Use of 3-D Seismic to Characterize Seafloor Hydrocarbon, Brine, and Sediment Expulsion from the Slope of the Northern Gulf of Mexico

Shedd, William ¹; Roberts, Harry S.^2
1 Resource Evaluation, Minerals Management Service, New Orlaeans, LA.
² Coastal Studies Institute, Louisiana State University, Baton Rouge, LA.

The MMS proprietary 3-D seismic data base has been used to interpret over 76,000 square miles of the seafloor on the upper and lower slope of the Gulf of Mexico (GOM) in U.S. waters. These interpretations have been used to locate several dozen groundtruthing dive sites since 2000. The bathymetric geometry and amplitude responses from the seismic data indicate a very active seepage of hydrocarbon, brine, and sediment that profoundly effect the geology and biology of the deepwater seafloor. The bathymetric geometry and amplitude response of the various seafloor features are controlled by pre-existing slope, the expulsion components, and the flux rate at the expulsion site.

Positive bathymetric features, such as mud volcanoes, occur where high flux rate seeps with significant sediment expulsion occur on relatively low slopes, whereas sediment flows occur on steep slopes. Mud volcanoes generally have low amplitude response on the seismic data due to lack of authigenic carbonate. Sediment flows can have either high or low amplitude depending on the composition of the flow and/or the presence of carbonate lithification of the flow. Brine seeps result in pools where they are expelled into closed depressions or brine flows down slopes. Low to moderate expulsion rate sites result in mounds often covered with authigenic carbonates (formed by chemosynthetic bacteria) which, if swept by bottom currents, will be exposed and become suitable substrate for chemosynthetic megafauna, corals, anoemones, etc. Low to moderate expulsion rates in pre-existing bathymetric depressions result in authigenic hardgrounds that do not get exposed and current swept. These are not suitable substrates for chemosynthetic megafauna due to soft sediment cover over the carbonate.

In much of the upper slope and parts of the lower slope, rapid and possibly explosive gas expulsion has occurred. In these areas, sediment has been removed, creating roughly circular depressions, or pockmarks. Many of these pockmarks show active vertical migration on seismic cross-sections, indicating continuing migration. Small (1-1.5 meter wide, >4 meter deep) “expulsion tubes” have been observed in the vicinity of pockmarks. The inner walls of these tubes were made up of interbedded carbonate and mud, and the seafloor in their vicinity was littered with carbonate rubble, presumably ejected from the tubes. No chemosynthetic megafauna or corals were observed near these tubes or in the pockmarks.

AAPG Search and Discovery Article #90090©2009 AAPG Annual Convention and Exhibition, Denver, Colorado, June 7-10, 2009