International Conference on Gas Hydrates (ICGH) (6th : 2008)

FORMATION PROCESS OF STRUCTURE I AND II GAS HYDRATES DISCOVERED IN KUKUY, LAKE BAIKAL Hachikubo, Akihiro; Sakagami, Hirotoshi; Minami, Hirotsugu; Nunokawa, Yutaka; Yamashita, Satoshi; Takahashi, Nobuo; Shoji, Hitoshi; Kida, Masato; Krylov, Alexey; Khlystov, Oleg; Zemskaya, Tamara; Manakov, Andrey; Kalmychkov, Gennadiy; Poort, Jeffrey Jul 31, 2008

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Proceedings of the 6th International Conference on Gas Hydrates (ICGH 2008), Vancouver, British Columbia, CANADA, July 6-10, 2008.  FORMATION PROCESS OF STRUCTURE I AND II GAS HYDRATES DISCOVERED IN KUKUY, LAKE BAIKAL Akihiro Hachikubo∗, Hirotoshi Sakagami, Hirotsugu Minami, Yutaka Nunokawa, Satoshi Yamashita, Nobuo Takahashi and Hitoshi Shoji Kitami Institute of Technology, 165 Koen-cho, Kitami 090-8507, JAPAN Masato Kida Advanced Industrial Science and Technology, 2-17-2-1, Tsukisamu-Higashi, Toyohira-ku, Sapporo, Hokkaido 062-8517, JAPAN Alexey Krylov All-Russia Research Institute for Geology and Mineral Resources of the Ocean, 1 Angliyskiy Prospekt, 190121 St.Petersburg, RUSSIA Oleg Khlystov and Tamara Zemskaya Limnological Institute, SB RAS, 3 Ulan-Batorskaya St., Irkutsk 664033, RUSSIA Andrey Manakov Nikolaev Institute of Inorganic Chemistry, SB RAS, 3 Akad. Lavrentiev Prospekt, Novosibirsk 630090, RUSSIA Gennadiy Kalmychkov Vinogradov Institute of Geochemistry, SB RAS, 1-a Favorsky St., Irkutsk 664033, RUSSIA Jeffrey Poort Renard Centre of Marine Geology, Ghent University, Krijgslaan 281-S8, Ghent B-9000, BELGIUM ABSTRACT Structure I and II gas hydrates were observed in the same sediment cores of a mud volcano in the Kukuy Canyon, Lake Baikal. The sII gas hydrate contained about 13-15% of ethane, whereas the sI gas hydrate contained about 1-5% of ethane and placed beneath the sII gas hydrate. We measured isotopic composition of dissociation gas from both type gas hydrates and dissolved gas in pore water. We found that ethane δD of sI gas hydrate (from -196 to -211 ‰) was larger than that of sII (from -215 to -220 ‰), whereas methane δ13C, methane δD and ethane δD in both hydrate structures were almost the same. δ13C of methane and ethane in gas hydrate seemed several permil smaller than those in pore water. These results support the following idea that the current gas in pore water is not the source of these gas hydrates of both structures. Isotopic data also provide useful information how the “double structure” gas hydrates formed. Keywords: gas hydrate, stable isotope, Lake Baikal, isotopic fractionation, crystal structure ∗  Corresponding author: Phone: +81 157 26 9522 Fax +81 157 26 9534 E-mail: hachi@mail.kitami-it.ac.jp  INTRODUCTION Gas hydrates in Lake Baikal were first obtained from sub-bottom depths of 121 and 161m in the Baikal Drilling Project (BDP) well at the southern basin in 1997 [1]. The gas concentration was mainly methane, and δ13C values (from -68.2‰ to -57.6‰) suggested that they are gases of microbial origin. Recently, those near the lake bottom were discovered in the area of the mud volcanoes named Malenky at the southern basin [2-4] and Kukuy K-2 at the central basin [5-7]. These sites are in the zones with elevated relief and characterized by flow escapes [8]. While hydrate gas retrieved from these areas contained mainly microbial methane, high concentration of ethane (13.4%) in the dissociated gas was found in Kukuy K-2 [6], which indicated gas hydrate of mixed genesis (microbial + thermogenic) and suggested the presence of structure II (sII) gas hydrates [7]. An appropriate composition of methane and ethane forms sII gas hydrates. They appeared from 0.6-0.8% to 25-27.8% of ethane composition in vapor phase at 274.2K [9-10]. CP-MAS 13C NMR spectroscopy was applied to determine their crystal structure [11]: the hydrate samples that obtained in Kukuy K-2 and contained about 1415% of ethane belonged to sII, whereas the sI gas hydrate contained about 3% of ethane and placed beneath the sII gas hydrate in the same cores. The coexistence of sI and sII gas hydrates