Methane Migration Mechanisms for the GC955 Gas Hydrate Reservoir, Northern Gulf of Mexico.

Abstract:

In marine continental margins, high saturations of natural gas hydrate are observed within the gas hydrate stability zone, but the gas migration pathways to the hydrate reservoir are not often clear. We focus on a site in Green Canyon Block 955 (GC 955) in the northern Gulf of Mexico, with a water depth of 2000 m. At GC 955, gas hydrate occurs in a 35 m-thick reservoir with saturations of 79-93% in numerous cm-thick sandy silt layers and little to no gas hydrate in the interbedded cm-thick clayey silt layers. Different sources of information suggest different types of potential methane migration mechanisms at GC 955. For example, relatively high particulate organic carbon (POC) content at the reservoir depth and the microbial methane source indicated by gas chromatography suggest methane may be generated locally and is diffused or advected with saturated aqueous phase to local hydrate deposits. In contrast, the visible deep-rooted faults and interpreted gas pockets along the faults on seismic data that intersect the gas hydrate reservoir suggest that free gas may migrate directly into the reservoir.

We test three different mechanisms to form gas hydrate at GC 955: short-range methane diffusion, short-range methane diffusion coupled upward water advection, and free gas flow. Our numerical simulations show that both the short-range migration and the upward water advection are not enough to form high saturation gas hydrate at GC 955, even with extremely high POC content deposited at the seafloor and high upward water advection velocity. Gas hydrate of 20-94% in saturation is formed in the sandy silt layers by free gas flow. The results indicate that free gas flow is required to form the high saturations of gas hydrate observed at GC 955.