Evidence of gas hydrate from downhole logging data in the Ulleung Basin, East Sea

The Gas Hydrate Research and Development Organization (GHDO) of Korea successfully accomplished both coring (hydraulic piston and pressure coring) and logging (logging-while-drilling, LWD, and wireline logging) to investigate the presence of gas hydrate during the first deep drilling expedition in the Ulleung Basin, East Sea of Korea (referred to as UBGH1) in 2007. The LWD data from two sites (UBGH1-9, UBGH1-10) showed elevated electrical resistivity (>80 Ω-m) and P-wave velocity (>2000 m/s) values indicating the presence of gas hydrate. During the coring period, the richest gas hydrate accumulation was discovered at these intervals. Based on log data, the occurrence of gas hydrate is primarily controlled by the presence of fractures. The gas hydrate saturation calculated using Archie’s relation shows greater than 60% (as high as ∼90%) of the pore space, although Archie’s equation typically overestimates gas hydrate saturation in near-vertical fractures. The saturation of gas hydrate is also estimated using the modified Biot-Gassmann theory (BGTL) by Lee and Collett (2006). The saturation values estimated rom BGTL are much lower than those calculated from Archie’s equation. Based on log data, the hydrate-bearing sediment section is approximately 70 m (UBGH1-9) to 130 m (UBGH1-10) in thickness at these two sites. This was further directly confirmed by the recovery of gas hydrate samples and pore water freshening collected from deep drilling core during the expedition. LWD data also strongly support the interpretation of the seismic gas hydrate indicators (e.g., vent or chimney structures and bottom-simulating reflectors), which imply the probability of widespread gas hydrate presence in the Ulleung Basin.     

Evidence of gas hydrate from downhole logging data in the Ulleung Basin,East Sea

The Gas Hydrate Research and Development Organization (GHDO) of Korea successfully accomplished both coring (hydraulic piston and pressure coring) and logging (logging-while-drilling, LWD, and wireline logging) to investigate the presence of gas hydrate during the first deep drilling expedition in the Ulleung Basin, East Sea of Korea (referred to as UBGH1) in 2007. The LWD data from two sites (UBGH1-9, UBGH1-10) showed elevated electrical resistivity (>80 Ω-m) and P-wave velocity (>2000 m/s) values indicating the presence of gas hydrate. During the coring period, the richest gas hydrate accumulation was discovered at these intervals. Based on log data, the occurrence of gas hydrate is primarily controlled by the presence of fractures. The gas hydrate saturation calculated using Archie’s relation shows greater than 60% (as high as ∼90%) of the pore space, although Archie’s equation typically overestimates gas hydrate saturation in near-vertical fractures. The saturation of gas hydrate is also estimated using the modified Biot-Gassmann theory (BGTL) by Lee and Collett (2006). The saturation values estimated rom BGTL are much lower than those calculated from Archie’s equation. Based on log data, the hydrate-bearing sediment section is approximately 70 m (UBGH1-9) to 130 m (UBGH1-10) in thickness at these two sites. This was further directly confirmed by the recovery of gas hydrate samples and pore water freshening collected from deep drilling core during the expedition. LWD data also strongly support the interpretation of the seismic gas hydrate indicators (e.g., vent or chimney structures and bottom-simulating reflectors), which imply the probability of widespread gas hydrate presence in the Ulleung Basin.     

Fault system and thermal regime in the vicinity of site NGHP-01-10, Krishna–Godavari basin, Bay of Bengal

