Sediments with gas hydrates: Internal structure from seismic AVO

Journal Article · Tue Sep 01 04:00:00 EDT 1998 · Geophysics

DOI:https://doi.org/10.1190/1.1444462· OSTI ID:669964

Ecker, C; Dvorkin, J; Nur, A ^[1]

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1. Stanford Univ., CA (United States). Dept. of Geophysics

The authors interpret amplitude variation with offset (AVO) data from a bottom simulating reflector (BSR) offshore Florida by using rock-physics-based synthetic seismic models. A previously conducted velocity and AVO analysis of the in-situ seismic data showed that the BSR separates hydrate-bearing sediments from sediments containing free methane. The amplitudes at the BSR are increasingly negative with increasing offset. This behavior was explained y P-wave velocity above the BSR being larger than that below the BSR, and S-wave velocity above the BSR being smaller than that below the BSR. The authors use these AVO and velocity results to infer the internal structure of the hydrated sediment. To do so, the authors examine two micromechanical models that correspond to the two extreme cases of hydrate deposition in the pore space: (1) the hydrate cements grain contacts and strongly reinforces the sediment, and (2) the hydrate is located away from grain contacts and does not affect the stiffness of the sediment frame. Only the second model can qualitatively reproduce the observed AVO response. Thus inferred internal structure of the hydrate-bearing sediment means that (1) the sediment above the BSR is uncemented and, thereby, mechanically weak, and (2) its permeability is very low because the hydrate clogs large pore-space conduits. The latter explains why free gas is trapped underneath the BSR. The seismic data also indicate the absence of strong reflections at the top of the hydrate layer. This fact suggests that the high concentration of hydrates in the sediment just above the BSR gradually decreases with decreasing depth. This effect is consistent with the fact that the low-permeability hydrated sediments above the BSR prevent free methane from migrating upwards.

• OSTI ID: 669964
• Journal Information: Geophysics, Journal Name: Geophysics Journal Issue: 5 Vol. 63; ISSN GPYSA7; ISSN 0016-8033
• Country of Publication: United States
• Language: English

Related Subjects

03 NATURAL GAS
FLORIDA
FLOW BLOCKAGE
GAS HYDRATES
GEOLOGIC STRUCTURES
METHANE
NATURAL GAS DEPOSITS
PERMEABILITY
SEISMIC SURVEYS