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METHANE HYDRATES AND BSR MODELS

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. They are stable under certain pressure and temperature conditions (Figure 1). Thus, the occurrence of bottom simulating reflectors is restricted to two distinct regions: deep oceanic and polar. In deep oceanic regions, BSRs are found in outer continental margins of slopes and rises where cold bottom water is present. In polar regions, the BSRs are normally associated with permafrost, both onshore in continental sediments and offshore in sediments of the continental shelves.

phase
phase
Figure 1.
Phase diagram showing the boundary between methane gas and methane hydrate. Redrawn after Kvenvolden (1993).
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Two models have been proposed to account for gas hydrate formation and thus the development of BSRs. The first one assumes the local generation of methane from organic material at the depth of the hydrate. Gradually thickening and thus deepening of the methane hydrate zone causes it eventually to subside into a temperature region where hydrate is unstable. Consequently, free gas can be present in this region (Kvenvolden and Barnard, 983a). The BSR is caused by the impedance contrast at the base of the hydrate layer and the top of the gas layer. A second model, on the other hand, supports the formation of methane hydrates 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 microbially at depths below the hydrate stability zone but not at depths sufficient for the formation of thermogenic methane. Thus free gas does not have to be present below the BSR. In this case, the BSR can be the consequence of the impedance contrast between overlaying sediments containing substantial amounts of high-velocity hydrate and underlaying normal velocity brine sediments.


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Next: PREPROCESSING STEPS Up: Ecker & Lumley: AVO Previous: Introduction

2015-03-10