This paper describes a system for classifying porosity and fractures in reservoir rocks, as revealed by megascopic and microscopic examinations of cores and thin sections. In addition, there is information on mineral composition, texture, arrangement and distribution of cementing minerals, kind and distribution of alteration products, and other features revealed by the study of more than 450 cores.

The extensive use of the acidizing and fracturing processes to stimulate wells has resulted in development of numerous well-treating materials and techniques. Research to determine the most effective and economical methods of improving wells with chemicals has been under way for many years. One phase of this research program, begun in 1951, centered on a detailed study of cores and rock samples. This paper contains a report on some of this work.

The writers have devised a system of classifying natural porosity and fractures. It was found that the classification of porosity types could be based on vugs and fossils. In the study leading to the grouping of fracture types, the writers used a system based on orientation of the fractures and whether they were open, partly filled, filled, or closed.

Photomicrographs and bore-hole photographs are used to illustrate the various types of porosity and fractures. Examples are given to show how the classification systems can be used.