Zhu Youhai (1963-), male, researcher, mainly engaged in the research of salt deposits and nonmetal deposits in E-mail:zyh@mx.cei.gov.cn.

Note: This article was published in the 3rd issue of Marine Geology and Quaternary Geology in 2005, with some modifications.

Institute of Mineral Resources, Chinese Academy of Geological Sciences, Beijing 100037.

Abstract: ODP (Ocean Drilling Plan)-1 146 station is located in the small rift basin in the south of dongsha islands. The analysis results of high-altitude air and acidolysis hydrocarbon show that the volume fraction of hydrocarbon gas is low at 0 ~ 250 m (BSF stands for the depth under the seabed, unit: m), but there are obvious high hydrocarbon anomalies at 390~600 m (bsf), especially at 550~590 m (bsf). This high hydrocarbon anomaly may be related to natural gas hydrate, which is the result of the lateral migration of high hydrocarbon fluid released by the decomposition of natural gas hydrate along interlayer cracks or faults in adjacent areas. The measurement results of methane carbon isotope show that the δ 13C 1 value is -24.0 ‰ ~-37.8 ‰ (PDB standard), and the molecular ratio of combined hydrocarbon gas is C1(C2+C3), 1 146 stations.

Keywords: hydrocarbon gas; Stable isotope; Natural gas hydrate; the South China Sea

Geochemistry of hydrocarbon gas at ODP 1 146 station in South China Sea and its significance.

Zhu Youhai and Wu Bihao

CAGS Institute of Mineral Resources, Beijing 100037.

1 146 station, ODP 184 segment, is located in a small rift basin (Jianfengbei basin) on the southern slope of dongsha islands, with a water depth of 2,092 meters. The maximum depth of this rift basin is 607 meters below the seabed, ending in the Lower Miocene. We analyzed the concentrations of methane, ethane, propane and n-butane in the headspace gas and "extraction" gas (gas released by acid treatment of sediments) at 1 146 site and the carbon isotope composition of methane. Between 390 and 600 mbsf intervals, especially between 550 and 590 mbsf intervals, a relatively high concentration of hydrocarbon gas was observed, which may be caused by the decomposition of natural gas hydrate nearby and migrated to 1 146 along the fault or layered plane. The carbon isotope composition (δ 13C value is -37.8‰~-24.0‰) and molecular ratio of methane show that the gas below 400 mbsf is composed of thermogenic gas or mixed gas. However, some microbial gases may appear above 400 mbsf.

Keywords: hydrocarbon gas; Isotope; Gas hydrate; South China sea

Introduction to 0

Hydrocarbon gases in sediments mainly include microbial gas, pyrolysis gas and their mixtures. Microbial gas is the main gas source of most shallow gas and natural gas hydrate, while pyrolysis gas is the main gas source of conventional natural gas reservoirs [1].

Natural gas hydrate is a white crystalline substance composed of gas molecules and water, which is mainly produced in seabed sediments and onshore permafrost zones. Because its methane resource is (1.8 ~ 2.1) ×1kloc-0/6m3, which is twice the total carbon content of fossil fuels such as oil, natural gas and coal known in the world [2]; Countries all over the world and some international organizations attach great importance to the investigation and study of geological disasters and environmental problems caused by its development, which is the research hotspot of international geoscience today.

There are thick Mesozoic and Cenozoic sediments in the South China Sea, rich in organic matter, and many oil and gas fields have been discovered, especially in the northern and southern shelf areas, and a number of large and medium-sized natural gas fields have been discovered, such as Yinggehai Ya- 13 gas field and Palawan gas field. At the same time, the continental slope area of the South China Sea also has good conditions for the formation of natural gas hydrate, and a series of prospecting indicators have been found, such as simulated seabed reflection layer (BSR), hydrocarbon gas anomaly and satellite thermal infrared sea surface warming anomaly. [3-8].

