THE GROWTH AND DECAY OF HYDRATE ANOMALIES IN MARINE SEDIMENTS

Abstract:

Natural gas hydrates, stored in huge quantities beneath permafrost, and in submarine sediments on the continental shelf, have the potential to become a vital clean-burning energy source. However, clear evidence is recorded in coastal sediments worldwide that past changes in environmental conditions have caused hydrates to become unstable and trigger both massive submarine landslides and the development of crater-like pockmarks, thereby releasing methane into the overlying seawater and atmosphere, where it acts as a powerful greenhouse gas. Arctic permafrost is thawing, and environmental changes can alter ocean circulation to warm the seafloor, causing hydrates to dissociate or dissolve in the sediments beneath. Decades of focused research provide a firm understanding of laboratory conditions under which hydrates become unstable and dissociate, and how hydrate reserves form when microbes convert organic material into methane, which can also dissolve and be carried by pore waters into the hydrate stability zone. Despite these advances, many key questions that concern both the resource potential of hydrates and their role in causing environmental geohazards, are intimately tied to the more poorly understood behavior of hydrate anomalies, which tend to be concentrated in the large pores of sand layers and form segregated lenses and nodules in muds. We present simple models designed to unravel the importance of the diverse physical interactions (i.e. flow focusing, free-gas infiltration, and pore-scale solubility effects) that help control how hydrate anomalies form. Predicted hydrate distributions are qualitatively different when accumulation in anomalies is supplied primarily by: 1. aqueous flow through sediments with enhanced permeability, 2. free-gas transport high above the three-phase stability boundary, or 3. diffusive transport along solubility gradients associated with pore-scale effects. We discuss examples that illustrate each of these distinct generation modes, in hopes of providing a framework for interpreting field observations of hydrate anomalies and their geomechanical properties in terms of the history of environmental forcing that led to their development.