R/V MARION DUFRESNE
NORTHERN GULF OF MEXICO
JULY 1 - 18, 2002
September 25, 2002
The northern Gulf of Mexico hosts numerous seafloor (<7 m subbottom) occurrences of gas hydrate. The seafloor is dominated by salt-tectonic basin structures, high sedimentation rates (about 40 cm/yr), and complex late Neogene stratigraphy with common seafloor failures. Natural oil and gas seeps are abundant, usually associated with fault conduits resulting in numerous hydrocarbon vents, often capped by gas hydrate when the seeps are within the hydrate stability zone. While gas hydrate is relatively common at the seafloor, the lack of bottom simulating reflections on seismic records suggest that gas hydrate at depth is largely absent. Thus, it is unknown if there are significant gas hydrate accumulations in reservoir sediments away from faults. To address this question a cruise was conducted with the IMAGES (International Marine Past Global Changes Study) and PAGE (Paleoceanography of the Atlantic and Geochemistry) programs aboard the Marion Dufresne in July 2002.
Seventeen giant piston cores, up to 38 m long, four giant box cores up to 10 m long, and four gravity cores up to 9 m long, were recovered along seismic reflection transects in widely different geologic environments in water depths ranging from about 600-1300 m. The transects were designed to extend from known seafloor gas hydrate occurrences across the adjacent basin to background sediments away from any gas venting sites. Gas hydrate was recovered in four cores from previously known venting sites in subbottom depths of about 3 to 9 m, but was not found in adjacent basins. Seventeen successful passive heat flow measurements to depths of 17 m were made in conjunction with hundreds of pore water chemistry measurements to better understand the thermal and geochemical regimes in sediments and their relation to gas hydrate formation and occurrence. Our results confirm the presence of gas hydrate in vent-related near-seabed sediments, however infer that gas hydrate is not common in adjacent sedimentary basins in the northern Gulf of Mexico.
Metadata from the cruise including navigation, personnel, core locations, etc., is available on the internet at the USGS web site: http://walrus.wr.usgs.gov/infobank/d/d102gm/html
A giant piston-coring cruise with multiple objectives was recently completed by a multinational group of scientists to better understand natural-gas-hydrate distribution across the continental slope of the northern Gulf of Mexico. The cruise, partly funded by the U.S. Department of Energy, originated in Cancun, Mexico, on July 1 and ended in Tampa, FL, on July 18. Gas hydrate, an ice-like crystalline solid containing high concentrations of methane, is a potential energy resource. It is also a hazard to hydrocarbon exploration and production, and may influence global climate change.
Although the amount of gas hydrate in the natural environment is inferred to be enormous, little is known about its distribution in sea-floor sediment or even exactly how it forms. Exploring these and other questions was among the goals of the recently completed coring cruise conducted jointly by The Institut Polaire Francais, Paul-Emile Victor (IPEV) and the USGS aboard the 120-m-long French research vessel Marion Dufresne.
The Gulf of Mexico is unique in the world for having significant amounts of both biogenic gas hydrate (hydrate formed in shallow sediment by microbial-sourced production of methane) and thermogenic-sourced hydrate (hydrate formed by natural gas seeping into the shallow subsurface sediment). Gas hydrate in the northern Gulf of Mexico has generally been recovered only from the upper few meters of sediment from shallow coring or submersible vessels, typically on sea-floor mounds.
The primary focus of this cruise was to determine if gas hydrate exists away from obvious sea-floor gas-hydrate mounds at fault conduits to sedimentary basins adjacent to structural features that conduct hydrocarbon fluids to the seabed. These results will be correlated with seismic records to assess the potential for using such records to locate sub-seafloor gas hydrate in the northern Gulf of Mexico.
