Characterization of in situ elastic properties of gas hydrate-bearing sediments on the Blake Ridge

Online Access: Get full text
doi: 10.1029/1999JB900127
Author(s): Guérin, Gilles; Goldberg, David; Meltser, Aleksandr
Author Affiliation(s): Primary:
Lamont Doherty Earth Observatory of Columbia University, Borehole Research Group, Palisades, NY, United States
Volume Title: Journal of Geophysical Research
Source: Journal of Geophysical Research, 104(B8), p.17,781-17,796. Publisher: American Geophysical Union, Washington, DC, United States. ISSN: 0148-0227
Note: In English. 56 refs.; illus., incl. 2 tables, sect., geol. sketch map
Summary: During Ocean Drilling Program Leg 164, shear sonic velocity and other geophysical logs were acquired in gas hydrate-bearing sediments on the Blake Ridge to characterize the very distinct seismic signature of such formations: anomalous low amplitudes overlying a strong bottomsimulating reflector (BSR). A comparison of the bulk moduli derived from the logs to standard elastic consolidation models shows that the sediments are overconsolidated above the BSR at 440 meters below seafloor (mbsf) because of the presence of hydrates. Below the bottom of the thermodynamic hydrate stability zone at ∼520 mbsf, the high compressibility of the formation and the attenuation of the monopole sonic waveforms are typical of sediments partially saturated with free gas. Between these two depths, gas hydrate and free gas seem to coexist. Within the Gas hydrate stability zone, we estimate the amount of gas hydrates using different models based on theories for wave scattering in multiphase media and for grain cementation. In close agreement with measurements made on discrete in situ samples, the latter describes most accurately the interactions between the matrix, the pore fluids, and the hydrates. This model indicates that 5 to 10% of the pore space is occupied by hydrates deposited uniformly on the surface of the grains. The comparison with Gassmann's model also show that the amount of free gas below the BSR never exceeds 5% of the pore space but is high enough to generate the BSR. The coexistence of free gas and gas hydrates below the BSR may be explained by capillary effects in the smaller pores or by remaining crystalline structures after partial hydrate decomposition. Copyright 1999 by the American Geophysical Union.
Year of Publication: 1999
Research Program: ODP Ocean Drilling Program
Key Words: 20 Geophysics, Applied; 29 Economic Geology, Energy Sources; Atlantic Ocean; Blake-Bahama Outer Ridge; Bottom-simulating reflectors; Capillarity; Cementation; Diagenesis; Elastic properties; Gas hydrates; Geophysical methods; In situ; Leg 164; North Atlantic; Ocean Drilling Program; Ocean floors; Petroleum engineering; Porosity; Reservoir properties; Saturation; Seismic methods; Thermodynamic properties; Velocity; Well-logging
Coordinates: N314708 N325901 W0752807 W0761128
Record ID: 1999067560
Copyright Information: GeoRef, Copyright 2017 American Geosciences Institute.