The thermal structure of the Dvurechenskii mud volcano and its implications for gas hydrate stability and eruption dynamics

The sediment temperature distribution at mud volcanoes provides insights into their activity and into the occurrence of gas hydrates. If ambient pressure and temperature conditions are close to the limits of the gas hydrate stability field, the sediment temperature distribution not only limits the occurrence of gas hydrates, but is itself influenced by heat production and consumption related to the formation and dissociation of gas hydrates. Located in the Sorokin Trough in the northern Black Sea, the Dvurechenskii mud volcano (DMV) was in the focus of detailed investigations during the M72/2 and M73/3a cruises of the German R/V Meteor and the ROV Quest 4000 m in February and March 2007. A large number of in-situ sediment temperature measurements were conducted from the ROV and with a sensor-equipped gravity corer. Gas hydrates were sampled in pressurized cores using a dynamic autoclave piston corer (DAPC). The thermal structure of the DMV suggests a regime of fluid flow at rates decreasing from the summit towards the edges of the mud volcano, accompanied by intermittent mud expulsion at the summit. Modeled gas hydrate dissociation temperatures reveal that the gas hydrates at the DMV are very close to the stability limits. Changes in heat flow due to variable seepage rates probably do not result in changes in sediment temperature but are compensated by gas hydrate dissociation and formation.     

Vodyanitskii mud volcano,Sorokin trough,Black Sea: Geological characterization and quantification of gas bubble streams

Vodyanitskii mud volcano is located at a depth of about 2070 m in the Sorokin Trough, Black sea. It is a 500-m wide and 20-m high cone surrounded by a depression, which is typical of many mud volcanoes in the Black Sea. 75 kHz sidescan sonar show different generations of mud flows that include mud breccia, authigenic carbonates, and gas hydrates that were sampled by gravity coring. The fluids that flow through or erupt with the mud are enriched in chloride (up to ∼650 mmol L⁻¹ at ∼150-cm sediment depth) suggesting a deep source, which is similar to the fluids of the close-by Dvurechenskii mud volcano. Direct observation with the remotely operated vehicle Quest revealed gas bubbles emanating at two distinct sites at the crest of the mud volcano, which confirms earlier observations of bubble-induced hydroacoustic anomalies in echosounder records. The sediments at the main bubble emission site show a thermal anomaly with temperatures at ∼60 cm sediment depth that were 0.9 °C warmer than the bottom water. Chemical and isotopic analyses of the emanated gas revealed that it consisted primarily of methane (99.8%) and was of microbial origin (δD-CH₄ = −170.8‰ (SMOW), δ¹³C-CH₄ = −61.0‰ (V-PDB), δ¹³C-C₂H₆ = −44.0‰ (V-PDB)). The gas flux was estimated using the video observations of the ROV. Assuming that the flux is constant with time, about 0.9 ± 0.5 × 10⁶ mol of methane is released every year. This value is of the same order-of-magnitude as reported fluxes of dissolved methane released with pore water at other mud volcanoes. This suggests that bubble emanation is a significant pathway transporting methane from the sediments into the water column.     

Gas hydrates and active mud volcanism on the South Shetland continental margin, Antarctic Peninsula

During the Antarctic summer of 2003–2004, new geophysical data were acquired from aboard the R/V OGS Explora in the BSR-rich area discovered in 1996–1997 along the South Shetland continental margin off the Antarctic Peninsula. The objective of the research program, supported by the Italian National Antarctic Program (PNRA), was to verify the existence of a potential gas hydrate reservoir and to reconstruct the tectonic setting of the margin, which probably controls the extent and character of the diffused and discontinuous bottom simulating reflections. The new dataset, i.e. multibeam bathymetry, seismic profiles (airgun and chirp), and two gravity cores analysed by computer-aided tomography as well as for gas composition and content, clearly shows active mud volcanism sustained by hydrocarbon venting in the region: several vents, located mainly close to mud volcanoes, were imaged during the cruise and their occurrence identified in the sediment samples. Mud volcanoes, vents and recent slides border the gas hydrate reservoir discovered in 1996–1997. The cores are composed of stiff silty mud. In core GC01, collected in the proximity of a mud volcano ridge, the following gases were identified (maximum contents in brackets): methane (46 μg/kg), pentane (45), ethane (35), propane (34), hexane (29) and butane (28). In core GC02, collected on the flank of the Vualt mud volcano, the corresponding data are methane (0 μg/kg), pentane (45), ethane (22), propane (0), hexane (27) and butane (25).     

