Mapping flow units in a heterogeneous carbonate reservoir : Reeves field, Yoakum county, Texas

Abstract

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 cyclic deposition and reservoir diagenesis. 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 comprised of mudstones, wackestones, packstones and grainstones, usually capped by tidal flat or supratidal facies. Although most of the parasequences are mud-dominated facies, those near the tops of individual parasequence sets tend to be grain-dominated. At least three parasequence sets bounded by major flooding or exposure surfaces are identified from stacking patterns. The San Andres strata at Reeves field have been affected by diagenetic processes that include dolomitization, leaching, and anhydrite emplacement. The interplay of these processes have resulted in significant modification of original depositional porosity and permeability. The principal reservoir at Reeves field occurs in the lower two parasequence sets that are correlatable over the entire field. Dominant pore types occur in association, and include intercrystalline, grain moldic and intergranular pores. Permeability varies more widely than porosity and it is generally linked with the extent of dolomitization. grain dissolution, and anhydrite pore-filling/replacement. There is a strong association between facies and porosity-permeability combinations. Highest values of porosity and permeability occur in the wackestones of thicker, subtidal parasequences within each set. Lower values are associated with the bases and tops of parasequences capped by intertidal/supratidal facies. The Reeves reservoir can be divided into seven flow units based on association of rockfabric, porosity-permeability combinations and pore types. Flow units are more layered and segregated in the western and northern half of the field than in the central and southern half of the field. Flow unit quality is related to combined depositional and diagenetic rock fabric in a parasequence, and the systematic stacking of parasequences that resulted in vertical stratification of flow units.