Drilling/coring activities onboard JOIDES Resolution for hydrate resource estimation have confirmed gas hydrate in the continental slope of Krishna-Godavari (KG) basin, Bay of Bengal and the expedition recovered fracture filled gas hydrate at the site NGHP-01-10. In this paper we analyze high resolution multi-channel seismic (MCS), high resolution sparker (HRS), bathymetry, and sub-bottom profiler data in the vicinity of site NGHP-01-10 to understand the fault system and thermal regime. We interpreted the large-scale fault system (>5 km) predominantly oriented in NNW-SSE direction near NGHP-01-10 site, which plays an important role in gas hydrate formation and its distribution. The increase in interval velocity from the baseline velocity of 1600 m/s to 1750–1800 m/s within the gas hydrate stability zone (GHSZ) is considered as a proxy for the gas hydrate occurrence, whereas the drop in interval velocity to 1400 m/s suggest the presence of free gas below the GHSZ. The analysis of interval velocity suggests that the high concentration of gas hydrate occurs close to the large-scale fault system. We conclude that the gas hydrate concentration near site NGHP-01-10, and likely in the entire KG Basin, is controlled primarily by the faults and therefore has high spatial variability.We also estimated the heat flow and geothermal gradient (GTG) in the vicinity of NGHP-01-10 site using depth and temperature of the seafloor and the BSR. We observed an abnormal GTG increase from 38 °C/km to 45 °C/km at the top of the mound, which remarkably agrees with the measured temperature gradient at the mound (NGHP-01-10) and away from the mound (NGHP-01-03). We analyze various geological scenarios such as topography, salinity, thermal non-equilibrium of BSR and fluid/gas advection along the fault system to explain the observed increase in GTG. The geophysical data along with the coring results suggest that the fluid advection along the fault system is the primary mechanism that explains the increase in GTG. The approximate advective fluid flux estimated based on the thermal measurement is of the order of few tenths of mm/yr (0.37–0.6 mm/yr).     

Distribution and expression of gas seeps in a gas hydrate province of the northeastern Sakhalin continental slope, Sea of Okhotsk

Multidisciplinary surveys were conducted to investigate gas seepage and gas hydrate accumulation on the northeastern Sakhalin continental slope (NESS), Sea of Okhotsk, during joint Korean–Russian–Japanese expeditions conducted from 2003 to 2007 (CHAOS and SSGH projects). One hundred sixty-one gas seeps were detected in a 2000 km² area of the NESS (between 53°45′N and 54°45′N). Active gas seeps in a gas hydrate province on the NESS were evident from features in the water column, on the seafloor, and in the subsurface: well-defined hydroacoustic anomalies (gas flares), side-scan sonar structures with high backscatter intensity (seepage structures), bathymetric structures (pockmarks and mounds), gas- and gas-hydrate-related seismic features (bottom-simulating reflectors, gas chimneys, high-amplitude reflectors, and acoustic blanking), high methane concentrations in seawater, and gas hydrates in sediment near the seafloor. These expressions were generally spatially related; a gas flare would be associated with a seepage structure (mound), below which a gas chimney was present. The spatial distribution of gas seeps on the NESS is controlled by four types of geological structures: faults, the shelf break, seafloor canyons, and submarine slides. Gas chimneys that produced enhanced reflection on high-resolution seismic profiles are interpreted as active pathways for upward gas migration to the seafloor. The chimneys and gas flares are good indicators of active seepage.     

Gas hydrate saturation in the Krishna–Godavari basin from P-wave velocity and electrical resistivity logs

During the Indian National Gas Hydrate Program (NGHP) Expedition 01, a series of well logs were acquired at several sites across the Krishna–Godavari (KG) Basin. Electrical resistivity logs were used for gas hydrate saturation estimates using Archie’s method. The measured in situ pore-water salinity, seafloor temperature and geothermal gradients were used to determine the baseline pore-water resistivity. In the absence of core data, Arp’s law was used to estimate in situ pore-water resistivity. Uncertainties in the Archie’s approach are related to the calibration of Archie coefficient (a), cementation factor (m) and saturation exponent (n) values. We also have estimated gas hydrate saturation from sonic P-wave velocity logs considering the gas hydrate in-frame effective medium rock-physics model. Uncertainties in the effective medium modeling stem from the choice of mineral assemblage used in the model. In both methods we assume that gas hydrate forms in sediment pore space. Combined observations from these analyses show that gas hydrate saturations are relatively low (<5% of the pore space) at the sites of the KG Basin. However, several intervals of increased saturations were observed e.g. at Site NGHP-01-03 (S_h = 15–20%, in two zones between 168 and 198 mbsf), Site NGHP-01-05 (S_h = 35–38% in two discrete zone between 70 and 90 mbsf), and Site NGHP-01-07 shows the gas hydrate saturation more than 25% in two zones between 75 and 155 mbsf. A total of 10 drill sites and associated log data, regional occurrences of bottom-simulating reflectors from 2D and 3D seismic data, and thermal modeling of the gas hydrate stability zone, were used to estimate the total amount of gas hydrate within the KG Basin. Average gas hydrate saturations for the entire gas hydrate stability zone (seafloor to base of gas hydrate stability), sediment porosities, and statistically derived extreme values for these parameters were defined from the logs. The total area considered based on the BSR seismic data covers ∼720 km². Using the statistical ranges in all parameters involved in the calculation, the total amount of gas from gas hydrate in the KG Basin study area varies from a minimum of ∼5.7 trillion-cubic feet (TCF) to ∼32.1 TCF.     

