Seismic attributes for identifying gas-hydrates and free-gas zones: application to the Makran accretionary prism

Abstract

The most commonly used marker for gas-hydrates is a bottom simulating reflector or BSR on seismic section. The BSR is not a lithological interface but a physical boundary between the gas-hydrates bearing sediments above and free-gas saturated sediments below. From visual inspection of seismic section, it is hard to demarcate the zones of gas-hydrates and free-gas laden sediments. At many places in the world, BSRs have not been observed but gas-hydrates have been recovered by drilling. Again, BSRs have been identified but no gas-hydrates have been found by drilling. Therefore, we need to study certain attributes to ascertain whether a BSR is related to gas-hydrates or to identify gas-hydrates without a BSR. It is also necessary to demarcate the zones of gas-hydrates and free-gas bearing sediments. Here we compute the reflection-strength, instantaneousfrequency and seismic 'blanking' or reduction in seismic amplitudes from a seismic data set in the Makran accretionary prism and demonstrate that these attributes can be used as important indicators for the exploration of gas-hydrates and free-gas. Presence of gas-hydrates increases the seismic velocity and causes amplitude 'blanking' due to cementation. The landward decrease in 'blanking' window above the BSR implies that the thickness of gas-hydrates bearing sediments decreases landward. Underlying free-gas saturated sediments have lower velocities than host sediments and exhibit high reflection strength due to variation in gas saturation. Presence of free-gas is also indicated by 'shadows' in the instantaneous frequency plot. High reflectivity and low frequency 'shadows' over a large time window below the BSR show a large thickness of alternate gas-rich and gaspoor strata. Coupled with these attributes, the velocity anomaly superimposed on the seismic section shows the lateral extension of gas-hydrates and free-gas bearing sediments.