In the past, the study of natural gas geochemistry in the South China Sea was mainly concentrated in the shallow water shelf area, while the deep water slope area was almost blank. Based on the investigation of ODP voyage 184 in the South China Sea, the author collected surface air and sediment samples at 1 146 station, and analyzed the gas composition and methane carbon isotope. This paper will introduce the experimental results of acid hydrolysis of high-altitude air and hydrocarbons, and discuss the causes and significance of hydrocarbon gases.

1 samples and methods

1February 99911-April 12, JOIDES Resolution of ODP made its maiden voyage to the South China Sea, conducted the 184 voyage survey on "Evolution history of East Asian monsoon in the South China Sea and its global climate significance", and * * completed 6 stations in the South China Sea. Among them, 1 146 station is located in a small rift basin (Jianfengbei basin) in the south of dongsha islands, with a water depth of 2 092 m (Figure 1). The 1 146 * * station has completed the drilling tasks of 1 146A, 1 146B and 15438+046c with the well depths of 607.0 and 607.0 respectively. The maximum age of bottom-hole sediments is about 19 Ma, in which 1 146 station is located in the favorable area for gas hydrate mineralization and prospecting, and it is possible to encounter gas hydrate or find related anomalies. Based on this, the author applied to the ODP organization to participate in the research work of the station as a shore-based scientist, and collected some sediments and headspace samples for various geochemical analysis.

The scientist on voyage ODP- 184 led by Academician Wang * * collected 39 top air samples for the author, including 1 146A hole 13 and146c hole 26, and the sampling depth was 342.6 ~ 56c. The specific method is to cut 5 cm3 sediment samples immediately after taking out the core and put them in a special glass test tube for sealing. In order to eliminate the interference of on-site air, the scientists on board also packed 1 on-site air sample for the author. After the samples to be sealed are transported to the laboratory, samples are prepared according to the requirements of top air (tank top gas), and then the volume fractions of methane (CH4), ethane (C2H6) and propane (C3H8) and the carbon isotope value of methane are detected. The specific testing work is undertaken by Jiangling Research Institute of Sinopec.

The author also analyzed 47 sediment samples in 1 146. The sampling depth was 8. 15 ~ 604.92 m (BSF) and the sampling interval was about15 m. The specific sampling methods were as follows: after the sediment samples were transported to the laboratory, they were naturally dried, mashed and sieved (. According to the modified acidolysis device, slowly add 5N dilute hydrochloric acid to acidify the sample (heating in a water bath at 40℃ to speed up the reaction), and then absorb CO2 with 7.5N alkali solution, so that the remaining gas is mainly hydrocarbon gas. After the acidolysis is completed (until bubbles are no longer generated), a certain amount of gas is extracted with a micro syringe to test the hydrocarbon gas content and stable isotopes.

Figure1ODP-1146bsr station and its surrounding area distribution map

2 Analysis results

2. 1 top air

Due to gas escaping during sampling, transportation and sample preparation, the methane content measured by the author is only 1% ~ 33% measured by scientists on board, and some samples even escape completely. Therefore, the author's analysis results are difficult to reflect the real situation of hydrocarbon gas. In order to show the volume fraction change and abnormal characteristics of hydrocarbon gas in this station, the on-site analysis data on board are cited for discussion.

On the voyage of 184, the scientists on board systematically analyzed the volume fraction of hydrocarbon gas in the air sample at the top of 1 146A hole. The results show that when the temperature is 23 1 m (bsf) (

2.2 Acid hydrolysis of hydrocarbons

The analysis results of 47 samples show that the volume fraction of methane is 15.7 ~ 394.l μ l/kg, with an average value of 133.4μL/kg. The volume fraction of ethane ranged from1.2 to 92.5 μ l/kg, with an average of 25. 1μL/kg. The volume fraction of propane is 0.5 ~ 38.6 μ l/kg, and the average value is 10.7μL/kg. The volume fraction of n-butane is 0. 1 ~ 16.9 μ l/kg, with an average of 4.6μL/kg. The volume fraction of hydrocarbon gas at 1 146 station is higher than the corresponding shallow sediments in Xisha Trough and the whole South China Sea, and its average value is almost twice that of shallow sediments, indicating that the volume fraction of hydrocarbon gas in deep sediments is much higher than that in shallow sediments.

