The objective of this study is to analyze the effects of different modeling approaches and various scales of geological heterogeneity on water-flood recovery and volumetrics of an incised valley reservoir. Seismic, well-log, and core data are integrated with an incised valley facies model to create cross sections used to perform two-phase 2-D (two-dimensional) fluid-flow simulations, Core observations and probe-permeameter data are acquired to perform a geopseudo upscaling exercise, which simulates the effects of small-scale sedimentary structures on fluid flow. Applying this method and incorporating small-scale sedimentary structures in 2-D fluid flow simulations have proved to make a significant difference in individual-well oil recovery (up to 8%) depending on the facies types involved in a well＇s drainage area. Incorporating variations in sand-body dimensions and connectivities has proved to have a major impact on field oil recovery (30% difference between extreme 2-D cases), whereas variations in incised valley size have the greatest impact on original oil-in-place values (21% between extreme 2-D cases). A layercake model of an incised valley reservoir results in optimistic performance compared to a geopseudo upscaled model(11% higher oil recovery for a 2-D case). In a highly favorable scenario, an incised valley reservoir indeed may behave like a layercake, but it is more likely that it will not per form as well. Not taking into account small-scale sedimentary structures, uncertainty in reservoir architecture, and incised valley size in reservoir simulation studies can introduce substantial errors in reserves estimation and production forecasting. Lessons learned from this 2-D study will be used in a future full-field three-dimensional waterflood simulation of the Countess YY pool.