We present numerical and experimental investigation on the existence of diffusion-induced compositional variations in a reservoir fluid in the absence of convective mixing. New pressure-decay experimental data are presented for ethane in contact with stock tank oil, and the values of diffusivity and solubility parameters are obtained by using the one-dimensional transient diffusion model and integral-based regression method. We show the variation in oil composition through composition-height measurements at the end of the experiment. Finally, the diffusion model with estimated parameters is used to predict the compositional variations, where comparisons show excellent agreement with the measurements, validating the parameter estimation technique and quantifying the compositional variations in the oil phase caused by diffusion.