Temperature-dependent viscoelastic compaction and compartmentalization in sedimentary basins

The near-surface compaction regime of most sedimentary basins is characteri zed 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, prec ipitation-dissolution processes lead to thermally activated Viscous deforma tion. 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 t he 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 ana lytical formulation of the compaction model provides a simple method of pre dicting both the depth at which permeability limits compaction, resulting i n top-seal formation, and the amount of fluid trapped beneath the top-seal. Fluid flow during hydraulically Limited compaction is unstable such that s edimentation rate perturbations or devolatilization cause nucleation of por osity 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 f luid pressure gradient and propagate through creation of secondary porosity in response to the mean stress gradient. The waves are a mechanism of epis odic fluid expulsion that can be significantly more efficient than uniform Darcyian fluid how, but upward wave propagation is constrained by the compa ction front so that the waves evolve into essentially static domains of hig h porosity following cessation of sedimentation. Yielding mechanisms do not appreciably alter the time and length scale of episodic fluid flow, becaus e fluid expulsion is ultimately controlled by compaction. The flow instabil ities inherent in viscous compaction are similar to, and a possible explana tion for, fluid compartments. (C) 2000 Elsevier Science B.V. All rights res erved.