A numerical compaction model of overpressuring in shales

Abstract

A one-dimensional, numerical model of sediment compaction has been developed using porosity, velocity of sediment particles, and depth of the evolving basin as master variables. The governing set of nonlinear, partial differential equations are solved by a finite difference scheme devised to be stable for calculations involving tens of millions years and depths up to 4 km. Input parameters include a sedimentation function and a permeability-porosity function representative of the modeled sediment. Additional terms can be incorporated to mimic the effect of fluid volume generated by dehydration from clay mineral transformations and by temperature and pressure variations. Evolution of pressure, porosity, permeability, and fluid and sediment particle velocities are documented in a vertical sediment column as well as properties of a sedimentary package being successively buried.

Although this model has many potential applications, it is used here to demonstrate that the major cause of overpressuring in sediments accumulating along passive margins is nonequilibrium compaction. In general, smectite dehydration and aquathermal pressuring play minor roles in the development and sustenance of overpressuring. Comparison of model cases and Gulf Coast overpressured cases shows that sedimentation rates and strata permeability are the most important geologic factors in the formation of overpressured zones.