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ABSTRACT: The Mathematical Modeling of Compaction and Diagenesis in Sedimentary Basins

Previous HitFOWLERNext Hit, A. C., and D. MARC AUDET,* University of Oxford, Oxford, England

The gravitational compaction of shale in sedimentary basins is one of the important processes in forward basin modeling. The ability to model porosity versus depth profiles is crucial for predicting oil Previous HitmigrationTop patterns, abnormal pore pressure development, and kerogen maturation indices. We present a mathematical model for the porosity and temperature evolution in an accreting sediment layer. Diagenesis is modeled by considering the specific example of the dehydration of montmorillonite to illite. The governing equations that describe mass, momentum, and energy conservation are written in dimensionless form. Compaction is controlled by a dimensionless group, the sedimentation parameter, which represents the ratio of the pore fluld expulsion rate versus the rate of overburden accumul tion. A second dimensionless group determines the relative importance of diagenesis.

As a base case, we have studied the case of homogeneous sediments accumulating at a constant sedimentation rate. The results show that the porosity versus depth behavior is not a simple exponential and that for overpressured shales, the shape of the compaction curve is a sensitive function of time. A comparison with previous work shows that oversimplified models based on physically unrealistic assumptions give misleading predictions for the conditions leading to overpressuring. The onset of overpressuring is sensitive to the permeability and mechanical properties of the sediments and care must be taken in choosing realistic constitutive laws. By providing a theoretical basis for predicting sediment compaction curves as a function of time and stratigraphy, our results are immediately a plicable to large-scale basins analysis programs.

 

AAPG Search and Discovery Article #91012©1992 AAPG Annual Meeting, Calgary, Alberta, Canada, June 22-25, 1992 (2009)