Print this pagePredictions of Burial Diagenetic Reactions in Sandstones and Shales. Importance for Fluid Flow and Basin Modelling
BJORLYKKE, KNUT
Diagenetic reactions in shales are in principle very similar to those in sandstones. The main difference is the lower diffusion coefficients in the shales. If the same critical minerals are present the pore water will approach the same equilibrium in the sandstones and shales. The potential for mass transfer by diffusion between sandstones and shales is then very limited.
Examples from the North Sea and Haltenbanken, Offshore Norway, show that there is no evidence of mass transfer near the contact between sandstones and shales. Frequently there is a sharp contact between porous sandstones and shales without evidence of silica import.
During deeper burial the main diagenetic reactions in sandstones and shales are: (1) Formation of illite from kaolinite and K-feldspar and from smectite; (2) Dissolution and precipitation of quartz; (3) Precipitation of chlorite as coatings on quartz grains; and (4) Dissolution and precipitation of carbonate minerals.
Compaction and porosity reduction occurs at different rates in sandstones and shales and at 4 km burial depth the porosity and permeability in shales is very low reducing the potential for fluid flow. Calculations show that dehydration of clay minerals may add significantly to the pore water flux. Compaction of sediments below about 2 km depth is mostly controlled by the initial mineralogy and temperature and to a little extremely effective stresses.