Chalk: composition, diagenesis and physical properties

, Vol. 55, pp. 97–128. © 2007 by Geological Society of Denmark. ISSN0011–6297.Chalk is a sedimentary rock of unusually high homogeneity on the scale where physical propertiesare measured, but the properties fall in wide ranges. Chalk may thus be seen as the ideal startingpoint for a physical understanding of rocks in general. Properties as porosity, permeability, capil-lary entry pressure, and elastic moduli are consequences of primary sediment composition and ofsubsequent diagenetic history as caused by microbial action, burial stress, temperature, and porepressure. Porosity is a main determining factor for other properties. For a given porosity, the specif-ic surface of the sediment controls permeability and capillary entry pressure. As diagenesis progress-es, the specific surface is less and less due to the calcite component and more and more due to thefine-grained silicates, as a reflection of the coarsening and cementation of the calcite crystals. Theelastic moduli, which define sonic velocity, are for a given porosity mainly controlled be the degreeof pore-stiffening cementation, which may be quantified by effective medium modeling.Diagenetic processes include mechanical compaction, pore-stiffening cementation, dissolution,and pore-filling cementation. Processes involving clay, silica, and calcite are interlinked, but progressdifferently in different localities. This partly depends on primary sediment composition, includingorganic content, which may induce the formation of concretions by microbial action. The diageneticprocesses also depend on water depth, rate of burial, and over-pressuring. These factors cause thestress, temperature and pore-pressure to increase at different rates during burial in different local-ities.