Depositional and Diagenetic Porosity, Reeves Field, San Andres Formation (Permian), Yoakum County, Texas: Field-Scale Reservoir Characterization in a Sequence Stratigraphic Framework

Ketema Amare, W. M. Ahr

The Permian San Andres Formation of West Texas and New Mexico is a major hydrocarbon reservoir comprised mainly of anhydritic dolostones that reflect a history of cyclical deposition and diagenesis. Field-scale study of reservoir properties in a sequence stratigraphic context indicates that parasequences and their stacking patterns hold the key to understanding lateral and vertical variations in reservoir quality. San Andres strata at Reeves Field consist of shallowing-upward sequences of subtidal, intertidal, and supratidal facies stacked systematically to form parasequences and parasequence sets. Parasequences are shallowing upward peritidal cycles usually capped by tidal flat or supratidal facies. At least three parasequence sets are bounded by major flooding or exposu e surfaces that are identified from stacking patterns. Both the parasequences and parasequence sets thin upward. Although most of the parasequences consist of mud-dominated facies, those near the tops of individual parasequence sets tend to be grain-dominated. The principal reservoir at Reeves Field occurs in the lower 2 parasequence sets, which are correlatable over the entire field. Highest values for porosity and permeability occur in the thicker, subtidal parasequences within each set. Most pore characteristics attributable to depositional processes have been extensively altered or obliterated by dolomitization, leaching, or anhydrite replacement Dominant pore types are moldic (grin and crystal molds) and intercrystalline (between dolomite crystals) and they are limited to mud-domina ed facies. Permeability varies more widely than porosity and it is generally linked with the extent of dolomitization, grain-dissolution, and anhydrite pore-filling/replacement. Abundance of moldic porosity corresponds with parasequence geometry, thickness, and original grain/mud ratio. Abundance and effectiveness of intercrystalline porosity corresponds with presence/absence and size of compromise crystal boundaries in mud-dominated facies.

AAPG Search and Discovery Article #91020©1995 AAPG Annual Convention, Houston, Texas, May 5-8, 1995