Scale-dependent gas hydrate saturation estimates in sand reservoirs in the Ulleung Basin,East Sea of Korea

Through the use of 2-D and 3-D seismic data, several gas hydrate prospects were identified in the Ulleung Basin, East Sea of Korea and thirteen drill sites were established and logging-while-drilling (LWD) data were acquired from each site in 2010. Sites UBGH2–6 and UBGH2–10 were selected to test a series of high amplitude seismic reflections, possibly from sand reservoirs. LWD logs from the UBGH2–6 well indicate that there are three significant sand reservoirs with varying thickness. Two upper sand reservoirs are water saturated and the lower thinly bedded sand reservoir contains gas hydrate with an average saturation of 13%, as estimated from the P-wave velocity. The well logs at the UBGH2–6 well clearly demonstrated the effect of scale-dependency on gas hydrate saturation estimates. Gas hydrate saturations estimated from the high resolution LWD acquired ring resistivity (vertical resolution of about 5–8 cm) reaches about 90% with an average saturation of 28%, whereas gas hydrate saturations estimated from the low resolution A40L resistivity (vertical resolution of about 120 cm) reaches about 25% with an average saturation of 11%. However, in the UBGH2–10 well, gas hydrate occupies a 5-m thick sand reservoir near 135 mbsf with a maximum saturation of about 60%. In the UBGH2–10 well, the average and a maximum saturation estimated from various well logging tools are comparable, because the bed thickness is larger than the vertical resolution of the various logging tools. High resolution wireline log data further document the role of scale-dependency on gas hydrate calculations.     

南海北部陆坡东沙海域海底丘状体气体与水合物分布

海底丘状体在天然气水合物发育区是一种常见的微地貌,对丘状体的研究有助于理解海底流体渗漏模式以及水合物的赋存规律。本文研究南海北部陆坡东沙海域天然气水合物发育区海底丘状体的特征及其与水合物的关系。研究所用的数据包括准三维多道地震数据、多波束数据以及浅地层剖面数据。在多波束海底地形图上,丘状体表现为局部的正地形,直径大约为300 m,高出周围海底约50 m。浅地层剖面上存在明显的声空白以及同相轴下拉现象,指示了海底丘状体气体的分布以及流体运移的路径。丘状体周围明显的BSR表明局部区域可能发育有水合物,水合物钻探结果也证实了这一推测。三维多道地震剖面上,丘状体正下方存在空白反射区域,这与泥火山的地震反射特征类似。但空白反射区域内存在强振幅能量,而且丘状体正下方存在连续的反射层,这表明该丘状体并非泥火山成因。综合钻探结果以及三维地震成像结果,认为水合物形成过程引起的沉积物膨胀以及海底碳酸盐岩的沉淀是形成该丘状体的主要原因。     

