Detailed sample studies are necessary to evaluate the porosity and permeability characteristics of carbonate reservoirs. Depositional porosity fabrics and the resultant permeability are varied, and may range from highly porous impermeable chalk to less porous but more permeable intergranular porosity in carbonate banks, which may be composed of pellets, oolites, or admixtures of fragmental debris. The presence of fossil cavities, calcispheres, and 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.

Several of these types of porosity may be 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. Conversely, nonproducible oil may be trapped in the high-porosity-low-permeability deposits, and the more permeable fracture or intergranular porosity may be water bearing.

These varied porosity fabrics can be recognized by sample and core examination, and a method of qualitative and quantitative logging 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.