Abstract

Clear Fork Group reservoirs exhibit recovery efficiencies that are the lowest among carbonate reservoirs in the Permian Basin. It is widely assumed that the key factor contributing to these poor efficiencies is heterogeneity. However, the exact nature of this heterogeneity is poorly known. The superb outcrops of reservoir-equivalent Clear Fork rocks in the Sierra Diablo of West Texas offer an excellent opportunity to examine the stratigraphic and petrophysical heterogeneity that exists in these rocks and to develop improved models for interpreting subsurface reservoir successions.

The best outcrops of reservoir-equivalent Clear Fork strata are exposed in outcrops extending more than 2 mi (3.2 km) along the south wall of Apache Canyon. Lower Clear Fork rocks document two low-accommodation transgressive events: a lower symmetrical high-frequency sequence (HFS 2.1) composed of basal and capping tidal flats separated by intervening maximum flooding subtidal deposits. The upper sequence (HFS 2.2) contains a similar transgressive succession but no highstand regressive section because of forced regression at the end of lower Clear Fork Leonardian sequence 3 deposition.

The upper Clear Fork section comprises three HFS that document Leonardian sequence 3 transgression. The lower succession (HFS 3.1), whose base is made up of transgressive Tubb silt-rich carbonates, displays all of the major facies tracts and styles of cyclicity encountered in Clear Fork reservoirs: transgressive ramp, outer ramp, ramp crest, inner ramp, and tidal flat. The overlying sequence (HFS 3.2), which exhibits similar architecture, demonstrates important differences in cyclicity and facies development in higher accommodation settings.

Clear Fork sequence architecture and facies in Apache Canyon are consistent with data from subsurface Clear Fork reservoirs in the Permian Basin, indicating these outcrops are directly analogous. The concepts and interpretations derived from these excellent outcrop successions are fundamental tools that can be applied to subsurface reservoir successions to produce better models for more efficient exploitation of this important hydrocarbon play.