Gas hydrate, an ice-like solid compound composed of methane and water molecules, was “re-discovered” from ocean sediments in the mid-20th century, while it had been known as a chemical material to chemists and chemical engineers even in the early 19th century. Since the re-discovery of natural gas hydrate it has been attracting growing interest among geoscientists from the viewpoint of potential natural gas resources, possible impact on global environmental changes, and trigger of geo-hazards such as landslides and coastal erosion. The development of gas hydrate science has been marked by a rapid increase of studies in publications from 1991 to 1999, reflecting ODP expeditions to the mid-America Trench and Blake Ridge, where deep corings recovered solid gas hydrate samples. The number of papers in international journals has increased to 500 to 600 annually in the last few years. Recent development of marine geology and geophysics, in particular of the Ocean Drilling Program (ODP), has dramatically increased our knowledge of gas hydrate and related phenomena. Bottom simulating reflector (BSR) on seismic profiles corresponds to the base of the gas hydrate zone in sediments, and is considered to be a useful tool to identify the distribution of marine gas hydrates. The base of gas hydrate stability (BGHS) is determined from P-T conditions of sediments and water depth, and BSR is expected to occur at the depth of BGHS. However, BSR is not always consistent with BGHS; and, in some cases, even two BSRs are identified at around the depth of BGHS. These observations seem to imply that marine gas hydrate is not necessarily stable at the present position but represents ephemeral and transient conditions. Integrated research activities of scientific projects and industry exploration efforts have identified two types of gas hydrate in marine sediments. These are deep-seated, stratigraphic-type deposits and shallow/structural accumulation. Japan's long-term exploration project led by Ministry of Economy, Trade and Industry (METI) has been targeting the stratigraphic type in the Nankai Trough, where 40 tcf of methane has been estimated to occur as concentrated gas hydrate deposits. Shallow accumulations are usually associated with gas chimney structures, and are common throughout the marginal seas of the western Pacific. Massive accumulation of the shallow type seems to be promising for gas production from gas hydrate as well. Sudden and major changes to the earth's environment and mass extinctions are characterized by sharp negative excursions of carbon isotopic composition. Massive dissociation of C-13 depleted gas hydrate with δ13C of -40 to -100‰, is believed to have caused such global changes. The Paleocene-Eocene boundary event (PETM event) is the best-explained case of gas hydrate-induced biotic overturn. However, serious problems have recently emerged from considerations of thermal propagation through sediments. A sudden increase of methane concentration at the Last Glacial Maximum has also been considered to result from gas hydrate dissociation, but the response of gas hydrate was not so simple during the Quaternary, when low sea level during glacial periods possibly de-stabilized subsurface gas hydrate, unlike the PETM of ice-free ocean.
A number of extensive methane plumes and active methane seeps associated with large blocks of methane hydrates exposed on the seafloor strongly indicate extremely high methane flux and large accumulations of methane hydrate in shallow sediments of the Umitaka spur and Joetsu knoll of the Joetsu basin 30 km off Joetsu city, Niigata Prefecture. Crater-like depressions, incised valleys, and large but inactive pockmarks also indicate methane activities over the spur and knoll. These features imply strong expulsions of methane gas or methane-bearing fluids, and perhaps lifting and floating-up of large volumes of methane hydrate to the sea surface. High heat flow, ∼100 mK/m, deposition of organic-rich strata, ∼1.0 to 1.5%TOC, and Pliocene-Quaternary inversion-tectonics along the eastern margin of the Japan Sea facilitate thermal maturation of organic matters, and generation and migration of light-hydrocarbons through fault conduits, and accumulation of large volumes of methane as methane hydrate in shallow sediments. Microbial methane generation has also contributed to reinforcing the methane flux of the Joetsu basin. Regional methane flux as observed by the depth of the