Marine sediments in the northern Gulf of Mexico are likely too warm and salty to hold the amount of methane gas hydrates originally thought to exist, according to researchers from a Georgia Tech, Rice University, and Scripps Institution of Oceanography collaboration.
They reported in the Mar. 15, 2005, issue of Geophysical Research Letters (GRL) that high-resolution geophysical and geochemical data for two representative sites (Fig. 1) off the coast of New Orleans suggest that previous gas hydrates estimates for the region should be revised sharply downward.
“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, an associate professor of geophysics in the Georgia Institute of Technology’s School of Earth and Atmospheric Sciences. “The thermal properties of salt make the sediments hotter, and the heat, coupled with the presence of the salt in pore spaces, makes it harder to form gas hydrates.”
The National Science Foundation funded the collaboration research.
The researchers continue to analyze their data to obtain a quantitative estimate of the gas hydrates at these sites, but Rupple says the deposits are likely to be thin or non-existent.
Researchers collected their data during a 2-week research cruise in October 2002.
Methane gas hydrates
Methane gas hydrates are 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 deepwater petroleum production.
The study explained that methane is produced by the decomposition of organic material in the sediment or by thermal processes similar to processes 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.
Gas hydrates exist along continental margins worldwide, mostly in sediments tens to hundreds of meters below the seafloor in water deeper than 500 m. These hydrates exist as disseminated deposits, chunks several centimeters across and sometimes as concentrated layers, according to the study.
In the northern Gulf of Mexico, previous research on potential methane gas hydrates assumed homogeneous conditions, such as same temperature and geology, and did not consider the impact of salt on hydrate formation, Ruppel noted.
“The methods we used are very good at helping us understand the conditions in the sediment and make a prediction of what’s there,” she explained. “We found conditions that are not compatible with published estimates that imply large methane gas-hydrate deposits” in the northern gulf.
Multidisciplinary approach
Ruppel and her colleagues took a multidisciplinary approach, using overlapping methods to characterize the two sites they studied, she said. They used high-resolution seismic equipment from the lab of Georgia Tech Assistant Professor Daniel Lizarralde to image the seafloor and to find conduits through which fluids could flow.
Geochemist Gerald Dickens of Rice University worked with graduate students to analyze water samples from sediment cores extracted from the seafloor. They 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, according to the study.
Ruppel was responsible for high-resolution heat flow measurements to constrain temperature and the rate of fluid flow in the sediments.
Collaborators from Scripps Institution of Oceanography also collected data on fluid flux from the seafloor in the northern gulf, an important constraint on the hydrology of the system and its potential for hydrate formation, Ruppel noted.
These measurements were not incorporated into the analysis published in GRL.
“When you put all of this together, you get a good idea of the conditions in the gas hydrate reservoir, that is, the sediments that contain gas hydrates,” Ruppel added.
Ongoing gas-hydrates research
Research to characterize methane gas hydrates is ongoing throughout North America and the world, Ruppel noted. She and her colleagues have also studied hydrates at Blake Ridge off the South Carolina coast during normal oceanographic cruises, deep-sea drilling legs, and submersible dives.
On the Blake Ridge, they characterized hydrates both below and on the ocean floor. Through the international Ocean Drilling Program, other scientists have drilled or will soon drill boreholes to explore hydrates off the coasts of Oregon and Vancouver.
Other research focuses on hydrates as potential energy sources, as well as the safety issues related to drilling. These issues include the potential for seafloor destabilization that could occur as hot fluids are pumped up from deep sediments through the hydrate stability zone, Ruppel explained.
“Methane gas hydrates are like ice,” she added. “They can melt and cause the seafloor to collapse.”
The research group will focus on these questions when it participates in spring 2005 in a northern Gulf of Mexico drilling program funded by the US Department of Energy through a joint industry project with ChevronTexaco Inc.
“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. ✦
