ABSTRACT: Porosity --Effective stress relationships during chemical diagenesis

Swarbrick, Richard E. , University of Durham, Durham, England

Compaction of sediments, i.e. volume loss expressed as porosity, is controlled by both mechanical and chemical processes. Mechanical compaction of mudrocks can be described well using porosity--effective stress relationships, developed for the range of lithologies depending on grain size distribution, if the sediments are at maximum burial and normally pressured. Where disequilibrium compaction is the origin of any excess pore pressure (overpressure) these relationships remain valid as sediments remain on the loading curve for normal compaction. However there are examples where sediments fall off the loading curve in the field of low effective stress (high overpressure) and low porosity. Explanations for this field of data have been in relation to unloading due to uplift or fluid expansion mechanisms for overpressure. Where unloading has occurred the porosity recovery is small and the sediments must have had a previously higher effective stress, normally associated with greater burial stress. However chemical processes, mainly associated with cementation, can also create low porosity at low effective stress and represent an alternative explanation. Where diagenesis controls porosity the porosity--effective stress relationships are no longer valid. Unfortunately the range of porosity enhancing and reducing processes involved in chemical diagenesis are difficult to quantify. Case studies are used to illustrate examples of young mudrock dominated sediments which have compacted mechanically, and older sediments in which chemical processes contribute to their porosity evolution.

AAPG Search and Discovery Article #90913©2000 AAPG International Conference and Exhibition, Bali, Indonesia