Sediment Compaction and Rock Properties

Knut Bjorlykke and Jens Jahren
Geoscience, University of Oslo, Oslo, Norway

Compaction of sediments is driven towards lower porosities and higher densities as a function of increasing stress and chemical reactions. Mechanical processes are controlled by the effective stress and chemical compaction by dissolution and precipitation of solids. Chemical compaction is a function of thermodynamics and kinetics and silicate reactions are very slow and sensitive to temperature. Chemical compaction of siliceous sandstones is modelled based on the assumption that the precipitation of quartz and other cements are the rate limiting steps. This is therefore a function of temperature and nearly independent of the stress.

Quartz cementation is probably also important in the compaction of siliceous mudstones. The kinetics of carbonate precipitation is much faster and less dependent on temperature and probably more dependent on effective stress (pressure solution) and stylolite formation. Carbonate compaction is however poorly understood and the relation between stress and both mechanical and chemical compaction is difficult to model. Experimental compaction of artificial and natural samples provides valuable constraints on the mechanical compaction processes. The compressibility and the velocity vary considerably with changes in primary sediment composition, particularly in mudstones. Modelling of mechanical and chemical compaction requires detailed input about mineralogy and textural relations which are difficult to predict prior to drilling. We have therefore used well data and mineralogical analyses from the North Sea basin to establish compaction and velocity trends for different lithologies and burial histories. This provides a basis for prediction of porosity and density for basin modelling and velocity distributions for seismic modelling and interpretations.

AAPG International Conference and Exhibition, Cape Town, South Africa 2008 © AAPG Search and Discovery