in the same core was also confirmed by XRD technique [12]. In this study, gas compositions and isotopic ratios (13C and D) were taken from hydrate-bound gas and also from dissolved gas in sediments by a headspace gas method to understand the formation process of the different crystal structures of gas hydrate in Kukuy K-2, Lake Baikal. EXPERIMENTAL The cores of hydrate-bearing sediment were retrieved by using a gravity corer from two sites in Lake Baikal on September 2005 (VER05-03) and September 2006 (VER06-02). Kukuy K-2 site (water depth: 908-923m) and Malenky site (water depth: 1370-1375m) are located in the central and the southern basins of Lake Baikal, respectively (Figure 1). Gas hydrates were found in seven cores (90-330cm depth beneath the lake bottom) at Kukuy K-2 in 2006 and two cores (115-180cm depth beneath the lake bottom) at Malenky in 2005. Gas samples were obtained on board: dissociation gases from hydrate samples were separated from dissociation waters and filled into 5ml vial bottles,  55°N  54°N  Malenky Angara river Irkutsk  105°E  53°N 109°E  Kukuy K-2 Selenga river 52°N 107°E  100km  Figure 1 Map of the study area in Lake Baikal. and dissolved gases in sediments (mainly in pore water) were obtained by the headspace gas method. Gas hydrate samples were also stored in a dryshipper at liquid nitrogen temperature and dissociation gases were obtained from them in the laboratory of Kitami Institute of Technology. For the isotopic analyses of carbon and hydrogen, a continuous flow-isotope ratio mass spectrometer (CF-IRMS, DELTA plus XP; Thermo Finnigan) was used. Isotopic compositions are reported as δ values (δ13C and δD) given with reference to the VPDB and VSMOW standards, respectively. The analytical precision of δ13C is 0.1‰ and that of δD is 0.6‰. Gas compositions of C1-C4 in all gas samples were measured using a gas chromatograph (GC-14B; Shimadzu Corp.) equipped with a thermal conductivity and a flame ionization detector with a packed column (SunpakS, Shimadzu Corp.). RESULTS AND DISCUSSIONS Different relations between isotopic ratios and gas composition of the hydrates are plotted in Figure 2. Gas samples obtained from Kukuy K-2 are located in the field of mixed gas, whereas those obtained from Malenky are in the field of microbial gas (Figure 2a). Concentrations of ethane are 0.020.04% (Malenky), 1.2-4.5% (sI hydrates of Kukuy K-2) and 13.4-14.5% (sII hydrates of Kukuy K-2), respectively. C1/(C2+C3) data of sI and sII in Kukuy K-2 are clearly different from each other  100000  (a)  13  δ C of CH4 (‰)  Microbial gas  Kukuy K-2 st.I  10000  Kukuy K-2 st.II Malenky  -75  Thermogenic gas -70  -65  -60 13  -55  -50  -45  -40  -55  -50  -45  -40  -20  -15  -10  150 200  Hydrate Gas  250  Dissolved Gas  300  δ C of CH4 (‰VPDB)  350  -80  13  δ C of C2H6 (‰) -50 0  Microbial CH4 (CO2 reduction)  -70  Mixed & transition zone  Microbial CH4 (methyl-type fermentation)  -60 -50  -30  Kukuy K-2 st.II Malenky -300  -250  -200  -150  -25  Dissolved Gas  150 200  300 350  (b) 13  (c)  Figure 3 Isotopic profiles of (a) methane δ C 13 and (b) ethane δ C obtained from seven hydrate-bearing sediment cores in Kukuy K-2. Red circles: dissociated gas from hydrate, green circles: dissolved gas in pore water.  Kukuy K-2 St.I  Microbial CH4 and C2H6  Kukuy K-2 St.II  Microbial CH4 and thermogenic C2H6  Thermogenic CH4 and C2H6  -30 -20 -80  -30  -100  -70  -40  -35  250  -350  δD of CH4 (‰VSMOW) (‰VSMOW)  -50  -40  Hydrate Gas  100  Kukuy K-2 St.I  -20 -400  -45  50  Thermogenic CH4  -40  -60  (a)  (b)  Depth (cm)  -90  δ13C of CH4 (‰VPDB)  -60  100  Mixed gas  1  13  -65  50  100  10  δ C of C2H6 (‰VPDB)  -70  1000  Depth (cm)  C1 / (C2+C3)  -80 0  -70  -60  -50  -40  -30  -20  13  δ C of CH4 (‰VPDB)  Figure 2 Isotopic results of gas hydrate samples: 13 (a) relation between methane δ C and the ratio 13 C1/(C2+C3), (b) relation between δD and δ C of 13 methane, (c) relation between δ C of methane and ethane. The fields of gas origin in these graphs are according to [13-15]. and the former seems to be rather distributed widely from 13 to 80 whereas