Print this pageThe Formation of Overpressure and Nature of Pressure Transition Zones in Actively Subsiding Extensional Sedimentary Basins
MICHELSEN, JOHAN K. and KHAVARI KHORASANI, GANJAVAR
Disequilibrium compaction is generally held as the main cause for over-pressure generation in actively subsiding extensional basins. Present day basin models predict overpressure by such a mechanism, which have lead most authors to conclude that the models used, give a realistic picture of the dominant processes in such basins. Here, we take the view that disequilibrium compaction certainly is the key, but the physical formulations used in available models are so dissimilar to how compacting sediments probably behave, that results from available models are misleading. Present day models overlook existing works which strongly indicate that: (1) On a geological time scale, it cannot be justified to describe the sediment compaction with elastic rheological formulations. Sediments, behave closer to fluids, as dramatized by salt and shale diapirs. (2) The viscous forces between the rock matrix and pore water must be balanced tensorially. (3) Mineral dehydration reactions represent mass transfer from solids to pore water and neither are individually conserved.
More realistic physical formulations for sediment compaction explain geological "paradoxes" such as pressure transition zones cross cutting the entire range of lithologies, and pressure "compartments" unrelated to lithological boundaries. These effects originate in simulations as integral properties of the entire sediment package. The importance of understanding the dynamics of this behavior is emphasized with examples from deltaic settings.