The near-surface compaction regime of most sedimentary basins is
characterized by hydrostatic fluid pressures and is therefore determined
entirely by sediment matrix rheology. Within this regime, compaction is
initially well described by a pseudoelastic rheological model. With
increasing depth, precipitation-dissolution processes lead to thermally
activated Viscous deformation. The steady-state porosity profile of the
viscous regime is a function of two length scales; the viscous e-fold
length, related to the compaction activation energy; and a scale
determined by the remaining parameters of the sedimentary process.
Overpressure development is weakly dependent on the second scale for
activation energies >20 kJ/mol. Application of the steady-state model to
Pannonian basin shales and sandstones indicates a dominant role for
viscous compaction in these lithologies at porosities below 10 and 25%,
respectively. Activation energies and shear viscosities derived from the
profiles are 20-40 kJ/mol and 10(20)-10(21) Pa-s at 3 km depth. The
analytical formulation of the compaction model provides a simple method
of predicting both the depth at which permeability limits compaction,
resulting in top-seal formation, and the amount of fluid trapped beneath
the top-seal. Fluid flow during hydraulically Limited compaction is
unstable such that sedimentation rate perturbations or devolatilization
cause nucleation of porosity waves on the viscous e-fold length scale,
similar to 0.5-1.5 km. The porosity waves are characterized by fluid
overpressure with a hydrostatic fluid pressure gradient and propagate
through creation of secondary porosity in response to the mean stress
gradient. The waves are a mechanism of episodic fluid expulsion that can
be significantly more efficient than uniform Darcyian fluid how, but
upward wave propagation is constrained by the compaction front so that
the waves evolve into essentially static domains of high porosity
following cessation of sedimentation. Yielding mechanisms do not
appreciably alter the time and length scale of episodic fluid flow,
because fluid expulsion is ultimately controlled by compaction. The flow
instabilities inherent in viscous compaction are similar to, and a
possible explanation for, fluid compartments. (C) 2000 Elsevier Science
B.V. All rights reserved.