Fig. 2 Variation diagram of headspace hydrocarbon gas integral number and molecular ratio at ODP-1146 station (original data are quoted from reference [9]).

Profile: above 254.3 m (bsf), the volume fraction of methane is relatively low, ranging from 15.7 ~ 129.3μl/kg, with little change; It gradually increased downward, reaching a peak of 394.1μ l/kg at 393.5 m (bsf); Further down, there are two sub-peaks at 553 m (bsf) and 583.7 m (bsf), which are 286.3μL/kg and 282.5 μ l/kg respectively. Lower to the bottom again (Figure 3); Compared with the top air, the peak position of acidolysis hydrocarbon is obviously higher, but the positions of the two secondary peaks are basically the same as those of the top air. The trends of ethane, propane and n-butane are basically the same as those of methane, but n-butane has a peak at 342.5 m(bsf) (Figure 3).

Fig. 3 Variation diagram of hydrocarbon gas integral number (μL/kg) and its molecular ratio in acidolysis hydrocarbon at ODP-1146 station.

2.3 carbon isotope of methane

The carbon isotope of methane was measured in nine top air samples at 1 146 station. The results show that the δ 13C 1 value is -24.0 ‰ ~-37.8 ‰ (PDB standard, the same below), and the average value is -33. 1 ‰. The δ 13C 1 of the hole1/46a is higher, and the two samples are -24.0 ‰ and -26.4 ‰ respectively. However, the δ 13C 1 of the hole 1/46c is relatively low, ranging from-31.3 ‰ to-37.8 ‰ (table1). At the same time, the carbon isotope of methane in the acidolysis hydrocarbon sample of 1 146 station 16 was determined. The results showed that the δ 13C 1 value was -29.8 ‰ ~-36.2 ‰, with an average of -33.7 ‰.

3 results discussion

3. 1 hydrocarbon gas abnormal horizon

According to the analysis results of hydrocarbon gas at 1 146 station, the hydrocarbon gas integral number in high-altitude air and acidolysis hydrocarbon is low, and does not change much from 0 to 250 m (BSF). The downward hydrocarbon gas integral number gradually increased, and the first peak of acidolysis hydrocarbon began to appear at 393 m (bsf), and the top air also showed a rapid increase trend. In the range of 550~590 m (bsf), the hydrocarbon gas is abnormally high, and the top air peak appears at 563m(BSF)( 1 146 a hole) and 572.8 m (BSF) (1146b hole). Further down, the volume fractions of hydrocarbon gas and acidolysis hydrocarbon in the air decrease (Figures 2 and 3). That is, there is obvious high hydrocarbon anomaly at 390~600 m (bsf).

The causes of abnormally high hydrocarbons include strong in-situ hydrocarbon generation performance, external replenishment and gas hydrate decomposition. There is no high organic carbon anomaly at 390~600 m (bsf), but the mass fraction of organic carbon at 1 146 station tends to decrease gradually from top to bottom [9], so in-situ hydrocarbon generation is unlikely to lead to high hydrocarbon anomaly in this section. If hydrocarbons are more likely to be generated in the middle and lower parts because of geothermal gradient, the volume fraction of hydrocarbon gas should be gradually increased, rather than reaching the peak in the middle part. In the case of deep recharge, the volume fraction of hydrocarbon gas should also gradually increase from top to bottom, and there should be no abnormal high value at 390~600 m (bsf), and then gradually decrease downward.

Natural gas hydrate is a solid substance composed of light hydrocarbon molecules (mainly methane) and water. If the sediment is rich in hydrocarbon gas, it will be beneficial to the formation of natural gas hydrate, and once the hydrate is decomposed, a large amount of hydrocarbon gas will be released, thus increasing the volume fraction of hydrocarbon gas in the sediment. If hydrocarbon gas exists in the pores of sediments in a free state or is adsorbed on the surface of sediments in an adsorbed state, it can be detected by headspace method; If it enters authigenic carbonate minerals or cements as inclusions, it can be detected by acid hydrolysis. Therefore, the high hydrocarbon anomaly in the range of 390~600 m (bsf) at 1 146 station may be related to natural gas hydrate. Combined with this station, signs of decreasing Cl- content and authigenic siderite nodules rich in 18O were found in the range of 550 ~ 600 m (BSF).

