The factors controlling the abundance and migration of heavy vs. light oils, as constrained by data from the Gulf of Suez. Part II. The significance of reservoir mass transport processes

Reservoir flow processes have a major impact on the oil quality and distribution of heavy vs. light oils, both within reservoirs and on the petroleum system scale. In the Gulf of Suez (GOS) gravitational segregation of black oil columns is a common phenomenon. An understanding of the mass transport processes in these reservoirs is important for evaluating their impact on the composition of migrating petroleum. 2D simulation results show that pressure diffusion is a too slow process to be the main cause for the vertical compositional gradients. A reservoir filling model, which is a refined version of the geological model of Gussow, linked to the composition of the GOS expelled petroleum fluids and their PVT behaviour and constrained by the GOS oil field data, is presented. This model gives viable explanations for (a) formation of gravitationally segregated oil columns, (b) tar mat occurrences and their affinity to low maturity, heavy oils and (c) wide spread occurrences of heavy oils and their relationship with light oils. In this model, continuous density overturning, by compositionally controlled bulk flow is the prime cause for the formation of gravitationally segregated columns, occurring when (a) the charge to the reservoir displays a maturity related decrease in density with time and (b) the reservoir volume, relative to the charge volume, is large enough for the reservoir to retain a wide maturity range. Lateral compositional variability in many reservoirs is today interpreted to image the initial filling of the reservoirs. It is demonstrated that such lateral gradients can form effectively from re-orientation of vertical gradients, via ordinary diffusion. These gradients have symmetry elements relative to the petroleum mass distribution in the reservoir. Fill and spill, from gravitationally segregated columns, enhance the efficiency of the migration of heavy oils.