The origin of near-seafloor 'crust zones' in deepwater

摘要

In recent years, geotechnical site investigations for deepwater oil and gas developments have disclosed the presence of a thin "crust zone" of anomalous high strength immediately below the seabed in several deepwater regions around the world. The origin of the crust zone, however, has not been satisfactorily explained with conventional soil mechanics. The sediments within the crust do not have physical property values or definable trends in water content, density, plasticity and grain size distribution that explain why the crust exists. Previous marine biological studies have shown that biological activity or bioturbation affects geotechnical properties including soil shear strength. A variety of sedimentological and geochemical tests together with geotechnical tests were performed on cores recovered at two deepwater sites offshore Nigeria where a crust is present. This paper presents the test results that suggest that the "crust zone" is both a product of intense bioturbation and early geochemical alteration of burrow walls and pelletized burrow fills. The paper begins with a literature review on the effects of biological activity on sediment physical properties.