Gashydrate areas, gas discharge and methane flows in the Okhotsk Sea

Pestrikova N.L.

V.I. Il’ichev Pacific Oceanological Institute feb ras, Vladivostok, Russia

natap81@mail.ru

The discovery of natural gashydrates (overland and in the sea), their high energy capacity, understanding their significant role in the global and regional natural processes attract interest of scientists of the whole world. At present great attention is attended to the regularity of gashydrate formation and decomposition and possibility their use as non-traditional hydrocarbon raw.

Nowаdays in the Sea of Okhotsk there are known two areas of gashydrate occurring in the bottom sediments [12, 4, 1, 2, 10, 5, 3, 7, 8]: the eastern Sakhalin slope and the north-western slope near Paramushir Island (the south-eastern part of Golyginskii flexure). These geological structures are characterized by sufficiently high potential of hydrocarbon generation. Gashydrates are associated with areas of gas vents located near by active deep faults.

Before 2009 in the north-western part of the Okhotsk Sea Area there are discovered about 400 natural gas vents (mainly, methane composition) from bottom sediments to the sea water and 11 structures where gashydrates were sampled. Gashydrates are revealed nearby seafloor layers of sediments in various forms like lenses, layers, interlayers and their fragments, for instance. Expedition research from 1998 to 2002 point to that all these natural gas vents are referred to the intersection zones of the faults of the north-western and north-eastern direction controllable by East Sakhalin fault zone, influencing the seismo-tectonic activity of north-eastern shelf and slope of the Sea of Okhotsk.

The methane concentration in the water of the northwestern part in the Sea of Okhotsk sharply increase when gashydrates are in the sediments located nearby active deep faults. Gas bubbles direct to the sea surface and create anomalies with maximum values 20 000 – 200 000 nl/l near seafloor and 500-1000 nl/l near sea surface. Whereas in the areas of oil-and-gas structures as a rule methane concentrations in the bottom water are about 2500 nl/l when background values in the region are: seasurface water – 70-90 nl/l, subsurface sea water (75-100 m) – 100-120 nl/l, and water near by seafloor (1000 m and deeper) - about 15-20 nl/l.

Research of the gas parts of sediments in the Sea of Okhotsk shows: in the cores where gashydrates are not found as a rule methane concentrations increase from a depth of 200 cm (0,1 – 1 ml/l and more), probably, it is explained by the presence of sulfate-reduction zone over the depth. In the sediments containing gashydrate this regularity is not observed because gashydrate intervals can contain sufficiently a huge volume of methane and locate at different subbottom depths. In the cores where gashydrates are found independently of depth methane concentrations increase up to 500 ml/l and more, sometimes reaching 3000 ml/l.

In 2009 a new area of subwater gas discharge was studied in the Sea of Okhotsk located souther earlier discovered gashydrate and gas natural vents areas in the north-western part of the Sea [11]. Methane anomalies are fixed about 1000-2000 nl/l in the water near seafloor. In this new area there are revealed parts of seafloor potentially productive to gashydrate presence (increase methane values in the sediments, presence of curdy structure when sediments are observed on the board and other). However, in distinction from earlier discovered northern area here the features of gashydrate presence are attracted to the more deeper sediment layers and occurrences of methane values are not so much. Perhaps, it is related with deeper deposit of gashydrates here.

In 2010 the sediments containing gashydrates are discovered in the new area of subwater gas discharge - 3 stations [12]. In the seafloor sediments, which were sampled in the area of gas vents, anomaly methane concentration is fixed here (200 ml/l). And in the seafloor water methane concentrations are 2000-4000 nl/l, exceeding a background in 100 and more times. Observed gashydrate layers in the sediments are revealed at a subbottom depth of about 2 m. Thickness of gashydrate layers was about 1 m. In addition, the structures with gas fluxes and the disturbance of seafloor surface shaped possible by gas-fluid flows from a depth of sediment cover are remarked by geophysical survey.

Ecological aspects of gashydrate accumulations in the Sea of Okhotsk

By special research in the Sea of Okhotsk with use of submersible inhabited craft in the area nearby Paramushir Island [4] and Ocean Floor Observation Systems [1, 2, 3] in the northeastern Sakhalin slope the seafloor surface was studied in detail. Received data show that methane fluxes impact on the ecosystem structure. Rapid development bacterial mates and sharply expressed prevalence of some benthos (molluscs, crustacea) have been fixed in comparison with neighbour territories where seawater have background methane concentrations.