Gas and fluid venting at the Makran accretionary wedge off Pakistan

The Makran accretionary complex shows a distinct bottom-simulating reflector, indicating a thick gas-hydrate-bearing horizon between the deformational front and about 1350 m water depth which seals off the upward flow of gas-charged fluids. A field of presently inactive mud diapirs with elevations up to 65 m was discovered in the abyssal plain seawards of the deformation front, suggesting that in the past conditions were favorable for periodic but localized vigorous mud diapirism. Regional destabilization of the gas hydrate leading to focused flow was observed where deep-penetrating, active faults reach the base of the gas-hydrate layer, as in a deeply incised submarine canyon (2100–2500 m water depth). At this location we discovered seeps of methane and H₂S-rich fluids associated with chemoautotrophic vent faunas (e.g., Calyptogena sp.). Driven by the accretionary wedge dynamics, the landward part of the gas-hydrate layer below the Makran margin is being progressively uplifted. Due to reduced hydrostatic pressure and rising ocean bottom-water temperatures, gas hydrates are progressively destabilized and dissociated into hydrate water, methane and H₂S. Sediment temperatures lie outside the methane stability field wherever water depth is less than 800 m. Above this depth, upward migration of fluids to the seafloor is unimpeded, thus explaining the abundance of randomly distributed gas seeps observed at water depths of 350 to 800 m. Received: 14 June 1999 / Revision accepted: 6 February 2000     

Association among active seafloor deformation, mound formation, and gas hydrate growth and accumulation within the seafloor of the Santa Monica Basin, offshore California

Seafloor blister-like mounds, methane migration and gas hydrate formation were investigated through detailed seafloor surveys in Santa Monica Basin, offshore of Los Angeles, California. Two distinct deep-water (≥ 800 m water depth) topographic mounds were surveyed using an autonomous underwater vehicle (carrying a multibeam sonar and a chirp sub-bottom profiler) and one of these was explored with the remotely operated vehicle Tiburon. The mounds are > 10 m high and > 100 m wide dome-shaped bathymetric features. These mounds protrude from crests of broad anticlines (~ 20 m high and 1 to 3 km long) formed within latest Quaternary-aged seafloor sediment associated with compression between lateral offsets in regional faults. No allochthonous sediments were observed on the mounds, except slumped material off the steep slopes of the mounds. Continuous streams of methane gas bubbles emanate from the crest of the northeastern mound, and extensive methane-derived authigenic carbonate pavements and chemosynthetic communities mantle the mound surface. The large local vertical displacements needed to produce these mounds suggests a corresponding net mass accumulation has occurred within the immediate subsurface. Formation and accumulation of pure gas hydrate lenses in the subsurface is proposed as a mechanism to blister the seafloor and form these mounds.     

Gas hydrate saturation in the Krishna–Godavari basin from P-wave velocity and electrical resistivity logs

During the Indian National Gas Hydrate Program (NGHP) Expedition 01, a series of well logs were acquired at several sites across the Krishna–Godavari (KG) Basin. Electrical resistivity logs were used for gas hydrate saturation estimates using Archie’s method. The measured in situ pore-water salinity, seafloor temperature and geothermal gradients were used to determine the baseline pore-water resistivity. In the absence of core data, Arp’s law was used to estimate in situ pore-water resistivity. Uncertainties in the Archie’s approach are related to the calibration of Archie coefficient (a), cementation factor (m) and saturation exponent (n) values. We also have estimated gas hydrate saturation from sonic P-wave velocity logs considering the gas hydrate in-frame effective medium rock-physics model. Uncertainties in the effective medium modeling stem from the choice of mineral assemblage used in the model. In both methods we assume that gas hydrate forms in sediment pore space. Combined observations from these analyses show that gas hydrate saturations are relatively low (<5% of the pore space) at the sites of the KG Basin. However, several intervals of increased saturations were observed e.g. at Site NGHP-01-03 (S_h = 15–20%, in two zones between 168 and 198 mbsf), Site NGHP-01-05 (S_h = 35–38% in two discrete zone between 70 and 90 mbsf), and Site NGHP-01-07 shows the gas hydrate saturation more than 25% in two zones between 75 and 155 mbsf. A total of 10 drill sites and associated log data, regional occurrences of bottom-simulating reflectors from 2D and 3D seismic data, and thermal modeling of the gas hydrate stability zone, were used to estimate the total amount of gas hydrate within the KG Basin. Average gas hydrate saturations for the entire gas hydrate stability zone (seafloor to base of gas hydrate stability), sediment porosities, and statistically derived extreme values for these parameters were defined from the logs. The total area considered based on the BSR seismic data covers ∼720 km². Using the statistical ranges in all parameters involved in the calculation, the total amount of gas from gas hydrate in the KG Basin study area varies from a minimum of ∼5.7 trillion-cubic feet (TCF) to ∼32.1 TCF.     