Thermal anomalies associated with shallow gas hydrates in the K-2 mud volcano, Lake Baikal

Thermal measurements and hydrate mapping in the vicinity of the K-2 mud volcano in Lake Baikal have revealed a particular type of association of thermal anomalies (29–121?mW?m^–2) near hydrate-forming layers. Detailed coring within K-2 showed that hydrates are restricted to two distinct zones at sub-bottom depths exceeding 70–300?cm. Temperature data from stations with hydrate recovery and degassing features all display low thermal gradients. Otherwise, the thermal gradients within the mud volcano are generally increased. These findings imply a more complicated thermal regime than often assumed for mud volcanoes, with important roles for both fluids and hydrates. The coexistence of neighbouring low and high thermal anomalies is interpreted to result from discharging and recharging fluid activity, rather than hydrate thermodynamics. It is suggested that hydrates play a key role in controlling the fluid circulation pattern at an early stage. At a later stage, the inflow of undersaturated lake water would favour the dissolution of structure I hydrates and the formation of structure II hydrates, the latter having been observed on top of structure I hydrates in the K-2 mud volcano.     

Mud volcanoes and gas hydrates in the Black Sea: new data from Dvurechenskii and Odessa mud volcanoes

Meteor cruise M52/1 documented the presence of gas hydrates in sediments from mud volcanoes in the Sorokin Trough of the Black Sea. In a mud flow on the Odessa mud volcano, a carbonate crust currently forms in association with anaerobic methane oxidation. Dvurechenskii mud volcano (DMV), a flat-topped mud pie-type structure, appeared to be very active. Pore water in sediments of DMV is enriched in several constituents, such as ammonium and chloride, which seem to originate at depth. High sediment temperatures of up to 16.5 °C in close contact to the ambient bottom water of 9 °C also suggest strong advective transport of material from greater depth. Steep temperature gradients indicate a high fluid and/or mud flux within DMV, which is confirmed by the shape of the pore water profiles. Active fluid expulsion sites are evidenced by direct seafloor observation, and a potential flux of methane from the sediment to the bottom water is indicated by water-column methane measurements.     

Active mud volcanism in the West Alboran Basin: Geochemical evidence of hydrocarbon seepage

The West Alboran Basin was previously classified as a mud volcanic province consisting of two mud volcano (MV) fields that are inactive at the present day: the Northern (Spanish) and the Southern (Moroccan) fields. The discovery of the first active mud volcano (Carmen; cruise TTR-17) in 2008, along with several pockmarks at the central part of the basin, motivates more careful geological and geochemical analysis of previous data and comparison to new observations.Gas bubbling from the crater of Carmen MV was observed and recorded using an underwater TV-system and a large TV-grab sample. The gas mainly consisted of methane with less than 1% wetness. However, all sets of homologues up to pentane were detected in the mud breccia of Carmen MV. Both molecular and stable carbon isotopic compositions, and their distribution along the core length, suggest a deep thermogenic source of hydrocarbons (HCs). Composition of the pore water from Carmen MV also points to a deep source of mud volcanic water. The isotopic results indicate that the source of mud volcanic water is the dehydration of clay minerals in the thermal zone of the smectite-to-illite transformation. Our observations allow us to infer the presence of structure II gas hydrates in mud breccia on the top of Carmen MV.High HC gas saturation in sediments in some pockmarks accompanied with live chemosynthetic fauna directly indicates the strong seepage activity of these structures. For the first time, authigenic carbonate crusts and chimneys with associated living chemosynthetic bivalves and tubeworms were sampled from a seep site in the West Alboran Sea. Authigenic carbonates consist of aragonite and calcite, and are characterized by a light carbon isotopic signature, up to −37.2‰ PDB, which points to their methane-derived origin.     

