The effect of lithologic heterogeneity on diffusive methane migration in hydrate systems

Abstract

Short-range diffusive gas migration has been proposed as a means of supplying methane to coarse-grained, hydrate-bearing sediments. In this scenario, methane is generated microbially in fine-grained sediments and moves diffusively into adjacent coarse-grained sediments due to a dissolved methane concentration gradient caused by pore size contrasts. Previous work has shown that short-range diffusion is sufficient to supply thin, isolated sands (<1 m thickness) with enough methane to achieve high hydrate saturations (>80%) in systems with a high ratio of clay to sand. Several notable hydrate reservoirs such as those encountered at Daini-Atsumi Knoll in the Nankai Trough, and Walker Ridge Block 313 and Green Canyon Block 955 in the Gulf of Mexico exhibit tens of meters of alternating sand/silt and clay layers that are decimeters in thickness. The coarse-grained layers in these settings are typically filled with hydrate to saturations exceeding 80%. While short-range diffusion is capable of filling thin, isolated sands with hydrate, thicker sands within more heterogeneous systems require mechanisms other than, or in addition to, short-range diffusion. Using 1-D numerical simulations, we demonstrate that short-range diffusive migration of microbial methane is insufficient for reaching consistently high hydrate saturations in such systems. This work shows that a different mechanism for gas transport, such as advective migration, is necessary to achieve these observed hydrate saturations in layered systems.