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A common assumption is that hydrocarbon charge homogenizes with the petroleum already in a trap: thus, compositional gradients reflect only subsequent segregation of the petroleum under the combined influences of gravity, temperature and diffusion. Since such homogenization would entail an unfavourable generation of potential energy, a more plausible hypothesis is that hydrocarbons stack into traps roughly in the density sequences in which they arrive. The commonly accepted model suggests that homogeneous and gravitationally unstable petroleum columns move towards graded equilibrium ones, whereas the hypothesis preferred here implies that they gradually diffuse towards equilibrium from the opposite direction, i.e. poorly mixed initial states.According to this hypothesis, trends of oil GOR and bubble point (Pb) are controlled by either (a) the charge GOR or (b) the evolving PT conditions at the GOC, depending on the gassiness of the charge. By contrast, API gravity trends mainly reflect the integrated maturity histories of source rock kitchens during the trap filling regardless of the charge gassiness. Because trap and kitchen histories are usually (very) different, a wide spectrum of API and GOR (and Pb) combinations is possible, from minor API gradients coupled with large GOR gradients and vice versa.