A very large monotonic variation of asphaltenes and viscosity has been measured by downhole fluid analysis (DFA) in crude oils in five-stacked sandstone reservoirs in the northern part of the Barmer Basin, northwest India, undergoing active biodegradation; each of the five-layered sand bodies shows overlaying fluid gradients with depth providing replicate validation of the measurements. Fluid data from four wells across the field shows that the gradients are uniform across the formation. The crude oil in the upper half of the oil column exhibits an equilibrium distribution of asphaltenes matching predictions of the Flory-Huggins-Zuo Equation of state (FHZ EoS) with the gravity term only using asphaltene nanoaggregates of the Yen-Mullins model. However, the bottom half of the reservoir reveals a large asphaltene gradient approximately three times larger than the equilibrium predictions from the FHZ EoS. This increase in asphaltenes creates a very large (8x) viscosity gradient and is a major production concern. In addition, these shallow reservoirs are undergoing active biodegradation at temperatures of 55-61 degrees C. A simple diffusive model coupled with the FHZ EoS is shown to account for the entire observed asphaltene distribution in each of the five sand layers. Alkanes (and some aromatics) are rapidly consumed at the oil-water contact at the base of the oil column. The rate-limiting step is the diffusion of these compounds to the oil-water contact. The loss of these oil components decreases the oil volume yielding an increase in asphaltene concentration and a significant increase in viscosity. The limited geologic time of the oil in the reservoir limits the vertical extent of the diffusive process accounting for the observation of asphaltene equilibrium at the top of the column. Specifically, petroleum system modeling of this basin indicates that the oil commenced undergoing biodegradation approximately 50 million years ago, and this duration matches the analysis using the diffusion model plus the FHZ-EoS. Gas chromatography applied to the oils from the top to the bottom of the oil column provides detailed compositional confirmation of the diffusive mechanism proposed. In particular, all measured compositional properties of the oil column are shown to be consistent with this simple diffusive model. The ability to account for asphaltene and viscosity variations in the five stacked sand layers with a simple diffusive model coupled with the FHZ EoS and the Yen-Mullins model provides a robust model for improving efficiency of reservoir engineering and oil production.