Estimation Of Gas Hydrates And Free Gas Concentrations Using Seismic Amplitudes Across The Bottom Simulating Reflector

Abstract

Presence of gas hydrates in the marine sediments elevates both P- (VP) and S-wave (VS) seismic velocities, whereas even a small amount of underlying free gas decreases the P-wave velocity considerably keeping the S-wave velocity negligibly affected. The amplitude variation with offset (AVO) data from which seismic velocities can be extracted contains useful information for the quantitative assessment of gas hydrate and free gas across the bottom simulating reflector (BSR), an interface between the gas hydrates and free gas bearing sediments. Here we present two techniques based on two completely different approaches to the same multi-channel seismic (MCS) reflection data in the Makran accretionary prism in the Arabian Sea to provide an estimate of gas hydrates and free gas across the BSR at two CDP locations along the seismic profile. In the first approach, we calculate the VP and VS and hence the VP/VS ratios using the traveltime inversion followed by a constrained amplitude variation with angle (AVA) modeling of the MCS data, and then quantify the amount of gas hydrate as 7–9% and free gas as 3.5–4.0% based on the Biot–Gassmann Theory modified by Lee (BGTL). The second approach, based on AVO intercept (A) and gradient (B) plots, reveals 8–14% gas hydrate underlain by 2.5–5.0% free gas again by using the BGTL model. Both techniques are robust and easy to implement, and the comparable results show an effective strategy for quantifying gas hydrates and free gas across the BSR with more certainty.