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Abstract

The Lower Ordovician Ellenburger Group of West Texas is a prolific oil and gas producer in the Permian Basin of West Texas. Regional analysis of depositional and diagenetic fabrics within the Ellenburger show reservoir facies to be dominated by a variety of breccia fabrics. A descriptive classification of Ellenburger breccias, including fracture, mosaic, clast-supported chaotic, siliciclastic-matrix-supported chaotic, and carbonate-matrix-supported chaotic types, allows simplified but genetically significant characterization of these highly varied breccia types. Although undoubtedly Ellenburger breccias are of diverse origins, vertical sequences from unbrecciated Ellenburger upward through chaotic, mosaic, and fracture breccia types in sub-Simpson Group Ellenburger reservoirs are best interpreted in terms of a karst model of cave formation, infill, and collapse. Roof and lower collapse portions of these sequences form the best reservoir intervals with siliciclastic-rich cave fill sediments commonly acting as baffles to fluid migration.

Core from the Gulf McElroy St. No. 1 well illustrates the characteristic succession of breccia fabrics used to develop the karst model. Karst facies recognized in the McElroy St. No. 1 core are lower collapse, cave-fill, and cave-roof, fracture breccias are not well developed in this core. Additional core material from producing zones in other wells is also displayed to illustrate pore types and breccia fabrics most commonly associated with producing intervals.

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