Consolidation Characteristics And Excess Pore Water Pressures Of Mississippi Delta Sediments

Abstract

ABSTRACT The majority of sediment instability problems in the Mississippi delta area of the Gulf of Mexico are related to weak, under consolidated sediments that are subjected to movement by both gravity forces and bottom pressures from storm waves. Thirty-five sediment samples from five borings drilled up to a depth of 50 meters sub bottom were taken within the South Pass OCS lease area Blocks 28, 47 and 48. The samples were tested to determine the state of consolidation and the amount of excess pore water pressures within distal shelf, prodelta and interdistributary shelf sediments of the Mississippi delta. Under consolidated sediments may exhibit excess pore water pressures approaching the effective overburden pressure, computed assuming hydrostatic conditions, resulting in a shear strength of the sediment that is equal to origin cohesion. Calculated excess pore pressures from consolidation testing compare favorably with direct measurements of pore pressures in OCS Block 28. The maximum calculated pore pressures were found to be 92 percent of the effective overburden pressure for sediments in Block 28. High sedimentation rates and low permeabilities were found to be the primary agents responsible for the high degree of under consolidation displayed by the majority of sediments tested. It was also determined that sub aqueous deformational processes may affect the consolidation characteristics and related geotechnical properties to depths in excess of 30 meters sub bottom. INTRODUCTION The constantly increasing demand for petroleum has resulted in continued development of oil and gas reserves on the outer continental shelf, Gulf of Mexico. This development has led to an increase in research related to the emplacement of structures on the sea floor which are compatible with active geologic processes. Understanding the processes associated with a particular depositional environment is fundamental to the understanding of the engineering factors related to sea floor sediment stability, specifically the assessment of potentially unstable areas and the forces related to mass movement. The determination of geotechnical properties in these regions may enhance the interpretation of sea floor processes as well as allow the engineer to better evaluate bottom instability and predict excess pore pressures associated with particular depositional environments. The interrelationship between geotechnical properties and sediment processes is particularly important in the South Pass region of the Mississippi delta complex where there occurs a wide spectrum of geologic processes and a variety of bottom stability conditions. The Mississippi delta is a dynamic region characterized by the interaction of riverine and marine processes and a large discharge of bedload and suspended sediment. Colemanl has described the environments of deposition associated with this dynamic region and found them to be extremely complex and variable. The primary sub environments encountered in this study are the distal shelf, prodelta and interdistributary shelf deposits. The distal shelf facies consists of thinly laminated and massive clays which are generally well burrowed and contain both macro- and micro-faunal remains along with diagenetic features.