Gassy sediment occurrence and properties: Northern Gulf of Mexico

Abstract

Free gas is ubiquitous at shallow sediment depths of tt~e northern margin of the Gulf of Mexico. Gassy sediment patches are between 250 and 500 m in horizontal size. Often the gassy layers are within 100 m from the sea floor and arc only a few meters thick. Both biogenic and Ihermogenic gas hydrates have been recove,-ed. Stability values of temperature and pressure indicate that hydrates can exist in water depths less than 500 m. Gassy sediment geoacouslic parameter values ark not well constrained because of a lack of concurrent measurements of acoustic properties and sediment gas content. For Gulf of Mexico gassy sediment, some reported in situ values of sound speed are reduced hy an order of magnitude below values for watcr saturated sediments. More commonly, sound speed is reduced from water saturated sediment values by only 15 to 50 percent.

Mud volcanoes, gas chimneys, pockmarks and mounds in the Nile deep-sea fan (Eastern Mediterranean): geophysical evidences

“…Depending on their nature and maturity (Sager, MacDonald, & Rousheng, 2003) cold seeps are also characterized by distinctive back-scatter signatures. A general characteristic is that fluid emissions, and the authigenic carbonate crusts associated with cold seeps, generally scatter the acoustic energy, generating thus typical high back-scatter responses, often characterized by circular or flow-like shapes (Anderson & Bryant, 1990;Behrens, 1988;Blondel & Murton, 1997;Bryant et al, 1990;Johnson & Helferty, 1990;Sager et al, 2003). Analyses of cores, recently obtained by Sager et al (2003) in the Gulf of Mexico, tend to indicate that enhanced bottom returns result also from seep matter (oil and gas) introduced into the near seafloor sediments, increasing sound scattering of the seafloor or the uppermost sediment cover.…”

Northern Gulf of Mexico continental slope

“…Such gassy sediments have a very low sound speed and cause tremendous problems with the acquisition and processing of seismic data. In addition, gassy sediments are patchy in nature causing major problems in applying proper corrections (Anderson and Bryant 1990). In addition to gas in bubble phase, Brooks and others (1984) discovered gas hydrates in upper continental slope depths in nearsurface sediments.…”

Long‐range side‐scan sonar imaging of hydrocarbon seeps and sediment mass wasting on the Louisiana continental slope, Gulf of Mexico

“…Because many of our observations of carbonate crusts and hydrates are concentrated at active seep sites where chemosynthetic organism communities are found, it is unclear whether carbonates and hydrates are the cause most sonar bright patches. Interstitial gas may also cause strong reflections depending on the gas bubble size and concentration (Anderson and Bryant, 1990).…”