Compaction Processes from Shallow to Deep Burial: Future of Diagenetic Research

Knut Bjørlykke
University of Oslo, Norway

Sediment compaction involves mechanical compaction of grain frameworks and dissolution and precipitation of minerals resulting in increased density and higher velocities. Diagenetic processes are now in principle well understood. Mechanical compaction follows the rules of soil and rock mechanics and can be simulated in the laboratory. The problem for modelling and prediction is to assume the right initial grain size distribution, textures and mineralogy. Mechanical compaction and grain breakage change the intergranular volume and the grain surface properties. This will determine both the rate of quartz (or carbonate) cementation and the remaining porosity. The compressibility of carbonate grains is highly variable and ooids may be less compressible than quartz grains. Chemical compaction is driven by thermodynamics and kinetics and is mainly a function of temperature and time, particularly in siliceous sediments. Modelling these processes require accurate quantification of input parameters such as textures and mineralogy and also of the temperature history. Chemical compaction in carbonates includes pressure solution which is controlled by the effective stress. Some diagenetic processes are still poorly understood or need more quantification, but diagenetic modelling can in principle predict the porosity in deep reservoirs. The main problem is to predict the initial rock composition in terms of mineralogy, texture, temperature and also the pore pressure during burial prior to drilling wild cats. We must therefore focus more on the relationship between diagenesis and rock physical properties, particularly the seismic response. This will provide a better basis for inverting seismic attributes to rock properties like porosity.