The San Andres Formation (Permian, Guadalupian) of the Permian basin is representative of carbonate ramp reservoirs in that it has highly stratified character, complex facies and permeability structure, and generally low recovery efficiencies of 30% of original oil in place. The approach used here to describe carbonate ramp reservoirs such as the San Andres Formation produces detailed reservoir models based on integration of sequence stratigraphic analysis, petrophysical quantification through definition of rock fabric flow units, and fluid flow simulation. Synthesis of these subdisciplines clarifies which aspects of the geologic-petrophysical model are most significant in predicting reservoir performance and ultimately in understanding the location of remaining oil satur tion.

The San Andres Formation crops out along the Algerita escarpment, a long, oblique-dip, continuous shelf-to-basin exposure in the central Guadalupe Mountains. These outcrops provide a unique opportunity to study lateral relationships in geologic and petrophysical structure analogous to those occurring between wells in subsurface reservoirs. On the basis of sequence stratigraphic analysis, three scales of cyclicity are recognized: depositional sequences, high-frequency sequences, and cycles. Examination of the cycles in two detailed window areas provides a practical scale for petrophysical quantification and fluid flow simulation. An understanding of cycle position within the high-frequency sequence framework also provides predictive information. Petrophysical analysis revealed six rock fabric groups dominated by intergranular, separate vug, or dense intercrystalline pore types. Comparison of these rock fabric groups with facies descriptions produced a rock fabric flow unit model that honors the geologic structure of the cycle and sequence framework. Permeability data were averaged within rock fabric flow units using a geometric mean approach based on fine-scale fluid flow modeling of deterministic and stochastically generated permeability fields.

Two-dimensional black oil fluid flow models illustrate that (1) major differences in sweep efficiency and fluid flow performance are predicted when linear interwell interpolations are compared with actual interwell-scale geologic structure as determined by outcrop geologic and petrophysical mapping, (2) an understanding of static geologic/petrophysical conditions provides only a partial understanding of reservoir performance defined by the interaction of these static properties and dynamic properties of fluid flow interaction within the flow unit architecture, and (3) because of the orderly distribution of high- and low-permeability facies within cycle stacks of high-frequency sequences, this larger scale of geologic description can give a reasonable first-order approximation of fluid flow patterns and early breakthrough.

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