Methane Hydrate and Free Gas on the Blake Ridge from Vertical Seismic Profiling

Holbrook, W. S.; Hoskins, Hartley; Wood, W. T.; Stephen, Ralph A.; Lizarralde, D.

doi:10.1126/science.273.5283.1840

Cited by 498 publications

(323 citation statements)

References 22 publications

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“…They also note, however, that attenuation may be affected by hydrate at shorter wavelengths and with higher concentrations. Wood and Ruppel [2000] argue that the reduced reflectivity on the Blake Ridge is due to scattering and destructive interference rather than to the homogeneity of the formation, as suggested by Holbrook et al [1996]. If attenuation and scattering above the bottom simulating reflector (BSR) were high enough, this strong reflector might likewise be attenuated.…”

Section: Discussionmentioning

confidence: 99%

Sonic waveform attenuation in gas hydrate‐bearing sediments from the Mallik 2L‐38 research well, Mackenzie Delta, Canada

2002

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[1] The Mallik 2L-38 research well was drilled to 1150 m under the Mackenzie Delta, Canada, and penetrated a subpermafrost interval where methane hydrate occupies up to 80% of the pore space. A suite of high-quality downhole logs was acquired to measure in situ the physical properties of these hydrate-bearing sediments. Similar to other hydrate deposits, resistivity and compressional and shear sonic velocity data increase with higher hydrate saturation owing to electrical insulation of the pore space and stiffening of the sediment framework. In addition, sonic waveforms show strong amplitude losses of both compressional and shear waves in intervals where methane hydrate is observed. We use monopole and dipole waveforms to estimate compressional and shear attenuation. Comparing with hydrate saturation values derived from the resistivity log, we observe a linear increase in both attenuation measurements with increasing hydrate saturation, which is not intuitive for stiffening sediments. Numerical modeling of the waveforms allows us to reproduce the recorded waveforms and illustrate these results. We also use a model for wave propagation in frozen porous media to explain qualitatively the loss of sonic waveform amplitude in hydratebearing sediments. We suggest that this model can be improved and extended, allowing hydrate saturation to be quantified from attenuation measurements in similar environments and providing new insight into how hydrate and its sediment host interact.

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Section: Discussionmentioning

confidence: 99%

Sonic waveform attenuation in gas hydrate‐bearing sediments from the Mallik 2L‐38 research well, Mackenzie Delta, Canada

2002

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“…However, the sonic log we have examined did not include velocities for the depth interval from 0 to 134 m. Therefore we compare the predicted vertical ? wave speed Vp with an observed wave speed Vp øbs which is set equal to Vp vsp at 100 mbsf and kp løg at The predicted Vp is compared with log data [Paull et al, 1996] and VSP data [Holbrook et al, 1996]. Predictions are given both for a completely disordered ( The aspect ratio of the quartz inclusions is held constant at 1.…”

Section: Resultsmentioning

confidence: 99%

Elastic properties of hydrate‐bearing sediments using effective medium theory

Jakobsen¹,

Hudson²,

Minshull³

et al. 2000

J. Geophys. Res.

236

136

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Abstract. Accurate and detailed models of the seismic velocity structure of gas hydrate-bearing sediments may be determined by careful analysis of controlled source seismic data. However, interpretation of these velocities in terms of hydrate saturation of the pore space has hitherto relied on semiempirical formulae and/or simple effective medium theory. We develop a rigorous theoretical scheme to relate the seismic properties of a clay-rich hydrate-bearing sediment to its porosity, mineralogy, microstructural features and hydrate saturation. We consider separately the two possible end-members for the distribution of hydrate in the pore space: (1) hydrates are unconnected and located in the pore voids without appreciable grain contact and (2) connected hydrates are forming cement binding around the grains.The scheme is transversely isotropic, to allow for anisotropy due to alignment of clay platelets, and is based on a combination of a self-consistent approximation, a differential effective medium theory, and a method of smoothing for crystalline aggregates. We have applied the scheme to lithological and seismic velocity data from Ocean Drilling Program Site 995 on the Blake Ridge (southeastern U.S. continental margin) to make estimates of the hydrate saturation. It was found that the hydrates are probably unconnected, and their volume concentration varies between -• 0% at 100 m below the seabed and -• 9% at 400 m depth, just above the "bottom simulating reflector", if the clay platelet orientation distribution resembles the function we have used.

