Climatic Feedback Loop

Paleoclimatic records of the recent geological past gleaned from ice cores from Antarctica and Greenland (see, e.g., Jouzel et al., 1993 Petit et al., 1999) show that at the onset of glacial cycles there is a relatively gradual decrease in atmospheric carbon dioxide and methane (Fig.l). Climatic ameliorations at the end of the glacial cycles, on the contrary, tend to be more abrupt and are accompanied by equally rapid increases in greenhouse gases. While the onset of glaciations is now widely...

Seismic Reflection

This is the most common marine geophysical tool. It involves a discharge seismic source, often an array of airguns and a receiver (Figures 3 & 4). Sediments that contain hydrate will normally show Vp higher than in normal oceanic sediment. Free methane accumulating in the pore spaces beneath the HSZ will cause a significantly lower Vp. Seismic velocity normally increases with depth so this velocity inversion, or negative acoustic impedance contrast, may exhibit the distinctive BSR reflection...

Mackenzie Deltabeaufort Sea Region Canada

The Mackenzie Delta-Beaufort Sea region, as described by Procter et al. (1984), is composed in part of modern deltaic sediments and older fluvial deposits of Richards Island, the Tuktoyaktuk Peninsula and offshore areas extending out onto the continental shelf to a water depth of about 200 m. The post Paleozoic sedimentary rocks of the Beaufort Sea continental shelf are subdivided into two major sections pre-Upper Cretaceous and Upper Cretaceous to Quaternary strata. A major regional...

Electrical resistivity

Four equally-spaced electrical-resistivity probes are placed on opposite sides of the sample. A current is flowed through the outer probes, and the potential is measured between the inner probes at specified times. The direction of current flow is reversed after each reading to reduce the impact of ion concentrations. The electrical-resistivity setup presently being constructed is similar to typical four-point measurements that are performed for geological applications (Chaker, 1996). The...

Synthesis Procedures And Apparatus 21 Pure methane hydrate

Samples of polycrystalline methane hydrate can be efficiently synthesized by promoting the general reaction CH4 (gas) + 6H2O (sol. > liq.) > CHi*6H20 (sol.) (Fig. 1). This product is achieved by the mixing and subsequent slow, regulated heating of sieved, granular, H20 seed ice and cold, pressurized CH4 gas in an approximately constant-volume reaction vessel (Fig. 2). This seeding method for hydrate nucleation and growth permits successful synthesis of polycrystalline test specimens with...

Advection Diffusion and Dispersion

If the pressure field in the sediment deviates from a hydrostatic condition, then the pore water will flow en masse according to Darcy's Law Volumetric flow Q per unit sectional area A is proportional to the gradient in excess fluid pressure (total Pf - hydrostatic pressure), and inversely proportional to the viscosity (a property only of the fluid, for water this depends mainly on temperature and salinity dissolved gas and pressure have little effect). The coefficient k in eqn. 3 is the...

South Orkney microcontinent

The South Orkney microcontinent (Fig. 5) is the biggest continental element of the South Scotia Ridge (250 x 350 km), an E-W-trending submarine morphological feature along which the present-day Antarctica-Scotia plate boundary is identified. The exposed rocks on the South Orkney Island are mainly metamorphic in origin, closely resembling oucrops of metamorphic complexes of teh paleo-Pacific margin found on the Antarctic Peninsula. Fig. 5 - General bathymetric map of the South Orkney Islands and...

Sensitivity to Gas Hydrate Related Deformation

The potential for gas hydrate to alter the mechanical properties of sediment is not uniformly distributed with water depth. Obviously, the proper temperature and pressure conditions and the presence of gas are required. Gas hydrates are typically stable in continental margin sediments in a zone that can be viewed as a seaward-thickening prism (Figure 1). This prism has a distinct up-slope limit. The water column temperature structure associated with most mid- and low-latitude regions is such...

Cascadia Margin Northeast Pacific Ocean Hydrate Distribution from Geophysical Investigations

Chapman1, M. Riedel1, N. Edwards3 and J. Yuan3 2Pacific Geoscience Centre Geological Survey of Canada Sidney, B.C., Canada V8L 4B2 Natural gas hydrate was first recognized on the Cascadia margin in 1985 through the characteristic bottom-simulating reflector (BSR) on conventional multichannel seismic data (Davis and Hyndman, 1989, Davis et al., 1990). Since then, the Cascadia accretionary margin has received the most intensive studies of any convergent margin...

