The Frio Fluvial-Deltaic Sandstone (Vicksburg Fault Zone) oil play of South Texas has produced nearly 1 billion barrels (Bbbl)of oil, yet it still contains about 1.6 Bbbl of unrecovered mobile oil and nearly the same amount of residual oil resources. More than half of the reservoirs in this depositionally complex play have been abandoned, and large volumes of oil may remain unproduced unless advanced characterization techniques are applied to define untapped, incompletely drained and new pool reservoirs as suitable targets for near-term recovery. Interwell-scale geological facies models of Frio fluvial-deltaic reservoirs are being combined with engineering assessments and geophysical evaluations in order to characterize reservoir architecture and flow-unit boundaries and to determine the controls that these characteristics exert on the location and volume of unrecovered mobile and residual oil. These results will help identify specific opportunities to exploit these heterogeneous reservoirs for incremental recovery by completion and strategic infill drilling.