Summary

We have developed a modular set of equations which allow the time of seal formation and the history of seal migration to be deduced from porosity-depth profiles. We apply these equations to interpret porosity profiles in the Western Mediterranean Sea and to 89 porosity profiles in the South Eugene Island area of the offshore Louisiana Gulf of Mexico.

The modular equations assume disequilibrium compaction predominates from a few hundred meters to ~5 km depth in sedimentary basins. At temperatures typical of this basin depth range, deformation by mechanical compaction and chemical compaction by pressure solution are poro-plastic. Compaction of sand-shale mixtures begins at an uncompacted porosity which is a non-linear function of the shale fraction. The modular equations describe porosity in a series of depth intervals in which the top of overpressure has either been “fixed” to a particular strata since sealing or has “migrated” so as to maintain a constant depth. Application of the equations to porosity profiles from the Gulf of Mexico basin and the Western Mediterranean Sea shows the following: (1) Fixed and migrating seal profiles are both commonly observed. (2) The profiles are spatially coherent. (3) Fluid overpressure can be predicted inside each compartments. (4) the effective stress inside growing faults bounding the compartments is always very low. The properties of both fixed and migrating seals are related to the presence of hydrocarbons and free gas in the pore space, and results from capillary pressure effects. Active faults form lateral boundaries between compartments, and provide preferred pathways for fluid flow along the fault planes. This suggests that sedimentary basins are composed of pressure compartments which exchange fluids in a predictable way. The model provide a new way to interpret porosity profiles in terms of fluid pressure evolution and transport of fluids.

AAPG Search and Discovery Article #90937©1998 AAPG Annual Convention and Exhibition, Salt Lake City, Utah