Gas Hydrate in the Northern Gulf of Mexico Has Puzzling Characteristics and Could Pose a Hazard to Deep Drilling
Science is often full of surprises, which is what attracts many people to becoming scientists. Studying gas hydrates in the Gulf of Mexico is no exception: the gulf should be full of gas hydrate, given its combination of variablesabundant natural gas, the right water depths, and appropriate temperature regimes.
But strangely, the one generally universal indicator of gas hydrate on seismic-reflection records, a bottom-simulating reflection (BSR), is basically absent on the multitudes of seismic-reflection records from the northern gulf.
Also unusual is that mounds of gas-hydrate deposits are commonly observed on the sea floor during submersible dives, and gas hydrate, which is less dense than water, should float away.
These unexpected characteristics were part of the impetus for the U.S. Geological Survey (USGS) to conduct a recent cruise to collect high-quality seismic-reflection data at selected sites in the gulf.
Gas hydrate is a frozen form of gas and water that occurs in shallow sediment on most continental slopes and rises of the world, at moderate pressures and temperatures and ample gas saturation. The gas is most commonly methane (CH4) but can also be hydrocarbon gases of greater molecular weight, such as ethane (C2H6) and propane (C3H8).
Gas hydrates have the capacity to stiffen near-surface sediment by replacing water-filled pores with hydrate-filled pores. They have been linked to massive slope failures on continental margins (the Storegga Slide off west-central Norway, the largest known submarine slide in the world, is a good example), in which the melting of gas hydrate is suspected of contributing to the destabilization of sediment.
As drilling in the Gulf of Mexico has progressed from shallow-water shelf depths (less than 200 m) to deep-water slope depths (more than 1,000 m), wells have come to penetrate the gas-hydrate-stability zone.
Because drilling can bring warm fluids up from depth, potentially melting the shallower gas hydrate, many researchers and engineers anticipate that drilling through gas hydrate may pose a hazard to the stability of the well, the platform anchors, the tethers, or even entire platforms.
In an effort to help understand the unusual occurrence of gas hydrate in the northern Gulf of Mexico and to gather data relevant to understanding the possible consequences of drilling, the USGS conducted a 14-day cruise aboard Texas A&M University's research vessel Gyre in May 2003.
We collected high-resolution multichannel seismic (MCS)-reflection data. These data, in combination with the three-dimensional MCS data provided by industry, will enable us to map gas-hydrate indicators and interpret the geologic framework around two potential deep-water gas-hydrate sites that may be drilled in spring 2004 by the Joint Industry Program (JIP), an industry consortium led by Chevron-Texaco and partly funded by the U.S. Department of Energy (DOE). Chief scientists were Pat Hart (Menlo Park, CA) and Debbie Hutchinson (Woods Hole, MA).
High-quality data were collected along more than 1,000 km of trackline and were processed onboard during the cruise. Initial interpretation shows a well-developed BSR of restricted extent near the Keathley Canyon site and several potential gas-hydrate vent/mound sites in the Atwater Valley site.
Several notable USGS firsts were achieved at sea: all data were demultiplexed and processed through final stack at sea, providing near-real-time feedback on data quality and results. Also, thanks to the efforts of Pat Hart (Menlo Park), Ray Sliter (Menlo Park), Seth Ackerman (Woods Hole), Brandon Dugan (Woods Hole), and Erika Geresi (University of Mississippi), using two seismic-processing systems and a computer equipped with geographic-information-system (GIS) software, all geometry was defined in true geographic coordinates and merged with the seismic data during the processing sequence.
The Gyre cruise, which is part of a much larger program of gas-hydrate research in the northern Gulf of Mexico, was conducted in coordination with the JIP to assess the hazard that gas hydrates pose to deep-water drilling.
The two primary study areas for the cruise, lease blocks Keathley Canyon 195 and Atwater Valley 14, were selected from six sites that the JIP originally considered for drilling.
The cruise results will be combined with data from additional site-survey work scheduled for August 2003, using near-bottom instrumentation that includes the Deep-Towed Acoustic/Geophysical System (DTAGS), a multichannel seismic instrument from the U.S. Naval Research Lab; heat-flow sensors from Georgia Tech; shallow coring devices from the Naval Research Lab; and instruments for measuring electrical resistivity from the Woods Hole Oceanographic Institution.
Selected tracklines from the USGS Gyre cruise will be reoccupied by these specialized instruments to further characterize the geology and gas hydrates of the potential drillsites. This work also builds on a strong foundation of gas-hydrate research in the Gulf of Mexico that has been established by numerous academic research groups.
in this issue:
Gas Hydrate: Potential Drilling Hazard