Much of the northern Gulf of Mexico subbottom consists of late Neogene hemipelagic fine-grained sediment. It is an extremely complex region shaped by numerous geologic processes including: salt tectonics and diapirism, faulting, and high sedimentation rates. These processes have produced morphological features characterized by numerous basins formed by salt withdrawal with adjacent ridge features, mass wasting (slumps and debris flows), pockmarks, hydrocarbon vents, seafloor mud volcanoes, gas-hydrate mounds, authigenic carbonate outcrops, chemosynthetic communities, and submarine canyons and fans.
Gas hydrate has been inferred from Bottom Simulating Reflections (BSR's) in many continental margin areas around the world. However, they are noticeably absent in the northern Gulf of Mexico. This may be in part because of the complex geologic nature of the subsurface resulting in complex geothermal gradients and hypersaline pore waters.
Core Site Selection
The core locations for the Marion Dufresne cruise were selected using seismic records obtained from two previous DOE-funded USGS cruises (USGS cruise ID's: M1-98-GM and G1-99-GM) over the upper- and middle-continental slope of the northern Gulf of Mexico. The 1998 survey, conducted in the Mississippi Canyon area, collected high resolution multichannel seismic reflection data using an air gun and water gun with a 250-m long streamer (Fig 1). As much as 2 km of penetration was obtained with 5 m of resolution. A single-channel deep-tow Huntec boomer survey, with about 200 m penetration and 0.25 m resolution, was also conducted during this cruise.
A number of different systems were used during the 1999 cruise in the Garden Banks and Green Canyon areas of the western Gulf of Mexico. Multichannel high-resolution seismics were acquired using the same water gun and streamer as during the previous cruise. In addition, Huntec deep-tow boomer, deep-tow side-scan, and chirp-seismic data were also recorded. The chirp seismic data penetrated to about 40 m subbottom with a resolution of approximately 0.1 m.
We also used 3-D seismic data from the Minerals Management Service (MMS) center in New Orleans to select some sites. The cores typically were obtained along existing seismic lines that originated at known gas hydrate sites and extend into or along the nearby basin.
Fig. 1. Previous (1998 and 1999) cruise areas conducted by the USGS and 2002 cruise area conducted aboard the R/V Marion Dufresne.
Marion Dufresne Core Recovery
Coring from the Marion Dufresne was conducted within four regions of the northern Gulf of Mexico:Tunica Mound, Bush Hill and the east and west flanks of Mississippi Canyon, to investigate the effect of different geologic settings and subseafloor conditions on the formation and presence of gas hydrate (Table 1, Fig. 2). Seventeen giant Calypso piston cores of up to 38 m in length and 4 box cores were collected. 500 m of piston core was recovered and 32 m of box core sediment was obtained for USGS-related studies. In addition, gravity cores were obtained mainly to acquire heat flow information from sensors attached to the perimeter of the core barrel Detailed station location maps for each area are in Appendix A.
We also collected 9 m-long box cores from Pigmy and Orca Basins for measuring anthropogenic contaminant input to the northern Gulf of Mexico from the Mississippi River during the Holocene. In addition the Marion Dufresne obtained two cores in Tampa Bay for the USGS Tampa Bay project to elucidate the climate history of Tampa Bay.
Fig. 2. Core locations for 2002 Marion Dufresne cruise.
Marion Dufresne Scientific Activities
A hull-mounted 3.5 kHz chirp and a 12 kHz seismic system were used to collect data at all core sites. Real-time bottom reflection data was used to fine tune the core coordinates at sea. Multibeam bathymetry was also obtained at all core sites (Fig. 3).
Fig. 3. Multibeam bathymetric image produced at sea.
The Marion Dufresne has a unique, unobstructed starboard main deck that allows the deployment and recovery of IPEV’s “Calypso” corer. That piston-coring system, driven by a 6-ton weight stand (Fig. 4), has recently obtained cores as long as 64.5 m.
Fig. 4. Calypso piston corer.
A newly-designed 11-m long box core, 25 cm by 25 cm in area, was also used to recover large sediment samples from the shallow subbottom (Fig. 5). The box cores were driven into the seafloor using the same weight stand as the piston corer.
