Detailed sample studies are necessary to evaluate properly the porosity and permeability characteristics of carbonate reservoirs. The depositional porosity fabric and resultant permeability are varied in carbonates. These may range from highly porous impermeable, intergranular porosity present in carbonate banks, which may be composed of pellets, oolites, or admixtures of fragmental debris. The presence of fossil cavities, calcispheres, and a few reefoid deposits may modify the overall fabric. In addition to these depositional characteristics, tectonism can alter the basic porosity-permeability relations by means of fracturing, recrystallization, and (or) tectonic dolomitization.

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Examples are known where several of these various types of porosity are present in a single deposit and influence log analysis in either a negative or positive manner. A nonpermeable chalk or calcisphere porosity carrying high water saturation may produce oil if the fracture fabric or associated intergranular permeability is oil-bearing. Likewise the reverse may be true if nonproducible oil is trapped in the high porosity-low permeability deposits and the more permeable fracture and (or) intergranular porosity is water-bearing.

The presence of these varied porosity fabrics can be recognized in samples and a method of logging and estimating the importance of the different types is suggested. Utilization of well sample data coordinated with realistic log analysis can lead to successful completion in zones which might be overlooked in a cursory log analysis. Examples of these sedimentary types from the Mid-Continent area are discussed and illustrated.

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