Integrated Geologic and Engineering Determination of Oil-Reserve-Growth Potential in Carbonate Reservoirs
Authors: Holtz M.H. , Ruppel S.C. , Hocott C. 
Published: December 2007

Summary
Leonardian restricted-platform carbonate reservoirs in the Permian Basin in west Texas and southeastern New Mexico exhibit abnormally low recovery efficiencies. Cumulative production from these mature reservoirs is only 18% of the original oil in place (OOIP), or about one-half the average recovery efficiency of Permian Basin carbonate reservoirs. Low recovery efficiency is directly related to high degrees of vertical and lateral facies heterogeneity caused by high-frequency, cyclic sedimentation in low-energy, carbonate platform environments and by equally complex postdepositional diagenesis. Because of their geologic complexity, these reservoirs have high reserve-growth potential. Although the 84 reservoirs in this play currently contain only 338 million STB of reserves, an additional 2.5 billion STB of mobile oil remains. With conventional, incremental well-spacing-reduction development strategies, these reservoirs have a current reserve-growth potential of 0.7 billion STB, or 27% of the remaining mobile oil. Advanced secondary recovery methods based on integrated geologic and engineering reservoir characterization, however, can facilitate recovery of the remaining 1.8 billion STB of mobile oil. Finally, a residual oil volume of 4 billion STB is a target for EOR achieved through miscible gas displacement.

Introduction
As pointed out by Fisher, future oil production within the Lower 48 states increasingly depends on reserve growth within existing reservoirs. The importance of such intrareservoir reserve growth is already apparent in Texas. In the last decade, 97% of all reserve additions in the west Texas Permian Basin came from existing reservoirs. Drew and Schuenemeyer demonstrated similar trends in reserve growth in the Gulf of Mexico Miocene and Pliocene fields.
Tyler et al. showed that reserve-growth potential relates directly to reservoir heterogeneity, which, in turn, is a function of depositional environment and diagenesis. Geologically similar reservoirs exhibit similar degrees of heterogeneity and, therefore, similar reserve-growth potential. The process of comparing, analyzing, and grouping geologically similar reservoirs into plays on the basis of depositional systems and diagenetic history facilitates calculation of reserve-growth potential.
Carbonate reservoirs are especially important to the study of reserve-growth potential in existing reservoirs. Carbonate reservoirs, as Arps showed, exhibit lower average recovery efficiencies than do siliciclastic reservoirs. In the Permian Basin, carbonate reservoirs currently exhibit an average recovery efficiency of about 35% . Reservoirs producing from rocks deposited in restricted-platform carbonate settings are associated with even lower efficiencies. Leonardian restricted-platform carbonate reservoirs, for example, exhibit average recovery efficiencies of only 18%, reflecting the high degree of heterogeneity in these rocks. Because of their low recovery efficiencies and high degree of heterogeneity, these reservoirs are ideal for studying reserve-growth potential. Accordingly, we have focused our study on Leonardian restricted-platform carbonate reservoirs of the Central Basin platform in west Texas and New Mexico because of the low recovery efficiencies in these reservoirs.