Print this pageApplication of Modern Concepts of Carbonate Sedimentology and Petrophysics to Characterization of Remaining Oil Resource: South Cowden Field, West Texas
RUPPEL, STEPHEN C., and F. JERRY LUCIA
Recovery of the large remaining oil resource in mature shallow-water-platform carbonate reservoirs demands a harmonious marriage of cutting-edge concepts of geological and petrophysical characterization. The offspring of a successful union are improved models of permeability and oil saturation as well as specific scenarios for efficient exploitation. We used such an approach successfully to develop a reservoir model and to define infill drilling targets in South Cowden field.
Cycle stratigraphic analysis of the Grayburg reveals substantial, yet systematic, variations in depositional facies across the field caused by at least three hierarchies of relative sea-level rise and fall: high-frequency cycles, four high-frequency sequences, and one composite sequence. High-frequency cycles provide the fundamental control on original depositional heterogeneity and afford a rigorous basis for high-resolution intrareservoir correlation. Late episodes of carbonate and sulfate diagenesis, which are largely unrelated to the cycle stratigraphic framework, however, impart an even stronger control on reservoir heterogeneity and locally obliterate primary controls. Dolomitization has eliminated the petrophysical distinction between most depositional textures. Sulfate alteration increases porosity but does not alter porosity-permeability relationships. Total dissolution of sulfate, however, creates a touching-vug pore geometry dramatically improving permeability.
Although these relationships are readily apparent in cores, accurate permeability modeling requires using wireline logs to recognize rock fabrics, importantly the touching-vug fabric. Acoustic-porosity crossplots provide a powerful technique for distinguishing unaltered, altered, and dissolution sulfate fabrics. Saturation calculations show that waterflood response is cycle specific and that low-permeability cycles, as well as unperforated permeable cycles, are bypassed and are infill drilling targets.