The Northern Gulf of Mexico includes the passive continental margin, shelf, and slope of southern North America. As compared to other passive margins, two features of the northern Gulf make this area a prime location for hydrate formation: 1) the presence of the Mississippi River delta, and 2) salt tectonism.

Side-scanning radar image of the geologic interpretation of Gulf of Mexico deposits.
Courtesy of USGS, GLORIA mapping project.

The eastern portion of the Gulf, which includes the Florida platform and escarpment, receives very little clastic sediment, and is therefore characterized by slow rates of carbonate deposition. In contrast, the western Gulf is dominated by siliciclastic deposition, primarily from the Mississippi River delta, and contains thick and rapidly-accumulating sequences of sandstones, siltstones, and shales rich in organic material. In addition, the low-wave action in the Gulf, the concentration of sediment supply at a single point source (the Mississippi River delta), and oversteepening of seafloor gradients has increased the potential for coarse clastics to reach deep-water environments as compared to other passive margins.

Although the northern Gulf of Mexico represents a passive continental margin, salt tectonism has resulted in extensive deformation of the sedimentary layers. During the Jurassic, the Gulf became isolated from the oceans, and a thick sequence of salt (Louann Salt) accumulated as the Gulf's water evaporated. As sediments were later deposited in the basin, their weight caused the salt to mobilize, much as toothpaste is squeezed out of a toothpaste tube. Today, salt diapirs, pillows and stocks, and listric faults are all indicators of Louann Salt movement due to overburden stresses. This deformation provides numerous pathways for the migration of thermogenic methane into shallow zones where hydrate is stable. The Sigsbee Escarpment in the western portion of the Gulf marks the southern limit of salt movement within the basin.

Another feature of the Gulf that drives the current interest in hydrates is the presence of an active oil and gas exploration industry. Because hydrate dissociation has been linked to seafloor instability, oil and gas companies drilling in offshore environments are interested in detecting hydrates so they may take the necessary precautions to reduce the risks associated with drilling through the unstable hydrate layer.

In most marine locations where hydrates are known to exist, a well-defined BSR (Bottom Simulating Reflector) marks the base of the HSZ (Hydrate Stability Zone) on seismic reflection data. However, in the Gulf of Mexico, well-imaged BSRs are rare; the complex shallow stratigraphy and structure interferes with the seismic signature of the BSR.

Gulf of Mexico with block boundaries

Beginning in the 1990s, the U.S. Geologic Survey, Minerals Management Service, Department Of Energy, and the National Oceanic and Atmospheric Agency, collaborated with academia and oil and gas companies to work on methods for better hydrate detection. In 1997 and 1998 scientists collected over 1450 km of single and multi-channel high-resolution seismic data in Bryant and Mississippi Canyons. In addition, a local ocean-bottom seismometer survey was conducted in Mississippi Canyon. In 1999, 1400 km of single and multi-channel high-resolution data plus 500 km of side-scan and chirp seismic was collected in Green Canyon and Garden Banks. Analysis of this data led to improved geophysical methods for detecting hydrates beneath the sea floor. High amplitude “shingled” reflections and confined zones of disturbed sediments from recent slumps are thought to be good indications of hydrate presence.

Seismic line over possible mud volcano.
Reproduced from USGS Open-file Report 99-570, A. Cooper, D. Twichell,
and P. Hart investigators.

Hydrate mound on sea floor

The accumulation of gas hydrates in sea floor mounds is another distinctive occurrence in the Gulf of Mexico. Seismic data have shown high amplitude fault planes and seismically diffuse areas of gas migration beneath the mounds. These fault planes and gas chimneys provide a conduit for thermogenic gases from deeper formations to pass through the HSZ and form hydrate mounds directly on the sea floor. Samples of hydrate with thermogenic gas have been retrieved from the Jolliet Field, Green Canyon in the Gulf of Mexico.

Deep-sea dive with Alvin submersible	Yellow hydrate Courtesy of Michael Peccin i

Deep-sea dives with manned submersibles like Alvin have brought researchers face-to-face with hydrates on the sea floor. Hydrate mounds have been found in the Gulf of Mexico with various colors such as orange, white, yellow, or red. Although no one knows the origin of the colors, a likely hypothesis points toward inclusions of oil, bacteria, and minerals.

Ice Worm

Tubeworms

One of the most exciting discoveries in the brief history of hydrate science occurred on July 12th, 1997, when two researchers aboard the submersible Johnson SeaLink II discovered a new species of marine worm in 550 meters of water in the Gulf of Mexico. Informally known as the "ice worm" it was discovered living on a hydrate mound capping an oil and gas seep. The ice worm feeds off hydrocarbons and bores through hydrates for shelter and sustenance.

Subsequently, a variety of organisms have been found near oil and gas seeps. Some feed directly off hydrocarbons (i.e. ice worm and bacteria) and are known as chemosynthetic organisms. Other organisms, such as the tubeworm and mussels, form symbiotic relationships with the chemosynthetic bacteria. The host provides oxygen while the bacteria supplies food to the mussel.