ABSTRACT: Styles of Deposition and Diagenesis in the Monahans Clear Fork Reservoir: Implications for Improved Characterization of Leonard Reservoirs on the Central Basin Platform

RUPPEL, STEPHEN C., Bureau of Economic Geology, University of Texas at Austin, Austin, TX

The Leonard Series (Lower Permian) of west Texas contains a substantial hydrocarbon resource; the original oil in place in these predominantly carbonate rocks totaled about 14.5 billion bbl. Recovery of this resource has proven difficult, however. Current recovery efficiencies average about 20%, far below the 35% average for other Permian basin carbonate reservoirs. Detailed characterization of the Leonard in the Monahans field (Ward and Winkler counties, Texas) illustrates that poor reservoir performance in these reservoirs is the result of extreme lithologic heterogeneity resulting from cyclic rise and fall of relative sea level. Patterns of both depositional and diagenetic facies are a function of this cyclicity.

Three orders of Cyclicity are apparent in the Leonard: high-frequency, fifth-order cycles averaging 1-2 m in thickness, fourth-order cycles averaging 15-20 m in thickness, and third-order cycles averaging 200 m in thickness. Depositional facies stacking patterns vary due to the interaction of all three orders of cyclicity. Diagenetic patterns reflect control by fourth-order and third-order cyclicity. Both depositional and diagenetic trends are modified by local topography.

Porosity and permeability also manifest cycle-related trends. In the upper and lower parts of third-order cycles (late highstand and transgressive sequences, respectively), porosity is encountered in both cycle-capping, tidal-flat deposits and in subtidal, grain-dominated rocks. In middle third-order cycle deposits (subtidally dominated, early highstand sequences), porosity is developed in subtidal grain-dominated rocks. Permeability is associated primarily with subtidal, grain-dominated rocks throughout the sequence. However, nonfacies specific permeability is locally developed below major fourth-order cycle tops due to diagenesis associated with sea level fall.

Porosity and permeability exhibit opposite relationships to paleotopography. Porosity, which is encountered in tidal-flat and subtidal facies, is greatest on paleotopographic highs, whereas permeability, which is most commonly developed in subtidal facies, is most common on paleotopographic lows.

Preliminary investigation of Leonard carbonate sequences elsewhere in the Permian basin reveals analogous styles and patterns of facies development. The concepts and models developed in the Monahans field should help improve characterization of these sequences as well.

AAPG Search and Discovery Article #91018©1992 AAPG Southwest Section Meeting, Midland, Texas, April 21-24, 1992 (2009)