Seismic Geomorphology and Overpressure Variation in the Shallow Water Flow (SWF) Prone Sand Units in the North-Central Gulf of Mexico

Abstract

The north-central Gulf of Mexico received rapid deposition of a basin floor fan system consisting of thick interbedded muds, silts, and sand turbidite deposits during the late Pleistocene. The rapid burial rates exceeded the dewatering rates of sediment pore fluid resulting in overpressure at very shallow depths below the mud line. Two stratigraphic sequences contain significant overpressure and shallow water flow incidents from these units, which often correlates with a rapid increase in pore pressure. The area is influenced with salt uplifts forming deep basins and tall ridges. The geomorphology and overpressure variation of these units were identified using publicly available 3D seismic, 2D seismic, and well data. The lateral extent, thickness, depth below sea level, and overpressure of the previously known ‘Blue Unit’ was revised, which extends from the northern portion of the Mississippi Canyon (MC) protraction area to as far south as the Atwater Valley (AT) protraction area. The underlying ‘Green Unit’ lateral extent, thickness, depth below sea level, and pore pressure has been mapped for the first time during this study. This unit was found to extend further south than the ‘Blue Unit’ into the AT protraction area. The tops of both units are highly incised by post-depositional erosional systems; however, the base of each unit is well preserved. The top of the ‘Blue Unit’ below the seafloor varies from < 70 m in the north to as deep as 700 m in the south, while the top of the ‘Green Unit’ is as shallow as 300 m in the north to > 900 m in the south. The pore pressure gradient generally increases from hydrostatic to a maximum of 0.52 psi/ft (11.76 kpa/m) below the top of the ‘Blue Unit’ to the base of the ‘Green Unit’.

AAPG Datapages/Search and Discovery Article #90291 ©2017 AAPG Annual Convention and Exhibition, Houston, Texas, April 2-5, 2017