sulfate-methane interface (SMI) is significantly high, < 1 m to 3 m, when compared to classic gas hydrate fields of Blake Ridge, 15 to 20 m, and Nankai trough, 3 to 15 m. δ13C of methane hydrate and seep gases are mostly within -30 to -50‰, the range of thermogenic methane, while dissolved methane of the interstitial waters a few kilometers away from seep sites are predominated by microbial with δ13C of -50 to -100‰. Seismic profiles have revealed fault-related, well-developed gas chimney structures, 0.2 to 3.5 km in diameter, on the spur and knoll. The structures are essential for conveying methane from deep-seated sources to shallow depths as well as for accumulating methane hydrate (gas chimney type deposits). The depth of BSR, which represents the base of gas hydrate stability (BGHS), on the spur and knoll is generally 0.20 to 0.23 seconds in two-way-travel time, whereas the BSRs in gas chimneys occur at 0.14 to 0.18 seconds, exhibiting a sharp pull-up structure. The apparent shallow BGHS is due to the accumulation of large volumes of high-velocity methane hydrate in gas chimneys. The depth to BGHS is estimated to be 115 m on an experimentally determined stability diagram, based on an observed thermal gradient of 100 mK/m. Then the velocity of the sediments on the Umitaka spur is calculated to be 1000 m/s, which is anomalously low compared to normal pelagic mud of 1600-1700 m/s. This exciting finding leads to the important implication that sediments of the Umitaka spur contain significant amounts of free gas, although the sediments are well within the stability field of methane hydrate. The reasons for the existence of free gas in the methane hydrate stability field are not fully explained, but we propose the following possible mechanisms for the unusual co-existence of methane hydrate and free-gas in clay-silt of the spur. (i) High salinity effect of residual waters, (ii) degassing from ascending fluids, (iii) bound water effect and deficiency of free-waters, and (iv) micro-pore effect of porous media. All of these processes relate to the development of gas hydrate deposits of the Umitaka spur. (View PDF for the rest of the abstract.)
A remarkable reflection record of a deeper part (> 200 m) appeared on the fish finder. Gas bubbling was confirmed directly by the ROV (Hyper-Dolphin/Natsushima) diving off Joetsu in the eastern part of the Sea of Japan. Visualization, using geographical feature mapping and side-scan sonar images of the topography, was useful for investigating methane hydrate and associated carbonates at the sea bottom. Besides, a sub-bottom profiling study produced useful information for detecting buried methane hydrate and carbonates, because some of them had been identified previously by piston core sampling studies. DAI-PACK (Deep sea Acoustic Imaging Package) was set on the ROV (Hyper-Dolphin) and the survey extended to a length of approximately 20 km and covered a total area of 900,000. A detailed distribution of patch-shaped rough geographical features, some fault-like lineaments and sub-bottom profile records (about a maximum of 15 m deep), were obtained by five dives during the NT07-20 and the NT08-09 cruises. The round-shaped patches of rough geographical features are thought to have been formed by gas supplied to the surface through fault systems.
The METI (Ministry of Economics, Trade and Industry) Sado-oki Nansei 3D seismic survey was carried out in deep water southwest off Sado Island, Japan Sea. The survey area covered the Umitaka Spur, which features mounds and pockmarks. Other surveys sampled large masses of methane hydrate beneath the sea floor, discovered methane bubble plumes rising into the water, and investigated high resistivity anomalies below the sea floor. We applied 3D pre-stack time migration and continuous velocity analysis method to 3D seismic data, and investigated the geological and P-wave velocity structure below the sea floor of the Umitaka Spur and its surrounding area. This revealed high velocity anomalies, suggesting the methane hydrate occurrence below mounds and pockmarks. On the other hand, P-wave velocities below the sea floor in areas of the Umitaka Spur were lower than propagated in water layers. Therefore, it was suggested that concentrations of methane hydrates were limited below mounds and pockmarks, although some gas may be contained in low-velocity zones. The situation may be caused by the localization of methane supplies from deep layers.
