In a reservoir fluid column, the composition of the reservoir fluid varies from one depth to another owing to gravity forces. This problem is known as compositional grading. In the case of the plus fractions, the gravity field promotes a change in their overall composition and also their average characterization parameters will differ from one depth to another. We present a continuous thermodynamic framework for compositional grading calculations in hydrocarbon reservoirs using an equation of state. The effect of the gravity field on the segregation characteristics of the (continuous) heavy fractions of the oil is established analytically using the method of moments. This allows the molecular weight distributions of the heavy fractions to be described as a function of depth for both the oil and gas regions of a reservoir fluid column. Such monitoring is important for the case of an extreme segregation of the heavy fractions. The validity of the proposed method is demonstrated for a reservoir-fluid column where measured data is available.