Free gas at the base of the gas hydrate zone in the vicinity of the Chile triple junction

Bangs, N. L.; Sawyer, Dale S.; Golovchenko, X.

doi:10.1130/0091-7613(1993)021<0905:fgatbo>2.3.co;2

Cited by 182 publications

(129 citation statements)

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“…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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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

View full text Add to dashboard Cite

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“…It was also assumed that low-amplitude reflections observed above some BSRs [Shipley et al, 1979] could be used to assess the concentration of gas hydrate within the stability zone [Lee et al, 1992]. However, the use of differing seismoacoustic acquisition techniques, such as multichannel seismics, ocean bottom seismometers [Katzman et al, 1994;Vanneste et al, 2002], deep tow seismics [Gettrust et al, 1999], vertical seismic profiling [Bangs et al, 1993;Holbrook et al, 1996], and down hole logging [Guerin et al, 1999;Lee and Collett, 2001], has led to a greater understanding of the nature of the BSR. The BSR is now considered to result from free gas in sedimentary layers beneath the hydrate stability zone (HSZ) [Holbrook et al, 1996;Korenaga et al, 1997;MacKay et al, 1994;Minshull et al, 1994;Singh et al, 1993].…”

Section: Nature and Distribution Of Marine Gas Hydratesmentioning

confidence: 99%

A laboratory investigation into the seismic velocities of methane gas hydrate‐bearing sand

2005

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[1] Remote seismic methods, which measure the compressional wave (P wave) velocity (V p ) and shear wave (S wave) velocity (V s ), can be used to assess the distribution and concentration of marine gas hydrates in situ. However, interpreting seismic data requires an understanding of the seismic properties of hydrate-bearing sediments, which has proved problematic because of difficulties in recovering intact hydrate-bearing sediment samples and in performing valid laboratory tests. Therefore a dedicated gas hydrate resonant column (GHRC) was developed to allow pressure and temperature conditions suitable for hydrate formation to be applied to a specimen with subsequent measurement of both V p and V s made at frequencies and strains relevant to marine seismic investigations. Thirteen sand specimens containing differing amounts of evenly dispersed hydrate were tested. The results show a bipartite relationship between velocities and hydrate pore saturation, with a marked transition between 3 and 5% hydrate pore saturation for both V p and V s . This suggests that methane hydrate initially cements sand grain contacts then infills the pore space. These results show in detail for the first time, using a resonant column, how hydrate cementation affects elastic wave properties in quartz sand. This information is valuable for validating theoretical models relating seismic wave propagation in marine sediments to hydrate pore saturation.Citation: Priest, J. A., A. I. Best, and C. R. I. Clayton (2005), A laboratory investigation into the seismic velocities of methane gas hydrate-bearing sand,

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“…The negative polarity of the BSR indicates that the reflection results from a decrease in seismic velocity and/or density at this depth. Many studies have concluded that the reflection results from the combined effect of hydrate within the stability zone, which increases seismic velocity, and free gas below the stability zone, which decreases seismic velocity [e.g., Bangs et al, 1993;Singh et al, 1993;Tréhu et al, 1995;Holbrook et al, 1996;Korenaga et al, 1997;Yuan et al, 1999;Pecher et al, 2001;Holbrook, 2001].…”

Section: Introductionmentioning

confidence: 99%

Seismic and seafloor evidence for free gas, gas hydrates, and fluid seeps on the transform margin offshore Cape Mendocino

et al. 2003

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[1] Seismic data and seafloor samples indicate the presence of free gas, gas hydrate, and fluid seeps south of the Gorda Escarpment, a topographic feature that marks the eastern end of the Gorda/Pacific transform plate boundary southwest of Cape Mendocino, California. In spite of high sedimentation rates and high biological productivity, direct or indirect indicators of gas hydrate presence had not previously been recognized in this region, or along transform margins in general. Gas is indicated by a bottom simulating reflection (BSR) observed near the Gorda Escarpment, by ''bright spots'' and ''gas curtains'' scattered throughout the sedimentary basin to the south, and by d C and d18 O isotopes of carbonates, which are similar to those recovered from other hydrate-bearing regions. The BSR reflection coefficient of À0.13 ± 0.04 and interval velocities as low as 1.38 km/s indicate that free gas is present beneath the BSR. Local shallowing of the BSR toward the north facing Gorda Escarpment and beneath a channel near the crest suggests fluid flow toward the seafloor. Integrating these various observations, we suggest a scenario in which methane is formed in thick Miocene and Pliocene deposits of organicrich sediments that fill the marginal basin south of the transform fault. Dissolved and free gas migrates toward the escarpment along stratigraphic horizons, resulting in hydrate formation and in channels, slumps and chemosynthetic communities on the face of the escarpment. We conclude that the BSR appears where hydrate-bearing sediments are uplifted because of current triple junction tectonics.

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Free gas at the base of the gas hydrate zone in the vicinity of the Chile triple junction

Cited by 182 publications

References 0 publications

Distribution and Characters of Gas Hydrate Offshore of Southwestern Taiwan

Distribution and Characters of Gas Hydrate Offshore of Southwestern Taiwan

A laboratory investigation into the seismic velocities of methane gas hydrate‐bearing sand

Seismic and seafloor evidence for free gas, gas hydrates, and fluid seeps on the transform margin offshore Cape Mendocino

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