Sulfate, halogen, and radioactive 129I concentrations were determined in pore waters associated with massive gas hydrate deposits in shallow sediments along the boundary between the Amurian and Okhotsk plates in the Okhotsk Sea and Japan Sea. Because of the strong biophilic behavior of iodine and weaker behavior of bromine, in contrast to conservative chlorine, in the marine system and the presence of a long-lived radioisotope of iodine (129I), these analyses are useful for determining the age and nature of source organic materials responsible for hydrocarbons, mostly methane, in gas hydrates. Rapid sulfate decreases with depth reflect active methane migration toward the seafloor, particularly around gas hydrate-bearing sites at both locations. While salt exclusion from gas hydrates during crystallization is likely to have resulted in the downward increase of Cl, and potentially Br and I concentrations, gas hydrate dissociation at depth caused a gentle dilution of pore waters. Biophilic Br and I concentrations rapidly increase with depth, reaching 1500 and 400 μM at the core bottom, respectively, indicating that upwelling fluids are enriched in Br and I derived from marine organic materials degraded in deep sediments. The 129I/I ratios thus reflect the potential ages of source formations of iodine and associated methane, providing ∼35 Ma northeast off Sakhalin Island in the Okhotsk Sea and ∼30 Ma in the Umitaka Spur-Joetsu Knoll region in the eastern margin of the Japan Sea. These ages correspond well with the initial activities of the Amurian and Okhotsk plates and the subsequent opening of the Japan Sea, which formed the present geological setting of the northeastern margin of the Eurasian continent. Active plate motions led to the rapid accumulation of organic-rich sediments that are responsible for iodine and methane in gas hydrate occurring along the plate boundary.
The Umitaka spur and the Joetsu knoll in the eastern margin of the Japan Sea off Naoetsu are characterized by a high methane flux accompanied by the formation of methane hydrates. Several sediment cores were obtained from this region during the “Natsushima” NT-06-19 cruise. Various geochemistry analyses were carried out on these samples. ANME-1 and ANME-2 groups of archaea were distinguished by biomarkers. ANME-1 was found from the sediment sample covered by a bacterial mat with the maximum concentration of dissolved methane in pore water. The samples in which ANME was detected are characterized by a remarkably high sulphur content.
Benthic foraminiferal assemblages of surface sediments were studied in the methane seepage area in the eastern margin of the Japan Sea to identify the effects of methane seep on foraminiferal distribution. Samples were collected from three areas, each including a methane-seep site. Their water depths range from 882 m to 1006 m in the Japan Sea Proper Water. At the methane-seep sites, there were significantly fewer living (stained) foraminifera than at other sites, and ratio of agglutinated foraminifera is small, showing the dominance of calcareous forms. Planktonic foraminifera are very abundant at seep sites. These suggest higher alkalinity at the sediment-water interface of the seep sites. Cassidulina norvangi, Pseudoparrella takayanagii and Bolivina decussata increase while Trochammina pygmaea, Thalmannammina parkerae, Lagenammina tublata, Reophax cf. dentaliniformis, Saccammina huanghaiensis, etc., decrease at the seep sites. These species are usually common in the Japan Sea Proper Water. Their changes in abundance seem, in part, to relate to alkalinity because the former taxa are calcareous foraminifera and the latter are all agglutinated. There are fewer Bolivina pacifica, Stainforthia fusiformis, Nonionella globosa, and Globobulimina auriculata in the living population at the seep sites, although they are abundant in the total (living + dead) population. These taxa are well-known infauna and can survive in oxygen-depleted environments. This discrepancy may be explained by the mode of methane seepage; the present seeping is not severer than in the near past and is transient at each seep site. The effects of the present methane seeps extend only to less than 40-50 m from each methane-seep site.
