Physical properties of the shallow sediments in late Pleistocene formations, Ursa Basin, Gulf of Mexico, and their implications for generation and preservation of shallow overpressures

Understanding the evolution of abnormally high fluid pressures within sedimentary formations is critical for analysing hydrogeological processes and assessing drilling risks. We have constructed a two-dimensional basin model and have performed numerical simulations to increase the understanding of the history of fluid flow and shallow overpressures in the Pleistocene and Holocene formations in the Ursa basin, deepwater Gulf of Mexico. We measured physical properties of sediments, such as porosity and permeability, in the laboratory and estimated in situ pore pressures from preconsolidation pressures. We obtained porosity-effective stress relationships from measurements of bulk density, grain density and preconsolidation pressures in the laboratory. Porosity-effective stress relationships were also obtained from downhole density logs and measured pore pressures. The porosity-effective stress and porosity-permeability relationships obtained were applied in two-dimensional basin simulations. Results showed that high pore pressures developed shortly after sediment deposition. Peaks in pore pressure ratios were related to high sedimentation rates of mass transport deposits and the incision of the Ursa channel. Lateral flows from the area where the overburden is thick towards the area where it is thin have occurred at least since 30 ka. Present pore pressure and temperature distributions suggest that lateral flows play a role in re-distributing heat in the basin. (C) 2009 Elsevier Ltd. All rights reserved.