BSR and Methane Hydrates: New Challenges for Geophysics and Rock Physics

摘要

It is generally accepted that solid gas hydrates which form within the uppermost few hundred meters of the sea floor are responsible for so-called Bottom Simulating Reflectors (BSRs) at continental margins. Gas to solid volumetric ratio in recovered hydrate samples may be as large as 170. Consequently, huge amounts of compressed methane (more than twice all recoverable and non recoverable oil, gas, and coal on earth) may exist under earth's oceans. These hydrates are a potential energy resource, they influence global wanning and effect seafloor mechanical stability. It is possible, in principle, to obtain a quantitative estimate of the amount and state of existing hydrates by relating seismic velocity to the volume of gas hydrate in porous sediments. This can be done by linking the elastic properties of hydrated sediments to their internal structure. We approach this problem by examining two micromechanical models of hydrate deposition in the pore space: (1) The hydrate cements grain contacts and thus significantly stiffens the sediment; and (2) The hydrate is located away from grain contacts and only weakly affects the stiffness of the sediment frame. To discriminate between the two models we use the Amplitude Versus Offset (AVO) technique of seismic data processing. This approach allows us to estimate the amount of gas hydrates in the pore space, and also to tell whether the permeability of the hydrated sediment is high or low. The latter is important for determining whether free methane can be trapped underneath a BSR.