Anatoly Obzhirov

Hydrate‐bearing structures in the Sea of Okhotsk

Gas hydrates are natural gas reservoirs in ice-like crystalline solids, and are stable in pore spaces of submarine sediments in water depths greater than about 300–500 m. They have been recovered in many of the worlds oceans, both at larger sub-bottom depths (up to 450 m) by drilling and near the seafloor in shallow cores by gravity-coring. In the latter case, the gas hydrates are related to the sites of enhanced seepage such as cold seeps and mud volcanoes [Ginsburg and Soloviev, 1998]. Multidisciplinary field investigations during the two cruises have revealed new, large hydrate-bearing seepage structures in the Sea of Okhotsk, a northwestern marginal sea of the Pacific Ocean (Figure l). The Derugin Basin at the central part of the Sea of Okhotsk, the zone of intensive gas seepage and hydrate accumulation, was studied during two cruises of the R/V Akademik M.A. Lavrentyev (LV) of the Russian Academy of Sciences (RAS), in August and October 2003 within the framework of the CHAOS project (hydroCarbon Hydrate Accumulations in the Okhotsk Sea) supported by funding agencies in five nations.

Gas hydrate forming fluids on the NE Sakhalin slope, Sea of Okhotsk

Abstract An area of focused fluid venting off NE Sakhalin, Sea of Okhotsk, was investigated in 2003 during the 31st and 32nd international expeditions of R/V Akademik M. A. Lavrentyev within the framework of the CHAOS Project. More than 40 structures related to seafloor gas venting were discovered and gas hydrates were sampled from three of these: CHAOS, Hieroglyph and Kitami. Geochemical analyses were used to define the mechanisms of gas hydrate accumulation and the sources of fluids involved. Chemical and isotopic analyses of the interstitial and hydrate waters suggest that hydrates were formed from seawater (or in-situ pore water) and an ascending fluid enriched in salts. Hydrate formation occurs at locations of the most intensive saline water upflow, and this is probably a function of the gas solubility in water in equilibrium with hydrate. The water involved in gas hydrate formation consists of about 70% pore water derived from the host sediment and 30% from the ascending fluid. The overall isotopic composition of the ‘fluid’ taking part in hydrate formation was calculated as δ2H≈−11‰ and δ18O≈−1.5‰.

Seismic facies and specific character of the bottom simulating reflector on the western margin of Paramushir Island, Sea of Okhotsk

Seismic profiles from a venting area on the western margin of Paramushir Island (Sea of Okhotsk) reveal a local complex structure and an interesting, unusual pattern of the bottom simulating reflector (BSR). The BSR is gradual rising towards the venting area. The geothermal gradient and the bottom temperature confirmed the methane hydrate. The temperature appears to be the most important factor controlling the hydrate stability. A locally higher heat flow caused the upward migration of the hydrate stability field and the subsequent degradation of the hydrated sediments, causing gas vent formation and the flux of methane gas into the water column.

Distribution of methane in waters of the Okhotsk and western Bering Seas, and the area of the Kuril Islands

Investigations of distribution and possible sources of methane dissolved in waters of the Okhotsk and western Bering Seas, and in the areaof the Kuril Islands were conducted in summer 1993 during the 24th cruise ofR/V ‘Akademik Nesmeyanov’. In open waters, the background content ofmethane was 30–80 nl CH4 l−1. At somestations, subsurface maxima of CH4 up to 523 nll−1 (up to 683% of saturation), were registered. Theywere associated with biological production. Maximal concentrations ofmethane, up to 5710 nl CH4 l−1 (1363% ofsaturation), were observed in areas of discharge from anaerobic zones ofbottom sediments and oil gas deposits, destruction of crystallohydrates, andemission along fractures in the earths crust. Migration ofCH4 from bottom sediments resulted in the establishment of amaximum at intermediate depths.

Distribution of helium and hydrogen in sediments and water on the Sakhalin slope

During the geological survey and prospecting, helium and hydrogen are recognized indicators of minerals, deep-seated faults, seismic activity, and ascending deep fluids. Their anomalous concentrations also serve as a marker of metamorphic processes. Helium survey is applied for tracing deep-seated faults and mapping permeable zones. In this work, the first results of gas geochemical survey in marine sediments and water to study the distribution of helium and hydrogen and their relation with the seismic activity of some geological structures in the southwestern Sea of Okhotsk and northern Sea of Japan. Anomalies of these gases (He up to 60 ppm, H up to 120 ppm) were identified in the hydrate-bearing sediments in the fault zones. It was also found that helium concentrations in the water column of the Tatar Strait are higher than in the studied area of the Sea of Okhotsk, possibly, due to the difference in geological structure and seismotectonic activation. Moreover, coal layers could be a significant source of methane in the gashydrate-bearing layers of the Tatar Strait.

