Abstract

The importance of fractures and breccias to the productivity of tight and otherwise unproductive formations has long been recognized in the oil industry; yet commercial hydrocarbon deposits associated with these phenomena have largely been discovered by accident. These facts neccessitated this study of Lower Ordovician Ellenburger carbonate reservoirs in West Texas, production from which depends entirely on intersecting networks of fractures, in the absence of intergranular porosity or permeability. The objective was to determine the causes of these fractures and the nature of the associated breccias, as well as to classify these breccias in such a way that they become of practical utility to all geologists.

The two classification schemes proposed are based on microscopic and macroscopic examination of over 1,829 meters of core, of outcrop samples, and of thin sections.

Investigations reveal the presence of numerous genetic types of breccia. However, over 95 percent of the fractures and breccias that contribute to Ellenburger production are tectonic in origin. Tectonic breccia fragments are sharply angular with zones randomly distributed, both regionally and within the same well.