Porosity in Carbonates

Abstract

Porosity in carbonate rocks, most commonly limestones and dolostones, is of great importance to study since around half of world’s hydrocarbon reserves are made up of dolomite and limestone, which formed mostly in a shallow marine environment and usually close to where such sediments originate from the source rocks. Carbonates possess both primary and secondary porosities, which reduces with progressive burial leading to increasing rigidity of the rock. Several classifications of carbonate rocks are available. These are based on texture, depositional environments (the three kinds of carbonate factories), energy of the depositional environment, mud to grain ratio (volume-wise), grain to micrite ratio, porosity-permeability parameters, depositional-, diagenetic- and biological issues etc. Out of them, those by Folk and Dunham have been entered most of the text books on sedimentology. Carbonates more commonly display dissolution, cementation, recrystallization and grain replacement than the siliciclastic deposits. The porosity-permeability relation in carbonates may or may not be linear. Several schemes of classification of porosity of carbonates are available. Archi’s scheme (based on qualitative evaluation of texture and porosity), the Choquette-Pray scheme (utilizes depositional and diagenetic changes in the rock), the Lucia scheme (works on inter-relationship between porosity, permeability and the particle size) etc.