Geologic Controls on Gas Hydrate Accumulations and Associated Geohazards

Abstract Gas hydrates and shallow gas are common hazards encountered during drilling. They also are important components of the global carbon cycle and a potential energy resource. Predicting the occurrence of free gas and hydrates, particularly highly concentrated accumulations, is therefore important for scientific and safety purposes. Hydrates accumulate following principles that are analogous to conventional hydrocarbons: the hydrate system requires at least one gas source, migration pathways, a suitable reservoir, and a seal. Here, we apply the hydrate system framework to several high-concentration accumulations to demonstrate the geologic controls on the different aspects of the system and the overall propensity of certain locations to host abundant hydrate and free gas. We find that a deep source of gas is typically required in these settings, even if the gas has microbial origins. This gas may be highly pressured and can generate fractures that reach from the base of the hydrate stability zone to the seafloor. At the Cape Fear Slide offshore North Carolina, newly acquired high-resolution 2D multichannel seismic data show details of the association among gas, hydrates, gas venting, and slope failure. Overall our analysis demonstrates the importance of considering the site-specific details of the local hydrate system in any shallow hazard analysis.