Sediments with gas hydrates: Internal structure from seismic AVO

Ecker, Christine; Dvorkin, Jack; Nur, Amos

doi:10.1190/1.1826475

Cited by 35 publications

(57 citation statements)

References 8 publications

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“…The water‐saturated bulk and shear moduli are calculated using the Gassmann [1951] equation. The bulk and shear moduli of the dry frame of sediments without gas hydrate can be calculated from the modified Hashin‐Shtrikman‐Hertz‐Mindlin theory [ Dvorkin et al , 1999; Ecker et al , 1998]. To estimate gas hydrate saturation, it is assumed gas hydrate is part of the dry sediment matrix and will reduce the porosity of the matrix.…”

Section: Resultsmentioning

confidence: 99%

Elevated gas hydrate saturation within silt and silty clay sediments in the Shenhu area, South China Sea

Wang¹,

Hutchinson²,

Wu³

et al. 2011

J. Geophys. Res.

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[1] Gas hydrate saturations were estimated using five different methods in silt and silty clay foraminiferous sediments from drill hole SH2 in the South China Sea. Gas hydrate saturations derived from observed pore water chloride values in core samples range from 10 to 45% of the pore space at 190-221 m below seafloor (mbsf). Gas hydrate saturations estimated from resistivity (R t ) using wireline logging results are similar and range from 10 to 40.5% in the pore space. Gas hydrate saturations were also estimated by P wave velocity obtained during wireline logging by using a simplified three-phase equation (STPE) and effective medium theory (EMT) models. Gas hydrate saturations obtained from the STPE velocity model (41.0% maximum) are slightly higher than those calculated with the EMT velocity model (38.5% maximum). Methane analysis from a 69 cm long depressurized core from the hydrate-bearing sediment zone indicates that gas hydrate saturation is about 27.08% of the pore space at 197.5 mbsf. Results from the five methods show similar values and nearly identical trends in gas hydrate saturations above the base of the gas hydrate stability zone at depths of 190 to 221 mbsf. Gas hydrate occurs within units of clayey slit and silt containing abundant calcareous nannofossils and foraminifer, which increase the porosities of the fine-grained sediments and provide space for enhanced gas hydrate formation. In addition, gas chimneys, faults, and fractures identified from three-dimensional (3-D) and high-resolution two-dimensional (2-D) seismic data provide pathways for fluids migrating into the gas hydrate stability zone which transport methane for the formation of gas hydrate. Sedimentation and local canyon migration may contribute to higher gas hydrate saturations near the base of the stability zone.

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Section: Resultsmentioning

confidence: 99%

Elevated gas hydrate saturation within silt and silty clay sediments in the Shenhu area, South China Sea

Wang¹,

Hutchinson²,

Wu³

et al. 2011

J. Geophys. Res.

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“…Results from the OBC line in the Storegga area may indicate that a noncementation model should be applied to estimate the hydrate concentration of sediments at the base of the gas hydrate stability zone along this line. The equations suggested by Ecker et al [1998] and Dvorkin et al [2000] are used to tentatively estimate the concentration of free gas and of gas hydrate from a noncementing gas hydrate model. We assume in the modeling, for simplicity, and due to lack of petrophysical information from the study area, a sediment with 60% porosity, and a pore space with 75% clay and 25% sand (quartz).…”

Section: Discussionmentioning

confidence: 99%

“…Both theoretical and experimental rock physics modeling suggest that the effect of gas hydrate on physical properties of shallow unconsolidated sediment will depend on the microscale gas hydrate distribution within the sediment pores. If gas hydrate is located at grain contacts, it will act as a load‐bearing component of the dry frame to strongly increase the stiffness of the sediment, and even small amounts of hydrate will according Dvorkin and Nur [1993] and Ecker et al [1998] cause a drastic increase in both V p and V s . Gas hydrate which is located away from grain contacts as a part of the sediment pore fluid will not affect the stiffness of the dry frame, and will cause a more evenly, but not as dramatic increase in V p and practically no change in V s .…”

Section: Discussionmentioning

confidence: 99%

Multicomponent ocean bottom cable data in gas hydrate investigation offshore of Norway

et al. 2003

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[1] A multicomponent seismic ocean bottom cable (OBC) line from the northern flank of the Storegga Slide offshore of Norway is the first OBC line ever acquired over a gas hydrate-related bottom simulating reflector (BSR). The seismic data are of very good quality, and display strong P-S converted wave reflections for the whole sediment column. All P-S reflections follow the sediment layers and are therefore interpreted to be caused by increased shear modulus and density due to sediment compaction. No P-S reflections are associated with the BSR along this line, indicating that the gas hydrate-bearing sediments at the base of the hydrate stability zone are not stiff enough to increase the shear modulus of the sediments to produce P-S converted wave reflections. Seismic interpretation and seismic stratigraphic modeling suggest that the BSR in this area is a contrast in compressional wave velocity only, mainly caused by small amounts of free gas confined to dipping sediment layers beneath the BSR. Gas hydrate saturation above the BSR, tentatively estimated to be about 1-1.5% of the sediment pore space, is most likely too low to distinguish between different models for microscale gas hydrate distribution.

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“…A variety of theoretical models or experimental models have been proposed to estimate gas hydrate saturation, such as Wyllie et al [15] time average equation with the seismic velocity [16–18], the effective medium theory [19–23], Biot-Gassmann theory model [24–27], compression wave (P wave) velocity of thermal-elastic theory [28], the three-phase equation (TPE) [29–34], and velocity model theory based on the two-phase theory (TPT) model [35]. Moreover, Tinivella et al [36] made a research to compare the TPT model with the TPE model for evaluating gas hydrate saturations in marine sediments, and the comparison showed that the two theoretical approaches were in very good agreement.…”

Section: Introductionmentioning

confidence: 99%

Acoustic Velocity Log Numerical Simulation and Saturation Estimation of Gas Hydrate Reservoir in Shenhu Area, South China Sea

Xiao

Zou

Xiang

et al. 2013

The Scientific World Journal

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Gas hydrate model and free gas model are established, and two-phase theory (TPT) for numerical simulation of elastic wave velocity is adopted to investigate the unconsolidated deep-water sedimentary strata in Shenhu area, South China Sea. The relationships between compression wave (P wave) velocity and gas hydrate saturation, free gas saturation, and sediment porosity at site SH2 are studied, respectively, and gas hydrate saturation of research area is estimated by gas hydrate model. In depth of 50 to 245 m below seafloor (mbsf), as sediment porosity decreases, P wave velocity increases gradually; as gas hydrate saturation increases, P wave velocity increases gradually; as free gas saturation increases, P wave velocity decreases. This rule is almost consistent with the previous research result. In depth of 195 to 220 mbsf, the actual measurement of P wave velocity increases significantly relative to the P wave velocity of saturated water modeling, and this layer is determined to be rich in gas hydrate. The average value of gas hydrate saturation estimated from the TPT model is 23.2%, and the maximum saturation is 31.5%, which is basically in accordance with simplified three-phase equation (STPE), effective medium theory (EMT), resistivity log (Rt), and chloride anomaly method.

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Sediments with gas hydrates: Internal structure from seismic AVO

Cited by 35 publications

References 8 publications

Elevated gas hydrate saturation within silt and silty clay sediments in the Shenhu area, South China Sea

Elevated gas hydrate saturation within silt and silty clay sediments in the Shenhu area, South China Sea

Multicomponent ocean bottom cable data in gas hydrate investigation offshore of Norway

Acoustic Velocity Log Numerical Simulation and Saturation Estimation of Gas Hydrate Reservoir in Shenhu Area, South China Sea

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