In most clastic rocks a relatively simple relation exists among producibility, porosity, and permeability, depending on the degree to which size and shape of framework particles influence these factors.

Carbonate rocks do not exhibit such a simple relation. In addition to porosity between framework particles, the particles themselves may be porous. Carbonate rocks are subject to leaching, replacement, and recrystallization to a vastly greater degree than are clastic rocks. During the course of diagenesis of carbonate rocks, type and degree of porosity and permeability may be so altered that they no longer offer a satisfactory measure of the producibility of the rock. Two carbonate rocks may have identical porosities and permeabilities, with one forming a good reservoir rock whereas the other is incapable of storing or producing oil or gas. A study of the pore geometry of a carbonate rock commonly is necessary to determine whether it is capable of producing hydrocarbons.

By relating previously devised systems of classification of carbonate-rock particles, grain-size, porosity, and texture to mercury capillary-pressure measurements, a petrophysical classification has been devised which classifies carbonate rocks by producibility. Families of capillary-pressure curves are related to families of carbonate-rock types. Once such a classification has been made for a carbonate rock in a given area, it is possible to predict the shape and amplitude of its capillary-pressure curve from a visual examination of the rock.

By relating rock characteristics to depositional environments, maps may be made that predict what the producibility of rocks in an area may be. Such maps can help reduce the number of dry holes drilled in areas where anticipated closure is less than that dictated by the pore geometry required for the rocks to produce hydrocarbons.

The pore geometry of dolomitized rocks differs greatly from that of limestone. Work done in the past has led to the conclusion that dolomitization creates and then destroys porosity and permeability. In the field, rocks commonly are found which appear to invalidate this conclusion. Pore-geometry studies indicate that the time during diagenesis at which dolomitization occurs and the original petrographic characteristics are the critical factors that determine whether dolomites will develop into reservoir rocks capable of hydrocarbon production.

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