Characterizing lipid biomarkers in methanotrophic communities of gas hydrate-bearing sediments in the Sea of Okhotsk

We studied specific lipid biomarkers of archaea and bacteria, that are associated with the anaerobic oxidation of methane (AOM) in a cold seep environment as well as the origin of sedimentary organic matter on the continental slope off NE Sakhalin in the Sea of Okhotsk. The organic geochemical parameters demonstrated that most of the sedimentary organic matter containing hydrate layers could be derived from marine phytoplankton and bacteria, except for a station (LV39-29H) which was remarkably affected by terrestrial vascular plant. Specific methanotrophic archaea biomarkers was vertically detected in hydrate-bearing cores (LV39-40H), coinciding with the negative excursion of the δ¹³C_org at core depths of 90–100 cm below the seafloor. These results suggest that methane provided from gas hydrates are already available substrates for microbes thriving in this sediment depth. In addition, the stable isotope mass balance method revealed that approximately 2.77–3.41% of the total organic carbon (or 0.036–0.044% dry weight sediment) was generated by the activity of the AOM consortium in the corresponding depth of core LV39-40H. On the other hand, the heavier δ¹³C values of archaeol in the gas hydrate stability zone may allow ongoing methanogenesis in deeper sediment depth.     

Mass transport deposits and gas hydrate occurrences in the Ulleung Basin,East Sea – Part 1: Mapping sedimentation patterns using seismic coherency

Seismic coherency measures, such as similarity and dip of maximum similarity, were used to characterize mass transport deposits (MTDs) in the Ulleung Basin, East Sea, offshore Korea. Using 2-D and 3-D seismic data several slope failure masses have been identified near drill site UBGH1-4. The MTDs have a distinct seismic character and exhibit physical properties similar to gas hydrate bearing sediment: elevated electrical resistivity and P-wave velocity. Sediments recovered from within the MTDs show a reworked nature with chaotic assemblage of mud-clasts. Additionally, the reflection at the base of MTDs is polarity reversed relative to the seafloor, similarly to the bottom-simulating reflector commonly used to infer the presence of gas hydrates. The MTDs further show regional seismic blanking (absence of internal reflectivity), which is yet another signature often attributed to gas hydrate bearing sediments. At the drill site UBGH1-4, no gas hydrate was recovered in sediment-cores from inside a prominent MTD unit. Instead, pore-filling gas hydrate was recovered only within thin turbidite sand layers near the base of the gas hydrate stability zone. With the analysis of seismic attributes, the seismic character of the prominent MTD (Unit 3) was investigated. The base of the MTD unit exhibits deep grooves interpreted as gliding tracks from either outrunner blocks or large clasts that were dragged along the paleo-seafloor. Similar seismic features were identified on the seafloor although the length of the gliding tracks on the seafloor is much shorter (a few hundred meters to ∼1 km), compared to over 10 km long tracks at the base of the MTD. The seismic coherency attributes allowed to estimate the volume of the failed sediment as well as the direction of the flow of sediment. Tracking the MTD and extrapolating its spatial extent from the 3-D seismic volume to adjacent 2-D seismic profiles, a possible source region of this mass failure was defined ∼50 km upslope of Site UBGH1-4.     

High-resolution P-Cable 3D seismic imaging of gas chimney structures in gas hydrated sediments of an Arctic sediment drift

The newly developed P-Cable 3D seismic system allows for high-resolution seismic imaging to characterize upper geosphere geological features focusing on geofluid expressions (gas chimneys), shallow gas and gas hydrate reservoirs. Seismic imaging of a geofluid system of an Arctic sediment drift at the Vestnesa Ridge, offshore western Svalbard, provides significantly improved details of internal chimney structures from the seafloor to ∼500 m bsf (below seafloor). The chimneys connect to pockmarks at the seafloor and indicate focused fluid flow through gas hydrated sediments. The pockmarks are not buried and align at the ridge-crest pointing to recent, topography-controlled fluid discharge. Chimneys are fuelled by sources beneath the base of gas hydrate stability zone (GHSZ) that is evident at ∼160–170 m bsf as indicated by a bottom-simulating reflector (BSR). Conduit centres that are not vertically straight but shift laterally by up to 200 m as well as discontinuous internal chimney reflections indicate heterogeneous hydraulic fracturing of the sediments. Episodically active, pressure-driven focused fluid flow could explain the hydro-fracturing processes that control the plumbing system and lead to extensive pockmark formation at crest of the Vestnesa Ridge. High-amplitude anomalies in the upper 50 m of the chimney structures suggest formations of near-surface gas hydrates and/or authigenic carbonate precipitation. Acoustic anomalies, expressed as high amplitudes and amplitude blanking, are irregularly distributed throughout the deeper parts of the chimneys and provide evidence for the variability of hydrate and/or carbonate formation in space and time.     