Table 1 ODP- 1 146 Carbon Isotope Test Results of Methane

1 146 station has a geothermal gradient of 59℃/km and a seawater temperature of 2.88℃[9]. Accordingly, the bottom boundary of gas hydrate stability zone at 1 146 station is about 268 m (bsf)[ 10], so it is 390 ~ 600. Considering that BSR has been found in the neighboring area (Figure 1), Song Haibin and others [1 1] think that BSR also exists at144 station, and Guangzhou Marine Geological Survey has recently been at 1 148 station. During the decomposition of natural gas hydrate, the released high-hydrocarbon and low-chlorine fluid laterally migrates to 1 146 station along interlayer cracks or faults.

According to the preliminary research of the scientists on board 184 voyage, there is a T2 seismic reflection surface near 430 m (bsf) of 1 146 station, which may be Miocene/Upper Miocene interface, and the acoustic impedance of this interface is the lowest. Near 520~530 m (bsf), it is the reflection surface of T4 earthquake, which may be the interface of Lower Miocene/Miocene. There are also a series of lithologic and physical changes, such as the decrease of linear sedimentation rate (LSR), the decrease of magnetic susceptibility and the increase of natural gamma value. There is a fault [65438] about 1.852 km to the northwest. Therefore, the fault and two seismic reflection interfaces can completely provide a channel for the lateral migration of pore fluid, resulting in high hydrocarbon anomalies and other geochemical anomalies at1/46 station at 390~600 m (bsf).

3.2 Source of hydrocarbon gas

Hydrocarbon gas is the material basis of natural gas hydrate and conventional natural gas. Its origin and source not only affect the metallogenic mechanism and formation process of natural gas hydrate or conventional natural gas, but also affect its resource evaluation and specific prospecting methods. Generally speaking, hydrocarbon gases in sediments can be divided into organic gases and non-organic gases. Among them, organic gases are subdivided into microbial gases, pyrolysis gases and their mixtures. Microbial gas refers to the gas transformed from organic matter in sediments by bacteria, mainly including CO2 reduction and acetic acid fermentation, and is the main gas source of most shallow gas and natural gas hydrate. Pyrolysis gas refers to the gas formed by deep cracking of organic matter after its evolution to oil generation stage, including oil-type associated gas and non-oil-type associated gas (coal-type gas), which is the main gas source of conventional natural gas reservoirs.

Statistics show that 80% of the natural gas in the world is pyrolysis gas, 20% is microbial gas, and there are relatively few inorganic natural gas [1]. On the contrary, most natural gas hydrates are composed of microbial gases, such as Blake Ridge, South China Sea Trough and Hydrate Ridge. Only a few of them are composed of pyrolysis gas, such as Caspian Sea, Gulf of Mexico and Mallik area in Canada. In addition, some of them are composed of mixed gases, and a typical example is the hydrate near DSDP-570 Station in the Sino-American Trough [13- 14].

Using the molecular composition of hydrocarbon gas and the carbon isotope value of methane is helpful to judge the origin and source of gas. If δ 13C of methane is less than -60 ‰ and Cl/(C2+C3) is greater than 1 000, it is microbial gas; If δ 13C of methane is greater than -50 ‰ and c1(C2+C3) is less than 100, it is pyrolysis gas; The middle mixture is [1].

Scientists on ODP- 184 analyzed and tested 74 top air samples at 1 146 * *. The values of C 1/(C2+C3) could not be calculated because the contents of ethane and propane in the upper and middle samples were lower than the detection limit, but they began to appear below 500 m (bsf). This shows that the lower part of the station may be pyrolysis gas or mixed gas, and the upper part may contain microbial gas. The fact that the value of C 1/(C2+C3) gradually decreases also shows that with the increase of depth, its maturity gradually increases, and the proportion of pyrolysis gas is increasing.