When mud volcanoes sleep: Insight from seep geochemistry at the Dashgil mud volcano,Azerbaijan

The worlds >1500 mud volcanoes are normally in a dormant stage due to the short duration of eruptions. Their dormant stage activity is often characterized by vigorous seepage of water, gas, and petroleum. However, the source of the fluids and the fluid–rock interactions within the mud volcano conduit remain poorly understood. In order to investigate this type of activity, we have combined satellite images with fieldwork and extensive sampling of water and gas at seeping gryphons, pools and salsa lakes at the Dashgil mud volcano in Azerbaijan. We find that caldera collapse faults and E–W oriented faults determine the location of the seeps. The seeping gas is dominated by methane (94.9–99.6%), with a δ¹³C (‰ V-PDB) in the −43.9 to −40.4‰ range, consistent throughout the 12 analysed seeps. Ethane and carbon dioxide occur in minor amounts. Seventeen samples of seeping water show a wide range in solute content and isotopic composition. Pools and salsa lakes have the highest salinities (up to 101,043 ppm Cl) and the lowest δ¹⁸O (‰ V-SMOW) values (1–4‰). The mud-rich gryphons have low salinities (<18,000 ppm Cl) and are enriched in ¹⁸O (δ¹⁸O = 4–6‰).     

Characteristics of the bottom simulating reflectors near mud diapirs: offshore southwestern Taiwan

Single-channel seismic recording was carried out off the southwestern coast of Taiwan. Six characteristic seismic facies associated with bottom simulating reflectors (BSRs) and mud diapirs are identified. The existence of reflections which mimic the seafloor, the reverse polarity, weak amplitude blocks, and strong diffraction patterns around the mud diapirs all suggest that gas hydrates exist in the deep-water regions. The bases of the hydrate stability zones upturn in the vicinity of mud volcanoes. The high heat flows of mud volcanoes provide heat sources which destabilize the gas hydrates and upturn the BSRs. Received: 24 March 1999 / Revision accepted: 10 December 1999     

Acoustic investigations of mud volcanoes in the Sorokin Trough, Black Sea

The Sorokin Trough (Black Sea) is characterized by diapiric structures formed in a compressional tectonic regime that facilitate fluid migration to the seafloor. We present acoustic data in order to image details of mud volcanoes associated with the diapirs. Three types of mud volcanoes were distinguished: cone-shaped, flat-topped, and collapsed structures. All mud volcanoes, except for the Kazakov mud volcano, are located above shallow mud diapirs and diapiric ridges. Beyond the known near-surface occurrence of gas hydrates, bottom simulating reflectors are not seen on our seismic records, but pronounced lateral amplitude variations and bright spots may indicate the presence of gas hydrates and free gas.     

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.     

Hydrocarbon gases in deposits from mud volcanoes in the Sorokin Trough, north-eastern Black Sea

Hydrocarbon gases were determined in sediments from three mud volcanoes in the Sorokin Trough. In comparison to a reference station outside the mud volcano area, the deposits are characterized by an enrichment of high-molecular hydrocarbons (C₂–C₄), an absence of unsaturated homologues, a predominance of iso-butane in comparison with n-butane, and the presence of gas hydrate. The molecular composition of the hydrocarbon gases suggests their deep sources and thermogenic origin. In the pelagic sediments at the reference station, the methane concentration is relatively low (up to 49 ml/l); maximum concentrations are reached in deposits of the Dvurechenskii mud volcano (up to 400 ml/l). It was the first time that gas hydrate was sampled at the Dvurechenskii mud volcano. The gas extracted by dissociation of hydrate samples was dominated by methane (99.5%) with low amounts of ethane and propane (less than 0.5%). The isotopic composition of the methane varies between –62 and –66 PDB in ¹³C, and between –185 and –209 SMOW in D, indicating a mainly biogenic origin with an admixture of thermogenic gas.     

