Stability of Near-Surface Gassy Sediments, Amazon Continental Shelf
Authors: Segall M.P. , Figueiredo A.G. , Silva C.G. , Levey R. 
Published: October 2008

Gassy sediments in continental margin environments pose constraints for subsea installations because of their inherent instability under loads imposed by earthquake activity, excessive erosion and/or deposition, near-surface faulting or diapiric activity. Evaluation of processes that control sediment stability in near-surface soils are essential for determining their viability as foundation materials for seabottom cables, pipelines, concrete footings and oil-production facilities.

In modern continental shelf environments gassy sediments are encountered in rapidly-deposited deltaic environments and are concentrated beneath impermeable fine-grained deposits or trapped within thalwegs of relict (Pleistocene) channels. Highresolution shallow seismic reflection profiles depict several types of methane deposits. In each of these formations, gas may be released to the seafloor via adjacent porous sediments or through failure of adjacent or overlying strata. 1. Spatiallydiscontinuous methane deposits that are stratigraphically controlled may occur within several meters of the seabed. Continuous release of gas as bubbles through adjacent porous media results in a seabed that does not display evidence of deformational processes. 2. "Channel-bounded" deposits occur at depth within the seabed and may release a continuous supply of gas to the seafloor. Evidence of heave that can extend more than 300 m from the buried channel margins indicates that initial gas release resulted from failure in which the horizontal effective stress was greater than the vertical effective stress. 3. Subsurface "pockets" of gas frequently exceed 1-km diameter and 40-m thickness; they generally are buried more than 30 meters below the seafloor. These methane concentrations are spatially and temporally constrained and have no contact with the seafloor under ambient conditions. Sediments directly overlying the entrapped gas display evidence of heave and shallow rotational faults.

Shear strength profiles of the continental shelf in which gas is not continually released to the seafloor display intervals of normally-to overconsolidated strata overlying underconsolidated to normally-consolidated sediments that are methane enriched. These sediments are "stable" under current met-ocean conditions; however, changes in the local hydrodynamic regime through emplacement of subsea installations or loads imposed by offshore production facilities can result in the loss of soil support.

Introduction

Recent interest from the international community for hydrocarbon exploration in the mouth of the Amazon River and deep waters on the continental margin prompted concerns regarding seabed stability in this dynamic regime. Of particular interest are the accumulations of shallow gassy sediments on the continental shelf and the potential concentration of hydrates on the continental slope.

Shallow, gassy sediments are recorded in continental margin environments throughout the world. Some form overpressured horizons above deeper oil reservoirs such as the shallow shelf off Angola1 or occur as seeps associated with diapirs and faulted structures2. Others result from the active upward migration of fluids through sediments overlying gas hydrates; these areas usually contain focused fluid escape features associated with mud diapirs and mud volcanoes (i.e., Gulf of Mexico3 and the Niger Delta4).

Relatively recent organic-enriched intervals of modern sediments form in-situ gassy horizons through the action of interstitial microbes that use pore water sulfate as an oxidant:

Mathematical equation (1) (Available in full paper)