Possible variation in methane flux caused by gas hydrate formation on the northeastern continental slope off Sakhalin Island, Russia

The Sakhalin Slope Gas Hydrate Project (SSGH) is an international collaborative effort by scientists from Japan, Korea, and Russia to investigate natural gas hydrates (GHs) that have accumulated on the continental slope off Sakhalin Island, Okhotsk Sea. From 2009 to 2011, field operations of the SSGH-09, -10, and -11 projects were conducted. GH-bearing and -free sediment cores were retrieved using steel hydro- and gravity corers. The concentrations of sulfate ions in sediment pore waters were measured to investigate sulfate concentration–depth profiles. Seventeen cores showed linear depth profiles of sulfate concentrations. In contrast, eight cores and two cores showed concave-up and -down profiles plausibly explained by sudden increase and decrease in methane flux from below, respectively, presumably caused by the formation of gas hydrate adjacent to the core sampling sites.

New data on lineament control of modern centers of methane degassing in east Asian seas

This paper reports on the heaviest submarine fluxes of natural gas bubbles (flares) that have occurred in the Sea of Okhotsk, the Laptev Sea, and Lake Baikal. It has been concluded that there is a “gas-geochemical response” of geodynamic and seismotectonic processes in the interaction of lithospheric plates. Using the example of one of the lineaments of Eastern Asia, we have shown that lineament geotectonic structures, which compose the regmatic net of the Earth, control the formation and activity of the biggest centers of methane emission, which are indicators of tectonospheric activity and hydrocarbon accumulation. Changes in the activity of these centers are informative signals of seismic fluctuations. It is concluded that Russian studies play a leading role in gas hydrate forecasts in the seas of the Eastern Arctic.

Gas-geochemical evaluation of the the petroleum potential of the Birofeld graben of the Middle Amur sedimentary basin (Russian Far East)

The results of the gas-geochemical investigations obtained in the Birofeld graben of the Middle Amur sedimentary basin during several years are considered together with the coal prospecting and geological-geophysical information. Data on the gas composition of soil, snow cover, groundwater, and surface water are reported. Gas-geochemical and genetic indicators are used for revealing the hydrocarbon (HC) gases of different origins and establishing the principal regularities of their distribution, which allows the prospects of the petroleum potential of the graben to be preliminarily assessed.

Fluid-Controlling Significance of the Nosappu Fracture Zone and Conditions for the Formation of Methane Fluxes and Gas Hydrates (Sea of Okhotsk Region)

It has been observed that the intensity of underwater gas flares unexpectedly increased after the deep-focus (625.9 km) earthquake that occurred in the Sea of Okhotsk on August 14, 2012. In this regard, we have analyzed the data resulting from interpretation of the focal mechanism for the strike-slip earthquakes which occurred in the Benioff seismic zone of the subducting Pacific Plate within the Sea of Okhotsk region over the period from 1977 to 2010. The NNW sinistral and NE dextral faults are found to form a conjugate system due to the WNW stress field. We have established that the dextral faults are mostly common at a depth of about 200 km along the Kuril Islands extension, while the sinistral ones are concentrated in the Nosappu Fracture Zone and traced to the NNW down to a depth of 680 km. The area of the gas flare discharge and gas hydrate accumulations have the same (NNW) direction. Thus, we have revealed that the Nosappu Fracture Zone appears to be a structure which controls fluid fluxes, providing permeability of the subducting slab of the Pacific Plate for ascending fluids from the lower mantle.

Gas-Geochemical Anomalies of Surface and Ground Waters of the Birofeld Graben of the Middle Amur Sedimentary Basin (Far East of Russia)

Anomalous concentrations of dissolved gases, exceeding their background values by an order of magnitude or more, are recorded in the surface and ground waters of the Birofeld Graben of the Middle Amur sedimentary basin. The main geological factors that influence the formation of the gas composition and gas saturation of waters are determined. The gas-geochemical indicators for the dissolved hydrocarbon gases, which make it possible to estimate their genesis and prospects for the oil-and-gas contents of the Birofeld graben, are specified.