Geological Controls on Reservoir Development in a Leonardian (Lower Permian) Carbonate Platform Reservoir, Monahans Field, West

This report combines borehole core and geophysical log study with porosity and permeability data to define patterns of facies and permeability distribution in a typical upper Clear Fork reservoir in the Permian Basin. Key findings are that permeability, but not porosity, varies markedly with facies and that spectral gamma ray logs are critical for defining facies, facies architecture, and reservoir quality. Compared with similar reservoirs in younger (Guadalupian) San Andres and Grayburg Formations, reservoirs that developed in the Lower Permian (Leonardian) Clear Fork Group are typified by very low recovery efficiencies. Geological characterization of the Clear Fork reservoir at Monahans field demonstrates that although this situation may result partly from the innate heterogeneity of Clear Fork rocks, it can also result significantly from an overestimation of the mobile oil resource in these fields. The Clear Fork reservoir at Monahans comprises the upper Clear Fork, Glorieta, and lower San Andres, all of which are characterized by cyclic, multiple-hierarchy, shallow-water, dolomitized carbonates. The cyclicity of these rocks is an important guide to their architecture and correlation. Reservoir development in these rocks reflects the combined influence of depositional and diagenetic controls. Although strong evidence points to selective porosity preservation caused by early diagenesis at cycle tops, petrophysical differences are nevertheless dominantly related to depositional facies. Tidal-flat deposits, for example, display high porosity but widely variable and generally low permeabilities and oil saturations. Subtidal grain-dominated rocks, on the other hand, exhibit similarly high porosity but much more favorable permeabilities and saturations. Distinction between these two facies is thus critical to accurate calculation of permeability and oil saturation and, in turn, to effective recovery of the remaining oil resource in Monahans and similar fields. Although most wireline logs cannot be used to distinguish critical facies or define cyclicity, spectral gamma-ray logs, when calibrated to cores, provide an accurate picture of the distribution of poor-reservoir-quality, tidal-flat rocks and good-quality, subtidal rocks. At the same time they provide a fundamental basis for defining reservoir architecture by defining cycle boundaries. Both are necessary to develop an accurate picture of reservoir heterogeneity in Clear Fork reservoirs.