Geological Controls On Gas Hydrate Distribution 41 Sediment Deposition

The most significant control on hydrate accumulation appears to be sediment deposition rate. Of the four concentrations of hydrate noted above, three are associated with areas of high deposition. The northernmost (Fig. 3) occurs in a region of undisturbed recent sedimentation (Schlee et al., 1992) between the Hudson and Wilmington sea valleys. The valleys form two major channelways for transportation of sediments across the continental rise. The area between the valleys is a depocenter where...

Identification Of Hydrate

Identification of areas of hydrate in the vicinity of Japan (Fig. 2) is presently Figure 2. Map showing hydrate deposits around Japan as revealed by presence of hydrate gas interface BSR. NT, Nankai Trough. Hydrate locations compiled by Mr. Mikio Satoh, Geological Survey of Japan. After Aoki et al. (1983), Arato et al. (1996), Ashi and Taira (1993), Ashi and Tokuyama (1997), Ashi et al. (1996), Ashi et al. (1999), Baba and Uchida (1998), Honza (1078), Hyndman et al. (1992), Nakamura et al....

Seismic indications of hydrate on the Indian continental slope

BSRs are more prominent in the western offshore region than the eastern offshore area and are distributed discontinuously along the entire length of the western margin, from 8 to 22 N. BSRs and blanking zones originally were identified on nine seismic sections in the western offshore (NIO, 1999) whereas one location in the eastern offshore BSR could be identified in water depths more than 2000 meters. In the western offshore region, the BSR occurrences are confined to a bathymetric window of...

Gas Hydrate as Contact Cement

The third modeling possibility is the case where gas hydrate forms as an intergranular cement. The basis for this model is the contact cement theory (CCT) of Dvorkin et al. (1994), which calculates the bulk and shear moduli of a dense, random packing of spherical elastic grains with small amounts of elastic cement at the grain contacts. This model is appropriate for granular sediments whose original porosity is approximately that of a sphere pack, namely 36 to 40 . The original CCT theory was...

Deep Water Fluid Venting and Gas Hydrate

Geological-geophysical studies carried out in the Ocean during recent decades have revealed widespread indications of fluid discharge from the sea floor. We have found that gas hydrate formation in the near sea floor corresponds to the majority of these fluid discharges. The shallowest water depths in which structure II hydrates have been detected on the seafloor are 480 m in the Caspian Sea and 530 m in the Gulf of Mexico. The shallowest depth of water at which of structure I hydrate has been...

Barents Sea Margin and Svalbard Margin

The largest contiguous sedimentary prism in the North Atlantic is along the western Barents Sea margin (Myhre and Eldholm, 1988), especially in the Bear Island Fan, where sediments are up to 7 km thick (Vorren et al., 1998 Hjelstuen, et al., 1999). Because rapidly deposited sediments derived primarily from a continental shelf area constitute good source beds, the west Svalbard sediment prism is likely to contain significant amounts of organic material. Moreover, the ocean continent boundary of...

Possible pitfalls

As the above discussion demonstrates, BSRs are more complex and diverse than previously thought. What matters in the context of gas hydrate quantification, however, is that the presence of BSRs is a reliable indicator that a study area contains gas hydrates. However, there are some pitfalls. BSRs appear to be obvious features, but they may not be noticed if interpreters don't expect them. In particular, inappropriate seismic processing that focuses on deep sediment sections may destroy BSRs....

Occurrence Of Bsrs And Hydrate Localities 31 Norwegian Margin

Two provinces on the mid-Norwegian Margin show BSR. Whereas the reflectors on the outer Voring Plateau are related to diagenesis (Skogseid and Eldholm, 1989), the BSR in the vicinity of the Storegga slide are related to gas hydrates (Bugge, 1983 Mienert and Bryn, 1997, Mienert, et al, 1998, Bouriak et al., 2000). These BSRs are located at the seaward termination of a thick wedge of Plio- Pleistocene sediments, and it is likely that fast sedimentation caused burial of large amounts of organic...