the latter is about 6. Methane δ13C ranges from -59.1 to -56.5‰ in Kukuy K-2 and from -66.0 to -64.6‰ in Malenky. There is no difference in δ13C between sI and sII hydrates of Kukuy K-2. In the diagram using δ13C and δD of methane [13-15], the methane obtained from Kukuy K-2 appears in the field of microbial  origin produced by methyl-type fermentation (Figure 2b) as has been already reported by [11]. Samples in Malenky are also plotted in the same field and methane δD ranges from -314 to 294‰ for both sites. Ethane δ13C in Kukuy K-2 indicates its thermogenic origin (Figure 2c) and methane δ13C in Kukuy K-2 seems to be closed to thermogenic field. We could not measure ethane δ13C in Malenky because of low concentration of ethane. From these results it is reasonable to say that microbial methane is dominant in Malenky and thermogenic methane and ethane mixes partly with the microbial methane in Kukuy K-2. We would like to focus attention on isotopic composition of methane and ethane obtained from Kukuy K-2. δ13C profiles from seven hydratebearing cores are plotted against depth from the lake bottom in Figure 3. Methane δ13C of dissolved gas in pore water was small (-65‰) in the upper layer (35-70cm depth) and increased with depth and reached to -55‰. On the other  0  Concentration of C2H6 (%) 5  10  15  0  20  δD of CH4 (‰)  δ13C of CH4 (‰)  5  10  15  20  -290  -50 -55 -60 -65  (a) -70  -295 -300 -305 -310 -315  (b)  -320  0  Concentration of C2H6 (%) 5  10  15  0  20  Concentration of C2H6 (%) 5  10  15  20  -190  δD of C2H6 (‰)  -15  δ13C of C2H6 (‰)  Concentration of C2H6 (%)  -20 -25 -30  (c) -35  -195 -200 -205 -210 -215 -220  (d)  13  Figure 4 δ C and δD of methane and ethane plotted against concentration of ethane. 13 Ethane δD of sI hydrate was larger than that of sII, though δD of methane, δ C of methane and ethane in both hydrate structures were almost the same each other. hand, gas hydrate was found from 93 to 334cm. Methane δ13C of hydrate gas distributed from -59 to -57‰ and several permil smaller than that of dissolved gas in pore water. A profile of ethane δ13C of dissolved gas in pore water seemed to be uniform (from -26 to -24‰) for the all sediment cores, on the contrary, that of hydrate gas distributed from -30 to -24‰ and smaller than that of dissolved gas in pore water. “Double structure” gas hydrates were found in two sediment cores (VER05-03 St2GC1 and St2GC6) in 2005 as reported by [11], and six sediment cores (VER06-02 St2GC5, St2GC7, St2GC22, St2GC23, St2GC30 and St2GC32) in 2006 by the information of ethane concentration. Ethane-rich hydrates, which correspond to sII, located in the upper part of the gas hydrate layer in the all cores, and comprised hydrate granules (several mm in diameter). sI hydrates were massive and/or stratified, and found in the lower part of gas hydrate layer. A sediment layer sometimes separated these hydrates of different structures. Isotopic compositions are plotted against ethane concentration in Figure 4. Ethane δD of sI hydrates seemed to be larger than that of sII hydrates, whereas methane δ13C, ethane δ13C and  methane δD in both structures were almost the same each other. Besides this, ethane δD of sI hydrates were rather distributed widely from -196 to -211‰ and those of sII were concentrated from -215 to -220‰. DISCUSSTION Isotopic fractionation of carbon and hydrogen of hydrocarbon gases during the formation of gas hydrates has been investigated by the previous study: δD of hydrate-bound gas becomes several permil smaller than that of the original gas, whereas δ13C of hydrate-bound and original gases are almost the same value [16-17]. The differences in δD of methane and ethane between gas and hydrate phases at 274.2K were 4.8 ± 0.4‰ and 1.1 ± 0.7‰, respectively. Figure 3 shows that methane δ13C of hydrate gas distributed from -59 to -57‰ and several permil smaller than that of dissolved gas in pore water (about -55‰ below 140cm depth). In case of ethane, δ13C of hydrate gas seemed to be smaller than that of dissolved gas in pore water. Because of the undetectable fractionation in δ13C