Fig. 5. A box corer being deployed.
Core handling and gas hydrate recovery
We used a safety protocol on most of the 17 recovered USGS piston cores (Fig. 6) to insure that hazardous gas overpressures did not develop. As the piston core liner was being removed from the metal core barrel, the liner surface temperature was monitored using an infrared temperature sensor. Holes were drilled at about a 1-m spacing to relieve any potential hazardous gas pressures and to collect gas samples for later isotopic analyses. Digital temperature probes were inserted into the holes after the gas pressure dissipated. They were monitored to find low thermal anomalies suggestive of gas hydrate dissociation. Much of the recovered sediment was highly gas charged as evidenced by the abrupt expulsion of sediment from the holes drilled in the core liner (Fig. 7). The cores were also checked for hydrogen sulfide gas to insure safe levels were not exceeded on deck or in the inboard laboratories.
The 4 box cores (Fig. 8) did not develop overpressures, however they were also checked for hydrogen sulfide. Only one subsampled core had to be removed from a laboratory because H2S exceeded the safety threshold.
Fig. 6. Piston core liner removed from the core barrel. Notice the temperature probes used to determine potential thermal anomalies associated with gas hydrate dissociation.
Fig 7. Gas expansion pushed "mud worms" out of the core liner and onto the deck through holes drilled to relieve pressure.
Fig. 8. A box core being subsampled. Sediment in the foreground is being placed into a pore water squeezer vessel. This core, MD02-2563C2 located on a large diapir in the MC853 lease area was saturated with liquid hydrocarbons.
Gas hydrate was recovered in four different cores at a maximum subbottom depth of about 8.2 m. The gas hydrate in core MD02-2565 was disseminated within fine-grained sediment and was associated with the presence of nearby hydrocarbons, whereas the gas hydrate recovered in core MD02-2569 consisted of massive veins that filled the entire cross section of the core liner (Fig. 9). Those pieces were typically at least 2 cm thick. This implies that we cored continuous layers of gas hydrate of some unknown lateral extent.
Fig. 9. Gas hydrate chunks recovered from a Calypso giant piston core MD02-2569.
The generation of gas caused by hydrate dissociation was spectacularly demonstrated when the upper several meters of core 2565 blew vertically out the end of the core barrel when the weight stand was removed, flew at least 10 m into the air, and landed in the gulf waters next to the ship. The gas hydrate remained on the surface of the water because of its low density and floated away as it dissociated. The recovered hydrate samples were preserved in liquid nitrogen or a –80°C freezer for future shore-based laboratory testing.
Core procedures and sample analyses
The cores are being used to study the distribution of natural gas hydrate using geochemical analyses of pore water and gas samples, and physical property measurements obtained from the sediment.
Whole-round samples, 10-cm to 25-cm long, were cut from the ends of core sections every 1.5 m. Most of the sediment from the whole rounds were placed in two types of squeezers to obtain pore water for chlorine and sulfate ion analyses (Fig. 9). A total of 458 pore water samples were obtained during the cruise. Other sediment was frozen for microbiological studies. The remaining intact whole-round sections were stored at 4°C for later shore-based stress history and other geotechnical studies. Intact whole-round samples were also acquired for isotopic gas geochemistry analyses to determine the source of the gas.
Fig 9. Pore water squeezing laboratory.
After the whole-round samples were removed on deck, the remaining sections were measured for thermal conductivity after temperature equalization. Then the cores were longitudinally split. The archive half was stratigraphically described, recorded for color with a spectrophotometer, digitally photographed, and run through a Multi-Sensor Track (MST) for recording density, P-wave velocity, magnetic susceptibility, and electrical properties.
