Geophysical signatures of mud mounds at hydrocarbon seeps on the Louisiana continental slope, northern Gulf of Mexico

Sager, William W.; MacDonald, Ian R.; Hou, Rousheng

doi:10.1016/s0025-3227(03)00097-5

Cited by 87 publications

(71 citation statements)

References 34 publications

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“…A laser line scan mosaic ( Fig. 3a) was geo-rectified in the ER-MAPPER ® software environment Sager et al, 2003). The GC233 brine pool is situated on a mound, which is elevated 6 to 8 m above the surrounding seafloor and has a basal diameter of approximately 100 m (Fig.…”

Section: Geologic and Geophysical Settingmentioning

confidence: 99%

“…The GB425 site lies on the western edge of the Auger basin Fig. 4), an intraslope basin that contains economically significant hydrocarbons in the Auger, Cardamom, and Macaroni fields and is bordered to the west by tabular salt bodies (McGee et al, 1993;Sager et al, 2003). The GB425 mud volcano is situated at a small, active diatreme on the southern margin of a flat-topped mound located mid-way along a major fault.…”

Section: Geologic and Geophysical Settingmentioning

confidence: 99%

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Geophysical and geochemical signatures of Gulf of Mexico seafloor brines

et al. 2005

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Abstract. Geophysical, temperature, and discrete depthstratified geochemical data illustrate differences between an actively venting mud volcano and a relatively quiescent brine pool in the Gulf of Mexico along the continental slope. Geophysical data, including laser-line scan mosaics and subbottom profiles, document the dynamic nature of both environments. Temperature profiles, obtained by lowering a CTD into the brine fluid, show that the venting brine was at least 10 • C warmer than the bottom water. At the brine pool, thermal stratification was observed and only small differences in stratification were documented between three sampling times (1991, 1997 and 1998). In contrast, at the mud volcano, substantial temperature variability was observed, with the core brine temperature being slightly higher than bottom water (by 2 • C) in 1997 but substantially higher than bottom water (by 19 • C) in 1998. Detailed geochemical samples were obtained in 2002 using a device called the "brine trapper" and concentrations of dissolved gases, major ions and nutrients were determined. Both brines contained about four times as much salt as seawater and steep concentration gradients of dissolved ions and nutrients versus brine depth were apparent. Differences in the concentrations of calcium, magnesium and potassium between the two brine fluids suggest that the fluids are derived from different sources, have different dilution/mixing histories, or that brine-sediment reactions are more important at the mud volcano. Substantial concentrations of methane, ammonium, and silicate were observed in both brines, suggesting that fluids expelled from deep ocean brines are important sources of these constituents to the surrounding environment.

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Section: Geologic and Geophysical Settingmentioning

confidence: 99%

Section: Geologic and Geophysical Settingmentioning

confidence: 99%

Geophysical and geochemical signatures of Gulf of Mexico seafloor brines

et al. 2005

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“…The identification and delineation of seep sites on the ocean floor often arises from the detection of anomalously high acoustic backscatter on side scan sonar or multibeam echosounder recordings (Hovland, 1991;Hovland, 1992;Orange et al, 2002;Van Rensbergen et al, 2002;Johnson et al, 2003;Sager et al, 2003;Shoji et al, 2005;Gay et al, 2006;Klaucke et al, 2006;Rollet et al, 2006). High-acoustic backscatter is caused by the enhanced acoustic impedance or roughness contrast between certain regions of the seafloor and their surroundings (Blondel and Murton, 1997).…”