Cold seep communities in the deep eastern Mediterranean Sea: composition, symbiosis and spatial distribution on mud volcanoes

Two mud volcano fields were explored during the French–Dutch MEDINAUT cruise (1998) with the submersible NAUTILE, one south of Crete along the Mediteranean Ridge at about 2000 m depth (Olimpi mud field) and the other south of Turkey between 1700 and 2000 m depth (Anaximander mud field) where high methane concentrations were measured. Chemosynthetic communities were observed and sampled on six mud volcanoes and along a fault scarp. The communities were dominated by bivalves of particularly small size, belonging to families commonly found at seeps (Mytilidae, Vesicomyidae, Thyasiridae) and to Lucinidae mostly encountered in littoral sulfide-rich sediments and at the shallowest seeps. Siboglinid polychaetes including a large vestimentiferan Lamellibrachia sp. were also associated. At least four bivalve species and one siboglinid are associated with symbiotic chemoautotrophic bacteria, as evidenced by Transmission Electronic Microscopy and isotopic ratio measurements. Among the bivalves, a mytilid harbors both methanotrophic and sulfide-oxidizing bacteria. Video spatial analysis of the community distribution on three volcanoes shows that dense bivalve shell accumulations (mainly lucinids) spread over large areas, from 10% to 38% of the explored areas (2500–15000 m²) on the different volcanoes. Lamellibrachia sp. had different spatial distribution and variable density in the two mud volcano fields, apparently related with higher methane fluxes in the Anaximander volcanoes and maybe with the instability due to brines in the Olimpi area. The abundance and richness of the observed chemosynthetic fauna and the size of some of the species contrast with the poverty of the deep eastern Mediterranean. The presence of a specialized fauna, with some mollusk genera and species shared with other reduced environments of the Mediterranean, but not dominated by the large bivalves usually found at seeps, is discussed.     

Evidence of subsurface anaerobic biodegradation of hydrocarbons and potential secondary methanogenesis in terrestrial mud volcanoes

The assessment of gas origin in mud volcanoes and related petroleum systems must consider post-genetic processes which may alter the original molecular and isotopic composition of reservoir gas. Beyond eventual molecular and isotopic fractionation due to gas migration and microbial oxidation, investigated in previous studies, we now demonstrate that mud volcanoes can show signals of anaerobic biodegradation of natural gas and oil in the subsurface. A large set of gas geochemical data from more than 150 terrestrial mud volcanoes worldwide has been examined. Due to the very low amount of C₂₊ in mud volcanoes, isotopic ratios of ethane, propane and butane (generally the best tracers of anaerobic biodegradation) are only available in a few cases. However, it is observed that ¹³C-enriched propane is always associated with positive δ¹³C_CO2 values, which are known indicators of secondary methanogenesis following anaerobic biodegradation of petroleum. Data from carbon isotopic ratio of CO₂ are available for 134 onshore mud volcanoes from 9 countries (Azerbaijan, Georgia, Ukraine, Russia, Turkmenistan, Trinidad, Italy, Japan and Taiwan). Exactly 50% of mud volcanoes, all releasing thermogenic or mixed methane, show at least one sample with δ¹³C_CO2 > +5‰ (PDB). Thermogenic CH₄ associated with positive carbon isotopic ratio of CO₂ generally maintains its δ¹³C-enriched signature, which is therefore not perturbed by the lighter secondary microbial gas. There is, however, high variability in the δ¹³C_CO2 values within the same mud volcanoes, so that positive δ¹³C_CO2 values can be found in some vents and not in others, or not continuously in the same vent. This can be due to high sensitivity of δ¹³C_CO2 to gas–water–rock interactions or to the presence of differently biodegraded seepage systems in the same mud volcano. However, finding a positive δ¹³C_CO2 value should be considered highly indicative of anaerobic biodegradation and further analyses should be made, especially if mud volcanoes are to be used as pathfinders of the conditions indicative of subsurface hydrocarbon accumulations in unexplored areas.     

