Abstract

Karst-related processes were of fundamental importance in the development of Lower Ordovician Ellenburger reservoirs of the Delaware and Val Verde Basins. Several levels of cave formation related to subaerial exposure during the post-Sauk unconformity characterize the Ellenburger Formation in West Texas. Two levels of cave formation were distinguished in the Humble Mills Mineral Trust NO. 1 core in the Val Verde basin and other cores in the Delaware and Midland basins (Hammes et al., 1996). The upper level is characterized by a distinct tripartite division of breccias which is characteristic for cave deposits (Kerans, 1990; Loucks and Hanford, 1992). (1) The fracture and mosaic breccias of the cave roof, (2) the sandy to conglomeratic cave fill, and (3) the chaotic breccia represent the break down and collapse of the cave. The lower level typically is composed of stacked fracture and chaotic breccias and does not display any clastic cave fill. The different breccia levels are separated by undisturbed Ellenburger facies which are composed of grainy and sandy shoal sequences at the bottom of the Ellenburger sequence, grading into subtidal dominated lithologies in the middle part, and intertidal dominated facies in the upper part. These undisturbed parts are interrupted by occasional thin brecciated intervals.

Both cave levels formed during the pre-Simpson exposure of the Ellenburger carbonate platform. During that time an extensive karst terrain developed creating a multiphase cave system (Figure 1). The onset of karstification at approximately 260 feet below the Ellenburger unconformity implies that phreatic processes contributed to cave formation. Caves within 10s of feet of the Ellenburger unconformity probably formed by agressive, vadose waters. Typically, passages of vadose origin are formed by gravitational flow and trend continuously downward along the steepest available openings (Palmer, 1991). Consequently, vadose processes commonly form vertical shafts, solution chimneys, and other vertical solution features (White, 1988). These solution features are indeed present in cores as brecciated intercalations between undisturbed Ellenburger (Figure 1). These thin, brecciated intervals occur at all depths and cannot be correlated. In addition, karst-related features close to the Ellenburger unconformity are probably of vadose origin. In contrast, phreatic passages originate along routes of greatest hydraulic efficiency acquiring a subcircular, elliptical cross-section or are elongated along fractures (Palmer, 1991). Phreatic and water table related processes formed most of the extensive cave system on the Ellenburger carbonate platform. During the long exposure of the platform a vast karst aquifer developed that dissolved limestone and dolomite as deep as 1000 feet below the unconformity (Figure 1). Typically, cave passages located close to the unconformity are near the recharge area of the aquifer. This is manifested by observations in cores in the northern part of the Ellenburger platform with karst breccias at or within 10s of feet of the unconformity (e.g., Pegasus field). In contrast, cave formation in wells in the southern part of the platform (e.g., Brown-Bassett field; Humble Mills core), does not begin until >200 feet below the unconformity. As inferred from modern analogs (e.g., Ford and Williams, 1989), infiltration of meteoric water occurs at the recharge zone by allogenic streams or across the karst platform through fissures and fractures (Figure 1). Dissolution of limestone and dolomite by underground streams and vadose processes created multiple cave levels in the Ellenburger as the result of a multiphase cave system that evolved with a shift in base level due to continental erosion (Figure 1). Both the upper and lower cave levels are therefore related to the pre-Simpson unconformity. Other causes of brecciation such as intraformational exposure surfaces, deep burial, tectonic, or mixing corrosion have been considered but not enough evidence is present to support any of these mechanisms. With increasing overburden the caves collapsed and the different breccia types developed (Figure 2). Due to extensive tectonic movement during Middle Ordovician, Late Mississippian to Early Pennsylvanian, and Late Pennsylvanian to Early Permian time, tectonic fracturing overprints can be found.