Previous Talks

Wednesday, January 9th, 2008

Shallow Water Flow and Seafloor Failure in the Ursa Basin

Brandon Dugan - Rice University

Abstract

Multi-channel seismic, log, and core data in the Ursa region are used to identify mass transport complexes (MTCs) in a deepwater system with shallow overpressure. The seismic signature of the MTCs is low amplitude, non-continuous, internal reflections; however their basal reflections are continuous and have high amplitudes. From log and core data, I interpret that the basal reflections are controlled by the sharp contrast between the high density MTCs and low density underlying sediment. Increased density and decreased porosity within the MTCs corresponds to increased resistivity and visual observations of increased shear deformation. I model the resistivity-porosity behavior at three sites (IODP Expedition 308 Sites U1322, U1323, U1324) and conclude that the sediments in Ursa region follow one trend regardless of stress path (uniaxial versus shear deformation). I use the resistivity-porosity behavior to estimate how much consolidation occurs from uniaxial deformation during burial and how much occurs from shearing in MTCs. This model illustrates that shear deformation increases from west (U1324) to east (U1322). Consolidation during shear deformation also decreases permeability of mud-dominated MTCs. This permeability decrease affects the pore pressure in the basin.

Consolidation experiments on sediments above 30 meters below sea floor (mbsf) show overpressure at Sites U1322 and U1324 ranges from 25-50% of the hydrostatic effective stress. Direct pressure measurements document that by 200 mbsf, overpressure approaches 60% of the hydrostatic effective stress at Site U1324 and 80% of the hydrostatic effective stress at Site U1322. All pressure data indicate that overpressure at Site U1324 is less than that at Site U1322, which is inconsistent with higher sedimentation rates at Site U1324. Average sediment accumulation rates were 9.3 mm/yr (U1324) and 3.6 mm/yr (U1322). I use one-dimensional sedimentation-consolidation models based on measured properties to estimate the fluid sources and sinks required to simulate the observed overpressures. To simulate the observations, a fluid source of 2-7 mm/yr is required for Site U1322, whereas a fluid sink (10-12 mm/yr) is required for Site U1324. These one-dimensional results predict upward flow above 200 mbsf for both sites, upward flow at depth for Site U1322, but downward flow at depth for Site U1324. This flow scenario is consistent with a flow-focusing (or centroid) model where rapid, asymmetrical loading of a permeable sand body by a low permeability mud yields downward drainage where overburden is thickest (U1324) and then lateral fluid migration causes increased pressures where overburden is thin (U3122).

Speaker Biography

Brandon Dugan is an assistant professor of Earth Science at Rice University. His research focuses on basin-scale hydrogeology to understand overpressure development, slope stability, gas hydrates, and unconventional freshwater resources. Brandon has a bachelor’s degree in geological engineering from the University of Minnesota and a doctorate in geosciences from Penn State University. As a Mendenhall postdoctoral fellow at the USGS, Brandon studied natural gas hydrates and shallow water flows. Brandon started his appointment at Rice University in January 2005 where he works with undergraduate and graduate students studying marine geotechnical problems using field programs, laboratory experiments, and theory.