Gas-Driven Tensile Fracturing in Shallow Marine Sediments

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 have 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 showed that fracturing is favored in the shallowest 38 m of sediment when the clay-sized fraction is 0.2, but fracturing may be possible to a depth of 132 m below seafloor (mbsf) with a clay-sized fraction of 0.5 and to a depth of nearly 500 mbsf with a clay-sized fraction of 0.7. 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. Gas-driven tensile fracturing is probably a common occurrence in the upper 10-20 mbsf regardless of clay-sized fraction, does not require much gas (far less than 10% gas saturation), and is not necessarily an indication of hydrate dissociation.