C values (δ¹³C ≤ −55‰) are characteristic of a microbial origin, while low C₁/C₂₊ ratios (C₁/C₂₊ ≤ 100) and high methane δ¹³C values (δ¹³C ≥ −50‰) are characteristic of a thermogenic origin [4,5,6,7,8,9,10]. On the plot of C₁/(C₂ + C₃) − δ¹³C₁, 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 δ¹³C₁ − δD_CH4 (Figure 5) shows that the methane in all of the samples is of microbial origin and is derived from a CO₂ 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.