Characterization of Diagenetically Altered Carbonate Reservoirs, South Cowden Grayburg Reservoir, West Texas (1996)

Abstract Much of the difficulty in constructing carbonate reservoir models for fluid-flow simulation results from diagenetic overprinting of depositional permeability patterns. In the South Cowden field, diagenetic effects result in (1) low porosity and permeability in the western and northern areas due to reduction of porosity by means of dolomitization and post-dolomitization compaction, (2) elimination of the petrophysical effects of depositional texture resulting from changes in particle size due to dolomitization, and (3) creation of a touching-vug pore system due to anhydrite dissolution. The extent of anhydrite alteration can be mapped to show three distinct diagenetic areas: those dominated by unaltered, altered, or dissolved anhydrite. Each alteration type has a unique acoustic-porosity transform that can be used to map the diagenetic areas and to calculate porosity when only acoustic logs are available. A single porosity-permeability transform characterizes the areas having unaltered and altered anhydrite, and the depositional stratigraphy is useful in constructing a reservoir model. A more favorable transform characterizes the area of dissolved anhydrite, and depositional stratigraphy is not useful in constructing a reservoir model because of the large effect of the diagenetic overprint. Introduction The construction of reservoir models for fluid-flow simulation of carbonate reservoirs is difficult because such reservoirs typically have complicated and unpredictable permeability patterns. Much of the difficulty results from diagenetic overprinting, which is represented by post-depositional changes that alter the depositional permeability patterns. The degree to which permeability distribution can be predicted is partly related to the degree to which permeability can be related to depositional patterns. Thus, it is easier to construct a model of a limestone reservoir that has undergone only cementation and compaction than it is to construct a model of a karsted reservoir that has undergone cavern formation and collapse as well as cementation and compaction. Cementation and compaction usually do not significantly modify depositional controls on permeability, whereas karsting processes create their own permeability patterns. The Permian-age carbonate-ramp reservoirs in the Permian Basin, West Texas and New Mexico, are typically anhydritic dolomitized limestone. Because the dolomitization occurred soon after deposition, depositional fabrics and patterns are often retained and a reservoir model can be constructed using depositional concepts. Recent studies of the San Andres outcrop in the Guadalupe Mountains and the Seminole San Andres reservoir in the Permian Basin illustrate how depositional fabrics and patterns can be used to construct a reservoir model. The South Cowden field, Ector County, Texas, is a Grayburg (Permian) anhydritic dolomite reservoir similar to many other Permian reservoirs in the Permian Basin. However, the diagenetic overprint has advanced so that, in some parts of the field, depositional patterns and fabrics can no longer be used to predict and model permeability. In this paper we describe this advanced diagenetic overprint and present methods of predicting and modeling its effects. Field Location and History The South Cowden field, located in Ector County, Texas, at a depth of 4,000 to 5,000 ft, produces from Grayburg dolomites and Queen sandstones. It is part of a complex of fields that includes the Johnson, North Foster, and South Foster fields. This study includes only a part of the South Cowden field: the Moss unit operated by Unocal, the Emmons unit operated by Fina, the South Cowden unit operated by Phillips, and the Colbert unit that was operated by Fina and is now operated by Aghorn. The study area covers about 14 sections (Fig. 1). P. 883