Samples of vent gas and gas hydrate on the Gulf of Mexico slope were collected by research submersible (∼540 m water depth) and by piston coring (∼1060–1070 m water depth). Although gas hydrate that crops out is transiently unstable, the larger volume of structure II gas hydrate in the gulf is stable or increasing in volume because gas from the subsurface petroleum system is venting prolifically within the gas hydrate stability zone. Vent gas from gas hydrate shows no meaningful molecular evidence of gas hydrate decomposition. Gas hydrate fabrics, mainly vein fillings, are typical of ongoing crystallization. Once crystallized, most hydrocarbons are protected from bacteria within the crystal lattice of gas hydrate. A leaky petroleum system is proposed to be the main source of thermogenic greenhouse gases in the central gulf. Stable gas hydrate sequesters large volumes of greenhouse gases, suggesting that gas hydrate may not be a significant factor in models of climate change at present.