Porosity in Carbonate
Carbonate Rocks exhibit a different type of Porosity as compared to the Clastic Rocks. The Porosity are divided into Primary and Secondary Porosity on the Basis of their time of Formation.
Type of Porosity
Pore types in carbonate rocks can generally be classified on the basis of the timing of porosity evolution into:
- primary pores (or depositional porosity), which are pores inherent in newly-deposited sediments and the particles that comprise them. Such pore types include interparticle pores in, for example, carbonate sands (but also in muddy carbonates), intraparticle pores (within particles such as foraminifera or gastropod shells), fenestral pores (formed by gas bubbles and sediment shrinkage in tidal-flat carbonates), and shelter and growth- framework pores (common in reef buildups);
- secondary pores, which are those that form as a result of later, generally post-depositional dissolution. Such pore types include all of those mentioned above, but only when it can be demonstrated that primary pores which subsequently were occluded by cement later had all or some of that cement dissolved (resulting in the generation of exhumed pores -Figure 2), as well as vugs (large pores that transect rock fabric, that is, dissolution was not fabric-selective) and dissolution-enlarged fractures. Most of these primary and secondary pore types can readily be identified in cores, and with the possible exception of shelter and growth-framework pores, also in well cuttings samples.
Types of Primary Porosity
Porosity between particles. It is seldom preserved because of porosity loss by cementation.
Porosity within the particles of grains
Intra crystal porosity
Porosity within the individual crystals.
Types of Secondary Porosity
Lensed shaped or globular sparry carbonate cement formed chiefly by the decay of sediment covered algal mats, shrinkage during drying and accumulation of pockets of gas or water.
It is a type of primary interparticle porosity created by the shattering effect of relatively large sedimentary particles which prevent in filling of pore space beneath them by finer clastic particle.
Growth framework porosity
Primary porosity created by the in place growth of a carbonate rock framework.
A pore formed by selective removal by solution from an original shell oolite etc
Porosity Generated due to fracturing in the rock strata.