The quartz-rich sandstones of the Upper Cretaceous Vivian Formation constitute the most important reservoirs in the prolific Marañón foreland basin of northern Peru. The Vivian sandstones are largely fluvial in the northeast and transition to a marine shoreface and open shelf in the west. The formation consists of two sand-rich units (Lower and Upper Vivian) with better reservoir characteristics in the Lower Vivian. Porosity versus depth analysis for the dominantly fluvial Vivian sandstones shows a simple trend of decreasing porosity with depth, with facies-dependent variations for wave-reworked facies. This simple linear trend (correlation coefficient R2 > 0.77) indicates that overburden stress is the dominant factor that determines the porosity reduction. Further west, in the wedge-top Santiago Basin, the Vivian sandstones exhibit anomalously low porosities at relatively shallow depths, a distinct diversion from the regional trend. The depth difference between these low porosities and the porosities of the regional trend was used as a proxy for the uplift that the basin has suffered during the Andean deformation in the latest Miocene. The resulting values (up to approximately 2600 m [8530 ft]) are consistent with uplift estimates derived from other methods and suggest that parts of the foredeep have been partially uplifted.