Fluvial-deltaic sandstone reservoirs in the United States are being abandoned at high rates, yet they still contain more than 34 billion barrels of unrecovered oil. The mature Oligocene-age fluvial-deltaic reservoirs of the Frio Formation along the Vicksburg Fault Zone in South Texas are typical of this class in that, after more than three decades of production, they still contain 61 percent of the original mobile oil in place, or 1.6 billion barrels. This resource represents a tremendous target for advanced reservoir characterization studies that integrate geological and engineering analysis to locate untapped and incompletely drained reservoir compartments isolated by stratigraphic heterogeneities. The D and E reservoir intervals of Rincon field, Starr County, South Texas, were selected for detailed study to demonstrate the ability of advanced characterization techniques to identify reservoir compartmentalization and locate specific infield reserve-growth opportunities. Reservoir architecture, determined through high-frequency genetic stratigraphy and facies analysis, was integrated with production history and facies-based petrophysical analysis of individual flow units to identify recompletion and geologically targeted infill drilling opportunities. Each of the two reservoir intervals selected approximates a fourth-order genetic unit and contains four to six higher frequency genetic units that act as individual flow units. Flow units progress from aggradational fluvial (upper delta plain) settings at the base of the E interval through retrogradational, then progradational channel and delta front bar settings at the top of the E and base of the D intervals. Uppermost D units represent a return to aggradational fluvial reservoirs. Greatest heterogeneities and lowest recovery efficiencies exist in the strongly retrogradational and weakly progradational units. Estimates of original oil in place versus cumulative production in D and E reservoirs suggest that potential reserve growth exceeds 4.5 million barrels. Comparison of reservoir architecture and the distribution of completions in each flow unit indicates a large number of reserve-growth opportunities. Potential reserves can be assigned to each opportunity by constructing an SojZlh map of remaining mobile oil, which is the difference between original oil in place and the volumes drained by past completions. The methodology demonstrated in this study has direct application to other reservoirs in Rincon field, other fields in the Frio Fluvial-Deltaic Play of South Texas, and mature reservoirs across the U.S. formed in analogous depositional settings.