Abstract

Although Leonard Series rocks constitute the thickest succession of Permian carbonate platform deposits in the Permian Basin, they have contributed a proportionally small amount of the 15 billion barrels of oil produced from such deposits. This is the result of more complex variations in facies than typical of higher recovery-efficiency reservoirs like the Guadalupian San Andres and Grayburg formations. Complexity is also reflected in the myriad names applied to the Leonard throughout the basin. Despite these complexities, facies, rock fabrics, and reservoir quality display systematic relationships to accommodation, sequence stratigraphic setting, and, to a large extent, formation name.

Leonard rocks are best understood in terms of accommodation trends. Low accommodation carbonate facies comprise low continuity, mud-rich, peritidal rocks that accumulated during late HST to early TST and typically exhibit low permeability and oil saturation even though they may be highly porous. This is due to the dominance of separate vug and fine intercrystalline pores in these rock-fabric class 3 rocks. These “ugly” rocks include the Wichita, Tubb, and Glorieta formations and are commonly “bad” reservoirs except where diagenesis has intervened. Collectively, these rocks have contributed less than 20 percent of the total oil production from the Leonard.

High accommodation facies include grain-rich subtidal rocks that formed during maximum platform flooding and sealevel highstand. Although lateral facies variations are common, these rocks typically exhibit relatively high permeabilities and oil saturations that are a function of the larger particle sizes and better sorting typical of dominantly class 1 and 2 rock fabrics. These “good” Leonard rocks include the lower and upper Clear Fork and the Abo, and their equivalents. In most reservoir settings in the Permian Basin, lower Clear Fork rocks are by far the best of the “good” rocks, accounting for nearly half of all Leonard oil production.

Detailed resolution of Leonard facies and rock fabrics requires core study. However, some image and gamma ray logs can, if calibrated to cores, provide a critical first-level distinction between potentially good and bad reservoir rocks at the formation and, in some cases, high-frequency sequence scale. This distinction is fundamental to identifying facies and rock fabrics distribution patterns in Leonard platform carbonates.