8. Reservoir Characterization
Q Estimation From VSP Data in Methane Hydrate-bearing Sediments
Jun Matsushima
The University of Tokyo, Japan.
Abstract
In order to estimate the amount of methane hydrates (MHs) which form in marine sediments at water depths greater than a few hundred meters, using not only velocity information but also attenuation information can provide much more information about MH-bearing sediments. While the presence of MH increases seismic velocity in the host sediment, recent works on sonic logging data and cross-well seismic data show that sonic and seismic waveforms are also significantly affected by the presence of MH. It is important to accumulate attenuation measurements on MH-bearing materials in various frequency bands and in various geologic environments in order to determine the physical mechanisms responsible for these phenomena. In this paper, data from two vertical seismic profiles (VSPs) from the Nankai Trough exploratory well, offshore Tokai, central Japan, are used to estimate compressional attenuation in methane hydrate (MH)-bearing sediments at seismic frequencies of 30-110 Hz. We compare spectral ratio and centroid frequency shift methods for measuring attenuation. To isolate intrinsic attenuation from total attenuation, attenuation is computed from multiples using one-dimensional synthetic VSP data from sonic and density logs. The use of two different measurement methods and two VSPs recorded at just 100 m separation provides an opportunity to validate the attenuation measurements. Macroscopically, the peaks of highest attenuation in the seismic frequency range correspond to low-saturation gas zones. This interpretation is supported by raw-level data such as the centroid frequency. No significant compressional attenuation was observed in MH-bearing sediments at seismic frequencies. In contrast, high compressional attenuation zones in the sonic frequency range (10-20 kHz) are affected by the presence of methane hydrates in the same well locations. Thus, this study demonstrated the frequency-dependence of attenuation in MH-bearing sediments.
Last modified: Fri May 19 18:05:16 2006