The carbon isotope of methane in nine top air samples with a height of 406.5 m (bsf) below 1 146 station shows that the value of δ 13C 1 ranges from-24.0 ‰ to-37.8 ‰ (table 1), which obviously belongs to the pyrolysis gas range. Since only four samples have values of δ 13C 1 and C 1/(C2+C3) at the same time, it is found that they are all in the range of mixed gas, but closer to pyrolysis gas (Figure 4). The other five samples could not be plotted because ethane and propane contents were not detected, but they were extrapolated according to their δ 13C 1 data. Therefore, the top air below 400 m (bsf) at 1 146 station should be pyrolysis gas or a mixture dominated by pyrolysis gas.

Fig. 4 methane carbon isotope (δ 13C 1) and its molecular ratio C 1/(C2+C3) of hydrocarbon gas at ODP-1/46 station.

47 acid-decomposed hydrocarbon samples were analyzed and tested at 1 146 * * station. The results show that the value of C 1/(C2+C3) is 4 ~ 18, with an average value of 6, which obviously belongs to the pyrolysis gas range and also shows a downward trend (Figure 3). The results of carbon isotope determination of methane in 16 acid-decomposed hydrocarbon samples show that the δ 13C 1 value is -29.8 ‰ ~-36.2 ‰ (table 1), which also obviously belongs to the pyrolysis gas range. After mapping the δ 13C 1 and C 1/(C2+C3) values of these 16 samples, it is found that they are all within the pyrolysis gas range (Figure 4). It can be seen that the acidolysis hydrocarbon in 1 146 station belongs to pyrolysis gas.

To sum up, the hydrocarbon gas in ODP- 1 146 station should be pyrolytic gas or mixed gas dominated by pyrolytic gas, but there may be some microbial gas in the middle and upper part. That is to say, the gas hydrate in 1 146 station and its adjacent area should be pyrolysis gas hydrate. Although most natural gas hydrates in the world are composed of microbial gas, if pyrolysis gas is added, its gas volume fraction will be higher, which is more conducive to the formation of natural gas hydrates.

4 conclusion

Through the systematic test and analysis of hydrocarbon gas at ODP- 1 146 station, it is found that the content of hydrocarbon gas at 0 ~ 250 m (BSF) is low, with little change; The volume fraction of hydrocarbon gas gradually increased downward, and the peak value of acidolysis hydrocarbon began to appear at 393 m(bsf). The volume fraction of methane in the air at the top of the tower also increased greatly, and high molecular hydrocarbons such as ethane and propane began to appear. At 550~590 m (bsf), the top air has a peak value, while the acidolysis hydrocarbon has a sub-peak value. Further down, hydrocarbon gas and acidolysis hydrocarbon in the air are reduced. The high hydrocarbon anomaly of 390~600 m (bsf) may be the result of hydrocarbon-rich fluid migrating along interlayer faults or faults after the decomposition of natural gas hydrate in adjacent areas.

The results of carbon isotope determination of methane in nine headspace and 16 acidolysis hydrocarbon samples show that the δ 13C 1 value is -24.0 ‰ ~-37.8 ‰, which indicates1/in combination with the hydrocarbon gas molecular ratio c1(C2+C3). Among them, the acidolysis hydrocarbon samples all belong to pyrolysis gas, and the top air in the lower part is also pyrolysis gas or mixed gas dominated by pyrolysis gas, while the top air in the middle and upper part may contain microbial gas. Therefore, the natural gas hydrate in 1 146 station and its adjacent area should be pyrolytic natural gas hydrate.

Acknowledgement: This paper is supported by the National Natural Science Foundation Project (4047300 1) and the National Nansha Special Project. Special thanks go to all the scientists on voyage ODP 184 headed by Academician Wang for collecting top-level atmospheric samples for us, and also to Comrade Rao Zhu from the National Geological Experimental Testing Center and Comrade Li from Lanzhou Institute of Geology, Chinese Academy of Sciences for analyzing some samples for us.

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