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

Abstract

In October 1997, new vertical-incidence, wide-angle reflection and refraction data were obtained on the convergent continental margin off Pakistan. The most prominent feature across the entire margin into the Gulf of Oman is a bottom simulating reflector (BSR). This reflection generally coincides with 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 gas is trapped between stratigraphic layers. Joint inversion of wide-angle and refraction data yielded a ~200 m thick low-velocity zone at BSR depth. Using the Born expansion, band-limited impedance logs were calculated from deconvolved and scaled single-channel data, suggesting that velocity at the BSR drops by ~200 m s-1. Some 40 km towards the coast, CDP4400 of Minshull and White (1989) yielded a velocity inversion of about 600 m s-1, indicating that BSR characteristics vary significantly across the continental margin off Pakistan. Beneath the Gulf the BSR was sampled at a similar subbottom depth, where seismic bright spots occur. However, its reflection amplitude is unusually low and seismic stratigraphy beneath the BSR did not show enhanced reflectivities. Joint traveltime inversion of wide-angel 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 sediments incorporated into the accretionary prism. We believe that these variations are related to an evolutionary process due to tectonic uplift and ongoing sedimentation as sediments are incorporated into the accretionary prism. Sedimentation and uplift produce an upward migration of isotherms and therefore 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. CDP4400 was located farther landwards at a depth 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 stability field. In addition, tectonic dewatering may accumulate hydrate at the base of the stability field by advection of methane from depth.