Stratigraphic and Lithologic Zonation in the East Cowden Grayburg Unit, Ector County, Texas: Potential for Horizontal redevelopment of a Mature Waterflood?

By

Lee M. Petersen, Palo Pinto Exploration, Inc., Fort Worth, Texas and Robert S. Jacobs, Encore Operating, L.P., Fort Worth, Texas

Since its discovery in 1937 and subsequent initiation of water flooding in 1973, the E. Cowden Grayburg Unit (ECGU) has been one of the most prolific producers of oil from Permian-age carbonates on the Central Basin Platform. To date over 34 million barrels have been produced from this 720-acre unit and remaining proved reserves are currently estimated at 7 million barrels of oil.

Our detailed stratigraphic and lithologic interpretation results from core description and a modern logging program that pays particular attention to the effect of lithology on porosity. The identification of siltstones and the presence of sulfates have affected our understanding of the efficiency of the existing waterflood in this unit. We believe that the multiple deposition cycles and flooding surfaces allow for the possibility of concentrating water injection in zones of higher remaining oil saturation. The combination of vertical producing wells and horizontal (water injection) laterals drilled from existing vertical wells provides a viable way to recover oil in an economically responsible way. Samples, logs, pressures, and rotary sidewall cores in our initial five well vertical infill program have thus far confirmed our interpretation, and are likely to be followed by an initial horizontal re-entry program as we attempt to apply this modern drilling technique successfully to the Grayburg.

AAPG Search and Discovery Article #90010©2003 AAPG Southwest Section Meeting, Fort Worth, Texas, March 1-4, 2003