The working half of the core was brought into the physical properties laboratory for the determination of electrical resistivity, P-wave velocity, water content, and shear strength by mini-lab vane, torvane, and pocket penetrometer. Approximately 1100 water content samples were acquired during the cruise. Additional grain density, grain size, and mineralogy analyses will be conducted on land.
Heat flow measurements
Gravity cores with staggered temperature sensors and recorders (Fig. 10) were used to determine 17 deep (about 17 m) heat-flow profiles near the piston-core sites (Table 1). Results indicate that widely varying geothermal gradients exist across the northern Gulf of Mexico, an important observation for defining the subbottom extent of gas-hydrate stability.
Fig. 10. Gravity core with staggered outrigger heat flow sensors
Shipboard participation included: USGS (Menlo Park); USGS, (Woods Hole); USGS (St. Petersburg); Monterey Bay Aquarium Research Institute (MBARI); University of Victoria, BC, Canada; College of William and Mary; Moscow State University; University of Tokyo; and Texas A & M University.
Considerable at-sea help was provided by an international group of about 40 scientists under the IMAGES (International Marine Past Global Changes Study) and PAGE (Paleoceanography of the Atlantic and Geochemistry) programs . The IMAGES program is an international effort to understand the mechanisms and consequences of climatic changes using the oceanic sedimentary record.
Collaborations are ongoing with Institut Francais pour la Recherche et la Technologie Polaires on heat-flow studies, with IPEV on paleomagnetic studies, and with the Laboratoire des Sciences du Climat et de l’Environnement on physical-property profiles.
The USGS cores collected in the gulf have been archived at Texas A&M University.
Abbreviations and Nomenclature
BSR Bottom Simulating Reflection
C2 Calypso Squared, box corer, max. sample dimensions: 25 cm x
25 cm x 11m
Grav Gravity core, sample dimensions: 10 cm diameter x 10 m (approx.)
GHF Gravity Heat Flow corer, gravity corer with staggered welded outriggers for holding temperature sensors and recorders, used to acquire heat flow measurements
IMAGES International Marine Past Global Changes Study program
IPEV Institut Polaire Francais, Paul-Emile Victor
MBARI Monterey Bay Aquarium Research Institute
MMS Minerals Management Service
PC Calypso Piston Core, sample dimensions: 10 cm diameter x
64.5 m (max. recovered core length to date)
PAGE Paleoceanography of the Atlantic and Geochemistry program
USGS United States Geological Survey
Table 1. Core locations and information.
Core location maps
Core and sample list
Number of cores recovered:
Gravity heat flow: 21 penetrations at 9 stations
Length of core sediment recovered:
Box: 32 m (approx)
Gravity: 17 m
Gravity heat flow: 57 m
Piston: 500 m
Number of samples acquired:
Pore water: 458
Water content/geotechnical: 1100 (approx.)
CHIRP SEISMIC/ MULTIBEAM, VERY-HIGH-RESOLUTION SINGLE
CHANNEL, BRAND: THALES UNDERWATER SYSTEMS, MODEL:
SEAFALCON II, MEDIA: DIGITAL, CODE: S, COMMENT:
CHIRP SEISMIC/ MULTIBEAM, MULTIBEAM BATHYMETRY,
BRAND: THALES UNDERWATER SYSTEMS, MODEL: SEAFALCON II,
MEDIA: DIGITAL, CODE: M, COMMENT: HULL-MOUNTED SYSTEM
PISTON CORE, SEAFLOOR CORE, BRAND: CALYPSO, MODEL: ,
MEDIA: CODE: G
GRAVITY CORE, SEAFLOOR CORE, BRAND: , MODEL: , MEDIA: ,
CODE: GHF, COMMENT: HEAT FLOW MEASUREMENTS
BOX CORE, SEAFLOOR CORE, BRAND: , MODEL: , MEDIA: , CODE:
C2, COMMENT: NON-TRADITIONAL BOXCORER
available on the internet at the USGS web site: http://walrus.wr.usgs.gov/infobank/d/d102gm/html