On the Importance of Advective Versus Diffusive Transport in Controlling the Distribution of Methane Hydrate in Heterogeneous Marine Sediments

The occurrence of methane hydrate in marine reservoirs often correlates with the physical properties of the host sediments. High hydrate saturations (>60% of the pore volume) found in association with coarser-grained strata have been attributed to both enhanced advective transport through more permeable sediment layers and to perturbations in phase equilibrium related to pore space geometry that results in increased diffusive transport. To assess the relative importance of these mechanism in controlling hydrate occurrence, we develop a 1-D model for hydrate growth along dipping, coarse-grained layers embedded in a fine-grained sediment package. We explicitly account for pore size effects on methane solubility and permeability-driven variations in fluid flux. We show how the vertical distribution of hydrate varies in response to changes in grain size and rates of fluid advection, sedimentation, and in situ methane production. We then use our model to simulate centimeter-scale variations in hydrate saturation observed at Walker Ridge Block 313, Hole H in the Gulf of Mexico. We find that the largest concentrations of hydrate are controlled by diffusion, while increased advective methane supply favors more distributed growth throughout high-permeability regions. Our results hold promise for using well log-derived estimates of hydrate saturation to infer sediment properties and the sources and rates of methane supply during reservoir emplacement.