Characterizing the thermal regime of cold vents at the northern Cascadia margin from bottom-simulating reflector distributions, heat-probe measurements and borehole temperature data

Several cold vents are observed at the northern Cascadia margin offshore Vancouver Island in a 10 km² region around Integrated Ocean Drilling Program Expedition 311 Site U1328. All vents are linked to fault systems that provide pathways for upward migrating fluids and at three vents methane plumes were detected acoustically in the water column. Downhole temperature measurements at Site U1328 revealed a geothermal gradient of 0.056 ± 0.004°C/m. With the measured in situ pore-water salinities the base of methane hydrate stability is predicted at 218–245 meters below seafloor. Heat-probe measurements conducted across Site U1328 and other nearby vents showed an average thermal gradient of 0.054 ± 0.004°C/m. Assuming that the bottom-simulating reflector (BSR) marks the base of the gas hydrate stability zone variations in BSR depths were used to investigate the linkages between the base of the gas hydrate stability zone and fluid migration. Variations in BSR depth can be attributed to lithology-related velocity changes or variations of in situ pore-fluid compositions. Prominent BSR depressions and reduced heat flow are seen below topographic highs, but only a portion of the heat flow reduction can be due to topography-linked cooling. More than half of the reduction may be due to thrust faulting or to pore-water freshening. Distinct changes in BSR depth below seafloor are observed at all cold vents studied and some portion of the observed decrease in the BSR depth was attributed to fault-related upwelling of warmer fluids. The observed decrease in BSR depth below seafloor underneath the vents ranges between 7 and 24 m (equivalent to temperature shifts of 0.07–0.15°C).     

Gas hydrates in shallow deposits of the Amsterdam mud volcano, Anaximander Mountains, Northeastern Mediterranean Sea

We investigated gas hydrate in situ inventories as well as the composition and principal transport mechanisms of fluids expelled at the Amsterdam mud volcano (AMV; 2,025 m water depth) in the Eastern Mediterranean Sea. Pressure coring (the only technique preventing hydrates from decomposition during recovery) was used for the quantification of light hydrocarbons in near-surface deposits. The cores (up to 2.5 m in length) were retrieved with an autoclave piston corer, and served for analyses of gas quantities and compositions, and pore-water chemistry. For comparison, gravity cores from sites at the summit and beyond the AMV were analyzed. A prevalence of thermogenic light hydrocarbons was inferred from average C₁/C₂₊ ratios <35 and δ¹³C-CH₄ values of ?50.6‰. Gas venting from the seafloor indicated methane oversaturation, and volumetric gas–sediment ratios of up to 17.0 in pressure cores taken from the center demonstrated hydrate presence at the time of sampling. Relative enrichments in ethane, propane, and iso-butane in gas released from pressure cores, and from an intact hydrate piece compared to venting gas suggest incipient crystallization of hydrate structure II (sII). Nonetheless, the co-existence of sI hydrate can not be excluded from our dataset. Hydrates fill up to 16.7% of pore volume within the sediment interval between the base of the sulfate zone and the maximum sampling depth at the summit. The concave-down shapes of pore-water concentration profiles recorded in the center indicate the influence of upward-directed advection of low-salinity fluids/fluidized mud. Furthermore, the SO ₄ ^2? and Ba²⁺ pore-water profiles in the central part of the AMV demonstrate that sulfate reduction driven by the anaerobic oxidation of methane is complete at depths between 30 cm and 70 cm below seafloor. Our results indicate that methane oversaturation, high hydrostatic pressure, and elevated pore-water activity caused by low salinity promote fixing of considerable proportions of light hydrocarbons in shallow hydrates even at the summit of the AMV, and possibly also of other MVs in the region. Depending on their crystallographic structure, however, hydrates will already decompose and release hydrocarbon masses if sediment temperatures exceed ca. 19.3°C and 21.0°C, respectively. Based on observations from other mud volcanoes, the common occurrence of such temperatures induced by heat flux from below into the immediate subsurface appears likely for the AMV.     