Section: Introductionmentioning

confidence: 99%

Anomalous sea-floor backscatter patterns in methane venting areas, Dnepr paleo-delta, NW Black Sea

et al. 2008

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The relation between acoustic seafloor backscatter and seep distribution is examined by integrating multibeam backscatter data and seep locations detected by single-beam echosounder. This study is further supported by side scan sonar recordings, high-resolution 5 kHz seismic data, pore-water analysis, grain-size analysis and visual seafloor observations. The datasets were acquired during the 2003 and 2004 expeditions of the EC-funded CRIMEA project in the Dnepr paleo-delta area, northwestern Black Sea.More than 600 active methane seeps were hydro-acoustically detected within a small (3.96 km by 3.72 km) area on the continental shelf of the Dnepr paleo-delta in water depths ranging from -72 m to -156 m. Multibeam and side scan sonar recordings show backscatter patterns that are clearly associated with seepage or with a present dune area. Seeps generally occur within medium-to highbackscatter areas which often coincide with pockmarks.High-resolution seismic data reveal the presence of an undulating gas front, i.e. the top of the free gas in the subsurface, which domes up towards and intersects the seafloor at locations where gas seeps and medium-to high-backscatter values are detected. Pore-water analysis of 4 multi-cores, taken at different backscatter intensity sites, shows a clear correlation between backscatter intensity and dissolved methane fluxes. All analyzed chemical species indicate increasing anaerobic oxidation of methane (AOM) from medium-to high-backscatter locations. This is confirmed by visual seafloor observations, showing bacterial mats and authigenic carbonates formed by AOM. Grain-size analysis of the 4 multi-cores only reveals negligible variations between the different backscatter sites.Integration of all datasets leads to the conclusion that the observed backscatter patterns are the result of ongoing methane seepage and the precipitation of methane-derived authigenic carbonates (MDACs) caused by AOM. The carbonate formation also appears to lead to a gradual (self-)sealing of the seeps by cementing fluid pathways/horizons followed by a relocation of the bubble-releasing locations. KeywordsMethane seeps; acoustic seafloor backscatter; anaerobic oxidation of methane; bacterial mats; pockmarks; methane-derived authigenic carbonates

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“…Seafloor mounds with associated gas hydrate deposits have been detected and studied across the northern Gulf of Mexico continental slope using 2D and 3D seismic data, multibeam bathymetry, piston coring, and observations from manned submersibles (e.g., MacDonald et al, 2003;Roberts and Carney, 1997;Roberts, 2001;Sager et al, 2003Sager et al, , 2004Sassen et al, 1999). Roberts and Carney (1997) conclude that the mounds result from episodic, fault-controlled fluid venting and associated sediment expulsion.…”

Section: Introductionmentioning

confidence: 99%

A photographic and acoustic transect across two deep-water seafloor mounds, Mississippi Canyon, northern Gulf of Mexico

Hart

Hutchinson

Gardner

et al. 2008

Marine and Petroleum Geology

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a b s t r a c tIn the northern Gulf of Mexico, a series of seafloor mounds lie along the floor of the Mississippi Canyon in Atwater Valley lease blocks 13 and 14. The mounds, one of which was drilled by the Chevron Joint Industry Project on Methane Hydrates in 2005, are interpreted to be vent-related features that may contain significant accumulations of gas hydrate adjacent to gas and fluid migration pathways. The mounds are located w150 km south of Louisiana at w1300 m water depth.. New side-scan sonar data, multibeam bathymetry, and near-bottom photography along a 4 km northwest-southeast transect crossing two of the mounds (labeled D and F) reveal the mounds' detailed morphology and surficial characteristics. Mound D, w250 m in diameter and 7-10 m in height, has exposures of authigenic carbonates and appears to result from a seafloor vent of slow-to-moderate flux. Mound F, which is w400 m in diameter and 10-15 m high, is covered on its southwest flank by extruded mud flows, a characteristic associated with moderate-to-rapid flux. Chemosynthetic communities visible on the bottom photographs are restricted to bacterial mats on both mounds and mussels at Mound D. No indications of surficial gas hydrates are evident on the bottom photographs.Published by Elsevier Ltd.

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Geophysical signatures of mud mounds at hydrocarbon seeps on the Louisiana continental slope, northern Gulf of Mexico

Cited by 87 publications

References 34 publications

Geophysical and geochemical signatures of Gulf of Mexico seafloor brines

Geophysical and geochemical signatures of Gulf of Mexico seafloor brines

Anomalous sea-floor backscatter patterns in methane venting areas, Dnepr paleo-delta, NW Black Sea

A photographic and acoustic transect across two deep-water seafloor mounds, Mississippi Canyon, northern Gulf of Mexico

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