Consolidation State Of Upper Continental Slope Sediments, Northern Gulf Of Mexico

Abstract

ABSTRACT The stability of the Upper Continental Slope sediments of the northern Gulf of Mexico was examined by estimating the degree of consolidation of the sediments. Geotechnical properties of 14 piston cores coupled with analysis of deposition rates provided the data base for the interpretations. Degree of consolidation was estimated by using two independent methods. The first method was based on a comparison of measured and empirically derived shear strength values, where the derived values were representative of expected strengths in normally consolidated sediment. Disagreement, therefore, signified a departure from normal consolidation. The second method permitted direct computation of degree of consolidation by using the sedimentation rates in conjunction with consolidation theory. Results from both methods indicate that in general the sediment on the upper slope is normally consolidated to slightly underconsolidated, although examples of moderate underconsolidation and overconsolidation may be found. These deviations are rare, thus most areas are considered essentially stable with respect to the surficial sediment. Local variability precludes extrapolation of this conclusion to sites not covered in this study. INTRODUCTION As the petroleum industry has breached the shelf break and moved onto the upper Continental Slope, its offshore technology has been challenged by a new set of problems. Some of these problems, such as the greater water depths and increased gradients, can be solved without specific environmental data; others, such as the prospect of large or even smallscale deformation cannot. The most basic difficulty lies within this second category, for design factors, such as efficiency, safety, and pollution prevention, cannot be handled effectively from an engineering standpoint unless the circumstances are defined. As yet, such definition is rarely possible on the slope. This unfortunate circumstance was never more center-stage than in the northern Gulf of Mexico, currently the site of 25% of all the world offshore petroleum activity, where the encroachment onto the upper slope (200–1000 m water depth) is already well in progress. Emplacing various structures and pipelines in this "frontier" environment requires detailed information, particularly of the geotechnical variety; thus, this move to deeper water must mark time while site-specific data are gathered and evaluations made. A further complication is that this upper slope, through its variety of depositional environments, may represent a spectrum of bottom-stability conditions. The extremes of this spectrum are erosional areas, which may be very stable, and areas subjected td high rates of deposition, such as within the Mississippi depocenter, which may be considerably unstable. This local variability also implies that the potential for mass movement or deformation is likewise inconstant. Our research, therefore, sought to evaluate sediment stability at selected sites on the upper slope of the northern gulf. This evaluation is based on one facet of the stability problem: stability as related to consolidation states of the sediment mass. Other aspects, such as tectonic activity or conventional slope stability analysis, are not included in paper.