As a result of the seafloor research it was discovered that there is a bottom deformation with the formation of hills and holes which have diameter more than 10 meters and height more than 2 meters when gashydrates destruct. On the slopes the disturbance and the moving of sediments to down are observed. It is the significant change of bottom morphology. Therefore, it is necessary to take into account the opportunity of gashydrate decomposition with the subsequent bottom destruction, because the consequences of this event may be unpredictable. It is concerned, for instance, engineering-technical works: the lining of oil pipelines, the construction of derricks etc.

As regards the aspect of climate change: a huge amount of methane are released from the seafloor to the water and then from the sea surface to the atmosphere (methane is a strong “greenhouse gas”) when gashydrates decompose. And vice versa, when gashydrate accumulations are stable (seismo-tectonic activization is reduced), they can delay methane entrance to the environment.

Thus, at present it is very important to understand and determine the role of gashydrate accumulations in the global and regional nature processes.

References

1. Biebow N. and Huetten E. (eds.) KOMEX Cruise Reports I & II RV Professor Gagarinsky, Cruise 22, RV Akademik M.A. Lavrentyev, cruise 28. GEOMAR Report 82 INESSA. Kiel, Germany. 1999. 188 p.

2. Biebow N., Kulinich R., and Baranov B. (Eds.). Kurile Okhotsk Sea Marine Experiment (KOMEX II). Cruise Report: RV Akademik Lavrentyev, cruise 29. Leg 1-2. 2002. 190 p.

3. Dullo W.-Chr., Biebow N., and Georgeleit K. (Eds.). SO178-KOMEX Cruise Report: RV SONNE. Mass exchange processes and balances in the Okhotsk Sea. Germany. 2004. 125 p.

4. Ginsburg G.D., Soloviev V.A. Submarine gashydrates. Saint-Petersburg: VNIIOceangeologia, 1994. 199 p.

5. Matveeva T., Soloviev V., Shoji H., Obzhirov A. (Eds.). Cruise Report CHAOS-1: RV Academic M.A. Lavrentyev, cruises 31 and 32. S-Pb.: VNIIOkeangeologia, 2005. 164 p.

6. Obzhirov A.I., Astahova N.V., Lipkina M.I., Vereshchagina O.F., Mishukova G.I., Sorochinskaya A.V., Yugai I.G. Gasgeochemical zoning and mineral associations of the bottom of the Sea of Okhotsk. Vladivostok: Dal^,nauka, 1999. P. 184.

7. Obzhirov A.I. (Eds.) Complex geological, hydrological, gasgeochemical and geophysical research in the area of gashydrate accumulation in the Sea of Okhotsk: CHAOS-2 Cruise Reports, RV Akademik M.A. Lavrentyev, cruise 36. Vladivostok. 2005. 123 p.

8. Obzhirov A.I. (Eds.) Complex litological, hydrological, gasgeochemical and geophysical research in the area of gashydrate accumulation in the Sea of Okhotsk: CHAOS-3 Cruise Reports, RV Akademik M.A. Lavrentyev, cruise 39. Vladivostok. 2006. 62 p.

9. Obzhirov A.I., Salyuk A.N., Shakirov R.B., Druzhinin V.V., Mishukova G.I., Ageev А.А., Salomatin A.S., Pestrikova N.L., Veselov O.V., Kudel^,kin V.V. Methane flux and gashydrates in the Sea of Okhotsk // Science and technics in the gas industry. Moscow: Publishing house OOO “IRTS Gasprom”, 2004. № 1-2. P. 20-25.

10. Obzhirov A.I., Shakirov R.B., Mishukova G.I., Druzhinin V.V., Luchsheva L.N., Ageev A.A., Pestrikova N.L., Obzhirova N.P. Study of the gasgeochemical fields in the water environment of the Sea of Okhotsk // Vestnik FEB RAS. Vladivostok: Dal'nauka. 2003. № 2. P. 118-125.

11. Shoji H., Jin Y.K., Obzhirov A. and Baranov B. (Eds.) Operation Report of Sakhalin Slope Gas Hydrate Project 2009, R/V Akademik M.A. Lavrentyev Cruise 47. Published by the New Energy Resources Research Center Kitami Institute of Technology. Kitami, Japan, February, 2010. 136 p. P. 27-32.

12. Jin Y.K., Shoji H., Obzhirov A. and Baranov B. (Eds.) Operation Report of Sakhalin Slope Gas Hydrate Project 2010, R/V Akademik M.A. Lavrentyev Cruise 50. Published by Korea Polar Research Institute. Korea, January, 2011. 129 p. P. 34-37.