A numerical model for the formation of gas hydrate below the seafloor

We develop a numerical model to predict the volume and distribution of gas hydrate in marine sediments. We consider the environment of a deep continen tal margin where sedimentation adds organic material to the region of hydra te stability. Conversion of the organic material to methane by bacteria pro motes hydrate formation and depletes the supply of organic carbon. We deriv e mass balance equations for the volume of hydrate and gas bubbles in the s ediments and account for the changing concentration of dissolved methane an d salts in the pore fluid. The effects of sediment compaction and the assoc iated fluid flow are explicitly modeled. Allowances for deeper sources of f luid are also described, though we focus on the case of an idealized passiv e margin where carbon is input solely through sedimentation. The numerical calculations indicate that the key parameters in this model are the rate of sedimentation, the quantity and quality of the organic material, and a rat e constant that characterizes the vigor of biological productivity. Model p redictions for conditions that are representative of the Blake Ridge are co mpared with observations from Ocean Drilling Program Leg 164. We obtain a v ery good match to the observed chlorinity profile, including the region bel ow the stability zone, without invoking any extraneous sources of freshenin g. We also predict that hydrate is unlikely to occupy more than 7% of the p ore volume, in good agreement with observed estimates.