During methane hydrate exploration and research, remote and on-board acoustic surveying and monitoring of methane hydrate can be easily and economically conducted using a quantitative echo sounder. Simultaneously, the structure and the floating-up speed of methane plumes can be obtained from an analysis of acoustic data. We conducted a survey of methane plumes from 2004 through 2008 at a spur situated southwest off the coast of Sado Island (tentatively called Umitaka Spur) and at the Joetsu Knoll. In 2007 and 2008, we performed experiments by releasing methane hydrate bubbles and methane hydrate, and letting them float upward. Consequently, we demonstrated that acoustical reflection from the methane plumes correlates with water temperature and depth, that the floating-up speed is constant but depends on the conditions of methane hydrate, that the discharge of methane hydrate bubbles changes, and that there is a wide scattering of materials below the seafloor where methane plumes are located. Furthermore, the amount of methane hydrate bubbles seeping was estimated by a preliminary calculation. The method will be applied not only to basic research on methane hydrate but also to assessments of the environmental impact of methane hydrate exploitation.
Methane hydrates exist beneath the sea bottom near cold seeps NE off the Sakhalin in the Sea of Okhotsk. Multidisciplinary field operations were performed at a study area (approximately 16 × 20 km2) to investigate seepage characteristics and understand gas hydrate formation mechanisms. A continuous profiling survey was conducted to obtain a distribution map of seepage structures on the floor by using a deep-tow, side-scan-sonar equipment. The distribution map reveals that the dense area of seepage structures coincides with a sea-floor area of deformed sediments caused possibly by repeated sediment slumping and debris flows in the past. We speculate that this deformation may have created shallow faults that are suitable to conduits for the migration and discharge of gas and fluid. Three seepage structures were selected to study about their fluid-seep conditions around the sea floor level. Hieroglyph seepage structure is located at the northern end of the dense area of the structures. Kitami and Chaos structures are located about 2 and 7 km respectively apart from the Hieroglyph structure within the dense area. Large plumes on echograms and higher methane contents in the water column confirm gas seepage activities at the three structures. There observed at least two and four plumes at the Hieroglyph and Chaos structures, respectively. Each gas chimney image in seismic reflection profiles was traced to connect each BSR and seepage structure. Both pull-up and disturbed structures of BSR around the gas chimney images were interpreted as to be indications of significant heat flows caused by ascending fluid at both Kitami and Chaos structures. On the other hand, almost no pull-up/disturbance of BSR was observed at the Hieroglyph structure, suggesting little water seepage. The seep activity may vary with time off the Sakhalin. The Hieroglyph structure is located at the edge of a dense area of the seepage structures. It might serve as an indicator for the long-term activity of the fluid seepage system off the Sakhalin.
Differences in seepage activity among three gas-seepage structures including hydrate-bearing sites at the Derugin Basin, NE Sakhalin Island, Russia were investigated. Chemical analyses of pore-water geochemistry, water-content distribution and stable isotopes were conducted to describe the complicated geochemical seepage environments involving a flux of free-gas and/or gas-saturated water. Traces of deep ascending fluid were not found in the hydrate-containing Hieroglyph seep, but were suggested in the lower parts of cores from the CHAOS and Kitami seeps based on the presence of abnormally heavy deuterium.
New hydrate-bearing seepage structures off Sakhalin in the Sea of Okhotsk were investigated from 2003 to 2006 within the framework of the CHAOS project. We obtained samples of natural gas hydrate and measured the molecular and isotopic compositions of hydrate-bound gas. Methane δ13C and δD were in the range of -67 to -63‰ and -207 to -193‰, respectively. These results indicate a microbial origin produced by CO2 reduction according to Whiticar's diagram. Because ethane δ13C showed a thermogenic origin, hydrate-bound gas contains a small amount of thermogenic gas. The hydration numbers of the samples were estimated as 6.19 ± 0.02 using the Raman spectra of the C–H stretching mode and a thermodynamic calculation. Heat flow calorimetry revealed that the values for dissociation heat of the samples were 18.1 ± 0.3 kJ mol-1 (from hydrate phase to gas and ice phases) and 55.4 ± 0.4 kJ mol-1 (from hydrate phase to gas and water phases), which agree well with the values in of literature.