Potential distribution of methane hydrates in the world's oceans

Estimates of the magnitudes and spatial distribution of potential oceanic methane hydrate reservoirs have been made from pressure-temperature phase relations and a plausible range of thermal gradients, sediment porosities, and pore fillings taken from published sources, based on two major theories of gas hydrate formation (1) in situ bacterial production and (2) pore fluid expulsion models. The implications of these two models on eventual atmospheric methane release, due to global warming, are briefly examined. The calculated range of methane volumes in oceanic gas hydrates is 26.4 to 139.1 × 10¹⁵ m³, with the most likely value on the lower end of this range. The results for the bacterial model show a preferential distribution of hydrates at mid- to high latitudes, with an equatorial enhancement in the case of the fluid migration model. The latter model also generates a deeper and thicker hydrate stability zone at most latitudes than does the former. Preliminary results suggest that the hydrate distribution predicted by the fluid migration model may be more consistent with observations. However, this preliminary finding is based on a very limited sample size, and there are high uncertainties in the assumptions. The volume of methane hydrate within the uppermost l m of the hydrate stability zone and within 1°-2°C of the equilibrium curve, assuming in situ bacterial generation, is 0.93–6.32 × 10¹² m³, or 0.0035–0.012% of the maximal estimated hydrate reservoir. Nevertheless this volume, if released uniformly over the next 100 years, is comparable to current CH₄ release rates for several important CH₄ sources. Corresponding CH₄ volumes calculated using the fluid migration model are nearly 2 orders of magnitude lower.