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

Abstract

Abstract Itis 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 warming 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:The hydrate cements grain contacts and thus significantly stiffens the sediment; andThe 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 BSR. Introduction Methane hydrate is a solid substance consisting of gas and water molecules that chemically interact to form an ice-like crystalline structure. This structure is stable within the temperatures that prevail in the ocean floors at water pressures of tens of bars (water depth of a few hundred meters). Thus when methane is available, the pore space of shallow marine sediments will contain not only brine but also solid hvdrate. Gas to solid volumetric ratio in covered hydrate sarnple may be as large as 170 (Fig. 1). Because temperature in the sediments increases with depth, these hydrate become unstablea few hundred meters below the ocean floor (Kvenvolden, 1993). This lower boundary gives rise to the so-called bottom simulating seismic reflectors (BSRS) which parallel the seafloor (Fig. 2). These BSRS are most likely caused by a sharp elastic contrast between the sediments containing methane hydrate and the underlying sediments without them. Often free methane is trapped below the BSRS, enhancing or even dominating the seismic reflections, and implying that the hydrated zone must be impermeable. The amount of concentrated carbon tied up in earth's subsea methane hydrates significantly exceeds that in all other hydrocarbon deposits on earth. This is why gas hydrates are beginning to be recognized as a potential future energy source. Furthermore, being so abundant, hydrates may significantly contribute or even control the methane balance in the atmosphere thus affecting global climate and buffering its change (Kvenvolden, 1993).