We present a seismic-scale study of the marine gas hydrate accumulation in Green Canyon Block 955 (GC 955) in the northern Gulf of Mexico and propose a mechanism for the formation of high-concentration gas hydrate accumulations in relatively clay-free marine reservoirs. In GC 955, gas hydrate occurs at a high concentration within an anticlinal structure, in sandy silt layers that are capped by a muddy interval. We interpret that the high hydrate concentration was reached by sustained free gas flow into the reservoir and gradual growth of hydrate. We further interpret that a network of normal faults served as pathways for migration of free gas to the reservoir. These faults also compartmentalized the reservoir and created local leak points though the capping mud layers. Well logs indicate that the hydrate-bearing interval is directly underlain by a water-bearing reservoir in some reservoir segments, whereas in others, gas is present below the hydrate. We interpret that where water is present beneath the hydrate-bearing interval, there is a leak point within the hydrate stability zone that has drained the gas. Conversely, when gas is present below the base of the gas hydrate stability zone (BHSZ), the leak point is below the BHSZ. We interpret leak points for free gas within the hydrate stability zone, which implies that free gas movement continues within the reservoir above the BHSZ. We conclude that gas hydrate saturation increased gradually by sustained free gas migration into reservoir segments above the BHSZ.