Structure II gas hydrates found below the bottom‐simulating reflector

Paganoni, M.; Cartwright, Joe; Foschi, Martino; Shipp, R. Craig; Rensbergen, P. Van

doi:10.1002/2016gl069452

Cited by 71 publications

(44 citation statements)

References 75 publications

(125 reference statements)

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“…However, at structurally controlled systems with high‐methane flux (e.g., Hydrate Ridge) drilling through the BSR has shown that the BSR does coincide with the predicted BGHS [ Trehu et al ., ]. Similarly, a recent study from borehole data at thermogenic gas hydrate systems of the convergent margin of Borneo documents a BSR that coincides with the BGHS predicted for measured concentrations of >96% methane type II gas hydrates [ Paganoni et al ., ].…”

Section: Discussionmentioning

confidence: 99%

“…The presence of >5% ethane and other higher‐order hydrocarbons as well as methane carbon isotopic ratios of ~40 ‰ point toward a significant contribution of thermogenic gas sources at Vestnesa Ridge (Figure ) [ Paull and Ussler , ]. Even with the abundant C 2 + measured though, these samples collected near the seafloor (<6 m bsf) along Vestnesa Ridge (Table ) are likely to underestimate the contribution of higher‐order hydrocarbons in the hydrate system at greater depths (within 140–190 m bsf), as recently documented from borehole data at the margins of Borneo [ Paganoni et al ., ].…”

Section: Discussionmentioning

confidence: 99%

“…Essentially, the gas source for hydrates in continental margins can be a mixture of microbial methane, thermogenic methane, and larger hydrocarbons such as ethane and propane, or even abiogenic gas [ Johnson et al ., ; Rajan et al ., ]. The presence of ethane, propane, and butane in addition to methane is a critical factor influencing hydrate stability and thus the thickness of the GHSZ, particularly in margins with structural focusing of hydrocarbons [e.g., Milkov and Sassen , ; Paganoni et al ., ]. Gas hydrates with larger guest molecules such as ethane are more stable and form in wider P‐T regimes if compared to pure methane hydrate [ Sloan and Koh , ].…”

Section: Introductionmentioning

confidence: 99%

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Bottom‐simulating reflector dynamics at Arctic thermogenic gas provinces: An example from Vestnesa Ridge, offshore west Svalbard

et al. 2017

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The Vestnesa Ridge comprises a >100 km long sediment drift located between the western continental slope of Svalbard and the Arctic mid‐ocean ridges. It hosts a deep water (>1000 m) gas hydrate and associated seafloor seepage system. Near‐seafloor headspace gas compositions and its methane carbon isotopic signature along the ridge indicate a predominance of thermogenic gas sources feeding the system. Prediction of the base of the gas hydrate stability zone for theoretical pressure and temperature conditions and measured gas compositions results in an unusual underestimation of the observed bottom‐simulating reflector (BSR) depth. The BSR is up to 60 m deeper than predicted for pure methane and measured gas compositions with >99% methane. Models for measured gas compositions with >4% higher‐order hydrocarbons result in a better BSR approximation. However, the BSR remains >20 m deeper than predicted in a region without active seepage. A BSR deeper than predicted is primarily explained by unaccounted spatial variations in the geothermal gradient and by larger amounts of thermogenic gas at the base of the gas hydrate stability zone. Hydrates containing higher‐order hydrocarbons form at greater depths and higher temperatures and contribute with larger amounts of carbons than pure methane hydrates. In thermogenic provinces, this may imply a significant upward revision (up to 50% in the case of Vestnesa Ridge) of the amount of carbon in gas hydrates.

