Although the process of dolomitization is not well known at present, the requirement of conservation of mass of the chemical constituents of the rock and the interstitial solution places definite limits on the process. Most subsurface waters contain relatively small concentrations of bicarbonate compared to their Mg and Ca contents. For these waters, the process of dolomitization must proceed by a mole-for-mole exchange of Ca by Mg without macrotransport of carbonate. This can be expressed by the chemical equation Mg (super 2+) +2CaCO 3 -->MgCa(CO 3 ) 2 +Ca (super 2+) . The porosity of the rock is not only affected by the 13% volume shrinkage on mole-for-mole dolomitization, but it also depends on the amount of compaction which takes place. If a lime mud is dolomitized, the mud will be compacted as the dolomite crystals grow, resulting in a decrease in porosity. Once the dolomite crystals form a space-supporting framework, the rate of compaction will decrease considerably. Replacement of the remaining mud will then lead to a marked increase in permeability and may also result in a porosity increase.