Characterization of a hydrate reservoir

Christine Ecker

christin@sep.stanford.edu

ABSTRACT

I use marine seismic data from the Blake Outer Ridge region to characterize the lateral distribution of a methane hydrate reservoir. Detailed amplitude versus offset (AVO) analysis of the data, combined with velocity analysis and seismic impedance inversion is used to explore the extend and characteristic of the bottom simulating reflector (BSR) associated with the base of the hydrate stability field. The results suggest a strong correlation between strong BSR reflections and the presence of a low velocity zone. This is indicative of the presence of free gas beneath the hydrate. Weaker BSR amplitudes occur in areas of decreased velocity contrasts and ``fractured'' appearance of the BSR. The P-impedance inversion results in strong contrasts both at the seafloor and the BSR due to the significant velocity contrasts there. The S-wave impedance contrast, on the other hand, shows a stronger contrast in the vicinity of the BSR than at the seafloor. The BSR contrast has a rather irregular appearance and seems to be dominated by events cutting in from underneath. These events are significantly weaker in the P-impedance contrast and are partly not even visible. AVO analysis of the amplitudes at three different locations along the BSR resulted in decreasing P-wave velocities across the BSR and S-wave velocities that are either increasing, decreasing or unchanged. Possible reasons for these local variations of the shear wave velocity might be either patchy hydrate saturation or patchy gas saturation beneath the hydrate. Furthermore, the hydrate might be fractured, thus causing the properties of the hydrate to change locally.