Abstract: Multidisciplinary Study to Optimize Reservoir Management of the North Penwell (San Andres) Unit, Ector County, Texas

W. T. Siemers, D. R. Prezbindowski, V. L. Skinnider, L. C. Maple, M. G. Gerard, M. E. Nagaty, J. J. Howard

A reservoir characterization study of the North Penwell (San Andres) Unit was undertaken to optimize a waterflood expansion project. The Permian San Andres Formation in Penwell field is composed of dolostones deposited as repeated, shallowing-upward, peritidal, carbonate sequences on the eastern shelf of the Central Basin Platform, forming a reservoir with complex internal geometry. Late-diagenetic dissolution of anhydrite and dolomite, marked by the alteration of anhydrite to gypsum within the subtidal, peloidal/skeletal, grain-rich lithofacies, is the major contributor to good porosity development and determines the occurrence of improved reservoir quality. Ranges in reservoir quality are linked to variable aspect (pore/throat) ratios derived from a unique combination o diagenetically evolved polymodal pore-size distributions and unimodal throat-size distributions. The best quality reservoir rocks are dolomitized packstones and grainstones with a well-developed, solution-enhanced interparticle/grain-moldic pore system of intermediate pore size. The subtidal grain-rich intervals serve as major flow units within the reservoir because of their good lateral continuity and internally consistent flow characteristics.

A model based on an open-restricted-closed diagenetic system is proposed in which high-quality reservoir flow units grade into layers of anhydrite-cemented dolostones representing the hydrologically less active boundaries of the flow units. The flow units defined by the distribution of porosity and permeability identified from the petrologic study were utilized to construct a preliminary 3-dimensional geocellular model. A reservoir simulation pattern model was used to test the geological model by matching simulation results with field performance.

AAPG Search and Discovery Article #90957©1995 AAPG Mid-Continent Section Meeting, Tulsa, Oklahoma