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

Abstract

Abstract. 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.

Sonic waveform attenuation in gas hydrate‐bearing sediments from the Mallik 2L‐38 research well, Mackenzie Delta, Canada

“…seismic profile (VSP) data by Sakai [1999]. It is consistent with the results of the only previously recorded V s log in hydrate-bearing sediments on the Blake Ridge [Guerin et al, 1999].…”

“…By comparison, waveform amplitude analyses in hydrate-bearing sediments are rare. Guerin et al [1999] described the apparent attenuation of dipole waveforms in the sediments of the Blake Ridge but rough hole conditions and low hydrate concentration (<15% of the pore space) limited interpretation to a qualitative discussion. They observe dipole wave amplitudes to be lower in hydrates, but there is no apparent effect on monopole waveforms.…”

“…Both methods rely on time-honored models and agree overall on the hydrate concentration around the Mallik well. We use a ''quick look'' Archie method similar to the method described by Collett [1998] and Guerin et al [1999] to calculate the hydrate saturation values in Figure 2f. In this method the resistivity data recorded in formations without hydrate are used as a baseline profile (R 0 (z)) from which any increase is attributed to the presence of hydrate.…”

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

“…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].…”

Large gas hydrate accumulations on the eastern Nankai Trough inferred from new high‐resolution 2‐D seismic data

“…The porosity was extracted from the JNOC hole porosity log [Matsumoto, 2002]. Unless grain contact cementation by hydrate is hypothesized to occur at a low hydrate concentration (less than 10%), most recently developed models, either empirical or based on physical theories yield comparable variations of Vp and Vs with hydrate concentration [Guerin et al, 1999]. The Tinivella [1999] approach was used to compute Vp and Vs for partially hydrate bearing or gas saturated sediments (Table 1).…”