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  • 1
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    An effective medium inversion algorithm for gas hydrate quantification and its application to laboratory and borehole measurements of gas hydrate-bearing sediments (2007)
    Blackwell Publishing
    Details
    Publication Date: 2022-05-25
    Description: Author Posting. © Blackwell, 2006. This article is posted here by permission of Blackwell for personal use, not for redistribution. The definitive version was published in Geophysical Journal International 166 (2006): 543–552, doi:10.1111/j.1365-246X.2006.03038.x.
    Description: The presence of gas hydrate in marine sediments alters their physical properties. In some circumstances, gas hydrate may cement sediment grains together and dramatically increase the seismic P- and S-wave velocities of the composite medium. Hydrate may also form a load-bearing structure within the sediment microstructure, but with different seismic wave attenuation characteristics, changing the attenuation behaviour of the composite. Here we introduce an inversion algorithm based on effective medium modelling to infer hydrate saturations from velocity and attenuation measurements on hydrate-bearing sediments. The velocity increase is modelled as extra binding developed by gas hydrate that strengthens the sediment microstructure. The attenuation increase is modelled through a difference in fluid flow properties caused by different permeabilities in the sediment and hydrate microstructures. We relate velocity and attenuation increases in hydrate-bearing sediments to their hydrate content, using an effective medium inversion algorithm based on the self-consistent approximation (SCA), differential effective medium (DEM) theory, and Biot and squirt flow mechanisms of fluid flow. The inversion algorithm is able to convert observations in compressional and shear wave velocities and attenuations to hydrate saturation in the sediment pore space. We applied our algorithm to a data set from the Mallik 2L–38 well, Mackenzie delta, Canada, and to data from laboratory measurements on gas-rich and water-saturated sand samples. Predictions using our algorithm match the borehole data and water-saturated laboratory data if the proportion of hydrate contributing to the load-bearing structure increases with hydrate saturation. The predictions match the gas-rich laboratory data if that proportion decreases with hydrate saturation. We attribute this difference to differences in hydrate formation mechanisms between the two environments.
    Description: This work was funded by European Commission under the contract EVK3-CT-2000-00043 (HYDRATECH). JAP and AIB were partly funded by Natural Environment Research Council.
    Keywords: Attenuation ; Elastic wave theory ; Gas hydrate ; P waves ; S waves
    Repository Name: Woods Hole Open Access Server
    Type: Article
    Format: application/pdf
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    PAPER (OA)
  • 2
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    Structural controls on seepage of thermogenic and microbial methane since the last glacial maximum in the Harstad Basin, southwest Barents Sea (2018)
    Elsevier
    In:  Marine and Petroleum Geology, 98 . pp. 569-581.
    Details
    Publication Date: 2021-02-08
    Description: The Harstad Basin is a structural block on the continental shelf of SW Barents Sea where gas hydrates likely occurred below the grounded ice-sheet during the last glaciation and it hosts active gas seepage at numerous seafloor sites. We present an integrated study of fluid flow systems in the Harstad Basin by combining seismic profile interpretations and gas flare mapping data with the geochemical results obtained on seafloor seeping gas and methane-derived carbonate crusts. More than 190 acoustic gas flares were registered in water column, many of them in association with pockmarks and carbonate crust fields. However, weak or absent seepage observed during remotely operated underwater vehicle transects across many pockmarks and crust fields suggests that seepage activity may have decreased since the last deglaciation. In the western Harstad Basin, seeps of microbial methane occur mainly above Tertiary formations that are pinching out below the glacial sediments. High amplitude seismic anomalies suggest the presence of gas pockets at the base of the glacial sediments and within Tertiary deposits. In contrast, gas seeping in the eastern Harstad Basin originates from a biodegraded thermogenic source tentatively connected to the deeply faulted Mesozoic rocks occurring below glacial sediments. This spatial variability in fluid sources is also recorded in the carbon isotope data of seafloor carbonate crusts, with δ13C values typically between −55 and −42‰ and −40 and −20 ‰VPDB for carbonate crusts associated with microbial and thermogenic fluids, respectively. U-Th chronology combined with the stable isotope data suggests that this discrepancy in fluid sources over a distance of about 20 km has been stable since the last glaciation and highlights the significance of regional underlying geology in mediating fluid supply to the seafloor
    Type: Article , PeerReviewed
    Format: text
    DOI: 10.1016/j.marpetgeo.2018.07.010
    Location Call Number Limitation Availability
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    OceanRep: Article in a Scientific Journal - peer-reviewed
  • 3
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    Gas hydrate and free gas detection using seismic quality factor estimates from high-resolution P-cable 3D seismic data (2016)
    Society of Exploration Geophysicists
    In:  Interpretation, 4 (1). SA39-SA54.
