Stratification testing in late 1977 proved that water coning is not operative through the Middle Ellenburger zone in the University Block 13 Ellenburger field in Andrews County, TX. The ensuing deepening and development drilling program has proved the existence of deeper reserves and has revised the plan of depletion for this field. Introduction The Ellenburger horizon in west Texas was deposited in a shallow marine environment nearly 500 million years ago during the early Ordovician period. The end of the Ellenburger deposition was marked by folding, faulting, and erosional activity. Extensive geological analysis of cores, drill cuttings, logs, and insoluble residue analysis data has shown that the Ellenburger may be divided into distinct upper, middle, and lower zones. Historically, most Ellenburger reservoirs were assumed to have an active bottomwater-drive producing mechanism, with vertical fractures through the thick carbonate being responsible for the vertical fluid movement. Consequently, numerous Ellenburger reservoirs were developed with a majority of the wells completed at or very near the top of the formation to prolong water-free production and minimize the water coning problem associated with an active bottomwater-drive mechanism.Since its discovery in 1960, the University Block 13 Ellenburger field, a west Texas carbonate reservoir found at a depth of approximately 11,000 ft (3353 m), was considered to have a bottomwater-drive mechanism; consequently, completions had been made only in the top of the Upper Ellenburger to minimize water coning as long as possible and to maximize recovery. When the four upstructure Ellenburger wells began producing water in 1977, approximately 8 years earlier than predicted by previous flood-out analysis, a testing program was initiated to determine whether aggravated coning through 140 to 240 ft (43 to 73 m) of oil column or edgewater encroachment through the Upper Ellenburger was responsible for early water production.Special testing, workover, and development drilling activity has resulted in a revised concept of the reservoir drive mechanism in this field. Stratification testing in late 1977 proved that water coning is not operative through the Middle Ellenburger in this reservoir and that the Ellenburger may be divided into upper, middle, and lower zones with restricted vertical communication between zones. To date, four of six deepenings and two development wells have been completed successfully at or near field top allowable of 285 BOPD (45 m3/d oil) per well in the Middle or Lower Ellenburger zones to recover reserves that may have been trapped under former completion practices.Various techniques have been used to evaluate zonation of the Ellenburger in this field. These geological techniques, along with various reservoir analyses, may aid in identifying potential deep zone reserves in other west Texas Ellenburger reservoirs. JPT P. 1452