COMPOSITE CLASSIFICATION OF FRACTURED AND BRECCIATED CARBONATE ROCKSB EXAMPLES FROM THE ORDOVICIAN ELLENBURGER GROUP, WEST TEXAS

Abstract

The attention paid to rock fracturing in recent years is a result of the discovery of large volumes of hydrocarbons in fractured rock reservoirs. This regional study was conducted in order to facilitate a better understanding of the performance of fractured reservoirs, and to select the proper procedures and well intervals for artificial fracturing.Five classification schemes are proposed for fracture, breccia, porosity and permeability developments, as revealed by mega‐ and microscopic examination of cores, thin‐sections and outcrop samples. A composite classification combines the attributes of the other schemes in an attempt to predict residual porosity and permeability on a reservoir scale. Various genetic types of breccia were encountered in this study, but those of tectonic origin predominate and control hydrocarbon production.At the initial time of fracturing, porosities range from low in crackle breccia, to very high in random and pheno‐breccias. With moderate cementation, random and pheno‐breccias remain the most porous and permeable structures, However, with a high degree of cementation, porosity and permeability become negligible in crackle breccia, and very low or absent in pheno‐breccia. The degree of cementation which controls residual porosity depends on the availability of mineral‐laden solutions.