High-resolution geophysical and geochemical data collected at two focused flux sites in the northern Gulf of Mexico reveal significant heterogeneity in salinities, thermal regimes, and fluid flow. Upward advection beneath mud mounds carries large amounts of gas, along with heat and dissolved ions, to shallow levels. Despite the ample gas supply, gas hydrate formation is inhibited by local thermal and salinity regimes. Published estimates of gas hydrate abundance in this region should be revised sharply downward in the vicinity of focused flux features.