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“…5). The blanking (or whitening) effect has been attributed to the presence of gas hydrate in the sedimentary strata above the BSR, but may as well reflect areas of naturally low and uniform reflectance (Holbrook et al 1996), and has not been directly and unambiguously attributed to the occurrence of gas hydrate (Vanneste et al 2000). On the other hand, the strong reflections beneath the BSR are common features of free gas accumulation beneath a gas hydrate-bearing layer (e.g., Singh and Minshull 1994;Wood et al 1994), and have been correlated to the presence of free gas at drilling sites (e.g., Bangs et al 1993;Holbrook et al 1996;Wood and Ruppel 2000).…”

Section: Bsr Distributionmentioning

confidence: 99%

Distribution and Characters of Gas Hydrate Offshore of Southwestern Taiwan

Liu¹,

Schnürle²,

Wang³

et al. 2006

Terr. Atmos. Ocean. Sci.

149

108

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ABSTRACT1 Institute of Oceanography, National Taiwan University, Taipei, Taiwan, ROC 2 Central Geologic Survey, Taipei, Taiwan, ROC 3 Exploration Department, Chinese Petroleum Company, Miaoli, Taiwan, ROC * Corresponding author address: Prof. Char-Shine Liu, Institute of Oceanography, National Taiwan University, Taipei, Taiwan, ROC; E-mail: csliu@ntu.edu.tw Bottom simulating reflector (BSR) is a key indicator for the presence of gas hydrate beneath the sea floor. Widely distributed BSRs have been observed in the area offshore of southwestern Taiwan where the active accretionary complex meets with the passive China continental margin. In order to better understand the distribution and characters of the gas hydrate in the region, closely spaced (1.86-km line spacing) multichannel seismic reflection surveys have been conducted in recent years under the support of the Central Geological Survey, ROC. Over 10000 km of multichannel seismic reflection profiles have been collected that cover an area of about 10000 km 2 offshore of southwestern Taiwan. BSRs can be identified along 50% of the seismic profiles that we collected. A newly compiled BSR distribution map suggests that gas hydrates are distributed both in the passive margin of the China continental slope as well as in the submarine Taiwan accretionary wedge, from water depths of 500 to over 3000 m. Gas hydrates are most concentrated underneath anticlinal ridges in the accretionary wedge, and underneath the slope ridges of the passive continental margin that were formed due to sedimentary processes. Active fluid activities are evident from various features observed on seismic reflection and chirp sonar profiles, such as mud volcanoes, gas plumes and gas charged shallow sedimentary layers. Fluid migration model has been established from a set of pseudo 3D seismic reflection data. The predicted locations of high fluid Terr. Atmos. Ocean. Sci., Vol. 17, No. 4, December 2006 616 flux correlate well with those interpreted from geochemical analyses that show very high methane concentrations and very shallow sulfate-methane interfaces (SMI). This demonstrates the importance of structural control over gas hydrate emplacement. From the observed gas hydrate distribution and characters, the area offshore of southwestern Taiwan provides an ideal place to study and compare the formation and migration of gas hydrates under different tectonic settings.

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Methane Hydrate and Free Gas on the Blake Ridge from Vertical Seismic Profiling

Cited by 498 publications

References 22 publications

Sonic waveform attenuation in gas hydrate‐bearing sediments from the Mallik 2L‐38 research well, Mackenzie Delta, Canada

Sonic waveform attenuation in gas hydrate‐bearing sediments from the Mallik 2L‐38 research well, Mackenzie Delta, Canada

Elastic properties of hydrate‐bearing sediments using effective medium theory

Distribution and Characters of Gas Hydrate Offshore of Southwestern Taiwan

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