at the formation of gas hydrate [16], we can say that the current gas dissolved in pore water is not the  source of these gas hydrates of both crystal structures. The gas hydrate crystals in Kukuy K-2 formed from methane (δ13C: -58‰) and ethane (δ13C: -30‰) of dissolved gas, and then the δ13C of dissolved gas in pore water increased several permil in VPDB scale by some geological activities of the mud volcano; routes of gas supply and/or mixing process of thermogenic and microbial gases might be changed. Although information how long the gas hydrates keep their isotope ratio may provide the formation period of them, it is an important subject of a future study. Figure 4 shows that ethane δD of sI hydrates seemed to be larger than that of sII hydrates. These results may provide the following hypothesis: the sII gas hydrates firstly formed and consumed a lot of ethane, and the sI gas hydrates then formed from the low concentration of ethane and increased ethane δD of hydrate phase. Although methane δD in hydrate phase must be also smaller than that in dissolved gas, the difference of methane δD between both structures seemed to be negligible due to abundant methane compared with ethane. In a ternary system of methane, ethane and water, ethane is enriched into hydrate phase at the formation of hydrate and conspicuously decreases the equilibrium pressure [18]. At their equilibrium state, ethane composition of hydrate phase is always larger than that of gas phase. Assuming that the gas supply from deeper sediment layer by ascending fluid was insufficient to keep the ethane concentration of pore water in the surface layer of lake-bottom sediment, we can say the following story: Nucleation of sII hydrate (about 14%C2) occurred and rapidly formed from dissolved gas in pore water (about 3%C2) due to low equilibrium pressure. The ethane concentration of dissolved gas then decreased and became less than the critical concentration of 0.6-0.8%C2 [10]. Because 0.6%C2 of dissolved gas was small to make sII hydrate, sI hydrate (about 4%C2) started to form slowly due to rather high equilibrium pressure. Both the ethane concentrations of sI hydrate and dissolved gas decreased due to preferential consumption of ethane into hydrate phase. The range of the ethane concentration in sI hydrate became larger than that in sII hydrate. Isotope results supports this story: light ethane (C2H6) was first concentrated in the hydrate phase (sII) and heavy ethane (C2H5D) remained relatively in the dissolved gas, which was consumed by the next stage of hydrate formation (sI).  On the other hand, an opposite idea for the formation process of double structure gas hydrates is also applicable [12]: sI gas hydrate (about 3%C2) firstly formed, dissociated and then sII gas hydrate (about 13%C2) formed from the dissolved gas in pore water (about 3%C2). This hypothesis is rather simple and can explain the case that sII hydrates exist adjacent to sI hydrates. It is possible that ascending fluids that transport gases stopped by some geological activities, sI hydrates partly dissociated and reconstruct sII hydrates. CONCLUSION We conclude that the current gas dissolved in pore water is not the source of these gas hydrates of both crystal structures in Kukuy K-2 mud volcano in Lake Baikal. We proposed an idea to explain the formation process of the double structure gas hydrates discovered in Kukuy K-2, Lake Baikal by using isotope data. However, it needs further discussion about the time scale of their formation, size effect of gas hydrate formation on the isotopic fractionation and geological evidence to support the above idea. ACKNOWLEDGMENTS This work was supported by funding agencies: Japan Society for the Promotion of Science KAKENHI 17550069, 18206099 and 19550077, the Ministry of Education, Culture, Sports, Science and Technology KAKENHI 19740323, Kitami Institute of Technology Presidential Grant, Flemish Fund for Scientific Research (FWOVlaanderen), Bilateral Flemish-Russian Federation Project, and the Integration project of RAS SB 58. 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