Exponential approximations to compacted sediment porosity profiles

Sediment compaction and corresponding porosity variations can be modeled by a simple exponential with depth. The porosity solution is derived analytically as a complicated function of pore water pressure, but the underlying form is shown to approximate an exponential except near the surface where the behavior is linear. Even though the analytical simplifications ignore some of the detailed effects of sediment lithology, a large world-wide collection of porosity data from compacted shales, silts and sandstones supports the general exponential trend. For comparison, a numerical compaction routine (with fewer assumptions than the analytical solution) is used in conjunction with a previously developed process-based model of marine sedimentation. Together, the compaction and sedimentation models show that in a realistic depositional environment, porosity varies exponentially with depth, in agreement with both the empirical data and the analytical approximation. (C) 2001 Published by Elsevier Science Ltd.