    Details
    Publication Date: 2019-02-01
    Description: We have estimated the seismic attenuation in gas hydrate and free-gas-bearing sediments from high-resolution P-cable 3D seismic data from the Vestnesa Ridge on the Arctic continental margin of Svalbard. P-cable data have a broad bandwidth (20–300 Hz), which is extremely advantageous in estimating seismic attenuation in a medium. The seismic quality factor (Q), the inverse of seismic attenuation, is estimated from the seismic data set using the centroid frequency shift and spectral ratio (SR) methods. The centroid frequency shift method establishes a relationship between the change in the centroid frequency of an amplitude spectrum and the Q value of a medium. The SR method estimates the Q value of a medium by studying the differential decay of different frequencies. The broad bandwidth and short offset characteristics of the P-cable data set are useful to continuously map the Q for different layers throughout the 3D seismic volume. The centroid frequency shift method is found to be relatively more stable than the SR method. Q values estimated using these two methods are in concordance with each other. The Q data document attenuation anomalies in the layers in the gas hydrate stability zone above the bottom-simulating reflection (BSR) and in the free gas zone below. Changes in the attenuation anomalies correlate with small-scale fault systems in the Vestnesa Ridge suggesting a strong structural control on the distribution of free gas and gas hydrates in the region. We argued that high and spatially limited Q anomalies in the layer above the BSR indicate the presence of gas hydrates in marine sediments in this setting. Hence, our workflow to analyze Q using high-resolution P-cable 3D seismic data with a large bandwidth could be a potential technique to detect and directly map the distribution of gas hydrates in marine sediments.
    Type: Article , PeerReviewed
    Format: text
    DOI: 10.1190/INT-2015-0023.1
    Location Call Number Limitation Availability
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    OceanRep: Article in a Scientific Journal - peer-reviewed
  • 4
    facet.materialart.
    Unknown
    Structural controls on seepage of thermogenic and microbial methane since the last glacial maximum in the Harstad Basin, southwest Barents Sea (2018)
    Elsevier
    In:  Marine and Petroleum Geology, 98 . pp. 569-581.
    Details
    Publication Date: 2021-02-08
    Description: The Harstad Basin is a structural block on the continental shelf of SW Barents Sea where gas hydrates likely occurred below the grounded ice-sheet during the last glaciation and it hosts active gas seepage at numerous seafloor sites. We present an integrated study of fluid flow systems in the Harstad Basin by combining seismic profile interpretations and gas flare mapping data with the geochemical results obtained on seafloor seeping gas and methane-derived carbonate crusts. More than 190 acoustic gas flares were registered in water column, many of them in association with pockmarks and carbonate crust fields. However, weak or absent seepage observed during remotely operated underwater vehicle transects across many pockmarks and crust fields suggests that seepage activity may have decreased since the last deglaciation. In the western Harstad Basin, seeps of microbial methane occur mainly above Tertiary formations that are pinching out below the glacial sediments. High amplitude seismic anomalies suggest the presence of gas pockets at the base of the glacial sediments and within Tertiary deposits. In contrast, gas seeping in the eastern Harstad Basin originates from a biodegraded thermogenic source tentatively connected to the deeply faulted Mesozoic rocks occurring below glacial sediments. This spatial variability in fluid sources is also recorded in the carbon isotope data of seafloor carbonate crusts, with δ13C values typically between −55 and −42‰ and −40 and −20 ‰VPDB for carbonate crusts associated with microbial and thermogenic fluids, respectively. U-Th chronology combined with the stable isotope data suggests that this discrepancy in fluid sources over a distance of about 20 km has been stable since the last glaciation and highlights the significance of regional underlying geology in mediating fluid supply to the seafloor.
    Type: Article , PeerReviewed
    Format: text
    DOI: 10.1016/j.marpetgeo.2018.07.010
    Location Call Number Limitation Availability
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    OceanRep: Article in a Scientific Journal - peer-reviewed
  • 5
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    Unknown
    Occurrence and Distribution of Bottom Simulating Reflections in the Barents Sea (2022)
    Springer
    Details
    Publication Date: 2022-05-20
    Description: The Barents Sea, located close to the Arctic Ocean, is a petroleum province featuring an extensive occurrence of gas hydrates and shallow gas in compacted sediments. Glacial erosion and uplift have contributed to the migration of gas originating from deeper rocks to the shallow sediments of this region, resulting in hydrates with higher-order hydrocarbons in addition to methane. This article documents reported gas hydrate indications and major controls on hydrate stability in the Barents Sea.
    Type: Book chapter , PeerReviewed
    Format: text
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    OceanRep: Book chapter
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