ABSTRACT
Petroleum reservoirs show variations in pressure, temperature, and composition with depth. This is illustrated in Figure 14.1, which schematically shows pressure, temperature, and C
mole fraction at three different depths. Pressure and temperature increase with depth. The concentration of lighter components decreases with depth, whereas the concentration of heavier components increases. Hirschberg (1988) reports as an extreme manifestation of compositional grading the presence of a tar mat essentially consisting of asphaltenes (Chapter 12) at the bottom of a reservoir. Also, there is evidence of large compositional variations for reservoir fluids close to the saturation point (Fujisawa et al., 2004).
