The mechanisms governing distribution of natural gas hydrates in marine sediment systems are not well understood. We focus on a hole in the Gulf of Mexico, Walker Ridge Block 313 Hole H, where a 2.5-m-thick gas hydrate-bearing sand occurs within a 152-m-thick fine-grained mud interval containing gas hydrate in fractures. The gas hydrate-bearing sand is surrounded by two distinct hydrate-free zones (10-m-thick above and 3-m-thick below). We hypothesize that microbial methane generated within the hydrate-free zones diffused into the sand and formed gas hydrate. We show that the amount of methane produced in the hydrate-free zones is likely enough to explain the gas hydrate content of the sand layer. Additionally, we show that there is enough time for dissolved methane to diffuse from the hydrate-free zones into the sand. We conclude that methane transport over significant distances via fluid flow is not required but that microbial methane could migrate short distances to form gas hydrate in the sand layer.