Seismic imaging of a fractured gas hydrate system in the Krishna–Godavari Basin offshore India

Gas hydrate was discovered in the Krishna–Godavari (KG) Basin during the India National Gas Hydrate Program (NGHP) Expedition 1 at Site NGHP-01-10 within a fractured clay-dominated sedimentary system. Logging-while-drilling (LWD), coring, and wire-line logging confirmed gas hydrate dominantly in fractures at four borehole sites spanning a 500 m transect. Three-dimensional (3D) seismic data were subsequently used to image the fractured system and explain the occurrence of gas hydrate associated with the fractures. A system of two fault-sets was identified, part of a typical passive margin tectonic setting. The LWD-derived fracture network at Hole NGHP-01-10A is to some extent seen in the seismic data and was mapped using seismic coherency attributes. The fractured system around Site NGHP-01-10 extends over a triangular-shaped area of ∼2.5 km² defined using seismic attributes of the seafloor reflection, as well as “seismic sweetness” at the base of the gas hydrate occurrence zone. The triangular shaped area is also showing a polygonal (nearly hexagonal) fault pattern, distinct from other more rectangular fault patterns observed in the study area. The occurrence of gas hydrate at Site NGHP-01-10 is the result of a specific combination of tectonic fault orientations and the abundance of free gas migration from a deeper gas source. The triangular-shaped area of enriched gas hydrate occurrence is bound by two faults acting as migration conduits. Additionally, the fault-associated sediment deformation provides a possible migration pathway for the free gas from the deeper gas source into the gas hydrate stability zone. It is proposed that there are additional locations in the KG Basin with possible gas hydrate accumulation of similar tectonic conditions, and one such location was identified from the 3D seismic data ˜6 km NW of Site NGHP-01-10.     

The characteristics of gas hydrates recovered from the Mount Elbert Gas Hydrate Stratigraphic Test Well, Alaska North Slope

Systematic analyses have been carried out on two gas hydrate-bearing sediment core samples, HYPV4, which was preserved by CH₄ gas pressurization, and HYLN7, which was preserved in liquid-nitrogen, recovered from the BPXA-DOE-USGS Mount Elbert Stratigraphic Test Well. Gas hydrate in the studied core samples was found by observation to have developed in sediment pores, and the distribution of hydrate saturation in the cores imply that gas hydrate had experienced stepwise dissociation before it was stabilized by either liquid nitrogen or pressurizing gas. The gas hydrates were determined to be structure Type I hydrate with hydration numbers of approximately 6.1 by instrumentation methods such as powder X-ray diffraction, Raman spectroscopy and solid state ¹³C NMR. The hydrate gas composition was predominantly methane, and isotopic analysis showed that the methane was of thermogenic origin (mean δ¹³C = −48.6‰ and δ_D = −248‰ for sample HYLN7). Isotopic analysis of methane from sample HYPV4 revealed secondary hydrate formation from the pressurizing methane gas during storage.     

Gas hydrate accumulation at the Håkon Mosby Mud Volcano

Gas hydrate (GH) accumulation is characterized and modeled for the Håkon Mosby mud volcano, ca. 1.5?km across, located on the Norway–Barents–Svalbard margin. Pore water chemical and isotopic results based on shallow sediment cores as well as geothermal and geomorphological data suggest that the GH accumulation is of a concentric pattern controlled by and formed essentially from the ascending mud volcano fluid. The gas hydrate content of sediment peaks at 25% by volume, averaging about 1.2% throughout the accumulation. The amount of hydrate methane is estimated at ca. 10⁸?m³ STP, which could account for about 1–10% of the gas that has escaped from the volcano since its origin.     