Deep Towed Seismic Detailed Structure at the BSR

High resolution multichannel seismic data were acquired in 1997 using the Deep-Tow Acoustics Geophysics System (DTAGS) in the vicinity of ODP Site 889 890 (Gettrust et al., 1999). The system was towed at a depth of about 1000 m in water depths averaging 1400 m. Because of its proximity to the seafloor, wide-angle reflection data may be recorded on the DTAGS streamer which has a relatively short length of 622 m. The wide angles facilitate velocity estimation and amplitude-versus-offset analysis....

Gas Hydrate Production Technology

Even though gas hydrate are known to occur in numerous marine and Arctic settings, little is known about the technology necessary to produce gas hydrate. Most of the existing gas hydrate resource assessments do not address the problem of gas hydrate recoverability. Proposed methods of gas recovery from hydrate usually deal with dissociating or melting in-situ gas hydrate by (1) heating the reservoir beyond hydrate formation temperatures, (2) decreasing the reservoir pressure below hydrate...

Estimate Of Methaneinplace In Hydrate

For each play area, an estimation of gas hydrate resources has been made (NIO, 1997) by assigning a probability from 5 to 95 . The various attributes have also been graded as a function the probabilities (Collett, 1996). The evaluated gas hydrate resource offshore India is restricted by a severely limited amount of relevant data, particularly in the continental slopes where both commercial data and existing marine geoscience data are minimal (Geoscience, 1998). No evaluation of an Indian...

Gibbs Free Energy Minimizations

Several methods to estimate the P-T conditions of methane hydrate stability by searching for the state which minimizes the Gibbs Free Energy of the system have been developed. These programs are computationally intensive and require sophisticated computer programming. However, computer programs to carry-out these calculations are now readily available. Sloan (1990, 1998) presents a detailed description of CSMHYD, a PC-DOS based computer program. His textbook includes a floppy disk with an...

Geological Framework

India is a cratonic shield area that has drifted northeast from an original position near the S W coast of Africa and Madagascar since the break-up of Pangea at the end of the Paleozoic. The northward overall movement of the oceanic plates in the Indian Ocean have driven the Indian craton into the Asian cratonic landmass and thrust it beneath, tectonically elevating the collision zone and the underplated zone and creating the highest mountains on Earth, the Himalayan mountains, that pass in an...

Northern Arabian Sea and Bay of Bengal

The total sediment thickness of the Bay of Bengal and the northern Arabian Sea can conveniently be divided into pre-collision (pre-Eocene) and post-collision (post-Paleocene) sedimentary sections (Curray, 1994 Fuloria, 1993). The younger sedimentary sequence represents the Bengal Fan, which began receiving sediments during the Eocene. The rate of sediment deposition may well have accelerated in the Early Miocene (Curray, 1991 1994). The sub-sea Bengal Fan is the largest in the world and has had...

Sea Level Rise

Figure 8 Methane release caused by relatively warm sea water that covers cold, hydrate-bearing coastal plain deposits in the Arctic. Figure 8 Methane release caused by relatively warm sea water that covers cold, hydrate-bearing coastal plain deposits in the Arctic. Evidence for dissociation of oceanic gas hydrate by pressure reduction seems clearer at present than for the effect of warming. When sea level drops, the pressure at the seafloor and down into the sediments decreases instantaneously....

Introduction

In the early 1820's, John Faraday, working in England, was investigating the newly discovered gas, chlorine. He easily repeated the earlier experiments of Humphrey Davy (Davy, 1811) in which gaseous chlorine and water formed solid chlorine hydrate upon cooling in the - late cold weather -. Faraday's lab curiosity chlorine hydrate has water as the host molecule, and chlorine molecules as the guest. These pioneering syntheses experiments are the first reported reference to a class of associative...

Tectonosedimentary Framework And Hosting Of Gas Hydrate

The margins of the northern sector of the South China Sea (SCS) displays a complex, tectonosedimentary framework. The NW margin of the SCS is a passive tectonic margin forming the boundary to the broad Chinese continental shelf. The eastern margin of the SCS is an active collisional margin of a prong of the circum-pacific plate. The Heng-Ch'un Ridge to the south of Taiwan (Fig. 1) is the southern continuation of the Japanese Ryukyu arc which passes through Taiwan. Figure 1. Simplified...

