3-D Stratigraphic Interpretation of Quaternary Mass-Transport Deposits in the Mensa and Thunder Horse Intraslope Basins, Mississippi Canyon, Northern Deep Gulf of Mexico

Jorge Diaz¹, Paul Weimer², and Geoff Dorn^2
1 PDVSA, Puerto La Cruz, Venezuela
² University of Colorado, Boulder, CO

The stratigraphic evolution of the Quaternary sediments in the Mensa and Thunder Horse intraslope basins, Mississippi Canyon, northern deep Gulf of Mexico, was interpreted based on based on 378 square miles 3-D seismic data. Water depths range from 5300 to 6500 feet. These shallow deposits pose shallow drilling hazards and illustrate the kinds of potential problems for drillers to avoid in deepwater.

Seven sequences were defined in the study area between 1.3 Ma to Present. Allochthonous salt systems had bathymetric expression and influenced the sediment thickness and the location of depositional systems. Six discrete seismic and geologic facies are present: channel-fill, overbank, deformed overbank, slides, mass-transport deposits, and hemi-pelagic sediments.

The three oldest sequences are characterized by channel-fill and overbank deposits. The four youngest sequences are characterized by extensive mass-transport deposits, overbank, and hemipelagic sediments. Mass-transports complexes overlie erosional boundaries-- up to 30 m of the underlying section has been eroded at the base of the deposits. These deposits consist primarily of chaotic, rotated, and thrusted seismic reflections with poor continuity. They vary in size and areal distribution from elongated to more equidimensional. Multiple sets of deposits have channelized into and stacked on one another. These deposits are interpreted to have been sourced primarily from the west, similar to the channels in the underlying sequences.

Potential drilling hazards occur primarily in (1) mass-transport deposits which cause slow drilling times, and possible pipeline ruptures associated with differential subsidence, and (2) channel-fill sediments, which can cause shallow flow problems.