Compaction and overpressuring in Pleistocene sediments on the Louisiana Shelf, Gulf of Mexico

Pleistocene shales on the Louisiana Shelf have accumulated at relatively high sedimentation rates (approximate to 1400 m/Ma), resulting in compaction disequilibrium and excess pore pressures. Mathematical models of gravitational compaction are useful tools for quantifying the rates at which various diagenetic processes affect porosity, effective stress and pore pressure. One of the major constraints in compaction modelling is characterising the mechanical compressibility of the sediments under one-dimensional loading. Soil mechanics theory provides a useful framework by quantifying sediment effective stress in terms of two mechanical parameters, both of which can be determined from uniaxial compression tests using a tri-axial apparatus. These mechanical parameters have been measured for natural and remoulded sediments, and many values are published in the geotechnical literature. If mechanical compaction is quantified accurately, then it is possible to distinguish and quantify other porosity reducing mechanisms in sedimentary basins. This paper re-examined published data from two wells in the Eugene Island area of the Louisiana Shelf in order to characterize the effective stress behaviour of shale sediments. Mechanical parameters were estimated and found to agree with values from consolidation tests on core samples recovered from nearby sites during Leg 96 of the Deep Sea Drilling Program. Using a continuum mechanics model for gravitational compaction, the estimated mechanical parameters were used to forward model the sudden increase in pore pressure at 2 km burial depth. The modelling results gave a lower bound on the effective permeability of the shallow, nearly normally pressured section, and an upper bound for the deeper, overpressured section. The effective permeability in the overpressured section has to be only about 5 times lower than that of the normally pressured section in order to account for the pressure contrast. Theoretical models for the effective permeability of sand-shale mixtures suggest that the permeability difference between the two sections may be due to variations in the shale fraction. (C) 1996 Elsevier Science Ltd.