Abstract

This paper critically examines the link between diagenesis and overpressure, how diagenetic reactions may produce overpressure, and conversely, how overpressure may influence diagenetic reactions. These questions are important, because they may hold a key to predicting overpressure and reservoir porosity in sedimentary basins. Our results are summarised below:

Effect of diagenetic reactions on overpressure

Smectite dehydration is unlikely to be a primary cause of overpressuring in sedimentary basins, because the volume of fluid released is small, and dehydration is inhibited by the build up of pressure. In addition, the exact reaction involved in the smectite to illite transition is not presently known. Thus it is not certain that there is an actual volume increase during the transformation. Quartz cementation is also unlikely to be a direct cause of overpressuring or underpressuring, because extensive cementation/dissolution requires an open system in which fluid is free to move and dissipate abnormal pressures. By contrast, the transformation of gypsum to anhydrite can potentially generate a fluid pressure significantly in excess of overburden pressure, depending on the rock permeability. The existence of basin-wide diagenetic pressure seals remains unsupported by direct evidence.

Effect of overpressure on diagenetic reactions

Overpressure inhibits pressure solution, and retards the development of late diagenetic quartz overgrowths, helping to preserve high porosities in reservoir sandstones during deep burial. The rate of quartz growth is fastest under conditions of high effective stress.