Forecasting Diffusive Formation of Free-gas Methane Layers in Sea Sediments

We study diffusive transport of aqueous methane in sea sediments, which is important for formation of methane hydrate and gas deposits and has even been claimed to be principal methane transport mechanism for some geological systems. The isothermal diffusion law (Fick's law), D X = − ∇ J , which is employed in the literature for investigations on methane hydrate, is shown to significantly deviate from the actual diffusion law, accounting for the non-Fickian contributions due to (i) temperature gradient (by virtue of thermodiffusion or “Soret effect”) and (ii) gravitational segregation. We focus our study on the mass transport in the bubbly (free-gas) zone and report that molecular diffusion (with non-Fickian contributions) can solely lead to formation of the free-gas zone. This free-gas zone can be formed independently of the presence of the hydrate stability zone and/or hydrate in it and can create bottom simulating reflector, not associated with hydrates. We calculate world maps of conditions for the formation of the free-gas zone in diffusion-dominated sediments for expected values of the thermodiffusion constant.