METHANE HYDRATE AND BSRS

A methane hydrate is an ice-like, crystalline lattice of water molecules in which gas molecules are trapped physically without the aid of direct chemical bonds. It forms only under certain pressure and temperature conditions (Figure ) which restrict the BSRs to two regions: polar and deep oceanic. In polar regions, the BSRs are normally associated with permafrost both onshore in continental sediment and offshore in sediment of the continental shelves. In deep oceanic regions, BSRs are found in outer continental margins in sediment of slopes and rises where cold bottom water is present.

 

hydrate
Figure 1 Pressure-Temperature relation for hydrate stability at different compositions. BOR indicates the conditions at the Blake Outer Ridge.

Two models have been proposed to account for gas hydrate formation and the development of BSRs. In the first, methane is assumed to be generated locally from organic material at the depth of the hydrate. As the zone of methane hydrate thickens and deepens, its base eventually subsides into a temperature region where the hydrate is unstable. In this region free gas can occur Kvenvolden and Barnard (1983a). As a consequence, the BSR is caused by the impedance contrast at the base of the hydrate layer and top of the gas layer. The second model assumes that the methane hydrates are formed through the removal of methane from rising pore fluids being expelled upwards from deeper in the sediment section Hyndman and Davis (1992). Most of the methane is generated at depths below the hydrate stability zone but not at depths sufficient for the formation of thermogenic methane. Therefore, the methane hydrates should be concentrated at the base of the stability zone and free gas does not have to be present below the BSR. In this case the BSR is the consequence of the impedance contrast between overlying sediments containing substantial amounts of high-velocity hydrate and underlying normal velocity brine sediments.

The approximate AVO amplitude responses of these BSR reflection models, based on linearized Zoeppritz reflection coefficients, are shown in Figure .

 

avo
Figure 2 Expected BSR AVO curves for hydrate overlaying brine sediment, and hydrate overlaying free methane gas.

A transition from overlying sediment containing methane hydrate to sediment containing free gas, having a low Poisson's ratio, gives a large decrease in Poisson's ratio and thus reflection amplitudes increase rapidly with increasing angle. On the other hand, sediment containing methane hydrate overlying brine sediment does not much change the amplitude with increasing offset. A comparison between this expected behavior with the one observed in this study should give a first order insight into the possible origin of the BSR in the data.