A giant (5.3 × 10 m) middle Miocene (c. 15 Ma) sediment mound (M1) above the Siri Canyon, Norwegian–Danish Basin: Origin and significance

A large-scale enigmatic mound structure (M1) has been discovered in middle Miocene strata of the Norwegian–Danish Basin, c. 10 km east and updip of the Central Graben. It is located about 1 km beneath the seabed and clearly resolved by a 3D seismic data set focused on the deeper, remobilised, sand-filled Siri Canyon. M1 comprises two culminations, up to 80 m high and up to 1400 m long, constituting a sediment volume of some 5.3 × 10⁷ m³. It is characterized by a hard reflection at the top, a soft reflection at the base, differential compaction relative to the surrounding sediments, and 10 ms TWT velocity pull up of underlying reflections, indicating a relatively fast mound fill, attributed to the presence of sand within the mound. Internal seismic reflections are arranged in an asymmetric concentric pattern, suggesting a progressive aggradation to the NW, downstream to a mid-Miocene contour current system. Numerous elongated pockmarks occur in the upper Miocene succession close to the mound and indicate that the study area was influenced by gas expulsion in the mid- and late Miocene.The reflection configuration, velocity, dimensions, regional setting, and isolated location can best be explained by interpreting the mound as a giant sand volcano extruded >1 km upward from the Siri Canyon during the middle Miocene (c. 15 Ma). The likely causes of this remarkable structure include gas charge and lateral pressure transfer from the Central Graben along the Siri Canyon reservoir. While this is the first such structure described from this part of the North Sea, similar-aged sand extrudites have recently been inferred from seismic observations in the North Viking Graben, thus suggesting that the mid-Miocene was a time of widespread and intense sediment remobilization and fluid expulsion in the North Sea.     

Discovery of a new hydrothermal vent based on an underwater,high-resolution geophysical survey

A new hydrothermal vent site in the Southern Mariana Trough has been discovered using acoustic and magnetic surveys conducted by the Japan Agency for Marine-Earth Science and Technology's (JAMSTEC) autonomous underwater vehicle (AUV), Urashima. The high-resolution magnetic survey, part of a near-bottom geophysical mapping around a previously known hydrothermal vent site, the Pika site, during the YK09-08 cruise in June–July 2009, found that a clear magnetization low extends ∼500 m north from the Pika site. Acoustic signals, suggesting hydrothermal plumes, and 10 m-scale chimney-like topographic highs were detected within this low magnetization zone by a 120 kHz side-scan sonar and a 400 kHz multibeam echo sounder. In order to confirm the seafloor sources of the geophysical signals, seafloor observations were carried out using the deep-sea manned submersible Shinkai 6500 during the YK 10-10 cruise in August 2010. This discovered a new hydrothermal vent site (12°55.30′N, 143°38.89′E; at a depth of 2922 m), which we have named the Urashima site. This hydrothermal vent site covers an area of approximately 300 m×300 m and consists of black and clear smoker chimneys, brownish-colored shimmering chimneys, and inactive chimneys. All of the fluids sampled from the Urashima and Pika sites have chlorinity greater than local ambient seawater, suggesting subseafloor phase separation or leaching from rocks in the hydrothermal reaction zone. End-member compositions of the Urashima and Pika fluids suggest that fluids from two different sources feed the two sites, even though they are located on the same knoll and separated by only ∼500 m. We demonstrate that investigations on hydrothermal vent sites located in close proximity to one another can provide important insights into subseafloor hydrothermal fluid flow, and also that, while such hydrothermal sites are difficult to detect by conventional plume survey methods, high-resolution underwater geophysical surveys provide an effective means.