Natural Gas as a Fuel

Once the methane potential of the hydrate deposits in the U.S. Exclusive Economic Zone (EEZ) is better described and methodologies for methane recovery developed, it will be reasonable to consider transition from a petroleum-based to a gas-based economy. Methane as a fuel offers clear advantages over oil or coal immense resource potential, ease of transport via in-place distribution infrastructure, less carbon dioxide release per unit volume burned, minimum release of exhaust pollutants from...

Invasion Percolation

Free gas moving through a sediment must first overcome the entry pressure and thereafter find the path of least resistance. Generally capillary forces outweigh the viscous forces since the flow is very slow and so the favoured path is the one which connects the largest throats through the medium however long and tortuous it may be. The mathematical concept underlying such flow through a network of pores and throats is termed percolation theory (Sahimi 1994). Its relevance to fluid flow, and...

Acoustic properties

P-wave signals are recorded at numerous times during testing (e.g., Fig. 4). P-wave velocities, Vp, are calculated routinely also. Figure 6 shows velocity changes induced in different materials by gas hydrate formation, freezing of the pore water, and by increasing consolidation stress. Knowledge of the material occupying the pore space and the consolidation stress are critical to understanding the implications of the various velocity measurements. Vp values typically increase with...

Methane hydrate formation from melting ice direct observations and factors for complete reaction

Hydrate formation and growth processes can be further investigated by optical microscopy, using a simple and versatile optical cell constructed from standard high-pressure valve bodies and Si02 capillary tubing (Fig. 1 in Chou et al, 1990). This design was used for observing methane hydrate growth from seed ice, by chilling the cell and loosely filling it with 200 p.m ice grains prepared identically to those used in standard samples. The pore volume was then evacuated and flooded with cold CH4...

Modulation of Gas Hydrate Stability by Bottom Water Temperature The Santa Barbara Basin

Greenland ice-core data has revealed that rapid increase in atmospheric methane coincided with an abrupt warming event at the end of Younger Dryas glacial episode (at ca. 11.6 k.y. B.P.) and occurred over relatively short interval of a few decades (Severinghaus et al., 1998). High-resolution nitrogen- and argon-isotope data from the Boiling transition (from stadial to interstadial) recorded in the Greenland ice also shows that methane concentrations rose over a period of about 50 years, but...

Pacific margin of the Antarctic Peninsula BSR

A prominent BSR has been described by Rebesco et al. (1997) on the steeper side of the continental rise sediment drifts, where it is rendered more visible than on the gentler side by the unconformity between seafloor and deeper reflectors (Fig. 8). The BSR is found consistently at a depth of 600-700 ms twt, approximately coincident with the lower boundary of the 'drift maintenance1 depositional stage. The reflector should be regarded as a surface of sharp change of sediment reflectivity, which...

Common gas laws

Boyle's Law Boyle's Law defines the relationship between pressure and volume. It states that at a constant temperature, the volume of a given mass of gas will vary inversely with the absolute pressure, or P Vi P2V2. Charles' Law Charles' Law concerns the relationship between temperature, volume and pressure. It states that 'if the pressure remains constant, the volume of a given amount of gas is directly proportional to the absolute temperature'. Dalton's Law Dalton's Law concerns the...

Costa Rica Margin

The first LWD data recorded in hydrate-bearing sediments were acquired on the Costa Rica margin during ODP Leg 170. Several holes were drilled into the sedimentary wedge upslope of the Costa Rica trench, penetrating Plio-Pleistocene age sediments that are mostly derived from the Central American peninsula. The wedge was cored, and where significant amounts of material was recovered for sampling, disseminated hydrate was observed in fractures, cemented ashes, and diffuse pore space Kimura, et...

Sh

The bulk modulus of the saturated sediment is calculated from those of the dry frame using Kf as the pore fluid bulk modulus in Gassmann's equations 5 . The shear modulus of the saturated sediment is that of the dry frame. The shear modulus of the pore fluid mixture is zero. This is the limiting factor on this model. It can only be used when the pore fluid does not affect the shear modulus of the overall sediment. This means the model is only appropriate when the gas hydrate does not fill the...

Selected References

References for each chapter have been amalgamated here. Because most of the contributors are actively publishing scientists, there are a number of publications referenced that are in press at the time that the book was transferred to the publisher. Many papers are referenced in more than one chapter. In a number of cases the year of publication has a suffic 'a' or 'b' as cited in at least one of the referencing papers. Other papers may reference the same citation without the suffix. The editor...