Depositional environment controls the architecture, heterogeneity, and ultimately the quality of oil reservoirs and is therefore one of the most important considerations in the development of any enhanced oil recovery program (EOR). Detailed characterization of the Pennsylvanian Bridgeport sandstone reservoirs in Lawrence Field, Illinois Basin, USA, has revealed juxtaposed deltaic and incised valley fill sediments that were deposited in the fluvial-tidal transition zone. Fluvial-tidal transition zone sediments are some of the most complex deposits known, but it has been shown through detailed facies analysis how two seemingly stratigraphically equivalent reservoirs can exhibit different reservoir properties depending on their position within the fluvial-tidal transition zone with important implications for operators considering EOR techniques, including chemical or carbon dioxide (CO2) EOR and geologic storage. The thinner, finer grained, and more compartmentalized Griggs sandstone was deposited in a low accommodation tidally influenced deltaic setting located in a seaward position within the fluvial-tidal transition zone. The thicker, coarser grained, and younger Robins sandstone was deposited as a fluvial system in an incised valley system that transitioned upward to estuarine conditions during transgression but remained in the landward portion of the fluvial-tidal transition zone. In the Robins sandstone, preserved primary intergranular porosity in the largely fluvial sandstone has resulted in high-quality, largely homogeneous reservoirs. Whereas in the Griggs sandstone, porosity and permeability are lower because of finer grained, more heterolithic deltaic reservoir facies that have had porosity and permeability reduced to a greater degree by diagenetic alteration. Only through detailed reservoir characterization were the differences in these reservoirs realized.