Natural gas hydrates on the continental slope off Pakistan: constraints from seismic techniques

In October 1997, new vertical-incidence, wide-angle reflection and refracti on data were obtained on the convergent continental margin off Pakistan. Th e most prominent feature across the entire margin into the Gulf of Oman is a bottom simulating reflector (BSR). This reflection generally coincides wi th the depth predicted for the base of the gas hydrate stability field. On the accretionary prism the BSR is a clear reflection with inverse polarity, and strata beneath the BSR show enhanced reflectivities, suggesting that g as is trapped between stratigraphic layers. Joint inversion of wide-angle a nd refraction data yielded a similar to 200 m thick low-velocity zone at BS R depth. Using the Born expansion, band-limited impedance logs were calcula ted from deconvolved and scaled single-channel data, suggesting that veloci ty at the BSR drops by similar to 200 m s(-1) Some 40 km towards the coast, CDP4400 of Minshull & White (1989) yielded a velocity inversion of about 6 00 m s(-1), indicating that BSR characteristics vary significantly across t he continental margin off Pakistan. Beneath the Gulf the BSR was sampled at a similar subbottom depth, where se ismic bright spots occur. However, its reflection amplitude is unusually lo w and seismic stratigraphy beneath the BSR did not show enhanced reflectivi ties. Joint traveltime inversion of wide-angle and refraction data revealed continuously increasing velocities, suggesting that an adequate supply of methane for forming free gas and gas hydrate is mainly confined to sediment s incorporated into the accretionary prism. We believe that these variation s are related to an evolutionary process due to tectonic uplift and ongoing sedimentation as sediments are incorporated into the accretionary prism. S edimentation and uplift produce an upward migration of isotherms and theref ore lead to a dissociation of gas hydrates, which releases methane gas and thus causes an enhancement of the BSR. Our survey imaged BSR properties of the abyssal plain and within the first slope basin at water depths of 3300 m and 2490 m, respectively. CDP3400 was located farther landwards at a dept h of 1730 m within a buried and uplifted and hence more evolved slope basin . We therefore suggest that, in the accretionary wedge, tectonic uplift and ongoing sedimentation cause hydrate recycling by upward migration of the s tability field. In addition, tectonic dewatering may accumulate hydrate at the base of the stability field by advection of methane from depth.