Abstract
The flow of gas through shallow marine sediments is an important
component of the global carbon cycle and affects methane release to the
ocean and atmosphere as well as submarine slope stability. Seafloor
methane venting is often linked to dissociating hydrates or gas
migration from a deep source, and subsurface evidence of gas-driven
tensile fracturing is abundant. However, the physical links among
hydrate dissociation, gas flow, and fracturing has not been rigorously
investigated. We used mercury intrusion data to model the capillary
drainage curves of shallow marine muds as a function of clay content and
porosity. We combined these with estimates of in situ tensile strength
to determine the critical gas saturation at which the pressure of the
gas phase would exceed the pressure required to generate tensile
fractures. Our work demonstrates that tensile fracturing is more likely
as clay content increases due to decreased pore sizes and increased
capillary pressure, but tends to be restricted to the shallowest portion
of the sediment column (<130 m below seafloor) except when the
clay-sized fraction exceeds 50%. Dissociating hydrate may supply
sufficient quantities of gas to cause fracturing, but this is only
likely near the updip limit of the hydrate stability zone, where release
of methane bubbles from discrete vents is to be expected due to the
combination of weak sediments and significant gas expansion. Gas-driven
tensile fracturing is probably a common occurrence near the seafloor,
does not require much gas, and is not necessarily an indication of
hydrate dissociation.