Sequence- and Cycle-Scale Stratigraphy of the Leonardian: Lessons from Outcrops and the Subsurface

Stephen C. Ruppel

ABSTRACT

Although Leonardian carbonate platform rocks were originally endowed with a large share of the total oil resource in the Permian Basin, they have historically been very stingy in giving it up. By comparison with major Guadalupian (San Andres and Grayburg) reservoirs, Leonardian rocks (Glorieta, Clear Fork, Tubb, and Wichita/Abo) have yielded only about half as much of their OOIP. This is somewhat surprising given the similarities in structural setting, facies, cyclicity, and diagenesis. However, Leonardian reservoirs are more heterogeneous stratigraphically, are composed of more mud-dominated facies, and are more difficult to correlate using conventional tools. Collectively, these attributes pose significant obstacles to effective reservoir characterization and exploitation.

Integrated studies of subsurface Leonardian reservoir successions and their outcrop equivalents in the Sierra Diablo and Guadalupe Mountains of West Texas provide important insights into the critical elements of reservoir heterogeneity. These studies define thicknesses, facies-stacking patterns, styles of cyclicity, and geometries of cycle-scale and sequence-scale architectural elements. These data, in turn, provide crucial models that can be used to define the basic elements of heterogeneity, the stratigraphic framework, and the vertical and lateral distribution of rock fabrics and porosity in Leonardian reservoirs. They also reveal fundamental contrasts in facies-dependent interrelationships among porosity, permeability, and saturation. Application of these integrated models to Leonardian reservoirs provides a more robust basis for defining reservoir properties and continuity for purposes of flow unit modeling and for redesign and implementation of advanced recovery operations.

Bureau of Economic Geology, The University of Texas at Austin, University Station Box X, Austin, Texas 78713; e-mail: stephen.ruppel@beg.utexas.edu.