James C. Matthews¹, Hans Martin Helset¹, Paul Reemst²

(1) Geologica, Stavanger, Norway
(2) Geologica

ABSTRACT: Shale Diagenesis Contributes to Fluid Overpressure: Sensitivity of Modeled Fluid Pressures to Permeability Reduction, Reaction Stochiometry and Chemical Compaction

Diagenesis is an important cause of porosity loss in sediments that contributes to overpressure generation. Diagenetic porosity loss (or chemical compaction) may result from two related processes; mineral dissolution under an applied stress and the precipitation of cements into pores. In addition to causing porosity loss, some mineral reactions release water into pore space. The effect of these diagenetic processes on fluid pressure depends on the relative rates of pore volume loss (due to mechanical and chemical compaction), fluid volume increase and fluid flow.

We use a fully coupled 1D pressure model to incorporate porosity loss resulting from both mechanical and chemical compaction in shale and sandstone lithologies. We model the illitization of smectite and kaolinite in shales. Clay mineral reaction progress is tied to shale permeability through a pseudo-surface area term in the Carmen-Kozeny relation. We assume that the reduction in solid volume due to the clay mineral reaction results in pore volume loss due to instantaneous compaction of the shale.

Many key assumptions related to modeling porosity loss in shales are poorly constrained and difficult to measure. Therefore, we have performed a number of sensitivity experiments in an attempt to understand the relative importance of various processes on porosity loss and fluid overpressure development in clastic rocks.

Case study results show that diagenetic porosity loss rates in shales are about one order of magnitude lower than the maximum porosity loss rates due to mechanical compaction alone. However, the diagenetic processes typically occur after sigificant mechanical compaction and their contribution to fluid overpressure is significant.

AAPG Search and Discovery Article #90906©2001 AAPG Annual Convention, Denver, Colorado