Gas hydrate associated with mud diapirs in southern Okinawa Trough

Many mud diapirs have been recognized in southern Okinawa Trough by a multi-channel seismic surveying on R/V KEXUE I in 2001. Gas hydrates have been identified, by the seismic reflection characteristics, the velocity analysis and the impedance inversion. Geothermal heat flow around the central of the mud diapir has been determined theoretically by the Bottom Simulating Reflectors (BSRs). Comparing the BSR derived and the measured heat flow values, we infer that the BSR immediately at the top of the mud diapirs indicate the base of the saturated gas hydrate formation zone (BSGHFZ), but not, as we ordinarily know, the base of the gas hydrate stability zone (BGHSZ), which could be explained by the abnormal regional background heat flow and free gas flux associated with mud diapirs. As a result, it helps us to better understand the generation mechanism of the gas hydrates associated with mud diapirs and to predict the gas hydrate potential in the southern Okinawa Trough.     

Physical properties of sediment from the Mount Elbert Gas Hydrate Stratigraphic Test Well, Alaska North Slope

This study characterizes cored and logged sedimentary strata from the February 2007 BP Exploration Alaska, Department of Energy, U.S. Geological Survey (BPXA-DOE-USGS) Mount Elbert Gas Hydrate Stratigraphic Test Well on the Alaska North Slope (ANS). The physical-properties program analyzed core samples recovered from the well, and in conjunction with downhole geophysical logs, produced an extensive dataset including grain size, water content, porosity, grain density, bulk density, permeability, X-ray diffraction (XRD) mineralogy, nuclear magnetic resonance (NMR), and petrography.This study documents the physical property interrelationships in the well and demonstrates their correlation with the occurrence of gas hydrate. Gas hydrate (GH) occurs in three unconsolidated, coarse silt to fine sand intervals within the Paleocene and Eocene beds of the Sagavanirktok Formation: Unit D-GH (614.4 m-627.9 m); unit C-GH1 (649.8 m-660.8 m); and unit C-GH2 (663.2 m-666.3 m). These intervals are overlain by fine to coarse silt intervals with greater clay content. A deeper interval (unit B) is similar lithologically to the gas-hydrate-bearing strata; however, it is water-saturated and contains no hydrate.In this system it appears that high sediment permeability (k) is critical to the formation of concentrated hydrate deposits. Intervals D-GH and C-GH1 have average “plug” intrinsic permeability to nitrogen values of 1700 mD and 675 mD, respectively. These values are in strong contrast with those of the overlying, gas-hydrate-free sediments, which have k values of 5.7 mD and 49 mD, respectively, and thus would have provided effective seals to trap free gas. The relation between permeability and porosity critically influences the occurrence of GH. For example, an average increase of 4% in porosity increases permeability by an order of magnitude, but the presence of a second fluid (e.g., methane from dissociating gas hydrate) in the reservoir reduces permeability by more than an order of magnitude.     

Anomalous seismic reflections related to gas/gas hydrate occurrences along the western continental margin of India

Gas hydrates along continental margins are commonly inferred from the presence of bottom simulating reflectors (BSRs) on reflection seismic records. Shale and mud diapirs are often observed in the proximity of BSR-inferred gas hydrates. Analysis of data from documented gas-hydrate occurrences suggests that the areas where mud volcanoes exist on the seafloor are promising locations for sediments with high gas-hydrate concentration. Along the western continental margin of India (WCMI), we have identified several anomalous reflections on single-channel, analogue seismic records in the proximity of BSRs, from which the presence of gas-charged sediments and gas seepages was inferred. These features characterize both the shelf-slope region of the WCMI and the adjoining deep-sea areas. The seismic records also reveal mud/shale diapiric activity and pockmarks near the gas hydrates.