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Press Release 05-046
Sediments in Northern Gulf of Mexico Not Right for Methane Gas Hydrate Formation
Areas off Louisiana coast are out of gas
March 24, 2005
Marine sediments in the northern Gulf of Mexico are likely too warm and salty to hold the amount of methane gas hydrates – a potential energy resource – originally thought to exist in the ocean floor there.
According to a recent report, high-resolution geophysical and geochemical data for two representative sites off the coast of New Orleans, La., suggest previous estimates for the region should be revised sharply downward.
Methane gas hydrates are touted as a potential new source of natural gas, but scientists are also studying them because they may contribute to global warming and could represent a threat to deep-sea petroleum production.
"We found that conditions are not favorable for the formation of methane gas hydrates at these sites because of the geology of the northern Gulf of Mexico, which consists of salt domes that one can think of as mushroom clouds of salt that rise buoyantly through sediments,” said Carolyn Ruppel, a geophysicist at the Georgia Institute of Technology and lead author on the paper. Ruppel is currently on leave and serving as a program director at the National Science Foundation (NSF), which funded the study.
“The thermal properties of salt make the sediments hotter there, and the heat, coupled with the presence of the salt in pore spaces, makes it harder to form gas hydrates,” she said.
The research was a collaboration among scientists at Georgia Tech, Rice University and the Scripps Institution of Oceanography. The researchers continue to analyze their data, which they collected during a two-week research cruise in October 2002, to measure the gas hydrates at the sites, but the deposits are likely thin or non-existent, Ruppel said.
Methane is produced during the decomposition of organic material in the sediment or by thermal processes similar to those responsible for the formation of oil. As the methane moves through the sediment, it combines with water at the low temperatures and high pressures beneath the ocean to produce an ice-like solid. Methane gas hydrates exist along continental margins worldwide, most in sediments tens to hundreds of feet below the sea floor in waters more than 1500 feet deep. These hydrates exist as disseminated deposits, chunks several centimeters across and sometimes as concentrated layers.
In the northern Gulf of Mexico, previous research on potential methane gas hydrates shared the same temperature and geology, but did not consider the impact of salt on hydrate formation, Ruppel noted.
She and her colleagues used overlapping methods to characterize the two sites they studied. They used high-resolution seismic equipment from the lab of coauthor and Georgia Tech scientist, Daniel Lizarralde, to image the sea floor and to find conduits through which fluids could flow.
Geochemist and co-author Gerald Dickens, of Rice University, analyzed water samples from sediment cores extracted from the sea floor. He and his coworkers developed chemical profiles that revealed, for example, salt and sulfate concentrations. Sulfate measurements are important for understanding the biology of the system, specifically the interaction of microbes that produce sulfate and methane.
Ruppel made high-resolution measurements of temperature and the rate of fluid flow in the sediments. Collaborators from the Scripps Institution of Oceanography also collected data on fluid flux from the sea floor--an important constraint on the hydrology of the system and its potential for hydrate formation, Ruppel noted.
“There’s a lot of research on hydrates going on,” Ruppel said. “Ultimately, these studies around North America and the world will shed more light on how much hydrate is out there. I hope that will get us closer to answering the question about whether hydrates are a viable energy resource. It’s going to take some time. If we do learn it’s a viable resource, then we’ll have to face a new set of issues on how to actually produce energy from this resource.”
Producing methane from gas hydrates faces some daunting challenges. A key question is whether it would take more energy to extract the gas hydrates than the gas may provide, Ruppel added.
The study was published in the March 15 issue of the journal Geophysical Research Letters.
-NSF-
Media Contacts
Cheryl L. Dybas, NSF (703) 292-7734 cdybas@nsf.gov
Jane Sanders, Georgia Institute of Technology (404) 894-2214 jane.sanders@edi.gatech.edu
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