Abstract

Karst and fracture-related processes were of fundamental importance in the development of reservoir heterogeneity in Ellenburger reservoirs of the Delaware and Val Verde Basins. Brecciation is associated with karst development below the pre-Simpson unconformity and extends up to 1000 ft below the top of the Ellenburger. Typically, two main levels of brecciation are recognized in >1000-ft-long cores from the Val Verde and Delaware Basins. The upper level is characterized by a tripartite subdivision of breccias related to cave formation: (1) fracture and mosaic breccias form the cave roof, (2) conglomerates and sandstones are related to cave fill, and (3) chaotic breccias are the product of cave collapse at the cave floor. The lower level exhibits fracture and chaotic breccias but contains no cave-fill sediments. Both the upper and lower levels of cave formation are interpreted to have developed during the pre-Simpson exposure of the Ellenburger carbonate platform, further contributing to reservoir heterogeneity. Historically, only the uppermost brecciated interval, just below the top of the Ellenburger, has been targeted for exploration. However, this study demonstrates that the lower zone is of equal importance as a hydrocarbon reservoir.

The Ellenburger caves and breccias formed as a result of a multiphase cave system that developed within the vast Ellenburger carbonate platform. Vadose processes contributed karst features close to the unconformity, whereas phreatic and water-table processes formed an extensive, multilevel karst system as deep as 1000 feet below the pre-Simpson unconformity. Therefore, karst features near the recharge area in the north of the Lower Ordovician platform appear very close to the unconformity, whereas karst features downdip to the south of the platform occur below 200-ft depth.