Thermal Constraints from Gas Hydrate Stability from Continental Margins

Jeffrey A. Nunn, Jeffrey S. Hanor, and Anna M. Belanger
Geology and Geophysics, Louisiana State University, Baton Rouge, LA

Gas hydrates are found at or just below the sediment-ocean interface in continental margins settings throughout the world. While gas hydrate occurrence is common, gas hydrates are stable under a fairly restricted range of temperatures. In a purely conductive thermal regime, near surface temperatures depend on basal heat flow, thermal conductivity of sediments, and temperature at the sediment-water interface. Thermal conductivity depends on porosity and sediment composition. Older margins with thin continental crust and coarse grained sediments would tend to be colder. Changes in temperature at the sediment-ocean interface due to ocean circulation or climate change propagate downward into the sediment with time. Short term changes, such as day to night, only penetrate a fraction of a meter. However, temperature changes on time scales of years or decades will penetrate much deeper. Another potentially important control on subsurface temperatures is advective heat transport by discharge of warm waters from depth. Discharge of meteoric waters occurs along many continental margins and can profoundly alter the thermal regime. For example, discharge of topographically driven groundwater along the coast in the Central North Slope of Alaska has increased surface heat flow in some areas by more than 50% over a purely conductive thermal regime. In some areas, such as offshore New England, fresh water has moved hundreds of kilometers offshore along shallow aquifers. Natural discharge of water along some continental margins has decreased over the last century due to overuse of ground water resources for agriculture and industry. Thus, shallow offshore sediments may be cooling with time.

AAPG Search and Discovery Article #90078©2008 AAPG Annual Convention, San Antonio, Texas