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Section: Discussionmentioning

confidence: 99%

Section: Discussionmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

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Bottom‐simulating reflector dynamics at Arctic thermogenic gas provinces: An example from Vestnesa Ridge, offshore west Svalbard

et al. 2017

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“…Thus, the term BGHSZ is to be interpreted as base of structure I gas hydrate, or in line with previous suggested terminology, BS I GHSZ [e.g., Boswell et al, 2016;Paganoni et al, 2016]. We further want to emphasize that our study is not concerning the detailed and local gas hydrate concentrations with depth and any localized variations therein.…”

Section: 1002/2017gc006805mentioning

confidence: 99%

“…6. Borneo (passive continental margin) i. Gumusut drilling project [Paganoni et al, 2016] [Suess et al, 1988]. 8.…”

Section: Introductionmentioning

confidence: 99%

Observed correlation between the depth to base and top of gas hydrate occurrence from review of global drilling data

Riedel

Collett

2017

Geochem Geophys Geosyst

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A global inventory of data from gas hydrate drilling expeditions is used to develop relationships between the base of structure I gas hydrate stability, top of gas hydrate occurrence, sulfate‐methane transition depth, pressure (water depth), and geothermal gradients. The motivation of this study is to provide first‐order estimates of the top of gas hydrate occurrence and associated thickness of the gas hydrate occurrence zone for climate‐change scenarios, global carbon budget analyses, or gas hydrate resource assessments. Results from publically available drilling campaigns (21 expeditions and 52 drill sites) off Cascadia, Blake Ridge, India, Korea, South China Sea, Japan, Chile, Peru, Costa Rica, Gulf of Mexico, and Borneo reveal a first‐order linear relationship between the depth to the top and base of gas hydrate occurrence. The reason for these nearly linear relationships is believed to be the strong pressure and temperature dependence of methane solubility in the absence of large difference in thermal gradients between the various sites assessed. In addition, a statistically robust relationship was defined between the thickness of the gas hydrate occurrence zone and the base of gas hydrate stability (in meters below seafloor). The relationship developed is able to predict the depth of the top of gas hydrate occurrence zone using observed depths of the base of gas hydrate stability within less than 50 m at most locations examined in this study. No clear correlation of the depth to the top and base of gas hydrate occurrences with geothermal gradient and sulfate‐methane transition depth was identified.

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Theoretical modeling insights into elastic wave attenuation mechanisms in marine sediments with pore‐filling methane hydrate

2017

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The majority of presently exploitable marine methane hydrate reservoirs are likely to host hydrate in disseminated form in coarse grain sediments. For hydrate concentrations below 25–40%, disseminated or pore‐filling hydrate does not increase elastic frame moduli, thus making impotent traditional seismic velocity‐based methods. Here, we present a theoretical model to calculate frequency‐dependent P and S wave velocity and attenuation of an effective porous medium composed of solid mineral grains, methane hydrate, methane gas, and water. The model considers elastic wave energy losses caused by local viscous flow both (i) between fluid inclusions in hydrate and pores and (ii) between different aspect ratio pores (created when hydrate grows); the inertial motion of the frame with respect to the pore fluid (Biot's type fluid flow); and gas bubble damping. The sole presence of pore‐filling hydrate in the sediment reduces the available porosity and intrinsic permeability of the sediment affecting Biot's type attenuation at high frequencies. Our model shows that attenuation maxima due to fluid inclusions in hydrate are possible over the entire frequency range of interest to exploration seismology (1–106 Hz), depending on the aspect ratio of the inclusions, whereas maxima due to different aspect ratio pores occur only at sonic to ultrasound frequencies (104–106 Hz). This frequency response imposes further constraints on possible hydrate saturations able to reproduce broadband elastic measurements of velocity and attenuation. Our results provide a physical basis for detecting the presence and amount of pore‐filling hydrate in seafloor sediments using conventional seismic surveys.

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Structure II gas hydrates found below the bottom‐simulating reflector

Cited by 71 publications

References 75 publications

Bottom‐simulating reflector dynamics at Arctic thermogenic gas provinces: An example from Vestnesa Ridge, offshore west Svalbard

Bottom‐simulating reflector dynamics at Arctic thermogenic gas provinces: An example from Vestnesa Ridge, offshore west Svalbard

Observed correlation between the depth to base and top of gas hydrate occurrence from review of global drilling data

Theoretical modeling insights into elastic wave attenuation mechanisms in marine sediments with pore‐filling methane hydrate

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