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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 1020‐1021 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, ~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 eYcient than uniform Darcyian fluid flow, 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. © 2000 Elsevier Science B.V. All rights reserved. |
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